Objective: We describe a case of pseudodystonia caused by palmaris brevis syndrome.

Background: Palmaris brevis (PB) syndrome is a pseudodystonia characterized by spontaneous, involuntary, nonpainful contractions of the PB muscle. It is a rare, benign condition that is believed to be related to focal irritation of motor fibers arising from the superficial branch of the ulnar nerve.

Method: Case Report:
A 21-year-old male was referred for evaluation of left-hand spasms occurring for 5-6 months. He initially noticed spasms and cramping over the hypothenar area when trying to sleep, but since then it had become more notable and frequent. Symptoms worsened towards the end of the day or when tired. There was no pain in hand or with spasms, and there were no relieving factors (no sensory tricks or positional variation).

Exam was significant for intermittent focal spasms of the left hypothenar and ulnar surface of hand causing hypothenar dimpling. Contractions were more prominent with fingers partially flexed or with percussion, and somewhat diminished with finger extension.

Results: Electromyography recorded a single myokymic discharge from the left abductor digiti minimi; otherwise, there were no abnormal findings in other muscles tested or evidence of an ulnar neuropathy. Neuromuscular ultrasound was performed 1 month later as symptoms had worsened, and was notable for repeated spasm of the PB muscle, with no abnormality of the superficial ulnar nerve branch or evidence of median or ulnar compression/neuropathy.

Conclusion: Palmaris brevis syndrome is a rare, benign pseudodystonia believed to be caused by focal irritation of motor fibers arising from the superficial branch of the ulnar nerve. It has been reported to occur with chronic compression of the proximal hypothenar eminence, such as prolonged use of a computer mouse and keyboard [1]. Similar to task-specific dystonias, it has been posited that it can also be triggered by extreme repetitive use [2]. Palmaris brevis syndrome can be treated with botulinum toxin injections, occupational therapy, and workplace modifications if occupation-related.

Our case highlights the utility of neuromuscular ultrasound in establishing the correct diagnosis of this syndrome.

References: [1] Liguori R, Donadio V, Di Stasi V, Cianchi C, Montagna P. Palmaris brevis spasm: an occupational syndrome. Neurology. 2003 May 27;60(10):1705-7. doi: 10.1212/01.wnl.0000064165.70159.93. PMID: 12771275.
[2] LeDoux MS, Xiao J. Palmaris Brevis Syndrome: A Treatable Pseudodystonia. Tremor Other Hyperkinet Mov (N Y). 2021 Oct 29;11:45. doi: 10.5334/tohm.659. PMID: 34754604.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/pseudodystonia-due-to-palmaris-brevis-syndrome-case-report/