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INTRA-OPERATIVE COMPLICATIONS

Author: Helen M. Manson, MB ChB MRCGP (UK)

PRONE POSITIONING OF PATIENTS ON OPERATING ROOM

ABSTRACT: This paper summarizes the complications related to positioning devices used to support patients during prone surgery. The surgical patient's position must allow the surgeon optimal access to the operative site, but it can unduly influence the patient's recovery time, morbidity and mortality. The types of complications associated with the prone position include: cardiopulmonary, central nervous system, pressure, ophthalmological, and oropharyngeal. These complications can have serious consequences for the patient, leading to pain, disability, paralysis, visual loss, or even death. The methods used to support the prone patient influences the occurrence and severity of complications. The design and ergonomics of the positioning equipment, in conjunction with staff education on proper techniques for positioning the patient, is vitally important.

"The successful outcome of surgery on the thoracic and lumbar spine is largely dependent on the proper positioning of the patient before the operation begins" (Iqbal Singh, M.D, The Prone Position: Surgical Aspects, 19871). This fundamental premise is important: careful positioning not only allows the surgeon optimal access to the operative site, but also influences the patient's recovery time, morbidity and mortality. Numerous commentators have described the range of complications resulting from improper prone positioning.1,2,3,4,5 Sengupta and Herkowitz write in Complications of Pediatric and Adult Spinal Surgery (2004): "Proper positioning of patients is one of the most challenging tasks in spinal surgery. Air embolism, peripheral nerve palsy, blindness, quadriplegia, compartment syndrome, pressure necrosis of the skin, excessive bleeding, and venous thrombosis are only some of the complications that may result from improper positioning."6 The prone position is a natural posture often adopted during sleep, when protective involuntary alterations in posture counteract postural atelectasis, ischemia, nerve compression and skeletal stress.1 Anesthesia leads to loss of these protective reflexes. Enforced immobility during surgery, often for a long duration of time, combined with drug-induced loss of tissue tone, changes in tissue mass associated with aging, co-existing medical conditions, the physiological consequences of being prone, and inadvertent pressure on the abdomen while prone, can lead to many complications. The patient's orientation on the operating table, the type of device used to support the patient in the operating room, and the way in which positioning devices are used by staff are also known to potentially influence the occurrence and severity of certain complications.2,3,4,5,6 Support devices include operating room tables or table frames, kneeling attachments, pads, rolls, blankets, safety belts, arm supports, mattress overlays and head supports. This paper summarizes the intra-operative complications related to positioning devices used to support patients prone, with particular reference to spinal surgery. This summary is based on an extensive literature search of Ovid Medline, conducted in January 2008. Search terms included combinations of the following terms: "prone", "position", chest", "breast", " implant", "rupture", "spine", "spine surgery", spinal surgery",

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"breast pain", "anesthesia", "complications", "pressure ulcers", "visual loss" and "nerve injury". The reference lists of those papers found in the initial search provided additional sources of information, and a library search was also conducted to find relevant textbooks. Several authoritative sources were found that provided an overview of prone complications. Martin has written an evidencebased summary on prone positioning, supplemented by his extensive experience as an anesthesiologist, in: Positioning in Anesthesia and Surgery (edited by Martin and Warner, 1997).2 Sengupta and Herkowitz describe these complications in detail in the context of spinal surgery, in Complications of Pediatric and Adult Spinal Surgery (edited by Vaccaro, 2004).6 Edgcombe et al have recently published a comprehensive review paper summarizing the evidence underlying the complications of anesthesia in the prone position (2008).3 The Association of periOperative Registered Nurses (AORN) has also characterized the complications of prone positioning, producing important guidelines to aid prevention.4 These sources describe the range of complications related to techniques used to support patients in the prone position, summarized in Table 1, and further discussed in this paper.

1. CARDIOPULMONARY EFFECTS OF THE PRONE POSITION

Prone positioning during anesthesia is associated with predictable changes in cardiopulmonary physiology. The way in which the patient is positioned, the type of support devices used, together with certain characteristics of spinal surgery can combine to magnify the impact of these physiological effects on the hemodynamic and respiratory systems. Inferior Vena Cava Obstruction In the prone posture, pressure on the abdomen compresses the inferior vena cava (IVC) and femoral veins, diverting blood from the distal parts of the body into perivertebral venous plexuses. This makes wound hemostasis difficult and obscures the surgical field. The great vessels and small bowel are at risk of being pressed against the lumbar spine by raised intra-abdominal pressure, increasing the risk of accidental iatrogenic injury to these organs during spinal surgery.2,3 IVC obstruction from pressure on the abdomen can therefore potentially reduce cardiac output and also contribute to blood loss. Different positioning devices have been designed to maximize operative access while minimizing pressure on the abdomen.

TABLE 1: THE RANGE OF COMPLICATIONS ASSOCIATED WITH PRONE POSITIONING DURING SURGERY

COMPLICATION

Hemodynamic effects Respiratory effects Central nervous system injury Pressure injury: Skin Pressure injury: Chest Pressure injury: Vascular supply Pressure injury: Bone & joints Pressure injury: Breasts & genitalia Pressure injury: Peripheral nerve

DESCRIPTION

Decreased cardiac index, inferior vena cava obstruction, venous gas embolism and nongaseous embolism. Decreased respiratory compliance (in the presence of abdominal compression). Ischemic cerebrovascular events (due to occlusion of the carotid or vertebral arteries); neurological deficits related to venous occlusion, air entrainment, pneumorrhachis, or cervical spine injury; triggering of symptoms from undiagnosed space-occupying lesions. Pressure ulcers, contact dermatitis, compression of the pinna, viscerostomy damage. Tracheal compression, mediastinal compression. Hepatic ischemia, pancreatitis, acute mesenteric ischemia; limb compartment syndrome and rhabdomyolysis; avascular necrosis of the femoral head. Shoulder injury, bone and joint pain. Pain, bleeding, implant rupture. Brachial plexus, ulnar, axillary, musculocutaneous and radial nerve compression; lateral femoral cutaneous nerve injury (meralgia paresthetica); facial nerve injury; penile nerve compression; supraorbital neuropraxia. Corneal abrasion, chemosis, ischemic optic neuropathy, central retinal artery occlusion, transient or permanent ophthalmoplegia, cavernous sinus thrombosis, central retinal vein occlusion, orbital hemangioma, orbital compartment syndrome, bilateral angle closure glaucoma, non-traumatic subperiosteal orbital hemorrhage, amaurosis, dislocated intraocular lens, fixed mydriasis, cortical blindness. Nasopharyngeal congestion, salivary gland swelling, macroglossia, oropharyngeal swelling. Hypothermia

