[Current methods of photorefractive keratectomy in correcting refractive errors].
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Astigmatism↗
Biomedical subjects
A Vannas
Publications and source records attributed to A Vannas.
Nutritional requirements of the corneal epithelium and anterior stroma: clinical findings.
PURPOSE: To monitor the health of the epithelium and the anterior stroma when porous membranes are implanted into the feline cornea and to determine membrane diffusivity characteristics needed to maintain corneal integrity. METHODS: Filtration membranes in a range of effective pore sizes of less than 15 nm (groups 1 and 2, n = 11), 25 nm (group 3, n = 8), 50 nm (group 4, n = 16), and 100 nm (group 5, n = 15) were implanted into an interlamellar corneal pocket of the stroma. The implanted membranes ranged in thickness from 6 nm to 15 nm and were between 8 mm and 12 mm in diameter. Animals were monitored for clinical signs of intolerance to the implants. RESULTS: At 1 month, thinning and ulceration had occurred in the epithelium and the anterior stroma of all animals in groups 1 and 2; epithelial changes, anterior stromal thinning, and ulceration had developed in 75% of animals of group 3; 50% of animals showed vascularization and only 7% showed epithelial degeneration in group 4; and local anterior stromal thinning was observed in 7% of animals in group 5, indicating clinical acceptance of the implanted membrane. In the long term (greater than 50 days), 30% and 73% of the group 4 and 5 corneas, respectively, were clinically quiet. Analysis of the failure times indicated an inverse relation between failure rate and pore size: less than 15 nm > 25 nm > 50 nm > 100 nm. The difference between the 100-nm and 50-nm membranes was significant (P = 0.03). CONCLUSIONS: A corneal implant must have a porosity greater than that provided by 50-nm membranes. The 100-nm membranes used in this study establish the porosity needed to satisfy the nutritional requirements of the cornea.
Animals↗
Tear plasmin activity with contact lens wear.
Plasmin, a proteolytic enzyme, has been detected in the tears of patients experiencing anterior ocular disease, and during contact lens wear. Using a radial caseinolysis procedure, we examined tear plasmin levels in 66 patients who were wearing soft and rigid lenses for daily and extended wear. Compared to non-contact lens wearers, patients wearing soft and rigid lenses for extended wear were significantly more likely to exhibit tear plasmin activity. Eight hours of open-eye thick HEMA lens wear did not induce tear plasmin activity in a group of 10 subjects. However, significant increases in tear plasmin activity were recorded after short-term (1 hour) eye closure with and without lens wear, and following overnight (8 hours) eye closure without lens wear. Overnight eye closure also resulted in significantly increased numbers of epithelial cells and leucocytes in the tear fluid. Our results suggest that increased tear plasmin activity during extended contact lens wear may be attributable to the effects of eye closure rather than hypoxia or the presence of the contact lens per se.
Adult↗
Incision depth affects the recovery of corneal sensitivity and neural regeneration in the cat.
To assess the effect of incision depth on the recovery of corneal sensitivity and neural regeneration, adult domestic cats underwent either 8-mm circular nonpenetrating keratotomies or penetrating autografts. The contralateral eye served as control. Corneal sensitivity was determined at various intervals after surgery. The depth of incision in the nonpenetrating keratotomies, assessed by optical pachometry, ranged between 49 and 91% of total corneal thickness. The animals were ranked based on the depth of incision and the average sensitivity within the keratotomy over the 1-yr recovery period. A significant negative correlation was found between incision depth and the recovery of sensitivity (Spearman rank-order correlation r = -0.84, P less than 0.05). Some recovery of sensitivity was found in the center and periphery of the incised zone when incision depth was less than 53% of total corneal thickness. With deeper incisions, the center of the incised zone remained insensitive throughout the measurement period, while the periphery of the incised zone showed a slight recovery of corneal sensitivity, proportional to incision depth. The recovery of corneal sensitivity was higher in a small annular region just distal to the incision site. When incision depth exceeded approximately 53%, gold chloride impregnation showed that the resultant reinnervation was confined to single intraepithelial axons or localized regions of irregular epithelial fibers. With shallower incisions, the deeper stromal trunks were spared, resulting in the persistence of a reduced subepithelial plexus and basal epithelial leashes. We have shown that when an incision severs all stromal trunks, the neural regeneration is insufficient for functional recovery of corneal sensitivity at the center of an 8-mm keratotomy.(ABSTRACT TRUNCATED AT 250 WORDS)
Animals↗
Heritability estimate for refractive errors--a population-based sample of adult twins.
