A technical improvement of the Haag-Streit pachometer. Short communication

Longterm follow-up of penetrating keratoplasty for keratoconus

PURPOSE

This study aims to describe the current visual and refractive status of patients who underwent penetrating keratoplasty (PK) for keratoconus > 20 years ago and to report on the current status of their grafts.

METHODS

A total of 138 eyes in 103 patients were grafted for keratoconus between August 1968 and December 1985. Patients who had not undergone retransplantation were invited to attend a clinical examination. Forty-eight patients (with 61 grafts) accepted the invitation and were examined.

RESULTS

The average length of time since PK was 26.9 years (standard deviation [SD] 4.2 years, range 20.8-38.0 years, n = 61). The average graft age at examination was 82.1 years (SD 19.9 years, range 41-115 years). A total of 80% (49 of 61 grafts) of the examined eyes had a clear graft and 46% (28 of 61 eyes) had best spectacle-corrected visual acuity (BSCVA) > or = 0.5. The mean endothelial cell density per mm(2) was 894 (SD 4.6, range 470-1775). The mean central corneal thickness of the clear grafts was 0.565 mm (SD 0.048 mm).

CONCLUSIONS

Penetrating keratoplasty for keratoconus has a good longterm prognosis; half of the eyes examined in this study had BSCVA > or = 0.5 at > 20 years after surgery.

Descemet's stripping automated endothelial keratoplasty and penetrating keratoplasty for Fuchs' endothelial dystrophy

PURPOSE

To compare the outcome of Descemet's stripping endothelial keratoplasty (DSAEK) to that of penetrating keratoplasty (PK) in patients with Fuchs' endothelial dystrophy.

METHODS

The first 20 patients who underwent DSAEK at the Department of Ophthalmology, Aarhus University Hospital were compared to 20 patients treated with classic PK. Best-corrected visual acuity, subjective spectacle refraction and corneal thickness were registered before surgery and 1, 3, 6 and 12 months after DSAEK surgery; they were also measured before surgery and 12 months and 2-3 years after PK. Endothelial cell density was measured 12 months after surgery in both groups.

RESULTS

Two primary graft failures were observed in the DSAEK group; no failures were seen in the PK group. Best spectacle-corrected visual acuity (BSCVA) at 12 months after surgery was significantly better in the DSAEK group (0.56 +/- 0.04) than in the PK group (0.33 +/- 0.06). At this time, 70% of the DSAEK-treated eyes but only 25% of PK-treated eyes had obtained a BSCVA of 0.5 or better. Two to three years after surgery, BSCVA was 0.5 or better in 55% of PK-treated eyes. Refractive ametropia and astigmatism were significantly smaller in DSAEK-treated eyes than in PK-treated eyes, even after suture removal and arcuate keratotomy. Endothelial cell density (cells/mm2) after 1 year was lower in DSAEK-treated (1.338 +/- 113) than in PK-treated eyes (1.610 +/- 124), but the difference was not statistically significant.

CONCLUSION

DSAEK seems to be superior to PK in treating Fuchs' endothelial keratoplasty, although primary graft failure may be more common. Visual recovery is faster, and major ametropia and astigmatism is not induced. Long-term follow-up studies are essential to assess whether this conclusion also holds true more than 1 year after surgery.

Corneal thickness: measurement and implications

The thickness of the cornea was reported in more than 100-year-old textbooks on physiological optics (Helmholtz, Gullstrand). Physiological interest was revived in the 1950s by David Maurice, and over the next 50 years, this 'simple' biological parameter has been studied extensively. Several techniques for its measurement have been described and physiological and clinical significance have been studied. In this review, the different methods and techniques of measurement are briefly presented (optical, ultrasound). While the corneal thickness of many animals are the same over a considerable part of the surface, in the human cornea anterior and posterior curvature are not concentric giving rise to a problem of definition. Based on this the precision and accuracy of determining the central corneal thickness are discussed. Changes in corneal thickness reflects changes in function of the boundary layers, in particular the endothelial barrier. The absolute value of thickness is of importance for the estimation of IOP but also in diagnosis of corneal and systemic disorders. Finally it is discussed to what extent the thickness is a biometric parameter of significance, e.g. in the progression of myopia or in the development of retinal detachment.

Human corneal thickness and its impact on intraocular pressure measures: a review and meta-analysis approach