Ophthalmological complications

Oropharyngeal complications Loss of body heat

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These support devices have varying effects on intra-abdominal pressure. Schonauer et al found, on literature review, that differences in intra-abdominal pressure and the extent of blood loss are linked to the patient's position, and vary according to the table or frame used during spinal surgery in prone patients.10 A rise in intra-abdominal pressure may be caused by factors other than the table or frame, such as sandbags, bolsters, pads, pillows, or the mattress of the operating table. The careful positioning of patients on operating support surfaces, and the use of support surfaces that minimize intra-abdominal pressure, are recognized as essential in order to reduce the risks of IVC obstruction.2,3,10 Decreased Cardiac Index Decreased cardiac index is a consistent physiological effect; an almost universal finding when a patient is moved from supine to prone.3 The specific prone position used influences the extent of hemodynamic effect. In one study of 21 patients undergoing lumbar surgery (1991), direct pulmonary artery and inferior vena cava (IVC) pressures were monitored: the flat prone position did not interfere with circulatory function, but use of a convex saddle frame decreased the cardiac index and stroke volume index, with no effect on IVC pressure.7 This was confirmed in a 2006 study, which used transesophageal echocardiography to compare different prone positioners.8 Cardiac output decreased with the Wilson and Siemens AG frames, while cardiac index and stroke volume decreased with the Andrews, Wilson, and Siemens systems. Cardiac preload decreased using the Andrews frame. The Jackson spine table and longitudinal bolsters had the least effect on cardiac performance. These studies demonstrate that the prone position can have an important hemodynamic impact, which varies with the type of patient support used. In certain variants of the prone position, such as the Tuck position, venous drainage can be inadvertently obstructed by knee or hip flexion, allowing pooling of blood in the dependent structures and reduced atrial filling and cardiac output. Spinal surgery is often associated with unexpected blood loss and hypovolaemia, which may itself cause cardiac arrest in susceptible patients. The prone position may exacerbate hypovolaemia by further reducing venous return and cardiac output.9 Embolic Complications Air embolism is one of the most serious complications in spinal surgery. Any open vein in which there is subatmospheric

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or negative pressure may draw in air intravascularly from a surgical wound. Efforts to minimize abdominal compression in the prone position can result in an increased negative pressure gradient between the right atrium and veins at the operative site. The low pressure in the IVC results in a negative pressure which could then move gas from the operative site to the right atrium. Air embolism has been reported infrequently following surgery in the prone position, described in the published literature as case reports.3,9,11,12,13 The precise incidence of air embolism during spine surgery is unknown, complicated by the varying sensitivity of detection methods.3 However it is known that the risk is dependent upon patient position and operative site. In a review of pediatric patients undergoing neurosurgical operations, there were two possible episodes of air embolism in 120 operations, an incidence of 1.7%.14 Edgcombe et al describe four case reports of fat embolism in patients undergoing spine surgery in the prone position.3 In one case, involving lumbar decompression and spinal fusion with harvesting of the iliac crests for bone grafts, the authors considered that the prone position had contributed to the embolic process, speculating that prolonged venous stasis had played some part in the release of multiple emboli from bone harvesting sites.15 Respiratory Effects As acknowledged by Edgcombe et al: "There are clear differences in respiratory physiology between the supine and prone position, including an increase in functional residual capacity and alterations in the distribution of both ventilation and perfusion throughout the lungs."3 Provided that the patient is supported with the abdomen free from pressure, prone positioning is known to improve ventilation/perfusion matching and consequently improve oxygenation in the surgical patient.2,3 However, if abdominal compression occurs, this can lead to decreased respiratory compliance. Inadvertent compression can happen if the patient is obese or incorrectly positioned on the operating frame. Under these circumstances, very high airway pressures may be required to ensure adequate ventilation. High airway pressures can in turn impair venous return to the heart, decreasing cardiac output and increasing systemic venous pressure. This in turn potentially affects spinal cord perfusion pressure, increasing the patient's risk for neurological complications.10 According to Martin and Warner: "The necessary high airway pressures and large tidal volumes needed to ventilate a patient who is improperly positioned prone can have several potential side effects that can be harmful, (including): pulmoCopyright 2009, Allen Medical Systems. D-770490-A1 Oct. 22, 2009

nary interstitial emphysema leading to mediastinal, retroperitoneal or subcutaneous emphysema; an exaggerated rise and fall of the patient's back, potentially affecting the surgical field; and high venous and cerebrospinal fluid pressures, visible as fluid fluxes in the wound." The authors recommend ensuring a position that does not compress the abdomen and using the least permissible lung inflation pressures to minimize venous congestion and cerebrospinal fluid surges in the operative field.2 Changes in pulmonary mechanics are also dependent on the type of prone position used, and on the equipment used to support the patient prone. For example, Palmon (1998) showed that pulmonary compliance decreased when patients were supported on chest rolls or on the Wilson frame, whereas the Jackson table was not associated with any change in pulmonary function.16 Summary Overall, it is evident that prone positioning of the patient, the type of patient support device used, and how the patient is placed on the support can have a significant impact on cardiac output, intra-abdominal venous pressure, respiratory compliance, extent of intra-operative blood loss and the risk of air or fat embolism.