The population-based Finnish Twin Cohort study was used to establish a heritability estimate for refractive errors especially for myopia. The twin cohort was derived from adult same-sexed twins in Finland. The total number of twins with both members alive in 1984 was 23,570. Of these, 3,676 twin pairs were monozygotic, and 8,109 pairs dizygotic. The sample for the present study was linked from the Finnish Police Force data base in 1984, where information of a person's possession of a driver's license and the obligation to wear glasses for far correction when driving a motor vehicle is recorded. Correlations in liability were estimated according to a multifactorial method of Smith. Falconer's heritability was 0.62 among males and 0.98 among females in the age group 28-29 years. When compared to previous twin studies of myopia, the proband concordance rates were higher for both MZ and DZ twin pairs.
Adult↗
Long-term changes in corneal endothelial morphology following wounding in the cat.
The cat eye was used to determine the long-term morphological changes in the corneal endothelium that occur after central endothelial wounding. Central corneal thickness was measured using ultrasonic pachometry. Specular microscopy and computer-assisted morphometry was used to quantify central and peripheral endothelial cell density (ECD), coefficient of variation (COV) and the mean and standard deviation of the shape factor (S) over an 18-month period. After endothelial wounding, there was a rapid increase in corneal thickness followed by a rapid nonlinear decline, reaching presurgical levels 35 days after wounding. Central cell density had decreased by 25% at 4 weeks after wounding. During the following 18 months, endothelial cell density in the central cornea increased slightly. The coefficient of variation had increased by 60% at 4 weeks after wounding. This recovered slowly and had reached control levels by 18 months. The mean shape factor was higher in the wounded eye throughout the 18 months, whereas the standard deviation of the shape factor recovered after 12 months. Peripheral ECD had decreased significantly by 12 months after wounding, while COV and the mean shape factor was not significantly affected. The standard deviation of the shape factor had also increased significantly in the peripheral cornea after 18 months. These findings suggest that following endothelial wounding in the cat, changes in endothelial morphology occur over the entire cornea. Endothelial cell density and the shape factor have not recovered to control values, even 18 months after wounding. This pattern of endothelial repair supports the mechanisms of cell movement suggested by Honda et al and confirm the similarities in endothelial response between cat and man.
Animals↗
Ophthalmic disease in twins: a nationwide record linkage study of hospital discharges and free medications for 16,067 twin pairs.
Record linkage of the Finnish Twin Cohort Study with the Hospital Discharge Registry and with the Registry of Rights to Free Medication kept by the Social Insurance Institution gave following numbers of twin pairs with an ophthalmic disease (one or both members of the pair had the disease) in each disease category: 98 with glaucoma simplex (5 concordant pairs), 38 with capsular glaucoma (no concordant pairs), 58 with iritis (no concordant pairs) and 149 with strabismus (2 concordant pairs). The number of concordant pairs in each disease category was small, except for glaucoma simplex in which concordant pairs could be broken down by zygosity. The ratio of observed to expected (based on association by chance) was 6.96 for MZ and 1.74 for DZ pairs. This result suggests that genetic factors play some role in the variability of prevalence of simple glaucoma. Main etiologic factors are still to be found in the environment. Data on occurrence of other diseases of ophthalmologic importance is presented.
Adult↗
Strategies for minimizing the ocular effects of extended contact lens wear--a statistical analysis.
In a study reported recently, the effects of long-term extended wear of soft contact lenses on the human cornea were found to include epithelial thinning, a reduction in epithelial oxygen uptake, induction of epithelial microcysts, stromal thinning, and an increase in endothelial polymegathism. A multiple regression analysis was performed on data from this study to identify lens or patient characteristics associated with these effects. Although generalization to the population of conclusions based on associations identified using a small sample should be treated with caution, results from this analysis suggest that lens-induced effects on the cornea may be minimized by fitting lenses that are thinner and more mobile, and by encouraging more frequent lens removal and replacement. Patients with thinner corneas and with high endothelial cell density and low polymegathism before commencing lens wear tended to show fewer effects from extended lens wear.
Cell Count↗
Conjunctival nerve pathology in multiple endocrine neoplasia. A case report.
