Ординатура / Офтальмология / Английские материалы / Orthokeratology Principles and Practice_Mountford, Ruston, Dave_2004

50 ORTHOKERATOLOGY

and corneal warpage (Holden et al 1988). However, the use of purely overnight-wear RGP lenses (as opposed to daytime wear) offers the orthokeratologist a number of advantages:

•Patients do not have adaptation problems. The absence of the blink reflex and reduced lens movement allows the patient to be comfortable.

•Overnight wear of RGP lenses does not result in significant lens deposition (Bennett et al 1986, Elie 1986). This is partly due to the fact that lenses are only worn during sleeping hours and due to the properties of modern contact lens materials.

•Rigid lenses offer superior handling compared to soft lenses (Key & Mobley 1989).

Unfortunately, no clinical studies relating to overnight wear of RGP lenses were found whilst compiling this review. However, important publications describing the complications of extendedwear RGP lenses will be discussed. The major complications of extended-wear RGP lenses arise because of the effects of oxygen depletion (hypoxia), lens adherence, and ocular infection. Each of these factors will now be considered in detail.

HYPOXIA

Basic physiology

Corneal tissue undergoes more metabolic activity than any other structure in the body. This high level of metabolic activity is principally responsible for maintaining corneal transparency. Oxygen, being a major requirement for any metabolic process, is derived from both the anterior and posterior surfaces of the cornea. Under normal circumstances, the cornea produces energy using aerobic respiration. This process effectively converts glucose into high levels of energy and basic waste products. When hypoxic stress is placed on the cornea (oxygen deprivation), the metabolic cycle used to convert glucose into energy is impeded. Instead, anaerobic respiration begins, producing less energy and a less basic waste product known as lactic acid. The presence of lactic acid encourages water to permeate the cornea by a process known as osmosis (where water moves from a low concentration to a high concentration). The inabil-

Figure 3.1 The effect of corneal hypoxia.

ity of the endothelial pump to remove water at the same rate at which it enters the cornea results in corneal edema (Fig. 3.1).

As well as the accumulation of lactic acid, carbon dioxide also builds up in the corneal stroma. Eventually, the pH of the cornea reduces (an acid shift). It is thought that 30% of corneal acidosis is because of the accumulation of carbon dioxide (hypercapnia).

Oxygen transmissibility and corneal edema

In a landmark study, the minimum oxygen transmissibility required of a contact lens in order to restrict overnight corneal swelling to 4% was

Figure 3.2 The relationship between oxygen transmissibility and corneal edema for extended wear (reproduced with permission from Holden & Mertz 1984© Association for Research in Vision and Ophthalmology).

derived (Holden & Mertz 1984). These authors discovered that a transmissibility of 87 x 10-9 (em x ml O2) / (s x ml x mmHg) was needed in order to achieve the same level of swelling following sleep (Fig. 3.2). Furthermore, a transmissibility of 24 x 10-9 (em x ml O2) / (s x ml x mmHg) was required for the daily wear of contact lenses. At present, none of the conventional soft contact lenses meet the "holy grail" oxygen transmissibility requirement of 87 x 10-9 (em x ml O2 ) / (s x ml x mmHg) required to limit corneal swelling to 4%. Therefore, use of these materials for extended wear poses a risk in compromising corneal health. Only the modern silicone hydrogels and rigid lenses are able to meet the physiological requirement for overnight wear.

The oxygen delivery through a rigid contact lens is not solely dependent on the material or lens design. The tear pump, lid tension, and rapid eye movements also play an important role in the transmission of oxygen (Koetting et aI1985). The tear pump is known to provide 16-19% tear exchange (PoIse 1979). However, during sleep or overnight wear, the tear pump cannot exert its effect. Tear stagnation plays an important role in the mechanism of ocular infection (Fletcher et al 1993a,b). Overnight wear of both hydrogel and rigid lenses reduces tear flow beneath a contact lens; the principal advantage of rigid lenses is that in the open-eye situation the pump will exert a greater effect and thus resolve edema at a faster rate (Holden et aI1988).

PHYSIOLOGICAL SIGNS

The ocular effects of extended-wear RGP lenses will be discussed in terms of the morphological changes that occur to anatomical structures of the eye and also with respect to recognized ocular conditions.