to head rotation during anesthesia in the prone position.3 For these reasons, it is generally accepted that careful positioning of the neck, and head support in a neutral position are essential to prevent neurological injury while prone.2,3,4 Venous Occlusion Edgcombe reports nine cases that involve prone patients and spinal surgery, in which hemodynamic changes have led to spinal cord ischemia.3 In a study by Bhardwaj et al , four patients developed new neurological deficits immediately after a cervical laminectomy (two had hemiparesis, one quadriparesis, and one paraparesis).19 These patients were prone, supported by chest rolls. The authors proposed that the use of chest rolls led to increased venous pressure, which, when combined with mild arterial hypotension, decreased the perfusion pressure in the spinal cord, causing ischemia. This paper states: "The use of frames that prevent abdominal compression, as well as avoidance of perioperative arterial hypotension, is important in maintaining adequate spinal perfusion during and after decompressive spinal cord surgery." A similar mechanism is thought to explain an additional case of quadriplegia after thoracolumbar decompression20 and two reports of thoracic level paraplegia after lumbar spine surgery.21 Two cases occurred in patients with abnormal venous anatomy. A patient with achondroplasia, who had stenosis of the jugular foramina (a recognized feature of achondroplasia), developed bilateral venous infarcts in the cerebellum. This occurred after a nine hour thoracolumbar operation, head-down on a Wilson frame, with high intra-thoracic pressures during positive pressure ventilation.22 Another case report describes a patient with an occipital meningioma who was placed prone on a horseshoe headrest: it is thought that the headrest caused compression of the anterior emissary veins, leading to venous stasis and rupture into the frontal extradural space.23 Air Entrainment Entrainment of air into the cranial cavity is common after neurosurgical procedures and occurs in any operative position.3 In one study, 16 of 28 patients (57%) undergoing posterior fossa or cervical spine procedures in the prone position experienced pneumocephalus, although this was asymptomatic and did not result in neurological deficit.24 Two cases of tension pneumocephalus have been reported, both in children undergoing intracranial surgery, placed prone.25,26 A single case report of air entrainment into the spinal canal (pneumorrhachis) after posterior fossa exploration, resulting in

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2. CENTRAL NERVOUS SYSTEM INJURY

The manner in which the patient is positioned can result in serious injury to the central nervous system. Although an uncommon complication, central nervous system injury can occur secondary to arterial or venous occlusion, air entrainment, cervical spine compression or from the triggering of symptoms related to an undiagnosed space-occupying lesion.3,6 Ischemic Cerebrovascular Events Excessive movement of the patient's neck during positioning can affect blood flow in the carotid and vertebral arteries. Wang et al describe a patient who had a fatal ischemic stroke after being positioned prone with the head rotated during spinal surgery. This patient had an unrecognized carotid artery stenosis.17 Another patient was reported as having a dissection of the left carotid artery, resulting in hemiparesis and aphasia, after spinal surgery.18 The dissection was thought to be related to extension or rotation of the neck during positioning. Occlusion or dissection of the vertebral arteries has been reported in at least four cases, linked to hypoperfusion secondary

quadriplegia, was thought to be related to the prone head-down position.27 The patient's position was thought to have contributed to the tracking of air through the foramen magnum into the cervical region. Spinal Cord Compression Prone positioning is known to be a high risk factor for patients with pre-existing spinal cord dysfunction. Two case reports describe postoperative paraplegia due to cervical spine injury, attributed to neck positioning during prone spinal surgery.3 An 18 year old patient had an eight hour operation to remove a cerebellar medulloblastoma, placed prone in the "Concorde" position with hyperextension of the neck: afterwards, he had a complete and permanent C5/6 sensory and motor deficit.28 It was postulated that the cervical cord was stretched in a narrow spinal canal, with an already-bulging C5/6 disc, resulting in ischemia. The other case involved a patient placed prone for two hours with head turned to the left side and cushion support, who developed a prolapsed intervertebral disc at C6/7.29 The authors concluded, "Careful neck positioning (is) mandatory for patients receiving surgery in the prone position". Undiagnosed space-occupying lesions Prone positioning can trigger neurological symptoms in patients with previously asymptomatic space-occupying lesions within the spinal canal or cranium. This is rare, reported in four cases.3 In each case, the proposed mechanism involved altered cerebrospinal fluid flow and epidural venous engorgement. One patient with neurofibromatosis had a neurofibroma in the posterior fossa fall anteriorly when positioned prone, compressing the medulla and pons and leading to bradycardia and fatal neurogenic pulmonary edema.30 Summary Positioning of the patient is of crucial importance to avoid serious or fatal neurological injury. The prone position can cause neurological deficits through: an increase in abdominal pressure leading to a decrease in spinal cord perfusion; provision of the "right" conditions for the introduction of air into the cranial cavity or spinal cord; and, by exacerbating a neck position that can occlude the arterial supply to the brain or compress the spinal cord. Patient support devices that minimize abdominal compression and maintain the neck and body in a neutral position can help prevent these neurological complications.

3. PRESSURE INJURIES

Injuries can occur in the prone position as a result of the direct application of pressure, or indirectly, as a result of compression of the vascular supply or nerves (Table 2).