A 31-year-old male with typical clinical features of multiple endocrine neoplasia type 2b (MEN 2b) was subjected to conjunctival biopsy. The patient had previously been operated for pheochromocytoma and medullary carcinoma of the thyroid. He came for ophthalmologic consultation because of redness and irritation of the eye due to dry eye syndrome. The conjunctival biopsy taken from his left eye revealed not only an increase in the number of conjunctival nerves but also an increase of their axons. The ultrastructure of the nerves was mostly normal, but the perineurium was often incomplete, and the interaxonal space appeared to be enlarged. Immunohistochemically, SP-, neurofilament- and, unexpectedly, leucine-enkephalin-positive nerve fibres were demonstrated in the conjunctival stroma. The rat is the only species in which enkephalin-like immunoreactivity has been described in the anterior segment of the eye. Thus, the presence of enkephalin-positive nerves in the conjunctiva of our MEN 2b patient may reflect a profound neural alteration.
Adult↗
Epithelial metabolism of the corneal graft is abnormal.
Both eyes of 16 patients who had undergone unilateral penetrating keratoplasty were examined in this study. The mean corneal oxygen uptake rate in the centre of the graft was 3.88 (SD 0.55) mmHg/s and in the control eye was 4.29 (SD 0.62) mmHg/s. In the temporal host periphery the results for the grafted and control eyes were 4.52 (SD 0.54) mmHg/s and 4.26 (SD 0.52) mmHg/s respectively. The differences in the oxygen uptake rate between the centre of the graft cornea and the contralateral control eye and between the centre and periphery of the graft eye were statistically significant (p less than 0.01). The central corneal touch threshold of the graft eye showed good recovery in four patients, while the remaining 12 had no or minimal recovery. The central and temporal epithelial and corneal thicknesses of the graft and the control eyes were not significantly different. The central endothelial cell density was 1383 cells/mm2 in the graft and 3053 cells/mm2 in the control eye. As corneal oxygen uptake reflects changes in aerobic epithelial metabolism, it is possible that there is a link between loss of neural integrity of graft epithelium and lowered epithelial aerobic metabolism following penetrating keratoplasty.
Adult↗
Long-term neural regeneration in the rabbit following 180 degrees limbal incision.
Penetrating 180 degrees superior limbal incisions were made on the right eye of four adult New Zealand albino rabbits. The contralateral eye served as control. Corneal touch thresholds (CTT) for the central, superior and inferior cornea (2-3 mm from limbus) were determined 3, 9, 15, 24 and 30 months after surgery. In all animals, the CTT was significantly elevated in the superior region of the cornea throughout the measurement period. CTT was elevated in the central and inferior cornea 3 months following surgery and was not affected in the inferior cornea on all other occasions. The animals were then sacrificed and the corneas subjected to histochemical demonstration of acetylcholinesterase corneal nerves. All rabbits showed a reduction in the number of histochemically detectable stromal nerve trunks in the operated region. These stromal nerve trunks showed regenerative changes including abnormally curved course and a subnormal number of axons within a nerve trunk. Epithelial nerve fiber defects included absence or distorted architecture of the basal epithelial plexus and intra-epithelial terminals. These results indicate that although extensive stromal reinnervation had occurred, the extent and quality of stromal nerves was inadequate to restore a normal epithelial plexus and corneal sensitivity.
Animals↗
Histochemical demonstration of adrenergic nerves in the stroma of human cornea.
Both fetal and adult human corneas were subjected to a variety of histochemical techniques for the demonstration of catecholamines. The techniques included both sodium-potassium-glyoxylic acid-induced fluorescence (SPG) and formaldehyde-induced fluorescence after freeze-drying (FIF) procedure. In fetal corneas the presence of adrenergic nerve fibers could be demonstrated by both SPG and FIF techniques. In contrast, in adult human corneas the adrenergic nerves could be identified only with the SPG technique, whereas with FIF the identification of specific catecholamine fluoresence was not possible due to extensive collagen autofluorescence.
Adrenergic Fibers↗
Distribution of fibronectin in human and rabbit corneas.
In order to study the possible role of fibronectin (FN) in corneal wound healing and the relationship between FN and sensory innervation, FN was demonstrated immunohistochemically in both normal and sensorily denervated rabbit corneas and in normal or tissue-cultured human corneas. The distribution of FN was the same in the groups examined: a thin subepithelial band of FN-like immunoreactivity was seen at the level of epithelial basement membrane and at the stromal side of Descemet's membrane. Epithelial abrasions were also performed in both normal and denervated rabbit corneas. The results were compared with those obtained from organ-cultured human corneas. Following abrasion of the corneal epithelium, FN was detected in the anterior margin of the denuded stroma 18 hr after the operation in the areas where the epithelium had not healed, but not 49 hr after. Sensory denervation did not affect the distribution of FN in normal, denervated or healing rabbit cornea. It is concluded that FN is probably not controlled by sensory innervation.
