The first description of the palmaris brevis muscle

Palmaris Brevis Syndrome: A Treatable Pseudodystonia

Background

Palmaris brevis syndrome, a pseudodystonia characterized by abnormal involuntary contractions of the palmaris brevis muscle which resides in the hypothenar eminence, is believed to be due to compressive irritation of motor fibers which arise from the superficial branch of the ulnar nerve.

Case report

Herein, we review the origins, differential diagnosis and pathophysiology of the palmaris brevis syndrome, and effective treatment of a patient with workplace modifications and injections of botulinum toxin type A.

Discussion

Prompt diagnosis of the palmaris brevis syndrome facilitates effective treatment and resolution.

Highlights

Like the task-specific hand dystonias seen in writers and musicians, palmaris brevis syndrome, a pseudodystonia, may be caused and aggravated by extreme repetitive use. Here, we report a case of palmaris brevis syndrome apparently triggered by high-volume use of a pipette and computer mouse and review relevant clinical facets from previously published cases. Treatment must include workplace modifications and may include injections of botulinum toxin.

Structural and functional anatomy of the palmaris brevis: grasping for answers

The palmaris brevis (PB) is a small cutaneous hand muscle that has been described as the most mysterious muscle from a functional and developmental perspective [Kaplan () Kaplan's Functional and Surgical Anatomy of the Hand]. Functionally, the PB is considered to deepen the hollow of the palm and to protect the neurovasculature of the ulnar canal. Although the function of the PB has been inferred from cadaveric observations, the electromyographic (EMG) activity of this muscle has not been explored systematically during specific movements of the hand. The purpose of this study was to record PB intramuscular EMG activity during dynamic grasping tasks, and to quantify the change in PB muscle length (M_L ) and thickness (M_T ) incurred during maximal contraction using ultrasound imaging. Intramuscular EMG was recorded from the PB in the dominant hands of 12 healthy participants (11 males, one female; age: 27 ± 4 years) during maximal abduction, flexion and opposition of the 5th digit, and two grasping tasks. Abduction of the 5th digit yielded the greatest EMG activity in most individuals (seven out of 11), and produced significantly less PB EMG activity when compared with grasping a cylindrical-shaped object (P = 0.003) but not a spherical-shaped object (P = 0.130). During maximal abduction of the 5th digit, PB M_L decreased in both the left (28 ± 11%; P = 0.002) and right (32 ± 5%; P = 0.002) hands. Similarly, a concomitant increase in PB M_T was observed in the left (68 ± 30%; P = 0.002) and right (85 ± 44%; P = 0.002) hands during the same contraction. These EMG results indicate that the PB is voluntarily activated during prehensile and non-prehensile movements of the hand with significant changes in muscle architecture. The study supports the preservation of the PB in surgical procedures based on its proposed protective role as a muscular barrier to the neurovasculature within the ulnar canal.