Significance of colour change in the cornea.

EVERY practising ophthalmologist meets with changes in the colour of the cornea, and it is indeed astonishing that this small gelatinous mantle, hardly more than half-a-millimetre in thickness, can register so many chromatic variations. Abnormal pigmentation may be congenital or acquired, transient or permanent, stationary or progressive, endogenous or exogenous, benign or malignant. It may be confined to the anterior or posterior surface, and may occupy any or all of the intervening layers. It may be disposed in a regular pattern or haphazardly scattered. Sometimes the whole corneal area will be tinged. In other instances we find abnormal coloration limited to the periphery or concentrated at the axis. Bearing in mind all these possibilities, we must admit that classification of corneal colour change is not easy, and our difficulty is enhanced by the fact that such change may be linked with accessory signs not only in the cornea itself, but also in other parts of the eye. Associated abnormality can also arise in the ocular adnexa and in more remote organs. Some observers have tried to draw a distinction between real and apparent changes in corneal coloration, and from one point of view their attitude seems logical. Nevertheless we do not always find, amidst kaleidoscopic conditions of practice, any sharp line of division between real and apparent colour. In the laboratory we can control the background so as to obtain valid answers from a colorimeter, but the living human cornea is backed by a screen of pigment which widely varies from patient to patient. Thus, for instance, the anterior corneal surface illuminated in a certain way can appear brown by virtue of its uveal background, without any corresponding structural modification in the cornea itself. Similarly, a tint may be imparted to the corneal periphery by changes in the limbal zone of the sclera or in the adjacent network of vessels. The concept of colour has become more and more complicated with the growth of knowledge, so that practising ophthalmologists cannot keep pace with all the modern techniques of investigation. We can nevertheless appreciate that there is a large subjective factor in most of our judgments concerning colour; and there is no call for bewilderment if we find the same structure changing its hue when we gaze at it from another angle, or with a different mode of illumination.