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REVIEW OF SOME OCULAR ANATOMY AND PHYSIOLOGY RELEVANT TO CONTACT LENS PRACTICE Gerald E. Lowther, O.D., Ph.D.

The following are some basic concepts of ocular anatomy and physiology that should be known in order to understood contact lens fitting and follow-up care. Lids and Blinking The eyelids are important for the protection of the eye and to maintain a smooth optical surface to the cornea. The eyelids are made up of several layers; the outer skin, a layer of connective tissue, Orbicularis muscle, levator palpebrae muscle, tarsal plate with meibomian glands (yellow streaks seen on underside of lower lid below-yellow arrow) and conjunctiva.

Blinking is important for spreading the tear film and removing tears and debris. The blink rate varies between individuals and under different conditions. An average blink rate is about once every 5 seconds (12 times per minute) but decreases significantly with visual tasks requiring concentration, i.e. using the computer or driving. Decreased blink rate can cause dryness symptoms and cause discomfort with contact lens wear. Irritation of the eye and psychological factors can result in a faster blink rate. During a blink the upper lid makes the major movement to close the palpebral aperture with very little upward movement of the lower lid. The lower lid does make a significant horizontal movement toward the nose on a blink. During a blink the eyeball moves back into the orbit about 1 mm. The total time for a complete blink is about 250 msec with the closing phase the fastest (about 80 msec). During a forced blink, or when the lids are held open and a patient tries to blink, the eyeball will rotate upward (Bell's Phenomenon) and the globe will retract. The conjunctiva of the lids and conjunctiva on the eye are continuous and form the upper and lower cul-de-sacs. If measured from the limbus (junction of the cornea and sclera) the upper and lower cul-de-sacs are 8-10 mm deep; laterally it is about 14 mm; and the conjunctiva nasal to the

limbus is about 7 mm. The upper and lower cul-de-sacs can be distended to 25-30 mm and 1416 mm, sufficient to hold a dislocated contact lens. Many patients do not understand the concept of the cul-de-sacs and believe that a contact lens can be lost behind the eye-their fear of this happening should be alleviated. Tear Film The tear film gives the surface of the cornea a smooth, optical surface. Without the tear film the ocular surface is opaque. The thickness of the tear film is thought to be 4 to 8 µm but there is controversy as to the actual thickness. The tear film is generally thought to be made up of 3 layers. The surface lipid or oily layer derived from the meibomian glands, the aqueous layer from the lacrimal gland and accessory lacrimal glands, and the mucoid layer from the goblet cells in the conjunctiva (some mucins are secreted by the epithelial cells). There is, however, a gradual transition from mucoid layer to the aqueous layer with mucoid material in the aqueous. The oily layer minimizes evaporation and prevent tears from spilling out onto the lids. The aqueous layer is the major portion of the tear film containing proteins, electrolytes, and other compounds but made up mainly of water . The mucoid layer covers the hydrophobic cell membranes allowing the aqueous layer to form a film. Without the mucoid layer the tears would just bead up and a film would not be formed. The mucoid layer is spread over the cornea by the wiping action of the lids on blinking. Failure of the wiping of the mucoid layer over an area of the cornea by the lids will result in epithelial cell damage, such as with a raised area near the limbus causing a dellen (ulcerated area near the limbus) or a thick rigid contact lens edge causing peripheral corneal staining. Normal tear production is about 1 µl/min. At this rate no tears reach the inferior meatus of the nose, with higher rates tears do reach the nose. If production is over 100 µl/min then tears flow over the lid margins (epiphora). During sleep and general anesthesia tear production almost ceases. This decreased production during sleep is likely the reason that contact lenses stick to the cornea with overnight wear and often feel dry and sticky on awakening. Likewise, many dry eye patients complain of discomfort on awakening. In addition to the film of tears over the cornea and conjunctiva, under normal conditions there is a volume of tears along the lid margins. This strip of tears is called the margin tear meniscus, margin tear strip or lacrimal lake. With dry eye this tear meniscus may not be continuous.

