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I. S. Kassem, C. Evinger, H. Pomeranz; Dry Eye, Blinking, and Hemifacial Spasm. Invest. Ophthalmol. Vis. Sci. 2009;50(13):4037.
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Hemifacial spasm (HFS) is a disorder characterized by involuntary facial muscle spasms that begin in one eyelid and progress to the ipsilateral lower facial muscles. Eyelid spasms result from closely spaced bursts of orbicularis oculi (OO) muscle activity that resemble blink oscillations with an extremely short interblink interval. The primary cause of HFS is unilateral pulsatile arterial compression of the facial nerve at the root entry zone. This pressure causes facial nerve weakness, resulting in poor tear film production that initiates an adaptive increase in the blink reflex. We investigated the effect of increased cornea afferent activity on the eyeblink and the incidence of eye irritation in HFS patients.
To investigate the effect of reduced tear production, four male sprague-dawley rats had surgical implantation SO nerve stimulation electrodes and OO electromyogram (EMG) recording electrodes. Rats were tested for several days prior to surgical removal of the exorbital lacrimal gland. We reduced cornea afferent activity in two dry eye model rats by anesthetizing the cornea with tetracaine. We also recorded OOEMG activity of two normal human subjects and increased cornea afferent activity using a simultaneous air puff and reflex blink stimulus. To investigate if eye irritation is more prevalent in HFS patients, we performed a retrospective chart review of 36 HFS patients and compared the incidence of their eye complaints with age-matched controls.
Lacrimal gland removal in rats resulted in blink oscillations accompanying innocuous reflex blink stimuli without any change in reflex blink threshold, blink amplitude, or trigeminal excitability. Reducing cornea afferent activity in these rats with corneal anesthetic reduced the probability of oscillations evoked by reflex blinks. In normal human subjects, increasing cornea afferent activity by pairing the reflex stimulus with an air puff increased the probability of blink oscillations. HFS patients also had more complaints of eye irritation compared to control patients.
Cornea irritation initiates the production of multiple blinks to a normally single blink-evoking cutaneous trigeminal stimulus. These blink oscillations may occur because of coincident cutaneous and elevated cornea afferent activity. The spasms of lid closure in HFS might be an exaggeration of these multiple blinks closely spaced together in time. Together, these studies show that eyelid spasm disorders such as HFS may be due in part to a disruption in the adaptation to cornea drying and irritation.
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