Preservation of the masseter reflex in Friedreich's ataxia.

In Friedreich's ataxia, the muscle stretch reflexes are absent or hypoactive in the upper and lower extremities because of pathologic involvement of the dorsal root ganglion cells. Paradoxically, the masseter reflex remains unaffected and may even be hyperactive in some cases. The masseter reflex is hypothesized as unique among stretch reflexes in that its afferent nerve cell body (the mesencephalic nucleus) is located intra-axially within the brainstem rather than in the craniospinal ganglia, where the afferent nerve cell bodies subserving the other stretch reflexes are located. This study supported this hypothesis by demonstrating that the masseter reflex is (1) usually absent in patients who are areflexic as a result of primary disorders of peripheral nerve axons or myelin, and (2) preserved in other disorders that primarily involve the dorsal root ganglion cells.