TABLE 2: Pressure Injuries Related To The Prone Position Injury From Direct Pressure Pressure ulcers Contact dermatitis Compression of the pinna Viscerostomy damage Tracheal compression Mediastinal compression Breast injury Injury to the genitalia Joint and bone pain, dislocation or fracture Source: Compiled from Martin, Edgcombe, Sengupta and Herkowitz, AORN.2,3,4,6 Injury From Indirect Pressure On Vascular Supply or Nerves Visceral ischemia: hepatic ischemia, pancreatitis, acute mesenteric ischemia. Avascular necrosis of the femoral head Limb compartment syndromes and rhabdomyolysis Peripheral nerve injury

Pressure on The Skin and Underlying Tissue Complications related to pressure on the skin and underlying tissue have been reported in published articles as contact dermatitis and pressure ulcers. Experienced clinicians have described other injuries related to prone positioning that are not presented as case reports or in case series in published articles, in particular, compression of the pinna and viscerostomy damage.2 Contact Dermatitis Edgcombe reviewed two case reports that describe contact dermatitis in patients positioned prone during surgery. One patient had their head supported in a flexible polyurethane foam positioner that supports the face during surgery by molding around the eyes, nose and mouth. The other case developed contact dermatitis in response to a monitor placed on the forehead. The skin reaction was thought to have been exacerbated by the prone position causing pressure against the electrode conductive gel.3 Compression of The Pinna Martin points out that improper positioning can cause all or part of the ear to be folded over. Pressure can result in cartilaginous damage that is potentially disfiguring.2

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Viscerostomy Damage Viscerocutaneous stomas can be compressed by prone supports, resulting in leakage of contents and operative contamination. Compression of the stoma can also potentially lead to ischemia of the stoma margins and a need for surgical revision.2 Pressure Ulcers It is widely recognized that surgery itself is a risk factor for the development of pressure ulcers because of prolonged immobility, unrelenting pressure on dependent parts of the body, and use of anesthetic agents. Therefore, all surgical patients are regarded as being at risk of damage to skin integrity, regardless of their body position during surgery. The incidence of intraoperativelyacquired pressure ulcers has been reported in studies to range from 8.5% to as much as 45%.31, 32,33,34 Since pressure ulcers that are initiated by surgery often do not appear until one to four days after an operation, these can be mislabeled, for example as a burn. This means that these are often not directly attributed to the surgical procedure and therefore the incidence of intraoperative pressure ulcers may be under-reported.32,34 The elderly, and patients with co-morbidity (such as diabetes, preoperative hypertension, respiratory disease or vascular disease), and sub-optimal nutritional status or small body size, are known to be at increased risk of developing an intra-operative pressure ulcer.31,33,34 Perioperative exposure of the skin to wetness (such as preparation solutions) and shearing, tearing or friction forces are recognized as contributory factors.4,31,33 Duration of anesthesia has also been implicated as a risk factor for skin ischemia in some studies. Aronovitch (1999) surveyed 1128 patients undergoing a procedure of at least three hours' duration and reported an incidence of 5.8% for surgery lasting three to four hours, increasing to 13.2% for surgical procedures lasting over seven hours.35 However, as noted by Price et al, this relationship between risk and duration of surgery has not been a consistent finding.34 Hoshowsky and Schramm describe the "interactive effects of patient risk factors", referring to the increased risk from a combination of contributory factors.36 How the patient is positioned, and the characteristics of the operating room support device used can exacerbate these macerating or tearing forces, or increase pressure on bony areas, resulting in damage to skin integrity.31,32,33,34 For example, the use of a standard operating mattress is known to increase capillary interface pressures over bony prominences beyond the capillary closing pressure, making ischemia more likely.35 Foam caused significantly more ulcers than a standard operating table in one study.37 Layering of materials such as blankets or padding under

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the patient can decrease the pressure-reducing effect of a mattress or overlay33, and the use of a warming blanket under the patient during surgery has been shown to increase risk of pressure ulcers.38 Although there is a need for additional studies to compare the pressure-reducing effects of various commerciallyavailable support surfaces, it is clear that patient positioning, and the support device used, are important factors in the prevention of intraoperative pressure ulcers. This is emphasized in the guidelines provided by the AORN and others4,31,33 which recommend: the tailoring of positioning devices to the surgical position used, and to the body habitus of the individual patient; the requirement for firm and stable devices to minimize shearing; the use of support surfaces resistant to moisture; and, the design of support equipment to distribute pressure evenly, over a large surface area. There are few studies assessing the incidence of pressure ulcers in patients positioned prone and no studies so far that evaluate the efficacy (in terms of ulcer prevention), of different support devices used for patients placed in the prone position. Despite the lack of specific data, it is evident that prone patients are at risk of pressure ulcer development, related to the increased risk inherent in undergoing a surgical procedure, the prolonged nature of spinal surgery, and the vigorous manipulation often required in this type of surgery (increasing the likelihood of shearing forces). As stated by Aronovitch: "All surgical patients undergoing prolonged procedures should be considered at risk for intraoperative ulceration."35 Many authors advise close attention in prone patients to pressure points at the forehead, chin, tip of the nose, ears, breasts, genitalia, anterior superior iliac spines, knees and feet.2,4,6 Sengupta and Herkowitz point out that pressure on the face is especially high during posterior surgery of the cervical spine and cranio-cervical junction, and recommend that Mayfield tongs are used, to reduce the effect of pressure on the face.6 Case reports in prone patients describe intraoperative pressure sores on the malar regions of the face, chin, eyelids, nose, tongue and iliac crests.3 Pressure ulcers are associated with significant costs, summed up by Schultz: "Pressure ulcers, regardless of their origin, represent negative outcomes for patients, including pain, additional treatments, and surgery, longer hospital stays, disfigurement or scarring, increased morbidity, and increased costs."33 Beckrich and Aronovitch calculated that, each year, approximately 1.6 million patients developed hospital-acquired pressure ulcers, at a cost of $2.2 to $3.6 billion (1998 figures).39 23% of these ulcers occurred in surgical patients undergoing procedures lasting more

