Corneal sensation is supplied by the fifth nerve. About 70 nerve fibres are present in the superficial layers of the cornea and they can often be seen when the cornea is examined with the slit-lamp microscope. They appear as white threads running mainly radially. Asking the patient to gaze straight ahead and then lightly touching the cornea with a fine wisp of cotton-wool can assess corneal anaesthesia. Care must be taken not to touch the lid margins when doing this. The blink reflex is then noted and it is also important to ask the patient what has been felt. In the case of elderly people, the blink reflex might be reduced, but a slight prick should be evident when the cornea is touched. Attempts to quantify corneal anaesthesia have led to the development of graded strengths of bristle, which can be applied to the cornea instead of cotton-wool.
Corneal anaesthesia can result from a lesion at any point in the fifth cranial nerve from the cornea to the brainstem. In the cornea itself, herpes simplex infection can ultimately result in anaesthesia. Herpes zoster is especially liable to lead to this problem and, because this condition can often be treated at home rather than in the ward, it will be considered in more detail here.
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