Changes in the tear film following extended wear

As the tear pump is not active during the closedeye situation, tear stagnation is present within overnight wear of RGP lenses. Tear flow beneath a contact lens is an important factor in the pathogenesis of contact lens-related ocular

OVERNIGHT AND EXTENDED WEAR OF RGP LENSES 51

inflammation. Tear stagnation results in decaying debris and bacteria becoming trapped between the posterior surface of the contact lens and the corneal epithelium (Mertz & Holden 1981, Zantos 1984). In addition, there is no doubt that the layers of the tear film are disturbed following contact lens insertion. Various investigators have demonstrated that one factor associated with Pseudomonas infection in contact lens wearers is an absence of the mucous layer of the tear film during contact lens wear (Fleiszig et al 1994a). In an eye not wearing a contact lens, Pseudomonas binding to the epithelium is inhibited by the mucous layer.

Changes to tear biochemistry following contact lens wear have been explored in detail and are beyond the scope of this chapter. However, some of the more fundamental changes associated with extended wear will be discussed.

--------------------COMPOSITION

Closed-eye lens wear has been associated with increased levels of inflammatory cells (Wilson et al 1989). The number of polymorphonuclear leukocytes (PMNs), however, is dependent on the level of adaptation of the contact lens wearer. Thakur & Willcox (2000) compared the number of PMNs and the levels of inflammatory mediators in tears following sleep in a noncontact lenswearing (NCLW) group, a nonadapted group (NACLW), and an adapted group (ACLW). Significantly higher levels of PMNs were found in the NACLW group compared with other groups. Furthermore, a significantly higher level of PMN chemoattractants was observed in the ACLW group compared to the neophyte and nonlens-wearing groups.

Changes in the concentration of certain protein in the tears can also be an indicator of inflammation. Typically, studies have analyzed the concentration of immunoglobulin A (IgA) in tears and ascertained that lower levels are found in adapted contact lens wearers - suggesting that the eye is a state of subclinical inflammation (Vinding et al 1987). Many more chemical changes occur in the tear film from the opento closed-eye situation and with the introduction of a contact lens. However, this area is more of academic interest.

52 ORTHOKERATOLOGY

TONICITY AND PH

When an unadapted subject inserts a contact lens (soft or rigid), reflex lacrimation dilutes the tear film, resulting in tears of lower osmolarity. However, once adaptation is complete, tear osmolarity is generally found to be higher. The most obvious reason for this would be reduced stimulation of tears due to reduced corneal sensitivity. Other explanations also exist, such as increased aqueous evaporation and deposits on the lens.

There is as yet little agreement as to the nature of pH shift in eyes wearing RGP lenses. Various studies have presented conflicting results. For example, Norn (1988) demonstrated a reduced pH shift in all lens types whereas Carney & Hill (1976) reported no change.

Epithelium (microcysts)

One of the first results of reduced oxygen levels at the anterior corneal surface is decreased cell production (Hamano et al 1983). A number of studies have shown that a reduction in the number of epithelial cells due to hypoxia results in epithelial cell enlargement, epithelial thinning, and, somewhat surprisingly, increased cell life (Bergmanson et a11985, Holden et al 1985a, Lemp & Gold 1986). Epithelial thinning may be explained by the fact that there is reduced epithelial mitosis in the presence of a rigid lens. Therefore, with cell loss exceeding cell production there would be a natural reduction in epithelial thickness (Ren et al 1999, Landage 2000, Bergmanson 2001). The effect of lid tension through the contact lens may induce squeeze pressure on the epithelial cells. Ren et al (1999) found increased epithelial cell flattening following RGP lens wear in rabbits.

An alteration in cell production causes the formation of epithelial microcysts (Fig. 3.3). Bergmanson (1987) suggested that microcysts represent an extracellular accumulation of brokendown cellular material in the basal epithelial layers of the corneal epithelium. Clinically, they appear as tiny translucent dots within the epithelium that are best detected with a slit lamp with high illumination and partial retroillumination (Zantos & Holden 1978). The numbers of microcysts show a correlation to the oxygen permeabil-

Figure 3.3 Epithelial rnicrocvsts.

Table 3.1 The relationship between the number of microcvsts and lens modality

Lens type/Ok/wear Number of microcysts

Dk, oxygen permeability; DW, daily wear; EW, extended wear; RGP, rigid gas-permeable.

ity (Dk) of the lens (Holden et al 1985a), the modality of wear (Holden et al 1985a), and the duration of wear (Fonn & Holden 1988).

