Few routine electrophysiologic tests are available to evaluate the cranial nerves and their proximal segments other than visual and brainstem evoked potentials. However, cranial nerves V (trigeminal) and VII (facial), along with their connections in the pons and medulla, can be assessed electrically with the blink reflex. The blink reflex is essentially the electrical correlate of the clinically evoked corneal reflex. Like the H reflex, the blink reflex is a true reflex, with a sensory afferent limb, intervening synapses, and a motor efferent. Blink reflexes are useful in detecting abnormalities anywhere along the reflex arc, including peripheral and central pathways. Accordingly, neuropathies or compressive lesions of the peripheral facial or trigeminal nerves may be detected, as well as central lesions in the brainstem, including those caused by brainstem strokes and multiple sclerosis.
Anatomy
The afferent limb of the blink reflex is mediated by sensory fibers of the supraorbital branch of the ophthalmic division of the trigeminal nerve (cranial nerve V 1 ) and the efferent limb by motor fibers of the facial nerve (cranial nerve VII). Just as with the corneal reflex, ipsilateral electrical stimulation of the supraorbital branch of the trigeminal nerve elicits a facial nerve (eye blink) response bilaterally. Stimulation of the ipsilateral supraorbital nerve results in an afferent volley along the trigeminal nerve to both the main sensory nucleus of V (mid-pons) and the nucleus of the spinal tract of V (lower pons and medulla) in the brainstem. Through a series of interneurons in the pons and lateral medulla, the nerve impulse next reaches the ipsilateral and contralateral facial nuclei, from which the efferent signal travels along the facial nerve bilaterally ( Figure 5–1 ).
The blink reflex has two components, an early R1 and a late R2 response. The R1 response is usually present ipsilaterally to the side being stimulated, whereas the R2 response is typically present bilaterally. The R1 response is thought to represent the disynaptic reflex pathway between the main sensory nucleus of V in the mid-pons and the ipsilateral facial nucleus in the lower pontine tegmentum. The R2 responses are mediated by a multisynaptic pathway between the nucleus of the spinal tract of V in the ipsilateral pons and medulla and interneurons forming connections to the ipsilateral and contralateral facial nuclei.
The earlier R1 response usually is stable and reproducible, with biphasic or triphasic morphology. In a small percentage of normal individuals, the R1 response cannot be reliably elicited on either side. The R2 responses, on the other hand, are polyphasic and variable from stimulation to stimulation. With repeated stimulation, the R2 responses tend to habituate.
Blink Reflex Procedure
The patient should be in a relaxed state, lying supine on the examining table, with the eyes either open or gently closed ( Box 5–1 ). Recording is performed simultaneously from both sides of the face using a two-channel recording apparatus. Surface recording electrodes are placed over the inferior orbicularis oculi muscles bilaterally ( Figure 5–2 ). For recording the compound motor action potential from the orbicularis oculi muscle, the active recording electrode (G1) is best placed below the eye just lateral and inferior to the pupil at mid-position. The corresponding reference electrodes (G2) are placed just lateral to the lateral canthus bilaterally. Alternatively, recording can be done with small concentric needle electrodes placed in the orbicularis oculi bilaterally. The ground electrode is placed on the mid-forehead or chin.
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The patient should be in a relaxed state, lying supine on the examining table, with the eyes either open or gently closed.
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Recording from both orbicularis oculi muscles is performed simultaneously.
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Active recording electrodes are placed below the eye just lateral and inferior to the pupil at mid-position, with the reference electrodes placed just lateral to the lateral canthus.
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A ground electrode is placed over the mid-forehead or chin.
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Sweep speed set at 5 or 10 ms/division.
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Sensitivity set at 100 or 200 µV/division.
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Motor filter settings are 10 Hz and 10 kHz.
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Stimulate each supraorbital nerve (preferably with pediatric prong stimulator) over medial eyebrow, recording orbicularis oculi bilaterally. Allow several seconds between successive stimulations to prevent habituation.
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For each side, 4–6 stimuli are obtained on a rastered tracing and superimposed to determine the shortest response latencies.
