A Review of Current Ocular Research

Here's a summary
of some of the latest, hot-off-the-press
journal articles in ocular research.

Marjorie Jeandervin, OD, MS
MARCH 1999

Sometimes there just isn't enough time to read through all of those journals that are most likely piling up on your desk. This article will help you catch-up on some of the most recent journal topics.

The Pathology of Dry Eye

What causative factor(s) initiates the sequence of events resulting in the clinical symptoms experienced by the patient? This question was the basis for a review of dry eye pathology by Michael Stern et al., (1998, Cornea). They described the lacrimal system as a functional unit composed of the ocular surface (the cornea, conjunctiva, accessory lacrimal glands and meibomian glands), the main lacrimal gland and interconnecting neural reflex loops. Figure 1 provides a brief summary of the neural pathways of the lacrimal system.

The normal eye protects against environmental influences such as air currents, low humidity and foreign bodies by producing tear proteins and growth factors. In addition, corneal epithelial cells and conjunctival goblet cells produce mucins to aid in lubricating the eye. As long as this system is functioning properly, dry eye syndrome should not occur.

In autoimmune disorders, activation of the T cells normally migrating through the lacrimal gland results in secretion of proinflammatory cytokines. This causes a disruption in nerve cell function and produces an alteration of the lacrimal functional unit which results in dry eye. In addition, recent findings indicate that apoptosis (programmed cell death) of lacrimal acinar cells, conjunctival epithelial cells and lymphocytes plays an important role in dry eye mechanisms.

Another proposed etiology of dry eye syndrome is the loss of systemic androgens in pre- and postmenopausal females, which results in a loss of support for lacrimal secretory function, producing an inflammatory environment which ultimately leads to lacrimal atrophy. Studies have shown that cyclosporine, an immunomodulatory drug well-known for its prevention of T cell-driven rejection after organ transplantation and treatment of several autoimmune diseases, reduces rose bengal staining of the ocular surface. Cyclosporine is also thought to prevent the production of inflammatory cytokines. Further studies of topical ophthalmic cyclosporine A for dry eye treatment are currently being conducted, and the systemic and topical use of androgens is also being investigated.

Chronic Epithelial Injury

Keratocyte apoptosis has been hypothesized to play a role in the pathogenesis of keratoconus. That's what Kim et al., (1999) reported in their recent article in Investigative Ophthalmology and Visual Science.

Apoptosis is thought to be involved in modulating the location of keratocytes in the stroma. An imbalance in the apoptosis process may be involved in keratoconus pathogenesis. The researchers investigated the corneal morphology changes associated with the epithelial injury noted in keratoconic patients by examining the effect of chronic corneal epithelial scraping on 20 eight-week-old white rabbits.

The rabbits were divided into two groups: a control group, whose corneas were left intact, and a treatment group, whose corneas were scraped. The investigators removed the entire corneal epithelium extending to the basement membrane, sparing approximately 1mm of the epithelium at the limbus. Immediately after scraping, the investigators instilled three drops of ciprofloxin and taped the lids closed for two hours. The thickness of the geometric center of the cornea was measured weekly using ultrasound pachymetry. Histologic analysis was performed at four, eight and 16 weeks.

A significant increase in the mean total thickness of the unwounded control corneas was noted at 16 weeks, but no significant change in mean total thickness in the chronically scraped corneas was noted. However, the mean total thickness of the chronically scraped corneas was significantly thinner than that of the unwounded corneas at eight, 12 and 16 weeks. As early as four weeks, epithelial hyperplasia developed in the central cornea of all chronically scraped eyes. The average thickness of the epithelium of the central cornea was approximately two times thicker in the chronically injured eyes than in the unwounded eyes at 16 weeks. Epithelial hyperplasia and a subepithelial acellular zone developed in the chronically injured eyes after four weeks of scraping.

In rabbits, chronic epithelial scraping leads to marked changes in corneal morphology and epithelial thickness. The authors found that the unwounded control corneas increased in total thickness and were thicker than the wounded corneas after 16 weeks. The total thickness of the wounded corneas remained the same, but calculated stromal thickness decreased with chronic corneal scraping. This decrease in stromal thickness, combined with the development of the subepithelial acellular zone and the epithelial hyperplasia, resulted in a relatively constant total corneal thickness. The authors hypothesize that these changes may be attributed to an activation of keratocytes beneath the acellular zone that produces growth factors. These growth factors are thought to stimulate epithelial cell proliferation and inhibit terminal differentiation, which ultimately results in epithelial hyperplasia.

Ongoing epithelial injury in keratoconus (in the form of chronic eye rubbing, poorly fit contact lenses, etc.) may produce effects similar to those found in the rabbit eyes. The pathophysiology of keratoconus is definitely complex, but perhaps we are one step closer to the answers.

Ultraviolet Radiation

Apoptosis has also been found in the crystalline lens following exposure to ultraviolet radiation. Condensation and fragmentation of the cellular content are the main characteristics of apoptosis in the lens. Ultraviolet radiation-induced apoptosis in rats peaked approximately 24 hours after exposure and involved the entire lens epithelium. Cells that fail to undergo repair processes following ultraviolet exposure undergo programmed cell death (apoptosis) instead.