Holden et al (1985a) counted the number of microcysts for various Dk lenses (low, medium, and high), lens type (RGP and soft) and usage (daily or extended wear). Table 3.1 summarizes the results.

A change in the metabolic activity of the cornea and those events that lead to tissue acidosis can compromise epithelial integrity. Metabolic corneal staining is generally induced by epithelial decompensation and microcysts that have broken through the epithelium (Fig. 3.4). Epithelial decompensation is clinically visible as diffuse punctate epithelial staining. In circumstances of extreme hypoxia, large full-thickness breaks may be observed (Efron 1999). This is because of the loosening of tight junctions and reduction in the

Figure 3.4 Metabolic epithelial staining.

number of hemidesmosomes (Bergmanson 1987, Madigan et a11987, Madigan & Holden 1992).

In one of the largest studies ever to investigate contact lens-related complications, Hamano et al (1985) recorded the prevalence of clinically significant corneal erosion and / or punctate staining in 66 218 patients. They found a prevalence of only 0.5% for the RGP lens-wearing group, 1.3% in the PMMA group, and 0.9% in the soft contact lens group.

Interestingly, the magnitude of the figures correlates with the level of oxygen transmissibility of the lenses in question, thus providing further indirect evidence that corneal hypoxia results in epithelial decompensation. It should be noted that epithelial microcysts are also found in the noncontact lens-wearing patients; however, they are significantly less.

Poise et aI (1987) conducted an extended-wear comparative prospective study over 6 months with 27 RGP wearers and nine soft lens wearers (oxygen transmissibility (Dk/L) 19.58 and 20.5 respectively). The incidence of epithelial microcysts was 22% for the RGP group and 56% for the soft lens group. It is interesting to note that both lenses had approximately the same degree of oxygen transmissibility; one explanation for this may be the effect of the tear pump that would be operational in the open-eye situation.

Later, in 1988, Poise et al evaluated the effects of extended wear in a group of 35 subjects who wore

OVERNIGHT AND EXTENDED WEAR OF RGP LENSES 53

RGP lenses of mean transmissibility of 37 x lO-y (ern x ml 02)/(S x ml x mmHg) for a period of 6 months. Subjects attended for six morning visits at monthly intervals to assess slit-lamp findings, corneal thickness, and refraction. Corneal swelling was found to be 5.9 ± 3.3% and microcysts were found in 6.5% of subjects. It is important to note that these rather modest complications were observed in subjects wearing lenses with a transmissibility of only 37 x 10-9 (em x ml 02)/(S x ml x mmHg). Currently, manufacturers can produce lenses with a Dk/L that exceeds the Holden-Mertz criterion for physiologically acceptable extended wear (87 x 10-9 (em x ml 02)/ (s x ml x mmHg)).

In another study, Key & Mobley (1989) studied the corneal response to extended-wear RGP lenses using the Paraperm EW lens (Dk 56) over 12 months in 372 subjects. Key & Mobley (1989) found mild 3 and 9 o'clock staining in 12.3%. MacKeen et al (1992) looked at 202 eyes over 12 months for extended wear using Menicon SF-P RGP lenses. Subjects wore lenses for an average of 6.2 days before cleaning. No significant complications were observed. Sigband & Bridgewater (1994) conducted a 3-month extended-wear trial on 81 subjects wearing FluroPerm 151 lenses (Paragon Vision Sciences, Arizona). The authors observed no physiological or pathological complications. Fifteen subjects discontinued due to poor comfort.

The epithelial effects of 24-h RGP lenses of varying transmissibility were determined in a study by Ichijima et al (1992). Rigid lens materials with transmissibility of 0, 33, 56, and 64 x 10-9 (em x ml 02) / (s x ml x mmHg) were placed on rabbit eyes for 24 h, after which the corneal epithelium was studied using the Tandem scanning confocal microscope (TSCM) and confirmed with scanning electron microscopy. TSCM showed no superficial epithelial cells but only the deeper exposed wing cells with the PMMA lens (Dk/L = 0). The lens with a transmissibility of 33 x 10-9 and 56 x 10-9 (em x ml 02)/(S x ml x mmHg) resulted in partial superficial epithelial cell desquamation. However, no epithelial swelling was detected with the 56 x 10-9 lens. The highest-transmissibility lens (Dk/L = 65) resulted in no epithelial swelling or epithelial desquamation.

