Blink Reflex Alterations in Various Polyneuropathies

An eyelid closure in response to some stimulus is a blink reflex (BR), which is normally isolated. In humans and primates, the closing is bilateral while in other animals, mostly those with eyes set laterally, the closure is frequently unilateral. In clinical practice, a BR is characteristically provoked by light corneal or eyelash touching or glabellar tapping. The British physician Overrend first elicited the blink reflex by tapping one side of the forehead. Kugelberg analysed the blink reflex electromyographically by electrically stimulating the supraorbital nerve. Since the original description of the blink reflex 100 years ago the study of this reflex has given understanding the central and peripheral mechanisms of the trigeminofacial pathways in normal and different disorder. Stimulation of the supraorbital nerve, a distal branch of the ophtalmic division of the trigeminal nerve , is the common technique in clinical neurophysiology to obtain a BR (Figure 1). Characteristically, an electrical stimulus on the supraorbital nerve induces two recordable responses in the orbicularis oculi muscles: an early one R1, ipsilateral to the stimulated side, and a later one, R2, which is bilaterally expressed. R2 response ipsilateral to the stimulus is frequently cited as R2i, and the R2c is the one obtained on the contralateral side. R1 component of BR has a rather stable latency. R2 typically shows relative variable latencies and larger magnitudes than R1 and its threshold is lower. R2 component is responsible for the eyelid closure of the blink. It has been observed that common motor unit potentials contribute to the buildup of both responses, that is, the same orbicularis oculi motor unit is depolarized after the stimulation at times corresponding to the latencies of R1 and R2 responses, the latter presumably in a repetitive reverberating pattern. Both response are cutaneous and nociceptive in origin. The classical findings, indicative of such a lesion are an afferent defect with prolonged latencies of R1, ipsilateral R2 and contralateral R2. The efferent type pattern occurs also with intraaxial lateral pontine lesions involving the pons at the trigeminal entrance zone. In facial nerve lesions there is a delay in the reflex latency only on the affected side, regardless of the side of stimulation. Such abnormalities were found in Bell’s palsy or other lesions of the facial nerve.