Publications

 

PURPOSE: To examine for epithelial thickness changes in response to compressive and shear forces associated with eye rubbing.

METHODS: The Holden-Payor optical pachometer was used to measure total and stromal-endothelial thickness, at central (right eye) and midperipheral (left eye) locations, for 12 subjects with normal corneas. Epithelial thickness was determined by subtraction. Baseline measurements were compared with measurements recorded immediately after 15 seconds of circular pattern rubbing over the cornea, using an index finger pad, and light to moderate force. Four subjects in each of 3 groups with mean ages of 19.5, 45.3, and 70.2 years were assessed. Measurement continued at 15-minute intervals until baseline thickness was restored.

RESULTS:An 18.4% reduction in epithelial thickness was recorded immediately after rubbing for central right eye and midperipheral left eye assessments. Both findings were statistically significant (P < 0.05). Recovery to baseline thickness occurred between 15 and 30 minutes centrally and between 30 and 45 minutes midperipherally.

CONCLUSIONS: These results indicate a statistically significant mechanical trauma to the epithelium after 15 seconds of light to moderate rubbing. With reference to the rheological properties of the cytoskeleton, an attempt to explain how the epithelium could lose and recover thickness has been made. Apart from cell flattening, displacement from the rubbed area of intercellular water, cytoplasm from ruptured cells, chains of wing cells, and mucin are suggested as possible thinning mechanisms. Extraneous eye rubbing may adversely alter the results obtained with pachymetry, videokeratography, wavefront aberrometry, refraction, and results obtained with orthokeratology contact lenses.

 

PURPOSE: To examine corneal responses to elevated intraocular pressure (IOP).

METHODS: For a sample of 10 normal subjects, noncontact tonometry was used to measure IOP elevations in response to scleral indentation from a standardized ophthalmodynamometer (ODM) force. Using the same ODM force, corneal topography was assessed for the same controls and a sample of 10 subjects with keratoconus (KC). It was assumed that the mean and range of IOP elevations were similar for both samples.

RESULTS: The ODM induced a mean IOP elevation for the control eyes of 99.4%. IOP elevation during topography was 15-20 seconds for both samples. With elevated IOP, there were no significant topographical changes for control subjects, but the mean values for steepest point of curvature and flat and steep simulated keratometry were significantly increased in subjects with KC [+1.84 (P < 0.029), +0.64 (P = 0.046), and +1.31 diopters (D) (P = 0.03), respectively]. The changes were significantly greater in subjects less than 30 years (P < 0.05). There were no significant topography changes from baseline after IOP elevation, for either control or KC samples.

CONCLUSIONS: Abnormal elastic (reversible) increased distensibility in some KC corneas is consistent with reduced corneal rigidity (lower elastic modulus and/or thickness). Abnormal distending responses may be increased when IOP elevations are higher and/or longer and/or more frequent. The results suggest that abnormal distending responses to elevated IOP in KC may reduce with age.

 

PURPOSE: To examine the corneal shape and ocular refraction under conditions of increased intraocular pressure (IOP) and to consider the possible influences of direct digital forces, corneal thickness, limbal and scleral rigidity, and the duration and frequency of IOP loading on the results.

METHODS: Forces applied digitally to the temporal sclera were used to obtain 15 optimized video-keratographic records for two subjects under different levels of IOP loading. Stability of refraction was also examined under the same conditions.

RESULTS: Relative to normal IOP, four levels of IOP increments were induced: 43% to 61% (from a normal 18.8 mm Hg to approximately 28.6 mm Hg), 119% to 153% (from a normal 12.2 mm Hg to approximately 28.8 mm Hg), 181% to 213% (from a normal 18.8 mm Hg to approximately 55.8 mm Hg), and 374% to 443% (from a normal 12.2mm Hg to approximately 62.1 mm Hg). Significant changes in response to IOP loading were few in number and of a minor degree. For example, the maximum simulated keratometry change was 0.20 diopters (D), and the maximum change in refractive error recorded was 0.625 D in cylinder.

CONCLUSIONS: Corneal shape and refraction were found to show only minor evidence of instability under conditions of IOP loading. The small number and minor degree of corneal changes recorded under IOP loading suggest that under these conditions of measurement, corneal stability may be the result of stress stiffening. However, mechanisms for corneal shape changes that depend on limbal and scleral rigidity may also contribute to corneal shape stability.

 

PURPOSE: To examine the reliability of an experimental method for increasing intraocular pressure (IOP) that could be used during videokeratography.

METHODS: Two levels of digital force (“light” and “firm”) were applied by each subject to the temporal sclera of the right eye through the adnexal skin. Noncontact tonometry was used to record IOP increments obtained for samples of 15 measurements with each level of digital force. The subject was advised of the IOP recorded after each measurement. Because “with-result-feedback” increments in IOP would not be possible during videokeratography, a second, “without-result-feedback” series was recorded 3 minutes later to examine the reliability of the induced IOP increments. The authors alternated the roles of subject and examiner.

RESULTS: The mean normal readings were 19.4 mm Hg (C.W.M.) and 11.7 mm Hg (G.C.B.). The mean IOPs recorded under with-result-feedback conditions for light and firm digital forces were 27.2 mm Hg and 58.3 mm Hg (C.W.M.) and 27.1 mm Hg and 52.1 mm Hg (G.C.B.), respectively. Coincidentally, light digital forces were found to induce IOP increments for G.C.B. that were similar to those that would be expected to occur during 5.5 g weighted Schiotz tonometry. The 95% confidence limits for IOP increments under with-result-feedback conditions for light and firm digital forces were 33% to 47% and 187% to 215% (C.W.M.) and 116% to 146% and 303% to 388% (G.C.B.), respectively. The means of the without-result-feedback IOP increments for the two levels of digital force, and for each subject, all fell within these 95% confidence limits.