Another objective study was conducted by Ichijima et al (2000), in which the concentrations of glucose and lactate were measured in the corneal epithelium, stroma, and aqueous humor using an enzyme assay. Lenses of four different transmissibilities (27, 43, 70, and 125 x 10-9 (em x ml O2) / (s x ml x mmHg)) were worn by eight rabbits in the right eye (the left eye was a control) for 1 month of continuous wear. The authors discovered that RGP lens-induced hypoxia is associated with an alteration in glucose-lactate metabolism. Extended wear of low-Ok/L lenses (27-70) resulted in increasing levels of lactate in the epithelium (as measured at day 7 and 1 month) and decreasing levels of lactate in the stroma and aqueous humor. However, the 125 x 10-9 lens showed no increase in epithelial lactate levels. The use of modern high- Ok materials is mandatory when performing overnight orthokeratology. Most modern orthokeratology lenses are made from materials with Oks of over 100 Barrers.

In summary, there is now widely accepted scientific proof that high-oxygen-transmissibility RGP lenses induce a reduced microcystic response compared to lower-transmissibility lenses. Furthermore, the incidence of microcysts increases with modality of wear, i.e., increased number of microcysts with extended wear (Holden et al 1987). More sensitive techniques such as TSCM have confirmed that ultrahigh-Ok lenses cause little or no effect on the corneal epithelium (Ichijima et al 1992). However, from a clinical standpoint the most sensitive test of corneal hypoxia available to practitioners is observation of epithelial microcysts. Generally, it is recommended that if more than 30 micro cysts are observed, clinical intervention is required (Efron 1999). The aim of treatment is to increase the oxygen transmission to the cornea; this may be achieved through increasing the Dk, reducing lens thickness, increasing the effectivity of the tear pump, reducing wearing time, and reducing the number of nights of wear.

Stroma

The principal effect of corneal hypoxia is corneal edema. The mechanism of edema formation was discussed earlier in this chapter. However, other

factors also play a role in lens-induced corneal edema. As well as hypoxia, hypercapnia, increased temperature, increased humidity and hypotonicity are known to induce edema (Sweeney & Holden 1991). However, of these factors, hypoxia is thought to be responsible for approximately 50% of all corneal edema (Sweeney & Holden 1991).

The long-term effects of edema and corneal acidosis were described by Holden et al (1985a). They found that, in a sample of extended-wear soft contact lens wearers who wore their lenses for 5 years, corneal edema reversed within a few days. However, stromal thinning continued for up to 10 days and was permanent. This thinning was attributed to the acidic shift in the cornea. Most clinicians unfortunately do not have a quantitative method of deriving corneal swelling, as with the studies discussed so far. However, the degree of corneal edema may be estimated by detecting subtle changes in the cornea using the slit lamp. In order to observe edema in a clinical setting, practitioners must detect striae. There is a general relationship where one stria approximately equates to 5% edema and endothelial folds around 10% edema (Woods 1989). Naturally, practitioners would also consider any other clinical indicators such as microcysts where the presence of microcysts indicates that normal corneal metabolism is being impeded.

Kamiya (1986), using the Menicon EX lens in 44 subjects worn on an extended-wear schedule for 1 year, found no signs of edema or ocular infection. In an important comparative study, PoIse et al (1987) found approximately equal levels of corneal swelling (7.8% for the RGP sample and 7.3% for the soft lens sample). The oxygen transmissibility of both soft and RGP lenses was also similar (20.5 and 19.58 respectively). Using a lens originally designed for extended wear (Paraperm EW, Ok 52), Key & Mobley (1989) found 8% corneal swelling that recovered to normal after 2 h. Kok et al (1992) confirmed these findings in a study where Quantum lenses (Ok 92) were worn on an extended-wear basis by 32 subjects (62 eyes). They also found corneal thinning over the first 6 months and corneal flattening over the first 3 months.

Corneal thickness was also measured by Iskeleli et al (1996) in a group of 57 eyes using the Hoya Hard EX contact lens (Ok 125). Lenses were

worn on average for a period of 5.28 ± 1.43 days over a 6-month period. Corneal thickness was measured at 1, 3, and 6 months. They found no statistically significant difference in corneal thickness at 1 month. However, a significant decrease in corneal thickness was observed at 3 and 6 months (P < 0.001). No serious complications were found in the course of the study and all 29 subjects continued RGP extended wear. The authors concluded that RGP extended wear provides an alternative to soft lens extended wear.

