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Images in Neurology
November 22, 2021

Tap-Elicited Clonus of the Biceps and Triceps Brachii

Author Affiliations
  • 1Department of Neurology and Neurological Science, Graduate School of Medical and Dental Sciences, Tokyo Medical and Dental University, Tokyo, Japan
JAMA Neurol. 2022;79(2):194. doi:10.1001/jamaneurol.2021.4273

A 43-year-old man with no family history of neurological disorders and no consanguinity in the family came to our hospital because of motor weakness, dysphagia, and dysarthria. He had been healthy at birth and started walking at 15 months old. However, the symptoms listed were noted in early to middle childhood and slowly progressed over 30 years. He was diagnosed with hereditary spastic paraplegia during the second decade of life. He was able to walk but became unable to speak or voluntarily move his tongue and upper limbs, except for minimal finger movements in his 30s. A percutaneous gastrostomy was performed at age 40 years. On admission to our hospital, a neurological examination revealed severe motor weakness in the face and upper limb muscles and moderate motor weakness in the lower limb muscles. Deep tendon reflexes were increased in all limbs, and the Babinski reflex was present on the right side. These findings, plus the tap-elicited clonus of the biceps and triceps brachii, all provided upper motor neuron signs (Video). The frequency of the clonus was 8.3 Hz, and the periodic bursts of the biceps and triceps brachii muscles were synchronous on surface electromyography. No cerebellar, extrapyramidal, or sensory signs were observed. A nerve conduction study in upper and lower limbs revealed compound muscle and sensory nerve action potentials with normal conduction velocity and amplitude. However, transcranial magnetic stimulation revealed either motor-evoked potentials with extremely prolonged latency or no motor-evoked potentials at all in the facial and upper or lower limb muscles. Needle electromyography revealed no explanatory abnormalities in the lower motor neurons. Magnetic resonance imaging of the brain and spinal cord revealed no abnormalities. These findings suggested that the slowly progressive degenerative loss of upper motor neurons caused his symptoms. He was diagnosed with an atypical form of primary lateral sclerosis. Direct sequencing revealed no genetic variation in any of the 34 exons or the 5′ and 3′ untranslated regions of alsin (ALS2) gene,1 the gene most often associated with juvenile primary lateral sclerosis.

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