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than three hours, representing up to 42% of costs of ulcers developing in hospital and an annual direct cost of $750 million to $925 million (1998 figures).39 Since pressure ulcers are considered preventable (according to the National Pressure Ulcer Advisory Panel Statement on Pressure Ulcer Prevention, 1992), damage to skin integrity during surgery is considered indicative of deficient quality of care.40 That skin integrity is a hallmark of quality nursing and a Quality of Care indicator is endorsed by many professional, federal and private health care organizations, such as the American Nursing Association41, the Agency for Health Care Policy and Research (AHCPR)42, American Medical Directors Association43, the Joint Commission on Accreditation of Healthcare Organizations44 and the ECRI Institute.32 It is widely-recognized that intra-operatively-acquired pressure ulcers expose nurses, surgeons, anesthesiologists and institutions to significant litigation risk.32, 39, 45, 46 According to the ECRI Institute (2006): "The mere existence of a pressure ulcer is often viewed as physical evidence of medical negligence."32 Summary Prone patients are at risk from intra-operatively-acquired skin damage because of the effects of anesthesia and the prolonged nature and physical forces required in surgical procedures that require prone positioning (such as spinal surgery). The consequences of pressure effects on the skin, in terms of morbidity and health care costs, are substantial. Intra-operative positioning and patient support surfaces that mitigate these pressure effects are important factors in the prevention of skin complications. This is recognized by professional and federal organizations such as the AORN and AHCPR, which call for active preventive measures, including adequate patient support surfaces, in published guidelines.4,42

between the spine and sternum while positioned prone. In three of the four cases, the problem was exacerbated by an underlying connective tissue defect, either Marfan's Syndrome,47,48 or tracheomalacia.49 Sternal or Mediastinal Compression Compression of the heart or great vessels has occurred in patients positioned prone during spinal surgery, in cases where there is an anatomical abnormality, such as scoliosis50 or pectus excavatum51,52, and after cardiac surgery.53,54 Alexianu et al described severe hypotension in a child with pectus excavatum, scoliosis and neurofibromatosis around the great vessels, when placed in a prone position on transverse bolsters for posterior spine surgery: sternal pressure was relieved when bolsters were placed longitudinally.51 Pressure On The Vascular Supply Indirect pressure effects on the vascular supply during prone surgery have been reported in the published literature as cases involving ischemia of the liver and bowel, compartment syndrome and rhabdomyolysis, and avascular necrosis of the femoral head. Visceral Ischemia Compression on the abdomen when prone under anesthesia can result in ischemia of abdominal organs. Hepatic ischemia and hepatic infarction have been described after prolonged surgery in the prone position.50,55,56 The authors of these case reports attributed the cause to the patient's position and the possibility of hypo-perfusion and ischemia of intra-abdominal organs. Edgcombe suggests that this complication may be more common than is realized: at least five other cases have been identified in a recent investigation by the United Kingdom National Patient Safety Agency.3 Mofredj et al describe a case of acute mesenteric ischemia following spinal surgery: prone position and hypotension were thought to have led to venous stasis and mesenteric vein occlusion in a patient with an inherited hypercoagulable state.57 Prone positioning has also been proposed as a cause of pancreatitis after spondylolisthesis surgery.58 Limb Compartment Syndrome and Rhabdomyolysis Flexion of the hips and knees whilst prone can lead to ischemia of the muscles in the lower limbs, especially if surgery is prolonged. Rhabdomyolysis is caused by muscle ischemia and re-perfusion, and is characterized by dark urine, muscle pain and generalized weakness. Severe hypoperfusion can lead to lacticacidosis and sometimes hyperkalemia, and acute renal failure.

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4. PRESSURE EFFECTS ON THE CHEST

Tracheal Compression Pressure on the trachea can result in serious consequences, since airway problems are difficult to manage in the prone patient and can result in cardiac arrest secondary to hypoxaemia.9 Four cases of tracheal compression have been reported during surgery in patients positioned prone.3 All of these patients had thoracic scoliosis, which is thought to have reduced the anterior-posterior diameter of the chest, causing the trachea to be compressed

There is evidence that the knee-chest and "Tuck" positions decrease blood flow in the posterior tibial artery, increase intramuscular pressure in the anterior compartment of the leg and release biochemical markers of muscle damage.3 In one study, 15 patients undergoing surgery for spondylolisthesis in the kneechest position all showed a significant increase in plasma creatine phosphokinase levels, and six patients also had myoglobinemia and myoglobinuria.59 Eight cases of compartment syndrome have been reported in patients undergoing spinal surgery in various prone positions that involved flexion of the hips and knees.3 In six of these cases, surgery lasted longer than three hours. Six patients required fasciotomy and three cases were complicated by acute renal failure, which was fatal for one patient. Rhabdomyolysis in the absence of compartment syndrome has been identified in four cases.3 Three of the patients had been placed prone in Jackson frames for prolonged spinal surgery. It was thought that muscle ischemia could have resulted either from compression of the large vessels in the abdomen (exacerbated by obesity), or from direct compression of the thigh muscles against the support device. Avascular necrosis of the femoral head Orpen et al describe three patients with pre-operative radiological signs of osteoarthritis, who developed collapse of the femoral head within two to eight weeks after decompression of spinal stenosis.60 These patients had been positioned prone on a Montreal mattress. The authors postulated that the combination of deliberate hypotension and prone positioning led to intraosseous venous congestion and ischemia of a compromised femoral head, leading to avascular necrosis. Pressure on bone and joints Martin, Ray, and Sengupta and Herkowitz express concern over patients placed prone who have pre-existing joint or bone problems, such as arthritis, osteoporosis and stiff joints (as in ankylosing spondylitis), or total joint replacements.2,6,61 The process of placing such patients prone, or the application of prolonged pressure on joints while prone can lead to joint or bone pain, or bone fractures. Sutterlin and Rechtine reported shoulder pain in one patient supported prone during spinal surgery.62 Two case reports describe shoulder dislocation occurring during in prone surgical patients.63,64

