Normal tendon reflexes despite absent sensory nerve action potentials in CANVAS: a neurophysiological study

CANVAS is a recently defined progressive ataxic syndrome with impairment of vestibular, somatosensory and cerebellar function due to atrophic degeneration of dorsal root ganglia and dorsal columns, of cranial nerve somatosensory ganglia, of vestibular ganglia and vestibular nerves and of cerebellar Purkinje cells. While all patients eventually develop sensory impairment in a non-length dependent pattern and lose sensory nerve action potentials, some retain their tendon reflexes. Here we study 5 CANVAS patients with absent sensory nerve action potentials but intact, even brisk Achilles tendon reflexes and, in 4, preserved H reflexes in the upper and lower limbs. These findings imply that dorsal root ganglion neurons subserving cutaneous afferents more vulnerable than those subserving muscle afferents. Our findings have a clinical message: preservation of the Achilles tendon jerk does not exclude a large fibre peripheral neuronopathy.