Tears are lost from the eye via: Evaporation Absorption by the conjunctiva of the lids and sclera Drained through the puncta Spilled over the lid margins When the lids close the upper and lower puncta match up and seal. With the lid muscular contraction during a blink fluid is forced from the canaliculi forming a partial vacuum in the canaliculi. When the lids open the last place the upper and lower lids touch is at the puncta. As the puncta separate the partial vacuum formed in the canaliculi sucks some of the tears from the tear meniscus into the canaliculi.

Tear film osmolarity (tonicity) The normal osmolarity of the tear film is about 315 mOsm/kg or equivalent to a 0.95% to 1% NaCl solution, greater than in the plasma. The osmolarity will increase due to tear film evaporation and with dry eye conditions. The osmolarity of the tear film is important in maintaining normal epithelial thickness and corneal hydration. If osmolarity decreases, such as swimming in fresh water (a hypotonic solution) or with excessive tearing, the corneal epithelium will become edematous. If a hypertonic solution (high osmolarity) is put on the cornea the epithelium will thin. This phenomenon is used to clear corneal edema with hypertonic solutions. With the closed eye the osmolarity decreases (more hypotonic) than with the open eye. This shift is probably due to the fact that there is no evaporation of the water with the closed eye.

Relative shift in osmotic pressure of the tear film from the open eye (100%) versus after a night's sleep (open circles). (From Hill RM. Curiosities of the Contact Lens. Professional Press 1981) Tear pH and buffering The pH of the tear film is alkaline in the range of 7.35 to 7.45. With prolonged eye closure, as with sleep, the pH of the tears becomes more acidic. This acidic shift may be due to the build-up of CO2 which causes carbonic acid to be formed.

pH change with the closed eye. Dashed line is open eye pH, open circles average tear pH after a night of sleep for each with arrow showing greatest change for each patient. (from Hill RM. Curiosities of the Contact Lens. Professional Press 1981) The tear film has bicarbonate ions and other compounds that are buffering agents which keeps the pH of the tears relatively constant. If a mild acidic or basic solution is introduced, the buffering capacity of the tears will neutralize the solution. This allows eye medications and contact lens solutions to be maintained at a pH different from the tear film but not create a problem when put on the eye. Such solutions can not be themselves highly buffered. Solutions with pH's in the range of 6.8 to 8.2 are usually tolerated. Tear film composition

The tear film has many components including numerous lipids, mucins, more than 60 proteins, numerous electrolytes, enzymes, vitamins and cells. Electrolytes Sodium and chloride ions are important for maintaining the epithelial cells and osmotic pressure of the tears. Potassium is critical to epithelial health and is higher in the tears by about 4X than in the plasma. Calcium and magnesium are also present and important for epithelial cell health Proteins Lysozyme, lactoferrin, -lysin and immunogloblins are proteins important in preventing bacterial infections. Cells Polymorphonuclear leucocytes are present in the tears and are important in immune reactions to prevent infection. Following sleep there is a great increase in the number of leucocytes indicating an inflammatory process. This can be a significant concern with the wearing of contact lenses overnight. There are sloughed off corneal and conjunctival cells in the tear film. These can collect under a contact lens during sleep. Tear film tests There are a number of tests used to test tear film quantity and quality. A common clinical test is the Tear Film Break-Up Time (TBUT or BUT). The test consists of instilling a small amount of fluorescein dye into the tear film and then viewing the tear film with a biomicroscope (with a cobalt filter and possibly a yellow filter over objective). One times the period between a blink and the first visible dark spot in the tear film indicating a break in the tear film. The BUT varies considerably for normal patients but averages between 20 and 30 seconds but many patients will have much longer BUT's. If it is below 10 seconds it is usually considered abnormal.