We determined the "normal" central corneal thickness (CCT) value in human corneas based on reported literature values for within-study average CCT values, and used this as a reference to assess the reported impact of physiological variables (especially age and diurnal effects), contact lens wear, pharmaceuticals, ocular disease, and ophthalmic surgery on CCT. With the expected CCT and its variance defined, it should be possible to determine the potential impact of differences in CCT in intraocular pressure (IOP) assessments, especially by applanation tonometry, using a meta-analysis approach. Some 600 sets of CCT data were identified from the worldwide literature over the period of 1968 through mid-1999, of which 134 included IOP measures as well. The within-study average CCT values and reported variance (SD) was noted along with the number of eyes and any special characteristics, including probable ethnic origin of the study subjects. Various sets of data were subjected to statistical analyses. From 300 data sets from eyes designated as normal, the group-averaged CCT was 0.534 mm. From 230 data sets where interindividual variance was reported, the group-averaged CCT was 0.536 mm (median 0.536 mm; average SD of 0. 031 mm, average coefficient of variation = 5.8%). Overall, studies using slit-lamp-based pachometry have reported marginally lower CCT values (average 0.530 mm, average SD 0.029 mm) compared to ultrasound-based studies (average 0.544, average SD 0.034 mm), which perhaps reflects the type of individual studied (non-surgical vs. pre-surgical patients) rather than the technique itself. A slight chronological increase in reported average CCT values (approximately 0.006 mm/decade) was evident, but a substantial chronological increase was evident for ultrasound pachometry studies (approximately 0.015 mm/decade). Within the meta-analysis-generated average and variance, age had no obvious impact on CCT measures for *whites, although an age-related decline in CCT is evident for non-whites. Any diurnal effects are likely concealed within the expected variance in CCT. Contact lens wear and pharmaceuticals generally produced changes in CCT that were well within the expected variance in CCT. Of the ocular diseases, only those associated with collagen disorders (including keratoconus) or endothelial-based corneal dystrophies (e.g., Fuchs) were likely to result in decreases or increases, respectively, of CCT beyond the normal variance. Routine contact lens wear and diseases such as diabetes seem unlikely to produce changes in CCT of a magnitude that would justify pachometry as a monitoring method beyond routine slit-lamp evaluation. Increases in CCT beyond the expected variance were reported after a range of intraocular surgeries (cataract operations, penetrating keratoplasty), whereas photorefractive surgery produces a measurable decrease in CCT. A meta-analysis of possible association between CCT and IOP measures of 133 data sets, regardless of the type of eyes assessed, revealed a statistically significant correlation; a 10% difference in CCT would result in a 3. 4 +/- 0.9 mm Hg difference in IOP (P </= 0.001, r = 0.419). The observed phenomenon was much smaller for eyes designated as healthy (1.1 +/- 0.6 mm Hg for a 10% difference in CCT, P = 0.023, r = 0. 331). For eyes with chronic diseases, the change was 2.5 +/- 1.1 mm Hg for a 10% difference in CCT (P = 0.005, r = 0.450), whereas a substantial but highly variable association was seen for eyes with acute onset disease (approximately 10.0 +/- 3.1 mm Hg for a 10% difference in CCT, P = 0.004, r = 0.623). Based on the meta-analysis, normal CCT in white adults would be expected to be within +/-11.6% (+/-2 SD) of 0.535 mm, i.e., 0.473-0.597 mm (95% CI, 0.474-0.596). The impact of CCT on applanation tonometry of healthy eyes is unlikely to achieve clinical significance, but for corneas of eyes with chronic disease, pachometry should be performed if the tonometry reveals IOP readi

Epithelial and corneal thickness measurements by high-frequency ultrasound digital signal processing

PURPOSE

The authors determine the mean central corneal and epithelial thickness in a group of normal human subjects using a new high-frequency ultrasound technique, incorporating digital signal processing.

METHOD

Both eyes of ten volunteers (age range, 23-44 years) were scanned through a normal saline standoff. Digitized ultrasonic echo data were mathematically transformed to produce a plot, the I-scan, which optimally localizes acoustic interfaces to provide improved measurement precision. System precision was determined by analysis of variance of repeated measures. Central epithelial thickness was obtained by averaging multiple measurements. Central corneal thickness was determined by fitting measurements of apparent corneal thickness in consecutive parallel B-scans to a mathematically modeled cornea. A speed of sound of 1640 m/second was used.

RESULTS

Epithelial pachymetric precision using A-scan and I-scan was 4.8 and 2.0 microns (standard deviation), respectively. The mean epithelial thicknesses for the right and left eyes were 50.7 +/- 3.7 microns and 50.3 +/- 3.4 microns, respectively. The mean corneal thicknesses in the right and left eyes were 514.6 +/- 38.4 microns and 516.2 +/- 37.8 microns, respectively. The root mean-square differences in epithelial and corneal thickness between the left and right eyes of each subject were 1.3 and 7.7 microns, respectively (neither was statistically significant).

CONCLUSION

This system provides a pachymetric precision superior to current optical and ultrasound methods. Epithelial and corneal pachymetry is obtained noninvasively by a method that is not limited to optically clear media.

The effect of topical steroid on postoperative corneal edema and endothelial cell loss after intracapsular cataract extraction

In a prospective study of 89 patients the postoperative corneal edema and endothelial cell count was studied after cataract extraction. A double blind investigation was performed as the patients during the first 6 postoperative days were treated with three different types of eye-drops: Maxidex, Pred-Forte 1% or placebo, all containing Benzalkonium Chloride 0.004%. The content of steroid in Maxidex and Pred-Forte was equivalent. We found no significant difference in estimates of endothelial cells in the three groups postoperatively. Corneal edema was less in the two groups treated with steroid. There was no significant difference between the two postoperative groups (p > 0.05) treated with Maxidex and Pred-Forte. The group treated with Pred-Forte showed significantly less postoperative corneal edema, calculated as the graphical area represented by the increase of central corneal thickness (CCT) after operation.

