Australian and New Zealand Journal of Ophthalmology 1988; 1 6 273-274

Editorial ASSESSMENT OF THE CORNEAL ENDOTHELIUM PRIOR TO CATARACT ANDCORNEALGRAFTSURGERY

There remains little doubt in the minds of most corneal observers and researchers, that the corneal endothelium plays a pivotal role in preserving corneal clarity. The relative role, however, of the number and shape of the endothelial cells versus their pumping function in maintaining this clarity remains speculative. To evaluate accurately the ability of the endothelium t o keep the cornea deturgessed and clear, would therefore need a measurement not only of their number, size and shape, but also of their pumping function. The former can be reasonably easily performed, while the latter, although performed in research situations with f luorophotometry , remains diff icul t , problematic, and entails major assumptions.

Because of the endothelium’s crucial role in keeping the cornea clear and its potential for damage during intraocular surgery such as cataract surgery and corneal transplant surgery, it would seem reasonable to assess the layer’s morphology by observing the cells for their size, number and shape preoperatively.

Whatever method we use t o d o this, it must be kept clearly in mind that we are unlikely to be sampling more than a few thousand cells out of half a million cells, and that the sampling is almost certain to take place only in the centre of the cornea where visualization is most easily undertaken.

The possible sampling errors in using this small sample in the centre of the cornea as an extra- polation of the entire endothelial population is of concern. The two methods which may be clin- ically used t o assess the corneal endothelium before operation are observation at the slit lamp

at high magnification, or some form of specular microscopy, usually wide-field contact specular microscopy with an instrument such as the Keeler Konan instrument.

The advantages of using the slit lamp are that it uses a n instrument which is uniformly avail- able t o all ophthalmologists, well understood, and will cost the doctor and patient nothing extra. It also saves the time involved in manoeuvring a patient to a new instrument, taking photographs, and measuring all parameters as is required with the contact specular microscope. Nonetheless, it does not readily permit you to take photographs or accurately count the number of cells in the area observed, and it takes a considerable degree of skill t o routinely observe the endothelium with the slit lamp.

With contact specular microscopes, it is rela- tively easy to observe and to photograph the cells, but i t involves applanating the cornea which can result in some punctate epithelial changes. It also means considerable photo- graphic or video recording, and time spent on analysing the photographs and obtaining cellular parameters, and most importantly the regularly available instrument costs approximately $60 OOO Australian.

Considering the possible problems with sampling, and the high cost of instrumentation and processing with specular microscopy, slit lamp observation should be the mainstay of endothelial morphological assessment by the regular ophthalmologist.

You might ask, having seen the endothelium, how d o 1 know how many cells there are and

EDITORIAL - ASSESSMENT OF THE CORNEAL ENDOTHtl IUM PRIOR TO CATARACT SURGERY 273

what shape they are? Only by experience in using pattern matching devices or cards which may be purchased very cheaply and by looking at many normals, is it possible reliably to produce a crude estimate of the endothelial density, such as the ability to differentiate an endothelium that has 3000 to 4OOO cells/mm’. from that which has only 1500 cells/mm2. This type of cell change can be assessed at the slit lamp. I regularly assess all preoperative patients, in fact all normal patients, by slit lamp observation of the endothelium and note whether I consider it to be in a normal range of approximately 3000 to 4OOO cells/mm2, of a grossly reduced range of around about 1500 cells/mm’, or in a pre-oedema range of approx- imately 500 cells/rnm2.

You may ask a further question. Why assess the endothelium at all when we have no mechanism for enhancing its function or increasing its mitosis, to prepare it better for the possible damage of intraocular surgery? Who would dare to suggest to ophthalmologists that the knowledge of the endothelial status in a particular cornea which is currently clear, is likely to change the type of surgery they are about to perform, such as whether to use an implant or not? Certainly, who would have the temerity to suggest that because the endothelium is reduced in number, that the individual ophthalmologist will take more care in doing the intraocular surgery, than he might normally use?

Nonetheless, I believe that observation of the corneal endothelium remains an important issue at the present moment, not only so that all ophthalmologists become routinely able to perform this test, but more importantly so that a grossly reduced endothelial mosaic will permit an appropriate informed consent to be obtained from a patient prior to cataract surgery. The knowledge of the increased risks of oedema after cataract surgery will save the ophthalmologist considerable postoperative surprise and the patient the equivalent postoperative anger should the cornea become oedematous. Should we ever be in the position to improve the pumping func- tion of the corneal endothelium, or to increase its numbers, then there will indeed be a very cogent reason for assessing the corneal endothelium of all patients. In contradistinction, the use of specular microscopy in the evaluation of donor material is well established and invalu- able in assessing corneas so that those with unsuspected corneal endothelial abnormalities can be discarded.

Until such time as we have methods to enhance endothelial function, special instrumentation such as specular microscopy to view the endothelium remains a valuable tool for researchers, but of doubtful worth for the prac- tising ophthalmic clinician.

L. W. Hirst

274 AUSTRALIAN AND NEW ZEALAND JOURNAL OF OPHTHALMOLOGY