Lateral Medullary Lesions Involving the Spinal Nucleus of the Trigeminal Nerve may Cause Ipsilesional Masseter Reflex Abnormalities

Background: Experimental data indicate a suprasegmental influence on the masseter reflex (MassR), which is mediated via the 5th nerve spinal nucleus. To the best of our knowledge, corresponding data in humans is still lacking. Patients and Methods: We retrospectively analyzed the files of 258 consecutive patients with clinical signs and symptoms of acute ischemia in the vertebrobasilar territory for unilateral infarctions caudal to the levels of the motor and main sensory nucleus of the 5th nerve. Biplane T₂- and echo planar diffusion-weighted magnetic resonance imaging with slice orientation parallel and perpendicular to slices of a stereotactic anatomical atlas were done in all patients. Individual slices were normalized and the lesions were projected into the corresponding slices of the anatomical atlas. Results: We identified 39 patients with unilateral infarctions caudal to the level of the 5th nerve motor and main sensory nucleus. Four of these patients had an abnormal delay and 2 a loss of the MassR on the side of the infarction. In all patients, the infarction involved the region of the 5th nerve spinal nucleus at least on one slice. Conclusions: Our findings indicate that medullary brainstem lesions involving the 5th SpN may cause ipsilesional MassR abnormalities. There is experimental evidence for an excitatory projection from the amygdaloid nucleus via the 5th nerve spinal nucleus to masseter motoneurons in the 5th nerve motor nucleus. An interruption of this pathway at the site of the 5th nerve spinal nucleus would be followed by an impaired excitation of masseter motoneurons, which would explain the MassR abnormalities in our patients.