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Pressure on breasts and genitalia Positioning devices can compress the scrotum and penis, and straps and seats to hold the female body in the kneeling prone position place pressure on the female perineum.2 The prone position may inadvertently damage breast tissue, resulting in chest wall pain, breast tenderness or bleeding of the nipples. Longitudinal positioning frames or rolls can damage breast tissue by direct compression.2 Prolonged pressure on a breast implant has the potential to result in implant rupture. Martin has also noted that extensive breast tissue can potentially threaten positioning, by forming an unstable and shifting platform when the patient is placed prone.2 Pressure on peripheral nerves Peripheral nerve injury is one of the most frequent causes of morbidity resulting from improper positioning intraoperatively.6 In one study, neurological injury related to positioning was found in 72 out of 50000 general surgery operations (0.14%), of which 38% were brachial plexus injuries.65 A review based on the American Society of Anesthesiologists closed claims database found an association between prone positioning and claims for nerve injury,66 however, reliable data on the incidence of peripheral nerve injury specifically in the prone position is lacking. Intra-operative injury to peripheral nerves occurs through stretching or compression of the nerve, or due to nerve ischemia. This is usually associated with prolonged surgery, although has been noted to occur in procedures lasting 45 minutes.67 Patients with diseased nerves (diabetes, peripheral vascular disease, alcohol dependency, pre-existing neuropathy) and anatomical abnormalities that change the pressure point distribution (such as scoliosis) have an increased risk of intra-operative peripheral nerve damage.3,68 Although there are other recognized causes of intra-operative nerve damage (i.e., retraction injury or brachial plexitis), there is no doubt that malpositioning of the patient and external compression from table surfaces, table edges and restraining straps are important causative factors.3,68 Upper limb nerve injury Brachial plexus injury is reported by Sengupta and Herkowitz as more common in the prone position than in the supine or lateral position.6 At least four cases of brachial plexus injury, and other cases involving ulnar, axillary, musculocutaneous and radial nerve injury, have been summarized as published case reports, related to intra-operative prone positioning.3

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Lateral femoral cutaneous nerve injury Compression of the lateral femoral cutaneous nerve (meralgia paresthetica) is a common complication of the prone position during spine surgery.65,69,70 Posts supporting the pelvis, or pillows under the patient can compress the nerve at the exit below the anterior superior iliac spine.69,71 The risk of nerve injury increases with higher Body Mass Index, longer surgical time and if the patient has a degenerative spinal disorder.70 In one prospective study, 23.8% of 252 patients on a Relton-Hall frame developed evidence of meralgia paresthetica after posterior spine surgery.70 Mirovsky and Neuwirth reported that meralgia paresthetica affected 20% of patients after posterior spine surgery on a Relton-Hall frame: in six of 105 patients, the injury was bilateral.71 This condition is usually associated with hypoesthesia in the anterolateral aspect of the thigh, and occasionally burning pain, hypersensitivity and dysesthesia.69 Although symptoms usually resolve within six months of surgery, meralgia paresthetica can rarely be associated with severe pain and restriction of activity.72 In the study by Mirovsky and Neuwirth, in 89% of patients, symptoms resolved within three months of surgery, but two patients still had pain and hypoesthesia of the anterolateral thigh one year after surgery.71 Facial Nerve Injury Several case reports relating to nerve injury of the head and neck have been described in Edgcombe's review, all attributed by the authors to nerve compression related to the prone position:

One case of injury to the lingual and buccal nerves during lumbar laminectomy;73 Three patients with supra-orbital nerve compression;74,75 One phrenic nerve injury in a diabetic patient, due to overextension or rotation of the neck;76 Recurrent laryngeal nerve damage due to the compression of the vocal cord and recurrent laryngeal nerve by the tracheal tube, and traction of the recurrent laryngeal nerve by rotation of the neck.77

Summary It is widely-acknowledged that positioning of the patient is an important causative factor in the occurrence of peripheral nerve damage, and that correct positioning, as well as the use of support devices that minimize pressure effects, are key to the prevention of these injuries. Although complete recovery can be expected in the majority of cases, some patients do experience a permanent loss of nerve function.

5. OPHTHALMOLOGICAL COMPLICATIONS

There is no doubt that eye complications are associated with the prone position during anesthesia, and the use of head support equipment.3,79 Compared with supine and lateral positioning, there is a ten-fold increase in eye injury associated with surgery while prone.79 Ophthalmological complications related to prone positioning during surgery range from keratoconjunctival injury, such as corneal abrasion and chemosis, to irreversible blindness. The review paper by Edgcombe et al provides a recent summary of the available evidence on visual loss following surgery in the prone position, concluding that: "Ophthalmic complications are well recognized in patients who have been prone under anesthesia, and can be devastating."3 This is acknowledged by organizations such as the AORN and the American Society of Anesthesiologists: these organizations have published guidelines for the prevention of postoperative visual loss (POVL).80, 81 Patients undergoing spinal surgery are known to have an elevated risk: a study of cases registered with the Scoliosis Research Society (1997) estimated that one eye complication occurs for every 100 spinal procedures.82 Corneal abrasion Corneal abrasion is a well-recognized, usually self-limiting, complication of anesthesia, regardless of patient position during surgery, resulting from incomplete closure of the eye, drying of the eyes, foreign bodies or other unintended materials contacting the eye during surgery.2,79 Rarely, abrasion can cause a corneal ulcer and partial or complete visual loss in the affected eye. Biswas et al found an incidence of 55% for conjunctival abrasions in prone patients undergoing cranial or spinal surgery in the prone position.83 According to Stambough: "The prone position is particularly implicated (in corneal abrasion) because direct or indirect eye pressure is more likely to occur."79 The AORN also notes that there is an increased risk for corneal abrasion when the patient is in the prone position.81

Penile Nerve Injury Injury to the dorsal nerve of the penis is described in two patients positioned prone on a fracture table.78 Operative positioning compressed the dorsal nerve of the penis between the pubic symphysis and the perineal post of the fracture table. As a consequence, both patients experienced difficulty in obtaining complete erections, which appeared to be permanent.

