Jaw Clonus and limb fasciculations: NEJM case report
A 24-year-old man presented to the neurology clinic with a 1-year history of progressive dysphagia, dysarthria, and weakness in his arms and legs. Three months before the onset of these symptoms, he had received an electric shock while repairing an electric fan; the contact point was the right hand, which had touched an uninsulated wire. The physical examination was notable for clonus of the jaw, brisk deep tendon reflexes in the arms and legs, and fasciculations of the tongue and of the muscles in the arms and legs (see video). The sensory examination was normal.

Nerve-conduction studies of the median, ulnar, tibial, and common peroneal nerves on both sides of the body showed diminished amplitude of compound muscle action potentials. Electromyography revealed neurogenic motor unit action potentials, with fasciculation potentials in the tongue and in all limbs. These findings were consistent with chronic motor denervation and reinnervation.

Magnetic resonance imaging of the brain and spinal cord revealed no abnormalities. Given the presence of both upper and lower motor neuron signs with sparing of the sensory pathways, a diagnosis of amyotrophic lateral sclerosis was made. Jaw clonus typically indicates damage to the upper motor neurons in the corticopontine tracts.

At follow-up 6 months after the diagnosis was made, the patient had worsening limb and bulbar muscle weakness and atrophy.

Source: https://www.nejm.org/doi/full/10.1056/NEJMicm1809249