The morphologic changes associated with apoptosis occur in stages, beginning with the formation of apoptotic bodies. This starts with chromatin condensation along the nuclear envelope of the cell. The adjacent cells then phagocytose these apoptotic bodies. Once inside the phagocytosing cell, the apoptotic bodies are further degenerated and digested until the cell is completely destroyed.

Long-Term Use of Antiglaucoma Medications

What happens to patients with long-term exposure of glaucoma medications? Liesegang (1998) provides a review of the extensive literature on the topic. The conjunctival response to long-term therapy can range from nonspecific toxic reactions to drug-induced cicatricial conjunctivitis (DICC). Although these adverse reactions may be due to the antiglaucoma medications, it's more likely that they're a result of the preservatives used.

The most commonly used preservative in antiglaucoma medications is benzalkonium chloride. This preservative has been found to produce severe changes in epithelial cells. Table 1 provides a list of adverse reactions to specific antiglaucoma medications. The subclinical effects of long-term therapy with multiple topical antiglaucoma medications include subclinical inflammation, loss of desmosomes and tight junctions, a pronounced increase in rough endoplasmic reticulum, abnormalities of the Golgi apparatus and a reduction in the number of microvilli. A subepithelial increase of lymphocytes, mast cells, macrophages and fibroblasts have also been reported in patients on 12 months or more of topical glaucoma therapy. Studies have shown a decrease in the number of conjunctival goblet cells with the use of two antiglaucoma medications for longer than three years. The opposite was found with long-term use of pilocarpine alone. Other reported epithelial changes include hyperplasia and keratinization.

Perhaps one of the most severe complications of long-term topical glaucoma therapy is DICC. Previous studies estimate the appearance of the first symptoms after 11 to 15 years of therapy. Table 2 lists the common features of various stages of the condition. The clinical signs and symptoms are similar to ocular cicatricial pemphigoid with an absence of the systemic features. DICC is usually nonprogressive and is predominantly seen in the lower fornix. Discontinuation of drug therapy does not lead to regression of tissue scarring.

These adverse reactions to the conjunctiva are significant when considering their implications in filtration surgery. The success rate of filtration surgery is reported to be lowered by long-term use of and the combination use of topical antiglaucoma medications. Although the use of B-blockers alone has not been implicated in the poor success of filtration surgeries, when used in combination with miotics or sympathomimetics it is a risk factor.

So do we send all glaucoma patients for filtration surgery and bypass topical medications? Perhaps a better conclusion is to monitor the patients who have been using topical antiglaucoma medications on a long-term basis and to keep in mind that "sympathomimetics and to a lesser extent, miotics, appear to have a more pronounced adverse effect compared with B-blockers, but preservatives and other factors, such as the duration of therapy, undoubtedly play a significant role. Long-term combination therapy, particularly that including miotics and sympathomimetics, should be considered a risk factor for failure of filtration surgery." Dr. Liesegang does point out that this review does not include many of the newer medications available.

Advances in Myopia Research

The search for a pharmaceutical treatment for myopia has led to studies of muscarinic acetylcholine receptors. These receptors have been implicated in the control of myopia in humans and in animal models. Determining which of the muscarinic receptors are involved in axial elongation is the focus of a study conducted by Lind et al, (1998). They evaluated the effects of the drugs atropine, pirenzepine, gallamine and 4-diphenylacetoxy-N-methylpiperidine methiodide (4-DAMP) on the chondrocytes of chick scleras.

Each of these drugs has a different site of action. Atropine is a nonselective muscarinic acetylcholine receptor antagonist. Pirenzepine is a selective M1 antagonist, gallamine is a selective M2 antagonist, and 4-DAMP is an M1 and M3 antagonist. Atropine has been found to reduce the progression of form-deprivation myopia in human and in animal studies. Unfortunately, atropine as myopia therapy is not without side effects. Pupil dilation and paralysis of accommodation are produced by atropine's effect on M3 receptors. It's important to determine which receptor type is involved in regulating the growth of the eye to find a drug with minimal side effects. A comparison of the effects of atropine, pirenzepine and 4-DAMP suggests that the M1 receptor type influences axial elongation in chicks. Gallamine had no effect. Although atropine was the most potent of the antagonists used in the study, pirenzepine was also effective and produced fewer side effects. Further research on the effects of muscarinic acetylcholine receptor antagonists will determine whether they can be used safely and effectively for myopia treatment in human beings.

Wine Consumption and ARMD

We have all heard that a glass of wine a day is good for your heart. What most likely started as an anecdotal remark has since been taken more seriously and has become the topic of clinical research. It has been reported that cardiovascular disease and age-related macular degeneration (ARMD) may have the same pathologic pathway and that moderate wine consumption decreases the risk of cardiovascular disease.

