CENTRAL THICKNESS IN CORNEAL DISORDERS

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A C T A O P H T H A L M O L O G I C A V O L . 5 6 1 9 7 8 Department of Ophthalmology (Head: N . Ehlers), Arhus Konamunehospital, University of Aarhus, Denmark CENTRAL THICKNESS IN CORNEAL DISORDERS BY NlELS EHLERS and THORKILD BRAMSEN 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 11), which showed a signifi- cantly reduced thickness. Reduced CCT was found in chronic degenera- tions 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 discussed. 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. Key words: corneal thickness - corneal dystrophy - corneal degeneration - stroma - endothelium. The maintenance of a normal and constant corneal thickness is important for stromal transparency, and a very limited variation of this dimension may be expected in the normal cornea. As regards the pathologically altered cornea, the thickness is known to be reduced in keratoconus, while a considerable increase may be evident in keratitis and in other acute conditions. In the various chronic disorders, we have found no data available. W e therefore report below the findings in a series of patients with chronic corneal diseases. The number of cases is limited, and the findings are more likely to show the possible value of pachometry, rather than document the significance of the differences. Received October 6 , 1977. 412 Thickness of Diseased Cornea Material and Methods The material comprises 54 patients showing various chronic corneal disorders. The diagnoses will appear in the section on results. Central corneal thickness was determined by the Haag-Streit pachometer, modified as previously reported (Ehlers 1974; Ehlers & Sperling 1977). When a perpendicular incidence of the light beam is optically secured, identical values are found in right and left eyes, and the previously used reference value of 0.520 mm in right eye and 0.524 in the left had to be altered. In a group of 45 conscripts a value of 0.5097 & 0.0041 for right eyes and 0.5100 & 0.0040 for left eyes (mean k SEM, N = 45) was found. The frequency distribution does not differ from the normal. Results Hereditary dystrophies Table I summarizes the observations made in patients with stromal dystrophies. The CCT was normal in granular dystrophy of Groenouw (type I), in lattice dystrophy (Haab-Dimmer), in crystalline dystrophy of Schnyder (the cases re- ported by Ehlers & Matthiessen 1973) and in central cloudy dystrophy of Fransois (the cases reported by Bramsen et al. 1976). Reduced thickness was found in macular dystrophy of Groenouw (type 11) (t-test, P < 0.001 when compared with the above mentioned value for young men). In dystrophies interfering with the endothelial barrier function (bullous keratopathy - Fuchs dystrophy) a pathologically increased thickness is an early Table I . Central cornea1 thickness in hereditary dystrophies. Type of dystrophy I ~~~~~s 1 Average 1 Range Granular (Groenouw I) 6 0.519 0.495-0.560 Macular (Groenouw 11) 4 0.409 0.380-0.435 Lattice (Haab-Dimmer) 3 0.505 0.480-0.525 Crystalline (Schnyder) 3 0.517 0.510-0.520 Central Cloudy (FranGois) 3 0.536 0.520-0.545 Fuchs Schlichting 20 increased 1 0.480 413 Niels Ehlers and Thorkild Brainsen Table I I . Central corneal thickness in stromal degenerations. :ration Average patients Luetic 3 0 .3 i3 0.340-0.400 Tuberculous (scrophulous) 6 0.470 0.400-0.530 Indefinite post-keratitic 2 0.472 0.420-0.525 Marginal (Fuchs-Terrien) 3 0.4% 0.430-0.4 7 5 sign. In these cases the CCT varies with the stage of the disease and no meaningful average can be given. The 20 cases referred to have been presented in another paper (Bramsen 8c Ehlers 1977). In one case of Schlichtings dystrophy low normal thickness was found, clearly distinguishing this condition from Fuchs dystrophy. Stromal degenerations Table I1 summarizes the observations made in patients with stromal degenera- tions. I t was found that all cases of long lasting degeneration had a reduced CCT. In the first three groups a reduced thickness could be seen in avascular corneae, corneae with ghost vessels, and corneae with blood filled vessels. Reduced