Excimer laser refractive keratectomy for high myopia. 6-month follow-up of patients treated bilaterally

Both eyes of twenty patients were treated for myopia with refractive keratectomy using an excimer laser (193 nm), and followed for six months. The 40 eyes were divided into Group I (22 eyes) with an attempted refraction change of 5-8 diopters; and Group I (18 eyes) with 9-12 diopters of attempted refraction change. In Group I the corrected visual acuity improved or was unchanged in 16/22 eyes. Two eyes with high preoperative corrected visual acuity had experienced a significant loss in corrected visual acuity after 6 months. In Group II, the corrected visual acuity decreased in 10/18 eyes, in 4 eyes significantly. Among the 40 eyes, 39 had a refraction change less than intended after 6 months. Overcorrection was seen in only 1 eye. In Group I, 20/22 eyes were corrected up to 2.5 diopters less than attempted. In Group II, 9/18 eyes were more than 2.5 diopters from the attempted correction. All eyes developed subepithelial opacification ('haze'), which, in spite of steroid treatment, was still present after 6 months. The haze was more severe in eyes treated with 9-12 diopters of attempted refraction change. The achieved refraction change in the two eyes of the same patient was found to be correlated, possibly due to an individual factor in corneal wound healing.

Epikeratophakia for high myopia

A cryo-technique for production of corneal epi-lenses has been developed. During the manufacturing procedure the anterior surface of the tissue is moulded and the posterior surface cut with a microtome. After cutting, the epi-lenses are stored for days to weeks in an organ culture medium. Twelve eyes in 10 patients have been treated with epikeratophakia for high myopia (greater than -7 D). In all eyes, epithelialization was complete within one week, and at follow-up 3-18 months after the operation all epi-lenses were clear. The average refractive change in spherical equivalents was 9.7 +/- 4.4 D. All eyes obtained better uncorrected visual acuity, and no eyes lost more than one line of the best corrected visual acuity. At the time of follow-up, the patients' native cornea had regained normal hydration, whereas the epi-lenses did not thin as much as expected.

A clinical comparison of optical and ultrasonic pachometry

Central corneal thickness (CCT) of normal and oedematous corneae was measured in a study comparing a modified Haag-Streit optical pachometer and an ultrasonic pachometer (Teknar Ophthasonic, preset velocity 1630 m/s). Sixty-eight patients were examined before and after cataract extraction with implantation of an anterior chamber lens. Mean values of CCT before operation were 531 +/- 4.9 (SEM) micron by optical pachometry and 524 +/- 4.7 microns when measured by ultrasound (not significantly different). On the first post-operative day the values were 618 +/- 8.4 and 602 +/- 7.6 microns for optical and ultrasonic measurements, respectively (significantly different, 2P less than 0.001). Correlation analysis showed a high dependence between the methods with coefficients of correlation being 0.955 before and 0.958 after the operation. Linear regression analysis revealed small, but significant differences between the techniques. The difference between the two methods increased with increasing corneal hydration, whereas it could not be ascribed to sex, age, or intraocular pressure. It is concluded that for clinical purposes optical and ultrasonic pachometry techniques are comparable.

The influence of donor age and post mortem time on corneal graft survival and thickness when employing banked donor material (A five-year follow-up)

Sixty-three consecutive corneal transplantations employing banked donor material were grouped according to donor age and post mortem time, respectively, and followed at regular controls for 5 years. Twenty-six cases (38% belonging to a risk group i.e. had earlier had corneal transplantation(s) and/or vascularized corneas) with a mean donor age of 55.8 years were compared with 37 cases (35% at risk) with a mean donor age of 75.7 years. The five-year graft survival was 72.0% in the first group and 86.3% in the second group, however, the difference was statistically insignificant. In 27 cases (26% at risk) with a mean post mortem time of 17.4 h a five-year graft survival of 81.0% was found compared with 80.3% for 36 cases (44% at risk) with a mean post mortem time of 40.5 hours. No difference in graft thickness was demonstrated between any of the groups, and in all cases with surviving grafts normal values for corneal thickness were achieved. It is concluded that neither donor age nor post mortem time are limiting factors when selecting donor material for organ culture and corneal grafting.

Comparison of sodium hyaluronate and methylcellulose in extracapsular cataract extraction

Extracapsular cataract surgery with posterior chamber lens implantation was performed in a total of 30 patients. These were randomly divided into two groups. In one group sodium hyaluronate 1% (Healon) was used as a tool in surgery, in the other group methylcellulose 2% was used. The viscous substances were irrigated out of the eyes at the conclusion of surgery. Central corneal thickness, intraocular pressure, and aqueous flare were recorded until one month post-operatively. There was no difference in corneal thickness between the two groups. The mean intraocular pressure was lower than pre-operatively in both groups with a lower pressure in the methylcellulose group compared to the sodium hyaluronate group during the first part of the observation period.

