Dry eye, blinking, and blepharospasm.

The primary purpose of the eyelids is to ensure corneal integrity. An important component of this protective function is maintaining the corneal tear film. Blink characteristics can determine tear film stability. Increasing blink amplitude thickens the lipid layer that overlies the aqueous layer of the tear film. This thickening reduces evaporation of the aqueous layer. Because each blink reforms the tear film, increasing blink frequency reduces tear film break-up. Both innocuous stimuli such as air across the eyelashes, and noxious stimuli such as touching the cornea, elicit trigeminal reflex blinks. If abnormal corneal afferent activity caused by corneal irritation acts as an “error signal” that adjusts the amplitude and frequency of reflex blinks evoked by both innocuous and corneal stimuli, then corneal irritation creates an adaptive blink response regardless of whether a corneal or an innocuous trigeminal stimulus elicits the blink. An investigation of the effects of aging on trigeminal blinks supports the hypothesis that corneal irritation modifies trigeminal reflex blinks evoked by innocuous stimuli. In people over age 40 years, a single innocuous, supraorbital (SO) nerve stimulus frequently evokes a reflex blink and additional blinks that occur at a nearly constant interval relative to the onset of the preceding blink, blink oscillations. Because blink oscillations are more frequent and larger than reflex blinks, this modification increases tear film stability. Peshori and colleagues propose that the development of consistent blink oscillations in many individuals over age 40 years is a blink adaptation that compensates for the subclinical reduction in corneal wetting that accompanies aging. This hypothesis predicts that innocuous SO stimuli should produce more blink oscillations in people with dry eye than in age-matched control subjects. We test this prediction by comparing the SO-evoked blinks of individuals with dry eye with those of age-matched controls. Given the frequent occurrence of dry eye at the onset of benign essential blepharospasm (BEB), blink modifications associated with dry eye may play a role in the origin of BEB. We present a hypothesis that links the adaptive processes initiated by dry eye with the origin of BEB.