Dry eye, blinking, and blepharospasm

Abstract

Keywords dry eye; blinking; blepharospasm; eyelid movements; blink oscillations; brainstemThe 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 breakup. [1][2][3][4] 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 5 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 5 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. 6 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), 7 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. METHODSMeasurements of eyelid movement and orbicularis oculi electromyographic (OOemg) activity were made on seven subjects, ranging in age from 44 to 72 years of age. Five female subjects had been diagnosed with dry eye at the SUNY Stony Brook Ophthalmology Clinic. The data from these subjects were compared with those from age-matched control subjects in another study. 5 Two normal subjects, a man and a woman, participated in an experimental paradigm described below. Apart from dry eye, no subject had any ocular disorders other than the need for corrective lenses. A previous study 8 detailed measurement of lid movements and OOemg used in the current study. Upper eye...

The Motor Cortex and Facial Expression:

“…Major structures would include the basal ganglia, thalamus, hypothalamus, periaqueductal gray, trigeminal complex, and reticular formation. 54,[115][116][117][118][119] This would be supported by the constellation of subcortical lesions that have been reported to give rise to deficits in either voluntary or emotional forms of facial expression. 107,[111][112][113]120,121 To learn more about the effects of subcortical lesions that give rise to these deficits, it will be a priority to map the subcortical systems that converge on the facial nucleus, including the various corticofacial pathways as they course through the corona radiata, internal capsule, and brainstem.…”

Spontaneous eye blinks are entrained by finger tapping

“…The functional role of spontaneous blinking is mainly to guarantee protection against corneal drying (Evinger, et al 2002) which is avoided by an appropriate tear film distribution over its surface (Evinger, 1995;VanderWerf, Reits, Smit, & Metselaar, 2007). While reduced blinking leads to tear film thinning, longer prevention of blinks can eventually result in the tear film breaking up (Holly, 1973).…”

Characterization of some morphological parameters of orbicularis oculi motor neurons in the monkey

“…However, recent efforts have focused on examining the role of the OO in neurological movement disorders affecting eyelid closure. Indeed, OO abnormalities occur in a host of neurological disorders including blepharospasm, hemifacial spasm, Meige syndrome, Parkinson's disease and Tourette's syndrome (Basso and Evinger, 1996;Hallett and Daroff, 1996;Smith et al, 1996;Schicatano et al, 1997;Jankovic and Tolosa, 1998;Peshori et al, 2001;Evinger et al, 2002;Hallett, 2002;Holstege, 2002). In many of these diseases there is a cumulative breakdown in the structural organization and functional regulation of the neural circuitry mediating OO function.…”