Corneal transplantation using 4-week banked donor material. Long-term results

Thirty-six corneal transplantations employing banked donor material maintained during prolonged storage (mean: 29.5 days) in organ culture at 31 degrees C were followed for 5 years. The overall graft survival was 80% , and graft failures were almost exclusively found in a pre-operatively defined risk group where the survival rate was 49.6% compared with 95.5% in the non-risk group. Five years after transplantation the mean central thickness of surviving grafts was 0.52 mm and the mean endothelial cell density 862 cells/mm2. All grafts were optically clear judged by slit-lamp appearance even though a higher degree of light-scatter was demonstrated compared with controls. The functional results were satisfactory with an over-all mean visual acuity of 0.67 cc. Excluding cases where low visual performance had non-corneal causes, 88% of patients obtained 0.5 cc or better. The median spherical equivalent of corneal curvature was 8.23 mm and the median astigmatism 4 diopters. Intraocular pressure was normal, and all grafts had obtained some degree of sensibility, although reduced compared with controls. The study demonstrates that banked donor material can be successfully employed for corneal transplantation achieving favourable long-term results.

Morphological changes and lens position 2 1/2 years after intracapsular cataract extraction with implantation of a semiflexible anterior chamber lens. A prospective re-examination

Morphological changes and lens position were examined in 51 patients 25 to 38 (mean 30) months after intracapsular cataract extraction (ICCE) with implantation of the semiflexible, 3M, style 70, anterior chamber lens. Morphological changes presumably associated with the ICCE proper were: iris transillumination defects (68.6%), rupture of anterior hyaloid membrane (20%), detachment of posterior vitreous (42.9%) and anterior synecchiae (13.7%). The implant-related morphological changes were caused by iris tucking (19.6%), small, mobile lenses causing endothelial disturbance (9.8%), iris-haptic adhesions and progressive ovalling of pupillary form. The present morphological changes were compared with a previously published examination of the same patients performed 4 months after surgery: 9 eyes without tissue changes around the lens feet at the previous follow-up now had iris-haptic adhesions involving from 1 to 4 lens feet. In 8 eyes the iris-haptic adhesions had progressed. Compared with the previous examination the fraction of eyes with tissue changes at the lens feet had risen from 47.1 to 64.7%, while the number of eyes with oval pupils had increased from 54.9 to 80.4%. Contact between lens haptic and iris root seems to constitute a constant stimulus for structural changes of the iris.

Determination of the corneal thickness profile by optical pachometry

A clinical method of measuring central-peripheral corresponding values of corneal thickness and measurement positions in the horizontal meridian is described. Employing a fitting procedure it is demonstrated that the relative increase in the corneal thickness from apex to limbus is proportional to the square of the chord distance. The proportionality factor is defined as the coefficient of thickness variation (TV). Tm is defined as the minimal corneal thickness. In relation to the visual axis the chord distance xm denote the temporal or nasal position of Tm. TV, Tm and xm are characteristic constants of a particular eye. Measuring the corneal thickness and the chord distance in mm pooled values of 80 eyes demonstrates (means +/- SD) Tm to be 0.575 +/- 0.027, and TV to be 0.0088 +/- 0.0020. With the normal interval in parenthes these results imply an enlargement of corneal thickness 0.9% (0.4-1.3%) 1 mm and 22% (12-32%) 5 mm from the visual axis. With respect to the position of Tm 21 eyes (26%) showed a median temporal displacement at 0.4 mm, 4 eyes (5%) showed a median nasal displacement at 0.3 mm and 55 eyes (69%) showed no significant displacement. This corresponds to the usual angle kappa value, which clinically expresses the often slight nasal decentration of the visual axis relative to the optic axis.

Some components affecting the precision of corneal thickness measurement performed by optical pachometry

With the Haag-Streit pachometer attached to a Haag-Streit 900 slit-lamp, 29 normal individuals were examined by repeated measurements on two separate days by the same observer. A statistical variance component model admitted three components of variance to be separated and estimated as SD: The biological day to day variation (0.006 mm), the variation attributed to the slit-lamp adjustment (0.005 mm), and the variation derived from the adjustment of the pachometer (0.013 mm). Knowing these components an optimum practical procedure of examination can be established.

No additive effect of tranexamic acid and naproxen on corneal deswelling. A study on oedema after uncomplicated intracapsular cataract extraction with anterior chamber lens implantation

The influence of systemic antifibrinolytic and antiprostaglandin medication on post-operative central corneal thickness was studied. Thirty patients underwent uncomplicated intracapsular cataract extraction with implantation of anterior chamber lens. The patients received either tranexamic acid, naproxen or both. The post-operative oedema did not differ significantly between the three groups studied. Models for studying changes in central corneal thickness are discussed. A review of the influence of drugs on corneal thickness is presented.

Corneal transplantation using long-term cultured donor material

Twenty-seven corneal transplantations with a mean duration of donor incubation of 14.3 days (group 1) and 36 transplantations with a mean duration of incubation of 29.8 days (group 2) were followed for 18 months. No significant difference was found in graft survival between the two groups (81 versus 80%). Almost all graft failures were found among pre-operatively defined high-risk cases, i.e. previously transplanted and/or vascularized recipient cornea. The profile of the corneal thickness curve was almost identical in the two groups, and normal values were reached in both. Visual acuity was 0.5 or better in 23% of the cases in group 1, and 35% in group 2. These figures were highly influenced by co-existing and complicating eye disease i.e. amblyopia, cataract, macular degeneration and glaucoma. The results show that 2 weeks and 4 weeks cultured cornea do not differ with respect to graft survival and thickness. The over all results indicate that long-term cultured donor material is suitable for corneal transplantation and fully comparable to material stored by other methods.

