There are a number of specific corneal dystrophies, most of which are inherited and most of which cannot be diagnosed without the aid of the slit-lamp microscope. For this reason, they will not be dealt with in any detail here. A list for reference is shown in Table 6.1.
Keratoconus (or conical cornea) is perhaps the commonest. It is still rare in the general population but is familiar to general practitioners looking after student populations because it tends to appear in this age group. The condition is bilateral and can be inherited as an autosomal recessive trait, although most patients do not have a positive family history. It should be suspected in patients who show a rapid change of refractive error, particularly if a large amount of myopic astigmatism suddenly appears. Often, but not always, there is an associated history of asthma and hay fever. The cornea shows central thinning and protrudes anteriorly. This can be observed with the naked eye by asking the patient to sit down and then standing behind
him so that one can look down on his down-turned eye. By holding up the upper lids, one can make an estimate of the abnormal shape of the cornea by noting how the cornea shapes the lower lid. Alternatively, the patient's cornea can be observed using Placido's disc. This ingenious instrument is simply a disc with a hole in the centre, through which one observes the patient's cornea. On the patient's side of the disc is a series of concentric circles,which can be seen by the observer reflected on the patient's cornea (Figure 6.13). Distortion of these circles indicates the abnormal shape of the cornea. Of course, more accurate assessment of the cornea can be made by observing it with the slit-lamp microscope and still more information can be obtained by keratometry or corneal topography, that is, using an instrument to measure the curvature of the cornea in different meridians. Keratoconus tends to progress slowly and contact lenses can be helpful. Sometimes a corneal graft is required. Less common corneal dystrophies include Fuch's endothelial, stromal and anterior dystrophies.
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