The oculo-auricular phenomenon. Findings in normals and patients with brainstem lesions.

The oculo-auricular phenomenon consists of bilateral coactivation of external ear muscles during lateral gaze. The electromyogram of the transverse auricular muscle was recorded in 25 healthy volunteers and 1186 patients. In normal subjects bilateral coactivation was observed with lateral gaze (96%), convergence (61%), active and intended head rotation (100%) and passive head rotation (50%). Uni- and bilateral labyrinthine excitation (60 and 80%) and proprioceptive input from the neck muscles (38%) are also effective. In patients with brainstem disease abnormal transverse auricular muscle coactivation is characterized by absence of activity in one or both ear muscles during lateral gaze in either or both directions. The most frequent abnormality was absence of transverse auricular muscle activity homolateral to right or left gaze (type Ia pattern). It was related to homolateral impairment of the blink reflex R1-response (90%) and the caloric response (90%), and to contralateral masseter reflex abnormality (70%). Electrophysiological data, clinical signs and imaging findings indicate that the type Ia pattern is caused by homolateral pontine or contralateral midbrain lesions. It is suggested that the supranuclear organization of the oculo-auricular phenomenon is based on descending tracts crossing at a mid-pontine level.