Generally, one may conclude that hightransmissibility lenses will result in less overnight edema. This statement can be substantiated by the work of Hideji et al (1989), who measured the corneal swelling following RGP lenses of various Dk values against high and low hydrogels and elastomer lenses before and after 24-h wear in rabbits. Greatest swelling was observed with PMMA lenses (21.6 ± 5.4%); conversely, least swelling was observed with the Menicon SF-P (or super EX) lens (Ok 116) (swelling = 2.9 ± 4.0%) - a value similar to that following overnight eye closure. The corneal swelling after elastomer lenses was also found to be similar to that found with the Menicon SF-P lens. With respect to modern-day orthokeratology, where lenses are worn only overnight and not during the day, there will be a greater rate of recovery and less hypoxic stress on the cornea. The very-high-Dk materials available today (up to 210) mean that there is less chance of observing the levels of corneal edema found in the aforementioned studies.

Swarbrick et al (1998) measured corneal and epithelial thickness in 11 eyes after orthokeratology treatment performed on a daily wear basis. Subjects wore OK704 lenses (Ok = 88, Contex Inc., Sherman Oaks, CA) for 28 days. The authors found central epithelial thinning (7.1 ± 7.1ILm) and mid-peripheral thickening approximately 2.5 mm from the corneal center (13.0 ± 11.1lLm) by day 28 (Figs 3.5 and 3.6).These measurements were found to be statistically significant. The authors proposed that the epithelium was either redistributed or compressed. Proposed mechanisms for the stromal changes were postlens tear forces, lens-induced edema, and pressure exerted by the steeper secondary curve on the lens. Unfortunately, the low subject numbers and the accuracy and repeatabil-

OVERNIGHT AND EXTENDED WEAR OF RGP LENSES 55

Figure 3.5 Change in corneal thickness following daily wearof reverse geometry lenses. Positive values represent thickness measurements performed on the nasal cornea.

Reproduced with permission from Swarbrick et al (1998) © The American Academy of Optometry.

Figure 3.6 Changes in epithelial thickness following orthokeratology. Reproduced with permission from Swarbrick et al (1998) © The American Academy of Optometry.

ity of the pachometer mean that further clinical studies need to be performed.

Endothelium

Essentially, two complications occur to the endothelium following extended wear of contact lenses; these are endothelial blebs and polymegathism.

The bleb response

Zantos & Holden (1977) were the first investigators to observe changes to the endothelium. The endothelial mosaic was found to contain localized dark areas similar to the appearance of cell "drop-out" among groups of endothelial cells. Zantos & Holden (1977) referred to this

56 ORTHOKERATOlOGY

appearance as endothelial blebs. This response can occur within minutes of inserting a contact lens (Zantos & Holden 1977). The number of endothelial blebs usually follows a chronological cycle: the greatest number of blebs is observed at 20-30 min and reduces to equilibrium at 4560 min. The equilibrated level lasts for the duration of contact lens wear (Zantos & Holden 1977).

Vannas et al (1984) performed histological studies of the endothelial bleb response. They found that blebs were not associated with cell loss but rather with individual endothelial cell edema. Fluid was also found between cells. The appearance of dark areas when viewed using the slit lamp therefore represents an optical phenomenon (which is concisely described by Efron (1999)). Individual cell edema causes the posterior endothelial cell wall (in contact with the aqueous) to bow. The anterior cell wall does not bow because it is in contact with the comparatively rigid Descemet's membrane. The principle of specular microscopy relies on light being reflected from the posterior endothelium to the microscope (the observation system). A change in configuration of the reflecting surface (in this case, the bowed posterior endothelium) changes the direction of the reflected path of light and thus it is not detected by the microscope (Fig. 3.7).

In order to investigate the etiology of blebs, Holden et al (1985b) assessed the effects of five stimuli on the corneal endothelium and corneal thickness. The stimuli were:

Figure 3.7 Demonstration of why blebs are not visible with specular reflection. Adapted from Efron (1999).

1.silicone elastomer contact lens

2.silicone elastomer contact lens in combination with anoxia

3.anoxia alone

4.thick hydroxyethylmethacrylate (HEMA) contact lens

5.gas mixture of 9.8% carbon dioxide, 20.5% oxygen, and the balance nitrogen.

