The Cornea Misunderstood

Because the cornea is the most superficial layer of the eye we may think that disorders of the cornea may be easier to diagnose than other ophthalmic disorders. However, many horses present with corneal disorders which are refractory to treatment or recur- rent. in these cases it is often difficult to know for sure what the problem is. at utrecht university Equine Clinic we’ve coined the phrase ‘the cornea misunderstood’ for these cases.

To get to full understanding of such cases it’s important to perform a complete ophthal- mic examination and to have a good understanding of corneal physiology and pathol- ogy. a very useful text for this purpose is Equine ophthalmology (BC Gilger 2nd Edition, Elsevier Saunders). apart from information on diagnostic procedures in general, there is a very useful chapter on ‘field ophthalmology’.

One tip that will prove useful to many is to make digital photographs of the eye that is being examined. By cropping off the unwanted parts of the picture and viewing the photographs on a computer screen, small details can be seen even better than live. photographs are also useful to point out to the owner what the problem is. Develop- ment over time can be assessed by comparing consecutive photographs. and finally, with photographs it’s possible to discuss a case with a distant colleague.

An important detail of corneal physiology is the fact that the cornea will only remain transparent if the stroma maintains its relatively dehydrated state. this means that the epithelium has to keep water from the tear film out and the endothelium pumps water from the stroma to the anterior chamber. With any case of corneal edema (appearing bluish) it is useful to assess whether the origin is endothelial (e.g. increased intraocular pressure, uveitis, blunt trauma) or epithelial (e.g. traumatic ulceration, epithelial infec- tion). to assess whether corneal opacities are epithelial, stromal or endothelial it is use- ful to use a slit lamp and magnification (e.g. loupe glasses). the slit lamp should illumi- nate a ‘slice’ of cornea at an angle to the examiner’s visual axis to allow us to distinguish the different layers of cornea. [...]