The thickness of the human cornea as determined by a specular method

The thickness of the human cornea was measured by a simple accurate method which entails measurement of the distance between the anterior and the posterior corneal reflections when the cornea is illuminated at an angle. As compared to methods measuring the width of the optical section, the advantage of this method is that the end-points of measurement are clearly defined, providing a better basis for an exact estimation of the corneal thickness. The optical principle of this specular method is presented in a general form. The influence of physiological variation in corneal curvature and index of refraction is analysed and shown to cause an insignificant error on the standard thickness estimate. Using this method, the corneal thickness was measured in a random population sample of 115 normal subjects, aged 10-90 years. The corneal thickness was found to approximate a normal distribution with a mean value (+/- SD) of 0.515 mm (+/- 0.033). A significant decrease in corneal thickness with age of 0.0045 mm per decade was found.

Long term results of corneal grafting in Fuchs' endothelial dystrophy

The long term results of grafting for Fuchs' endothelial dystrophy are reported in 25 patients with 33 penetrating keratoplasties, with a mean observation period of 50 months, 33% of the cases showed a visual acuity of 0.5 or better, while 42% had a visual acuity less than 0.1. The cause of visual acuity less than 0.1 was largely permanent corneal oedema, which was observed to develop in 39% of the cases. Some of the causes of oedema were accounted for by graft rejection, glaucoma and other obvious causes, but in a significant number of cases (12% of all cases), the oedema developed as a quiet, slowly progressive oedema without any obvious aetiology. It is tentatively suggested that the recurrence of oedema might be associated with the primary disease in patients with Fuchs' endothelial dystrophy.

Increased central cornea thickness in systemic sclerosis

Central cornea thickness (CCT) was measured in 32 patients with systemic sclerosis by the Haag-Streit pachymeter with improved centrality. Results were compared with measurements in 29 healthy adults matched with respect to sex and age. CCT was increased (P less than 0.001) in patients with systemic sclerosis (mean 0.56 mm, average SD right and left side 0.0297 mm) as compared to the controls (mean 0.51 mm, SD 0.0109 mm). CCT of right and left eye was increased in 69% and 72% of patients with systemic sclerosis as compared to controls (mean 0.51 mm +/- 2 SD). CCT increased during the first 8 years of the disease (correlation coefficient 0.593) reaching a plateau after 8 years (correlation coefficient 0.005). CCT did not increase during medical treatment with collagen inhibitors. Measurement of CCT may be useful as a supplement to other quantitative methods for diagnosis and control of systemic sclerosis.

Long term results of corneal grafting in keratoconus

Fifty-four corneal grafts in 45 keratoconus patients have been followed for 3 to 12 years (average 5.8 years). The corneal clarity, visual acuity, incidence of immune rejections, relation to HLA compatibility, corneal thickness and endothelial cell density are reported. At the follow-up 98% (53 out of 54) of the grafts were clear. Visual acuity was greater than or equal to 0.67 in 72% of the cases. Suboptimal vision was due to cataract or large astigmatism. Rejection had occurred in 5 cases (10%). The graft thickness was found to decrease steadily, reaching subnormal values about 1/2 year after the operation. Six years after grafting, however, the thickness did not differ from that of normal corneas. The endothelial cell density in uncomplicated cases ranged from 450 to 3000 cells/mm2, decreasing with length of post-operative period. The HLA compatibility could not be shown to influence the endothelial cell density.

The effect of the prostaglandin-inhibitor naproxen on the endothelial cell-loss after cataract extraction

The influence of the anti-inflammatory, prostaglandin-inhibitor naproxen on the endothelial cell-loss after cataract-surgery was studied. Twenty-six patients received the drug, and 27 comprised a control group. The cell-loss, estimated one year after surgery, was found to be significantly lower in the naproxen-treated group than in the control group.

Hearing loss in children with abnormal corneal thickness

Corneometry, i.e. measurement of the thickness of the centre of cornea, has been introduced as an objective method for the diagnosis of ecto-mesodermal insufficiency. A group of children, between 4 and 18 years of age with different types of hearing defects has been examined by this method and the results were evaluated separately for conductive and sensorineural hearing loss. Within the two groups, the cornea-positive children (i.e. children with abnormal corneal thickness) were analyzed in relation to the different diagnostic groups.