thickness in marginal degeneration is probably a premorbid condition. I t was noted in relatives of one of the patients. In this condition, as in other degenerations, increased thickness may be found with endothelial dysfunction, or in late cases with large ulcerations. Normal thickness is found in cases of arcus senilis but we have made no direct comparison between matched groups with and without this lipid deposition. Keratoconus In keratoconus the stromal thickness is reduced. This is a well known fact which need not be confirmed by pachometry. However, the thickness reduction is an early sign occurring before conus formation is visible in the slitlamp. In two cases of keratoconus posterior the CCT was normal outside the conus proper, illustrating the difference in pathogenesis of this condition compared to keratoconus. 414 Thickness of Diseased Cornea Discussion The most common corneal dystrophy, the arcus senilis or lipoides is not asso- ciated with any evident change in thickness. Similarly, most of the dystrophies studied have had a normal CCT. The only evident exception has been the macular dystrophy, Groenouws type 11, of which all the observed cases showed severely reduced thickness. Therefore, pachometry would seem to be a means of differentiating between the two types of classical stromal dystrophy. It may be recalled that the deposited substance is different in type I and type 11, in the former a hyaline, slightly PAS positive material, in the latter a mucoid material stained by Alcian blue and colloid iron. Histopathologically, the de- positions in lattice dystrophy resemble those of Groenouws type I which is in good accordance with the normal thickness in both conditions. Reduced thickness has been a constant finding in the chronic corneal de- generations of luetic, tuberculous and indefinite nature. The marginal degenera- tions were also found to have a reduced central thickness. The term corneal atrophy is appropriate, implicating some kind of abiotrophic process in the stroma. In this connection it is probably relevant to mention the minimum thickness obtained about 6 months after corneal grafting (Eblers 1974), which may also be related to a long term affection of the stromal metabclism. Pre- viously the corneal thinning syndromes were discussed by Etzine (1969), who, under this term, considered pellucid marginal dystrophy with or without loca- lized ectasia, keratoglobus and keratoconus. The pathogenesis of keratoconus is unknown and it would for the moment seem to be better to consider reduced corneal thickness after long-lasting degeneration as different from keratoconus. The normal or even reduced thickness in a vascularized cornea suggests that the vascular endothelium is impermeable to water. Fluorescein angiography might provide a clue to the understanding of thickness regulation in the vascularized cornea. As emphasized in the introduction, the limited number of cases studied does not permit definite conclusions to be drawn. The observations made must be ccnsidered as an illustration of the use of pachometry in the clinical study of corneal disorders. However, it provides sufficient basis for the statement that future reports on corneal disorders should include the measurement of the corneal thickness. References Bramsen T., Ehlers N. & Baggesen L. H. (1976) Central cloudy corneal dystrophy of Fransois. A c t a ophthal . (Kbh. ) 54, 221-226. 415 Niels Ehlers and Thorkild Bramsen Bramsen T. & Ehlers N. (1977) Bullous keratopathy treated by systemic tranexamic Ehlers N. (1974) Graft thickness after penetrating keratoplasty. Acta ophthal. (Kbh.) 52, Ehlers N. & Matthiesen M. E. (1973) Hereditary crystalline corneal dystrophy of Ehlers N. & Sperling S. (1977) A technical improvement of the Haag-Streit pachometer. Etzine S. (1969) Corneal thinning syndrome: Keratoleptynsis. Ophthalmologica 152, acid. Acta ophthal. (Kbh.) 55, 665-673. 893-903. Schnyder. Acta ophthal. (Kbh.) 51, 316-324. Acta ophthal. (Kbh.) 5,5, 333-336. 263-267. Authors address: Niels Ehlers, M. D., Department of Ophthalmology, Arhus Kommunehospital, DK-8000 Arhus C, Denmark. 416

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