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Chemosis Postoperative conjunctival edema (chemosis) is noted by Martin to be more likely to occur: "when the head of the prone patient is below the level of the atrium, or when the infusion volume of crystalloids is large."2 This adverse event may be asymptomatic or lead to conjunctival or corneal infection. Chemosis is a common postoperative finding in patients undergoing lumbar spine surgery in the prone position. Jeon et al reported an incidence of moderate and severe chemosis at 6% and 4%, respectively, for patients positioned prone in a Wilson frame during spinal surgery, with their head in a neutral position.84 In the head-down position, the incidence of moderate and severe chemosis increased to 31% and 7%, respectively. Positive fluid balance and duration of surgery increased the risk. Postoperative visual loss It is known that postoperative visual loss (POVL) is associated with spinal surgery and with prone positioning, although the pathophysiology, etiology, incidence, risk factors and prevention of POVL are still being characterized. Stambough and Edgcombe provide informative reviews of the latest research on POVL and the association of POVL with prone positioning and spinal surgery.3,79 Buono and Foroozan (2005) comprehensively describe the research on ischemic optic neuropathy and the relationship of this condition to spinal surgery.86 67% of all cases of post-operative visual loss registered on the American Society of Anesthesiologists Postoperative Visual Loss Registry occurred after prone surgery.85 A retrospective review of 3450 spinal operations in 1997 found that 0.2% of patients developed visual loss after spinal surgery.90 The incidence of visual loss after spinal surgery in the prone position at one institution91 was 1 in 1100, compared with the incidence in the general surgical population of 1 in 61000.92 In the prone, anesthetized patient, central retinal artery occlusion and ischemic optic neuropathy are the most common reasons for POVL.3 Central retinal artery occlusion Factors that increase intraocular pressure can lead to central retinal artery occlusion, retinal ischemia and POVL. The prone position itself has been associated with raised intraocular pressure (IOP).93,94 Cheng et al measured IOP in 20 patients scheduled for spine surgery in the prone position. IOP was noted to increase in prone patients, and during anesthesia.93 Hunt confirmed these results in patients undergoing spinal surgery and concluded that: "IOP increases when anesthetized patients are placed in the prone position."94

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Support devices can also affect IOP. One study found intraocular higher pressures for patients with heads placed in pillows, compared with head stabilization with Mayfield pins, although the study sample was small. Direct external pressure on the globes and periorbital structures, caused by a headrest or other support, is known to increase intraocular pressure, potentially resulting in central retinal artery occlusion and POVL (described as the "Hollenhorst Syndrome").3 Ironically, devices used to protect the eye can exert inadvertent pressure on the eye. Roth et al reported irreversible POVL in a 53 year old man secondary to use of protective goggles whilst in a prone position during spinal surgery.95 The authors of this study searched the Medwatch MAUDE database and found additional cases of patient injury from use of the goggles, all in patients placed prone in a foam headrest (including keloid scarring on the nose, skin abrasions, eyelid abrasions and neuropraxia of the supraorbital nerve). Ischemic optic neuropathy An increase in intraocular pressure or venous pressure, or a decrease in arterial pressure can diminish oxygenation of the optic nerve, leading to ischemic optic neuropathy (ION).3 ION can occur in the absence of external pressure from headrests, when the head is supported in pins, for example. As stated by Edgcombe, "A variety of factors influence intraocular pressure and some of these are clearly altered by prone positioning." Prone positioning and spinal surgery potentially raise intraocular pressure by: 1) increasing venous pressure and peak inspiratory pressure, thereby increasing intraocular pressure79; 2) increasing the risk of abdominal compression, reducing cardiac output and mean arterial pressure; 3) the use of a deliberate anesthetic technique that reduces arterial pressure to minimize blood loss. Other factors apart from hypotension that increase the risk of ION following spinal surgery include blood loss of 1000ml or greater, anesthetic duration of six hours or longer, complex instrumentation during scoliosis surgery, administration of large volumes of clear fluids, and pre-existing hypertension or vascular disease (such as atherosclerosis and diabetes).79,85 Buono and Foroozan describe the well-recognized phenomenon that a head-down and prone position can result in increased venous pressure, with facial and orbital edema, especially after prolonged surgery and large volumes of intraoperative fluid replacement.86 Occipital cortical infarct Cortical blindness is caused by an isolated stroke that selectively affects the visual cortex.79 Hypo-perfusion or embolism

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are the underlying factors, leading to occipital cortical infarct or cortical blindness. Stambough discusses this complication in his review article: Ophthalmic Complications Associated with Prone Positioning in Spine Surgery (2007).79 Other causes of visual loss Case reports have been published describing other visual complications after surgery in the prone position. These cases include: supraorbital neuropraxia (three patients), transient or permanent ophthalmoplegia (nine patients), and single case reports of cavernous sinus thrombosis, central retinal vein occlusion, orbital hemangioma, orbital compartment syndrome, bilateral angle closure glaucoma, non-traumatic subperiosteal orbital hemorrhage, amaurosis, dislocated intraocular lens; and fixed mydriasis.3,79 Some of these cases are described below:

CASE EXAMPLE: A 76 year-old patient had surgery for cervical spine stenosis, five hours' prone under anesthesia, with head supported on a Mayfield head clamp. On awakening, he reported unilateral visual loss. On examination, he had mild left proptosis with lid swelling and conjunctival chemosis. Pupil was mid-dilated and non-reactive. Fundoscopy showed retinal edema with a cherry-red foveal spot. A cerebral angiogram two weeks later showed a left cavernous sinus thrombosis. The authors attributed this complication, which resulted in irreversible long-term visual loss, to pressure from the headrest. Anand and Mushin, 2004.87

CASE EXAMPLE: An 80-year old man underwent an eight hour lumbar laminectomy for lumbar spinal stenosis. A silicone head rest was used. The patient experienced facial edema, corneal edema with an extensive corneal abrasion, a nonreactive pupil, advanced cataract and complete ophthalmoplegia, leading to complete and irreversible visual loss in the eye. The authors suggested that: "The progressive orbital edema secondary to the prone position, and possible unilateral direct pressure from the headrest device on periorbital structures resulted in congestion at the orbital apex, with a subsequent compartment syndrome and ischemic orbit." Leibovitch et al, 2005.88

CASE EXAMPLE: A 16 year old girl had scoliosis surgery in the prone position, her face resting on a padded, gel-filled horseshoe rest and eyes taped shut and padded with gauze. After surgery, she complained of visual loss in the right eye. Computed tomography showed a swollen medial rectus muscle with no other orbital pathology. The authors attributed the permanent visual loss to ocular compression against the headrest, caused by repositioning of the patient during surgery. Kumar et al, 2004.89

Summary Ophthalmological complications related to prone positioning range from common, self-limiting and mildly symptomatic corneal abrasions or chemosis, to rare, permanent and potentially devastating visual loss. Prone positioning on head support devices plays an important part in the causation and prevention of all of these ophthalmological complications.