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20

NUMBER OF EYES

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10

5

0

5-9 10-14 15-19 20-24 25-29 30-34 35-39 40-44 45-49 50-54 55-59 >60

BREAK-UP TIME (seconds)

Distribution of TBUT among a group of normal, young individuals (Redrawn from Rengstorff RH. Am J Optom 1974; 51:765) Other tests include: Corneal staining with fluorescein, rose bengal and lassamine green Schirmer Test Phenol red thread test Masurement of the lacrimal prism height Impression cytology Osmolarity measurement Fluorescein dilution test Lactoferrin immunoassay test More information can be found on these tests in Dryness, Tears, and Contact Lens Wear. Lowther GE. Butterworth-Heinemann, 1997. The Cornea Corneal dimensions (averages): Horizontal visible iris diameter (HVID): Vertical visible iris diameter (VVID): Center thickness: Peripheral thickness: Average front surface apical radius: Average back surface apical radius: Refractive index: Water content: 11.7 mm 10.6 mm 0.52 mm 0.67 mm 7.8 mm (43.27 D.) 6.5 mm 1.376 78%

The cornea is made up of 5 layers: Epithelium-about 0.05 mm thick (about 10% of the corneal thickness) Bowman's membrane Stroma-major portion of the cornea (approximately 90% of cornea thickness) Descemet's membrane-thin, elastic membrane Endothelium-single cell layer on inner surface to the cornea

The epithelium is made up of about 5 cell layers. The outer surface cells are squamous cells with the underlying cells being wing cells. The inner most layer of cells are the basal cells-these are the cells that divide and then migrate to the surface with the surface cells sloughing off. The life of an epithelium cell is about a week. Basal cells are attached to underlying basement membrane by hemidesmosomes and there are tight junctions (desmosomes) between epithelial cells. The tight junctions between cells and hydrophobic cell membranes prevent aqueous solutions from easily moving through the epithelium. Hydrophobic compounds, however, can easily penetrate the epithelial membranes and move through the epithelium.

The surface cells have microvilli, may have a function in stabilizing the tear film.

When the epithelium is damaged, it regenerates quickly by the migration of cells followed by mitosis. Several growth factors are involved. The cells migrate from the stem cells in the limbal region towards the center of the cornea. Small areas of damage may be healed in as little as 3-4 hours. With large areas of denudation the epithelial cells start to migrate into the area within

about 1 hour and if a large area is affected it may take several days for the epithelium to return to normal.

Migration pattern of the epithelium. During healing from a major epithelial loss a central pattern (solid line in diagram) may be visible. The epithelium has a thin basement membrane that is important in the adhesion of the epithelium. If it is damaged the epithelium is easily sloughed off causing what is called a recurrent erosion. Recurrent erosions commonly occur on awakening due to the lid adhering to the epithelium at night since there is little tearing at night. On awakening and opening the eye the loose epithelium is pulled off. Bowman's membrane is under the basement membrane. It is modified stroma and made up of fine collagen fibers. If damaged scarring will occur. The stroma makes up 90% (0.50 mm) of the corneal thickness. It is mainly collagen fibers arranged in about 200 to 250 parallel lamellae. The lamellae are arranged in a very orderly pattern resulting in good transparency. If the orderly arrangement is disturbed, for example by edema, the transparency is lost. There is a ground substance around the collagen made up of hydrophilic glycoaminoglycans. In addition, there are keratocytes, thin flat cells, between the lamellae.

From Hogan et al, Histology of the Human Eye, 1971, WB Saunders Co. Descemet's membrane is a thin, structureless, elastic membrane produced by the endothelim. It acts as the base membrane for the endothelium. Sometimes thickened areas form in the membrane and are called Hassall-Henle bodies. These are seen as dark spots with the biomicroscope.

(a-stroma, b-Descemet's membrane, c-endothelial cells) From Hogan et al, Histology of the Human Eye, 1971, WB Saunders Co.

The endothelium is made up of a single layer of hexagonal cells. These cells do not regenerate (or at least very slowly). If cell loss occurs surrounding cells will enlarge to cover the area of loss. This results in some cells being much larger than others, a condition called polymegathism. Endothelial cells are lost with aging. At birth there are approximately 4,000 cells/mm2 and decrease to about 2,000 cells/mm2 at 80 to 90 years of age. Long term hypoxia from low oxygen permeable contact lenses can cause cell loss and polymegathism.