Animals↗
Gyrate atrophy of the choroid and retina. A five-year follow-up of creatine supplementation.
Gyrate atrophy of the choroid and retina (GA) is an autosomal recessive chorioretinal degeneration with a 10-20-fold elevation of plasma ornithine due to deficient activity of ornithine aminotransferase. Type II fibres of the skeletal muscle are atrophic and contain tubular aggregates in electron microscopy. Deficient creatine and creatine phosphate formation have been postulated to be involved in the pathogenesis of GA. The five-year follow-up results of oral creatine supplementation in 13 patients are presented. Visual function tests and fundus photographs showed progression of GA during the treatment. The velocity of the progression varied considerably between individuals. Generally, the progression was rapid in the young patients and slow in the more advanced stages. Abnormalities in the skeletal muscle decreased or disappeared rapidly. They reappeared in the few patients who discontinued the medication. The difference in the therapeutic effect on the skeletal muscle and eye is discussed.
Adolescent↗
Histochemical evidence of limited reinnervation of human corneal grafts.
Acetylcholinesterase (AChE) positive nerve fibers were demonstrated histochemically in the normal human cornea and in 3 corneal grafts obtained after retransplantation. In the normal cornea AChE positive nerves form stromal nerve bundles, which divide into smaller branches contributing to the basal epithelial nerve plexus. Intraepithelial terminals are branches of this plexus. In a grafted cornea obtained 29 years after surgery the epithelium was innervated by a basal epithelial plexus, but only a few stromal nerve trunks had regenerated despite the long post-operative time. Corneal sensitivity had not returned to normal in this case. The remaining grafts, obtained less than 3 years after surgery, contained very few nerves. It seems that neither the architecture nor the density of corneal nerves fully regenerate in the graft cornea, and this probably explains why sensitivity does not return to normal.
Acetylcholinesterase↗
Evidence for sympathetic neural influence on human corneal epithelial function.
Patients with unilateral Horner's syndrome were studied to determine the influence of sympathetic innervation on the structure and function of the human cornea. There was no difference in total corneal thickness, corneal oxygen uptake rate or corneal touch threshold between the normal and Horner's eyes of these patients. Epithelial thickness, however, was slightly but significantly greater in the Horner's eye than in the normal fellow eye (P less than 0.025). When subjected to a hypoxic "Stress Test", patients with post-ganglionic lesions showed significantly more epithelial greying and microcystic oedema (P less than 0.025 and P less than 0.01, respectively) and reported seeing smaller and brighter haloes in the Horner's eye. The cornea of the affected eye of these patients also showed a significantly shower rate of deswelling in the first hour following the "Stress Test" (P less than 0.05). These findings suggest that both in the resting state and when the cornea is physiologically challenged, the sympathetic nervous system and in particular, the terminal sympathetic neuron, has a small but significant influence on the human corneal epithelium.
Adult↗
Corneal endothelium after photocoagulation in diabetic patients.
The corneal endothelium was photographed through a wide-field specular microscope in 38 eyes, in 20 successive diabetic outpatients receiving laser therapy because of underlying proliferative or background retinopathy. Areas of 100 individual endothelial cells from each central cornea were analysed using a digitizer. No statistically significant correlations were observed between mean cell areas or standard deviations of mean and total amount of previous laser energy received. Laser therapy or the type of diabetes did not seem to cause statistically significant changes in the endothelial cell areas examined.
Adult↗
Direct in vivo measurement of corneal epithelial metabolic activity using a polarographic oxygen sensor.
The component oxygen fluxes that constitute a single recording of corneal oxygen uptake when a polarographic oxygen sensor (POS) is placed in contact with the cornea were determined in the in vivo rabbit eye by removing the epithelium and injecting an air bubble into the anterior chamber. The relative contribution of epithelial oxygen consumption, stromal oxygen consumption and diffusion were approximately 55, 5 and 40%, respectively. It is demonstrated that differences in corneal oxygen uptake rates recorded with a POS primarily indicate differences in epithelial metabolic activity. Changes in epithelial metabolism can be determined by measuring both corneal oxygen uptake and corneal thickness. For example, if corneal thickness is constant, differences in epithelial oxygen flux (a) between two eyes of one person, or (b) before and after a procedure in the same eye, can be calculated by multiplying the difference in oxygen consumption measured with a POS by a factor of 1.8.
Animals↗