Sensorineural hearing loss and somatic insufficiency

Studies of clinical, audiometric and histological defects in patients with conductive otosclerosis, demonstrated the occurrence of subclinical manifestations in the hair, nails, joints and teeth. Light-microscopic examination of the skin in these patients has shown metachromasia of the ground substance and degenerative abnormalities in the collagen and elastic fibres. Structural abnormalities, in the organs developing from the ectoderm and mesoderm, led to the conclusion that otosclerosis is a universal disease characterized by ectomesodermal insufficiency, a somatic insufficiency. Optical measurement of the corneal thickness by corneometry has been introduced as an objective method for the diagnosis of somatic insufficiency. This method, now used routinely in our clinic, shows a high frequency of abnormal corneal thickness among patients with otosclerosis. Such a person is termed 'cornea-positive'. Cochlear otosclerosis with pure sensorineural hearing loss has been studied by X-ray in axial and semiaxial tomography. These patients were often X-ray positive and among them there were many cornea-positive individuals. Cases with noise-induced hearing impairment may also be cornea-positive, i.e. they may exhibit, somatic insufficiency indicating that their hearing organs are characterized by a decreased resistance to noise. Corneometry has shown its value in the diagnosis of insufficient somatic constitution typical of individuals at risk.

Clinical observations on the corneal thickness and the corneal endothelium in diabetes mellitus

The corneal thickness was measured by pachometry and the corneal endothelium was photographed by specular microscopy in 81 insulin-dependent juvenile diabetic outpatients. The corneal thickness of a normal group, diabetics without and with proliferative retinopathy was (mean +/- SD): 0.527 +/- 0.028, 0.544 +/- 0.028, and 0.566 +/- 0.027 mm, respectively (2p less than 0.01). As revealed in the specular photomicrographs, minute folds in the endothelial layer were found in 13 of the diabetics versus 1 of the normal group (2p less than 0.01). The cell density and the occurrence of dystrophic changes in the endothelium did not differ from those in normal persons. The augmented corneal thickness in the diabetic subjects is tentatively interpreted as minimal corneal swelling. It seemed to be present very early in the disease and may thus be one of the earliest clinically detectable changes off the diabetic eye.

Fresh and cultured corneal grafts compared by post-operative thickness and endothelial cell density

Thirty-nine corneas were removed within 6 h post mortem and stored in a moist chamber at 4 degrees C before grafting. The mean donor age was 33 years and the average time between death and grafting was 11 h. Thirty cadaver corneas were selected after trypane blue staining and cultured at 31 degrees C for 24 h before grafting. The mean donor age was 61 years and the mean time between death and culture was 18 h. During the first 10 postoperative days fresh grafts were thinner than cultured grafts. One year after the transplantation the two groups did not differ significantly in regard to the clinical result, corneal thickness, or endothelial cell loss. This indicates that corneas from old donors with extended post mortem time can be used for transplantation after individual evaluation and corneal culture.

Ophthalmological examination of patients in long-term treatment with tranexamic acid

Fourteen patients with hereditary angioneurotic oedema (Quincke' oedema) treated for an average period of 6 years (ranging from 15 months to 8 years) with tranexamic acid was given an ophthalmological examination in order to reveal any possible toxic damages caused by the treatment. As large doses of tranexamic acid given to animals have been shown to result in retinal degeneration, the examination was focused on checking the retinal function. A complete ophthalmic examination revealed no toxic effects in the eyes of any of the patients. No retinal damages were found that could have been caused by the tranexamic acid. The central corneal thickness was normal.

Transient changes in specular appearance of the corneal endothelium and in corneal thickness during anterior uveitis

Specular microscopic investigations on the corneal endothelium and optical measurements of the corneal thickness were carried out before and at intervals during treatment in 15 patients with acute anterior uveitis. The inflammatory induced changes in the specular appearance of the endothelium included a range of specular defects of which 4 types were distinguished: 1) large defects caused by large precipitates on the back of the cornea, 2) smaller defects also ascribed to precipitates, 3) gutta-like defects which were not associated with typical precipitates, and 4) defects smaller than one cell's width, the non-specular correlate of which was uncertain. The fractional area occupied by the specular defects in the acute phase was found to correlate significantly to the observed increase in corneal thickness. With a few exceptions, the endothelial changes disappeared completely in parallel with the other signs of acute inflammation. No cell loss was demonstrated in patients where a control count could be obtained. The maximum increase in corneal thickness ranged from 0.015 to 0.200 mm. During recovery the thickness showed a peculiar tendency to reach slightly subnormal values as a transient phenomenon. The clinical significance of these findings is discussed.

Corneal graft rejection probably mediated by antibodies

A clinical and laboratory analysis of a rejection episode is presented. A clear graft in a non-vascularized cornea from 1975 was rejected when the contralateral eye was grafted in 1979. Clinically this case was unique by showing no endothelial rejection line and no precipitates and, as seen after successful treatment, no endothelial cell loss. A test for direct cell mediated lympholysis (DCML) was negative while antibody dependent cell mediated cytotoxicity (ADCC) was positive at time of rejection, and negative three months later. This case suggests that humoral factors may be involved in corneal graft rejection.