Despite these findings, Obisesan et al., (1998) from the Department of Internal Medicine and Division of Ophthalmology at Howard University Hospital conducted a study which set out to examine the hypothesis that alcohol intake increased the risk of developing ARMD. Data were taken from the National Health Nutrition and Examination Survey (NHANES-1) ophthalmology data set and medical history questionnaires.

Included in this analysis was the ophthalmologic examination of 3, 072 people between the ages of 45 and 74. The mean age was 62. Cases were classified as having ARMD if they had been diagnosed with senile macular degeneration, senile disciform degeneration or senile circinate degeneration. Loss of macular reflex, pigment dispersion or clumping and drusen, and best corrected visual acuity of 20/25 or worse attributable to the disease were required for a diagnosis of ARMD.

The authors found a negative association between wine consumption and the development of ARMD. This was true for wine consumption alone, wine and beer combined and wine and liquor combined. In a multivariate analysis adjusting for the effect of age, gender, income, history of heart failure and hypertension, only the negative association of wine consumption alone and age-related macular degeneration was statistically significant (the odds ratio reported was 0.81).

 

TABLE 1:  Conjunctival damage reported as a result of long-term use of specific antiglaucoma medication PILOCARPINE, EPINEPHRINE, DIPIVEFRIN Chronic follicular conjunctivitis EPINEPHRINE, PILOCARPINE, GUANETHIDINE Epidermalization and goblet cell loss B-ADRENERGIC ANTAGONISTS Epithelization and keratinization TIMOLOL MALEATE Damage ocular surface by decreasing aqueous layer production rate B-BLOCKERS Delay corneal epithelial wound healing; corneal epithelial deficiency EPINEPHRINE, DIPIVEFRIN, PILOCARPINE, Drug-induced cicatrizing conjunctivitis B-BLOCKERS, ECHOTHIOPATE IODIDE

 

Table 2:  Characteristics of DICC EARLY STAGES Non-specific chronic papillary conjunctivitis LATER STAGES Foreshortening of the inferior fornix Shallowing of the canthal recesses Canthal keratinization Obliteration of the conjunctival folds Flattening of the plica and caruncle Conjunctival thickening Increased conjunctival vascularization WITH PROGRESSION Subepithelial fibrosis Punctal occlusion (especially inferior) with symblehara Conjunctival ulceration Trichiasis ADVANCED STAGES Complete cicatrization or epidermalization with accompanying secondary complications (infections and corneal opacification with vascularization)

References are available upon request to the editors at Contact Lens Spectrum. To receive references via fax, call (800) 239-4686 and request document #46. (Be sure to have a fax number ready).

Dr. Jeandervin is a graduate student at the Ohio State University. She is currently studying the effects of near work and myopia.

 

---

Report from BobFest

Robert B. Mandell, O.D., Ph.D. was honored for his substantial contributions to researching and teaching contact lenses, corneal physiology and corneal topography and for his 65th birthday at the BobFest, Corneal & Contact Lens Symposium, November 13th and 14th, 1998. Here is a sample of the continuing education provided by many of the people who participated: Charles Campbell, Ph.D. discussed the limits of resolution of current topography systems approaching 0.25D and 0.37�m of height and the value of repeated measures. Timothy Turner, Ph.D. reviewed corneal height map interpretation and drew an analogy to a reference sphere being like earth topography mapping and sea level. John Greivenkamp, Ph.D. presented the Zernike building blocks of understanding corneal topography. Cynthia Roberts, Ph.D. presented corneal maps that looked like Bob Mandell and peripheral thickening of the cornea in PRK related to hyperopic shift. Ray Applegate, O.D., Ph.D. discussed increased spherical aberration in eyes with PRK. Howard Howland, Ph.D. discussed measurement of coma and trefoil in vision research. Stanley Klein, Ph.D. reviewed corneal volume calculations and contact lens centration and motion. Steve Klyce, Ph.D. discussed the need for standardization in corneal mapping and linking excimer laser-corneal mapping intraoperatively, which was later demonstrated by Margarite McDonald, M.D. Brien Holden, Ph.D. presented an argument that corneal topography is important and refractive surgery is questionable. Robert Maloney, M.D. presented results of low contrast visual acuity associated with irregular astigmatism after refractive surgery. Loretta Szczotka, O.D., M.S. reviewed results of how valuable corneal mapping is to contact lens fitting. Barbara Caffery, O.D., M.S. reviewed lissamine green and rose bengal staining and lake of correlation with patient symptoms. Brian Levy, O.D., M.S. suggested surface chemistry, and not just more oxygen, will be a key for safer contact lens extended wear. Charles Leahy, O.D., Ph.D. and Carolyn Begley, O.D. discussed theories of lens comfort related to mucins, contaminated mucins acting as adhesives and immune response. Kenneth Polse served as the BobFest program chairman.

by Joseph T. Barr, O.D.

THE EYESSENTIALS Long-term combination therapy, particularly that including miotics and sympathomimetics, should be considered a risk factor for failure of filtration surgery. Studies have shown that cyclosporine reduces rose bengal staining of the ocular surface. Cells that fail to undergo repair processes following ultraviolet exposure undergo programmed cell death (apoptosis) instead.