Selective calf weakness suggests intraspinal pathology, not peripheral neuropathy

Four patients, referred as having peripheral neuropathy, were noted to be able to walk on their heels but not on their toes. In each, intraspinal disease was found: ependymoma of the filum terminale, spinal muscular atrophy, spinal stenosis, and meningeal carcinomatosis. By comparison, in 86 cases of hereditary motor and sensory neuropathy type 1, ankle plantar flexors were never weaker than ankle dorsiflexors. Patients with greater weakness in plantar flexors than in dorsiflexors should be suspected of having intraspinal disease rather than peripheral neuropathy. Physiologic and biomechanical factors may explain why muscles innervated by the peroneal nerve are weaker, or graded weaker, in peripheral neuropathy.