Corneal thickness in daily contact lens wear. A comparison of a low water content lens and a high water content lens

The corneal responses to daily wear of Scanlens 75 high water content lenses, average centre thickness 0.23 mm, and of Bausch & Lomb Soflens U3 low water content lenses, centre thickness 0.07 mm, were compared. Calculations of oxygen transmissibility indicated that corneal swelling might be less for the former lenses than for the latter lenses. This assumption was proved to be correct. The average increase in corneal centre thickness over the day (14 h) during a 4 week period was 1.6% for Scanlens 75 and 2.8% for Soflens U3. This difference was statistically highly significant (P less than 0.001). The thickness increase was greatest during the first week. Tests with thicker lenses showed that the thicker the lenses are, the more important it is to choose high water content lenses. The closed eye period increased corneal swelling in extended lens wear. The results favour a program based on certain high water content lenses and a daily wear schedule.

On the optical measurement of corneal thickness. II. The measuring conditions and sources of error

The optical measurement of corneal thickness based on oblique viewing of the optical section of the cornea is complicated by the finite width of the incident slit beam. In this report the theoretical and practical aspects of the effect of the slit width on the thickness reading are analysed. In practice, it was not possible to make slit-width independent thickness readings which were reproducible from one observer to another. In addition, the observed slit-width error was found to vary from one patient to another. The lack of reproducible estimate of the corneal thickness is attributed to difficulties associated with an exact definition of the edges of the visible bands of the optical section, which are determined by biological properties of the cornea as well as perceptive properties of the observer. Although inter-observer errors up to 0.02 mm were found, the intra-observer error amounted to only 0.005-0.006 mm (SD) between consecutive readings. Presumably this high intra-observer reproducibility is the result of the auxiliary pin-lights used. Changes in corneal thickness, measured by the same observer, can therefore be determined with great accuracy.

Central corneal thickness and intraocular tension in patients with acromegaly

In 27 patients with pituitary adenomas the central corneal thickness and the intraocular tension were measured. Thirteen of the patients were suffering from acromegaly, and in this group the central corneal thickness was 0.561 mm +/- 0.35 (mean +/- SD). In the 14 patients with pituitary adenomas but no acromegaly the central corneal thickness was 0.526 mm +/- 0.030 (mean +/- SD). This difference is statistically significant 0.01 greater than P greater than 0.001. In the 13 patients with acromegaly the intraocular tension measured by applanation was 16.9 mmHg +/- 2.3 (mean +/- SD) and in the control group 14.7 mmHg +/- 2.4 (mean +/- SD). This difference is statistically significant (0.05 greater than P greater than 0.02). When the applanation reading is corrected for the difference in the central corneal thickness, the patients suffering from acromegaly have an intraocular tension of 14.1 mmHg, which is of the same magnitude as the tension in the patients without acromegaly.

The effect of carbonic anhydrase inhibition on central corneal thickness after cataract extraction

The influence of acetazolamide (a carbonic anhydrase inhibitor) on central thickness of the human cornea in vivo was studied. Corneal thickness as measured after cataract surgery was significantly increased by acetazolamide, if the specular microscopy revealed central corneal guttae, pre-operatively. When no guttae were seen by specular microscopy, no effect could be demonstrated.

On the optical measurement of a corneal thickness. I. Optical principle and sources of error

The optical principle for the estimation of corneal thickness with a commonly employed method is considered. The theoretical relationship between the apparent and true corneal thickness is described in a simple manner. Based on the theoretical results it could be confirmed that physiological variations in refractive index and radius of the cornea induce an insignificant error in the corneal thickness estimate. The theoretical relationship between apparent and true corneal thickness was compared to the performance of two commercially available pachometers (Haag-Streit and Zeiss). The actual reading of the pachometers was found to be lower than the theoretical reading by a non-linear error amounting to about 0.010 and 0.100 mm at a true corneal thickness of 0.50 and 1.00 mm, respectively.

Corneal thickness and endothelial damage after intraocular lens implantation

The corneal thickness and the specular appearance of the corneal endothelium are reported in 100 patients with unilateral intraocular lens implantation. Post-operative time ranged from one to 42 months. An average central endothelial cell loss of uncomplicated cases of 46%, range 1 to 83%, with no correlation with time after the operation was found. A significantly higher cell loss was found in cases with technical complications, shallow anterior chamber or increased intraocular pressure post-operatively. No correlation was found between the corneal thickness and the endothelial cell loss. In two patients, however, with a cell density below 500 cells/mm2, a slight increase in corneal thickness was noted. Thirty patients presented a guttate endothelium. Irrespective of the occurrence of surgical complications the presence of a guttate endothelium was found to be a major determinant of the corneal thickness increase and could be ascribed as a cause of persistent corneal swelling in six of twelve patients with elevated corneal thickness. The progression of guttate changes occurred independently of the cell loss.

Influence of tranexamic acid and acetylsalicylic acid on the thickness of the normal cornea

In a masked cross-over study ten normal subjects were treated with tranexamic acid and acetylsalicylic acid one g three times daily for seven days. The central corneal thickness. was found to increase in response to acetylsalicylic acid and to decrease in response to tranexamic acid. This finding provides evidence for the involvement of the fibrinolytic system in the regulation of the normal corneal thickness. The endothelial morphology, as seen with the specular microscope, was unchanged during treatment with either drug.