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6. OROPHARYNGEAL COMPLICATIONS

Hans et al describe a case of bilateral painful swelling of the submandibular glands occurring after neurosurgery in the prone position, with the patient's head supported in a Mayfield holder.96 The authors determined that stretching of the salivary ducts, leading to stasis and acute swelling, explained the underlying etiology. Another paper describes six cases of bilateral parotid gland enlargement occurring after general anesthesia ("anesthesia mumps"), five of which occurred after prone surgery.97 Vascular congestion resulting from the patient's position during surgery was thought to have contributed to this complication. Three reports document macroglossia after surgery in the prone position, complicating a suboccipital craniotomy, a posterior cervical spine decompression and posterior fossa surgery.3 The authors identified obstruction to venous drainage, excessive flexion of the head, and/or the tracheal tube obstructing venous drainage from the lingual and pharyngeal veins as underlying mechanisms.

Humphreys et al described complications in a series of 107 pediatric patients undergoing upper cervical spine and posterior cranial fossa surgery in the prone position.99 A Relton frame with Mayfield or Gardner head holder was used. The authors reported the following as complications related to the positioning of the patient: a drop in temperature (in all 24 infants, and reduction of greater than 3oC in three of these patients), obstructed endotracheal tube, possible air embolus (two patients) and blood loss. However, the authors concluded that the prone position was safer than supine and sitting positions for pediatric patients who needed this kind of surgical treatment, since the risk of air embolism was less.

TABLE 3: Pediatric complications of the prone position Pediatric Complications Unintentional extubation Eye complications: corneal abrasions, conjunctival and periorbital edema of the dependent eye, retinal ischemia, post-operative visual loss due to ischemic optic neuropathy. Entangling of cables. Abdominal compression (leading to impaired ventilation, increased bleeding, and decreased cardiac output). Improper head and neck positioning (leading to venous and lymphatic obstruction). Macroglossia. Possibility of venous air embolism.

7. COMPLICATIONS OF THE PRONE POSITION IN PEDIATRIC PATIENTS

In children, complications linked to prone positioning are similar to those described for adults. However, there are particular risks in children related to: the hemodynamic consequences of positioning, blood loss, and the potential for vascular compromise of the spinal cord; underlying pathology (such as spinal deformity, or genetic syndromes with "hidden" organ structure, dysfunction or location); and, age-related anesthetic considerations, particularly hypothermia in neonates and small children.98,99 These complications are summarized in Table 3. Soundararajan and Cunliffe describe these challenges in their paper, Anesthesia for Spinal Surgery in Children (2007).98 The authors emphasize the importance of: "Careful positioning of the patient to prevent compression of the abdomen," especially with the aim of minimizing both spinal cord ischemia and compression on the spinal cord, whilst maintaining a bloodless surgical field. They also caution that: "Anesthetic management requires a meticulous approach to safety, positioning, and spinal cord perfusion, with maintenance of normothermia and normovolemia." Meridy et al reviewed pediatric patients undergoing neurosurgical operations and reported two possible episodes of air embolism in 120 operations, an incidence of 1.7%.14

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Source: Soundararajan and Cunliffe98

Being positioned prone on a support involving four pillars, such as the Relton-Hall table, increases the potential for heat loss in children, since the ventral surface of the patient is exposed to room air. LeBard100 and Soundararajan98 point out that hypothermia prolongs recovery from neuromuscular block, impairs platelet function, and leads to a higher incidence of wound infections. Meralgia paresthetica has also been noted after posterior spine fusion in pediatric patients Tejwani et al found that, of 56 children undergoing spine fusion for scoliosis (on a Jackson table with either the lower leg support table and thigh supports, or lower leg suspension sling), 10 (18%) developed meralgia paresthetica.101 This manifested as anterolateral thigh numbness without pain or weakness. Symptoms in all affected patients resolved in less than 6 weeks. Patients with meralgia paresthetica more often had idiopathic scoliosis, were positioned with the lower leg sling instead of the flat table support and tended to have longer surgery times.

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SUMMARY

The most common complications associated with the prone position include: ischemia of the skin at pressure points, peripheral nerve compression, and mild ocular injuries. These surgical outcomes can lead to prolonged hospital stays and increased health care costs, and have a significant impact on the patient's recovery time and quality of life. Many additional complications of prone positioning occur less frequently, but can have serious consequences for the patient, leading to pain, disability, paralysis, visual loss, or even death. Complications of prone positioning during anesthesia are related to the following factors, or to a combination of these factors: the physiological effects of being prone; the consequences of compression on the abdomen while prone; the nature of spinal surgery (such as prolonged duration, the need for a clear surgical field, manipulation and movement of the patient to allow instrumentation); pressure effects on different parts of the body from the equipment used to support the prone patient. The evidence presented in this summary illustrates the importance of the equipment used to support the patient. The review by Edgcombe et al concluded: "It is clear that the specific prone position and support system used influences not only the incidence of complications but also the alterations in cardiovascular and respiratory physiology which occur when a patient is moved from a supine to prone position in the operating theater."3 The way in which the patient is supported whilst prone influences the occurrence and severity of complications. The design of support equipment to minimize these complications, together with appropriate education of staff in how to optimize the preventive qualities of support devices, is vitally important. There is also a need for research that provides an evidence base demonstrating the efficacy and safety of different types of operating room support equipment.

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