CONCLUSIONS: For trained subjects, digital forces on the eye appear to allow an examination of the influence of IOP increments on anterior corneal surface curvature. The mean IOP increments for the two levels of digital force (+40% and +200% [C.W.M.] and +132% and +345% [G.C.B.]) appear to permit videokeratography at higher IOP levels and across a wider range than those previously examined in vivo.

 

BACKGROUND: This study was conducted to investigate the demographics of orthokeratology (OK) practice in Australia, to uncover any previously undocumented cases of serious adverse responses in OK, including microbial keratitis (MK), and to review the demographics of MK in OK in Australia.

METHODS: A questionnaire was sent to the 62 members of the Orthokeratology Society of Australia (OSA). Questions related to aspects of their OK practice, demographics of their OK patient base and any adverse responses to OK lens wear that they had encountered.

RESULTS: Thirty-three questionnaires (53 per cent) were returned. OSA members have been fitting OK lenses for a median of 7.5 years. OK patients were predominantly female, Caucasian, aged between 15 and 39 years and wearing lenses in an overnight modality. In addition to two cases reported previously, the survey uncovered seven further cases of MK in OK patients over an eight-year period. The infecting organism was Pseudomonas aeruginosa in four cases, Acanthamoeba spp. in two cases and unknown in three cases. There was no loss of visual acuity in seven cases. One case resulted in vision of counting fingers at one metre and another case resolved with 6/12 visual acuity. Non-compliance with instructions on lens care and after-care was reported in seven of nine cases of MK.

CONCLUSION: Overall, OSA members who responded to the survey have many years of experience in OK. The typical Australian OK patient is in young adulthood, female and Caucasian. A total of nine cases of presumed MK associated with OK have been reported in Australia over an eight-year period and seven of these were new cases uncovered by this survey. Our analysis suggests that the demographics of MK cases in OK reflect the demographics of the OK lens-wearing population.

 

BACKGROUND: One hypothesis for the pathogenesis of keratoconus includes teenage allergy, ocular itch and associated eye-rubbing.

METHODS: This study examined the prevalence of these factors for teenage and adult patients. The results for a sample of 53 subjects with bilateral keratoconus were compared with those for a control sample of non-keratoconus subjects, who also routinely wore RGP contact lenses. The strongest dominant hand and the eye with more advanced keratoconus were also determined, to examine for a relationship between them.

RESULTS: The keratoconic sample reported significantly higher levels of allergy, itch and rubbing as teenagers and as adults. However, all distributions were bimodal, consistent with the hypothesis that allergy, itch and rubbing are relevant in the pathogenesis of keratoconus only when the highest levels of these factors are present. For example, a significant relationship between the stronger dominant hand and the more advanced eye was evident only in subjects who reported the most severe rubbing.

CONCLUSIONS: This finding adds weight to the circumstantial evidence that rubbing contributes to the pathogenesis of keratoconus. Low levels of teenage rubbing by some keratoconic subjects suggest a non-rubbing pathogenesis and that emphasis on rubbing management is not warranted in these cases. However, high levels of adult rubbing reported by many keratoconic subjects indicate that the standard advice to avoid vigorous and prolonged rubbing is often not effective, even when repeated. There appears to be an indication for the need to improve the management of eye-rubbing for some patients with keratoconus or at risk of developing this disease.

 

We have examined the role of chloride ion channels as part of the control mechanism for adherence and growth of lymphatic endothelial cells. The chloride channel inhibitor N-phenylanthranilic acid (0.1 mM) inhibited the initial adherence of previously nonadherent cells in the period up to 3 hr following seeding onto gelatin-coated culture dishes. The potassium channel inhibitor 4-aminopyridine (1 mM) had no effect on the rate of cell adherence. N-Phenylanthranilic acid had no effect when added to confluent monolayers of cells that had been growing for 3 days. This, and the observation that N-phenylanthranilic acid did not affect the ultrastructure of the cells, suggested that plasma membrane chloride channels are involved in the initial signaling cascade triggered when lymphatic endothelial cells begin to adhere to a substrate.

 

Steroids are widely used in the prevention of corneal neovascularization in a wide range of natural and experimental situations. However, no information is available on their effect on the growth of the individual limbal blood vascular cells or of lymphatic cells involved in corneal neovascularization. In addition, tritiated thymidine labelling index is commonly used as an indicator of cell population but doubt exists as to whether it truly represents cell growth. Remote thermal cautery of the rat cornea was used to elicit corneal neovascularization. New cell growth was measured by tritiated thymidine uptake and by the number of cell nuclei per section. Cells investigated were the arteriolar, venular, capillary and lymphatic endothelial cells as well as the arteriolar and venular perivascular cells. A total of 89,320 blood vascular endothelial and perivascular cell nuclei and 12,075 lymphatic nuclei were counted. Thermal cautery elicited a significant increase in labelling index and cell population of all limbal vascular cell types. Steroid application elicited a significant short term inhibition or delay for all six cell types although this was not apparent for venular endothelial cells using labelling index as a growth indicator. At six days only the lymphatic endothelial cell population showed a significant (p < 0.001) increase associated with steroid use.