Corneal thickness and endothelial damage after intracapsular cataract extraction

In a prospective study corneal thickness and specular microscopic findings of corneal endothelium are reported in 37 patients undergoing intracapsular cataract extraction. Central endothelial cell loss was estimated six months after the operation and was found to correlate significantly to the immediate post-operative increase in central corneal thickness. A subgroup of patients showing slight endothelial dystrophy prior to the operation showed a significantly higher increase in corneal thickness fourth day after the operation. Six months after the operation a significant residual increase in corneal thickness was found for this group, while the rest of the patients had returned to near pre-operative levels. No correlation was found between cell loss and residual corneal thickness increase at this time. Six months after the operation a vertical difference in cell density was found. This difference could be correlated to the age of the patient, presumably indicating a less complete redistribution of the cell population in older patients.

Changes in the corneal endothelium after acute anterior uveitis as seen with the specular microscope

In order to investigate whether inflammation of the anterior uveal tract is accompanied by a loss of endothelial cells, 13 patients with previous unilateral attacks of anterior uveitis were photographed with the non-contact specular microscope. All patients had shown an increased thickness or frank oedema of the cornea during the acute phase of the inflammation. As compared to the healthy eye, only two patients (15%) showed endothelial cell densities on the affected side which were lower than was to be expected from the normal difference between counts from left and right eye in a control population of 36 subjects. The two patients with detectable cell loss also showed defects in the specular reflex, the significance of which is discussed.

The endothelial cell damage in acute glaucoma. On the corneal thickness response to intraocular pressure

In order to assess a possible damage to the corneal endothelium during pressure induced abnormal hydration of the cornea, 23 patients with a previous attack of unilateral acute glaucoma were photographed with a specular microscope. As compared to the healthy side, the endothelium of the affected side showed a mean decrease in cell density of 23.1%, range --4.8 to 68% (P less than 0.001). In retrospect this cell loss was found to correlate significantly to the increase in corneal thickness measured during the acute attack on first day of admission. At present examination mean central corneal thickness was identical in the affected and unaffected eye. A large variation was, however, found in the intraocular pressure of the previously attacked eye. If the subject inter-eye difference in intraocular pressure was related to the inter-eye difference in corneal thickness is significant negative correlation appeared. It is concluded that the intraocular pressure has a dual effect on the corneal hydration: if the endothelium is intact, the intraocular pressure decreases corneal thickness, whereas an increase is seen only if the endothelium is acutely damaged.

Early postoperative changes in graft thickness after penetrating keratoplasty. Influence of host corneal disorder on time course

In 172 patients the thickness of the corneal graft was followed with frequent measurements during the first 14 days after operation. Three different graft thickness time courses were observed. Patients with keratitis, stromal dystrophy and corrosion or mechanical lesion showed a secondary rise in graft thickness on the 6th postoperative day, while patients with keratoconus and those treated with tranexamic acid showed no rise on the 6th day. Patients with Fuchs' dystrophy differed from the other groups in not reaching the maximal thickness until the 3rd postoperative day. The possible correlation of these three time courses with changes in the fibrinolytic system is discussed.

A double-blind study on the influence of tranexamic acid on the intraocular pressure and the central corneal thickness after trabeculectomy for glaucoma simplex

The influence of tranexamic acid on the intraocular pressure and central corneal thickness has been examined by means of a double-blind trial in patients with glaucoma simplex. The material comprised 28 patients (14 pairs) who had all been operated on for glaucoma in one eye by trabeculectomy. Tranexamic acid was found not to influence the intraocular pressure, neither in the operated eye nor in the opposite eye. The possible antifibrinolytic effect on the outflow of the aqueous humour is discussed. As in previous studies an effect of tranexamic acid on the central corneal thickness was found in both the operated and in the non-operated eye. A possible mechanism behind this is discussed.

Central thickness in corneal disorders

In the single individual the central corneal thickness (CCT) shows only small variations. Therefore CCT has been studied in a number of corneal diseases in order to investigate if this dimension might contribute to the diagnosis or to the understanding of the pathogenesis. Normal CCT was found in hereditary dystrophies with the exception of the macular dystrophy of Groenouw (type II), which showed a significantly reduced thickness. Reduced CCT was found in chronic degenerations of leutic, tuberculous or indefinite nature. Marginal degenerations of Fuchs-Terrien type also showed reduced CCT. The possible role of abiotrophic processes in this corneal thinning is dicussed. Endothelial dysfunction is indicated by increased CCT. This occurs in bullous keratopathy and in many acute disorders. Vascularisation of the cornea does not preclude the occurrence of normal or even reduced thickness.

A double-blind study of the influence of tranexamic acid on the central corneal thickness after cataract extraction

A double-blind study of the influence of systemic tranexamic acid on the central corneal thickness after cataract extraction was performed in 17 pairs of patients. Apart from the cataract, no were present. A sequential statistics was used to show that the increase in central corneal thickness after operation was significantly less in the tranexamic acid treated group than in the placebo group. There was no significant difference in intraocular pressure between the tranexamic acid and the placebo treated group. The possible influence of tranexamic acid on the thickness controlling mechanism of the cornea is discussed, and studies concerning the fibrinolytic system, the complement system and the aqueous humour amino acid treated patients are mentioned.