The effect of contact lens wear on corneal sensation

The corneal nerves play an important role in the protection and maintenance of corneal health, and the corneal epithelium has the highest density of free nerve endings in the body. Contact lenses are increasingly used to correct refractive error or for cosmetic purposes. It is therefore important to study the relationship between these factors. Studies have revealed that contact lens wear can produce a reduction in corneal sensitivity, with the extent of sensation loss related to the type of contact lens, the material it is made from, and the frequency and duration of wear. In summary, as the time of wear increases, both in the short-term (days) and long-term (months), the greater the loss of sensation. Recovery to normal levels, with the cessation of lens wear, is also prolonged with extended durations of contact lens wear. Newer lens materials that have improved oxygen permeability have less of an effect. The two principle mechanisms by which the corneal nerves are affected are the mechanical action of the lens and interference with the metabolic function of the cornea, as a result of the reduced oxygen supply. The impaired metabolic function produces an increase in acidosis and a change in corneal pH as a result of hypercapnia. Both of these can alter nerve function, and so reduce corneal sensitivity