The resultant effect common to the stimuli that induced blebs was the ability of these stimuli to reduce the pH in or near the corneal endothelial layer, i.e., there was a local acidic shift. As discussed above, acidic shift in the cornea may be induced by hypoxia or by hypercapnia.

Clinically, the presence of endothelial blebs has no known adverse effects. Indeed, blebs have been detected in the noncontact lens-wearing eye - more so upon waking (Khodadoust & Hirst 1984). The diurnal variation in the appearance of blebs may again be accounted for by the fact that the cornea is mildly hypoxic upon waking. If, however, a repetitive exaggerated bleb response is observed, practitioners should increase the lens transmissibility in an attempt to reduce lens-induced hypoxia, which will otherwise lead to an acidic shift in the cornea.

Polymegathism

Polymegathism describes a variation in endothelial cell size (Fig. 3.8). Scientifically, the severity of polymegathism is measured using an index known as the coefficient of variation (COV) of cell size (see below).

Standard deviation of cell area in a COy = given field

Mean area of cells in a given field

The COY value has greater meaning when compared before and after lens wear in an individual subject. Furthermore, the COy is somewhat confusing in that the same value is possible for different mean cell areas (Doughty 1990). Other terms used to describe the endothelium are:

•endothelial cell density (number of cells per square millimeter)

•% hexagonal cells (polygonality).

Figure 3.8 Endothelial polymegathism. Courtesy of Topcon Corporation, Tokyo, Japan.

Endothelial cell density changes throughout life (Yee et a11985, Esgin & Erda 2000), therefore clinical studies must account for these age-related changes. MacRae et al (1985) conducted an agematched controlled clinical trial involving three groups of lens wearers:

•daily-wear soft contact lenses for an average of

6.3years

•long-term (> 20 years) users of hard contact lenses

•former users of hard contact lenses who had worn them for an average of 9.6 years but who had discontinued them for an average of

4.3years.

They found no significant difference in endothelial cell densities but discovered that there was a significant variation in cell size (polymegathism) and cell shape (polymorphism). Furthermore, the authors also found that the endothelial changes were similar in soft and hard contact lenses.

The effect of extended-wear rigid lenses with Oks of 71 and 92 was assessed in 16 patients by Nieuwendaal et al (1991). Subjects wore lenses for between 7 and 24 months. Morphometric analysis revealed no significant change in cell size or shape. The authors attributed this to the high Dk of the lenses used in the study.

Conflicting evidence exists with respect to the change in endothelial cell density and contact lens wear. For example, some investigators

OVERNIGHT AND EXTENDED WEAR OF RGP LENSES 57

(McMahon et a11996, Setaelae et a11998) found a significant reduction in endothelial cell density in PMMA wearers compared to control groups. Conversely, Chang et al (2001) found no change in endothelial cell density in daily wear soft contact lens wearers. Interestingly, Esgin & Erda (2000) found an increase in endothelial cell density in 97 unadapted eyes wearing high-Ok (92) daily-wear rigid lenses: the increase was dependent on the subjects' initial endothelial cell densities. Thus, subjects with cell densities less than the age-related norm observed significant increases in endothelial cell density within the first 3 months of lens wear. The authors explained this phenomenon as an adaptation to a new oxygen environment by cell migration and mitotic activity.

However, virtually all the studies agree that extended-wear lenses and low-Ok daily-wear lenses induce a change in cell shape and size. Bergmanson (l992) conducted a histopathological examination of the polymegethous human corneal endothelium. He found that the polymegethous endothelial cells had normal organelles and thus presumably functioned normally. However, ultrastructural examination of the lateral cell walls showed that they adopted a new oblique rather than straight appearance. Bergmanson used this clinical finding to interpret the pleomorphic changes observed with the specular microscope.

His thesis is that apparently shrunken cells observed with the specular microscope do not actually change in terms of their volume (Fig. 3.9). The cells' anterior face will have increased in size (the posterior face of the cell is viewed with the specular microscope).

Bergmanson (2001) concluded that the functional consequence of polymegethism has yet to

A B

Figure 3.9 (A) Graphic representation of a normal endothelial cell. (8) Polymegethous cell, where the inferior face (that viewed by the specular microscope) has shrunk but the superior face has increased its surface area.