Corneal Physiology The corneal epithelium utilizes oxygen from the atmosphere (diffuses through the tear film) and as a result of aerobic metabolism expels CO2 into the atmosphere. At sea level the air contents 20.9% oxygen and decreases with increasing altitude. With the closed eye, for example during sleep, the epithelium derives oxygen from the blood vessels of the conjunctiva reducing the oxygen level to about 7%.

Oxygen utilization and carbon dioxide expelled from the cornea under normal conditions.

Effect of altitude on oxygen and nitrogen content in the atmosphere. There is considerably less oxygen available to the front of the eye with increasing altitude. (From Hill RM. Curiosities of the Contact Lens. Professional Press 1981) There can be significant differences among individuals as to their corneal oxygen requirements. The graph below shows that there can be a 3-fold difference between individuals. This can be significant, especially for sleeping with lenses on the eye. The patient that needs more than the average oxygen may have adverse effects.

30 25 Number of Subjects 20 15 10 5 0 3 4 5

6 7 Oxygen Uptake

8

9

Data from Larke JR, Parish ST, Wigham: Apparent Human Corneal Oxygen Uptake Rate. Am J Optom 1981; 58:803. With 20.9% oxygen in the air at sea level, the following approximate oxygen levels are required for the indicated functions: Regular corneal function: To prevent suppression of mitosis To prevent corneal sensitivity loss To prevent depletion of glycogen stores 15% to 20.9% 13.1% 8.0% 5%

With corneal metabolism glucose is broken down in to obtain the required energy for the corneal cells. With sufficient oxygen present glucose is metabolized via the Embden-Meyerhof Pathway and the Tricarboxylic Acid Cycle (also know as Kreb's cycle or Citric Acid Cycle) to water and carbon dioxide resulting in 36 high energy ATP units. However, if sufficient oxygen is not present the glucose is degraded only to lactic acid with only 2 ATP's formed. Thus, under anaerobic conditions greater amounts of glucose must be metabolized for the same energy output.

About 60% to 70% of the glucose is used in the Hexose Monophosphate Shunt (also known as the Pentose Phosphate Pathway) which does not result in any net energy. It produces NADPH used as substrate for RNA and DNA synthesis. The majority of the glucose for epithelial metabolism is derived from the aqueous. It diffuses through the stroma. Very little comes from the tear film. Glucose is stored in the epithelial cells as glycogen. The photos below show normal epithelial glycogen stores (dark stain) in the top photo with the total loss of glycogen in the bottom section following a period of anoxia (oxygen deprivation).

With anoxia not only are the glycogen stores lost but due to the lack of energy water is not pumped from the epithelium and stroma resulting in edema. Also the lactic acid produced causes an influx of water. The photos below show a normal epithelium and epithelial swelling after a period of anoxia.

There is little metabolism in the stroma. Only the keratocytes require energy. The endothelial cells have a high metabolism. The endothelium is critical to maintaining the thickness and transparency of the cornea. Loss of the endothelium causes more corneal edema than loss of the epithelium. The endothelium receives glucose and most of its oxygen from the aqueous humor. The oxygen content of the aqueous is about 7.4%. Thus, luckily, contact lenses have only a minimal effect of the endothelium other then the polymegethism mentioned earlier. Suggested readings: There are numerous sources of further information on the anatomy and physiology of the anterior segment of the eye. Below are some possible sources if you want further information. Adler's Physiology of the Eye, 9th edition; Mosby 1992. chapters 1-3. Efron N. Contact Lens Practice. Butterworth-Heinemann 2002. Chapter 2. Phillips AJ, Speedwell L. Contact Lenses. Butterworth-Heinemann, 4th edition, 1997 chapter 2 Ruben M, Guillon M. Contact Lens Practice. Chapman & Hall Medical, 1994. Chapters 12-15.

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REVIEW OF SOME OCULAR ANATOMY AND PHYSIOLOGY

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