How can I clinically examine the palmaris brevis muscle?

Clinical Examination of the Palmaris Brevis Muscle

To examine the palmaris brevis muscle clinically, observe the hypothenar eminence for visible wrinkling or dimpling of the skin during forceful abduction of the fifth digit, which indicates active contraction of this superficial muscle. 1

Anatomical Localization

The palmaris brevis is a small, thin muscle located superficially in the hypothenar region of the palm, measuring approximately 2.1 × 2.1 cm with a thickness of only 0.2 cm. 1 The muscle lies at a depth of 3.7 to 6.9 mm from the skin surface (including 1.9 mm of skin thickness), making it accessible to clinical examination. 2

• The muscle inserts into both the hypothenar fascia and the dermis of the ulnar border of the hand 2
• It is positioned superficial to the ulnar nerve and artery, which run along its lateral (radial) side 2
• The muscle is the only structure frequently innervated by the superficial branch of the ulnar nerve 1

Clinical Examination Technique

Visual Inspection

• Ask the patient to forcefully abduct the fifth digit (little finger) while observing the hypothenar eminence 1
• Look for characteristic wrinkling or dimpling of the skin over the ulnar border of the palm, which occurs when the muscle contracts and pulls the skin medially 1, 2
• Note that the palmaris brevis is not under voluntary control in most individuals, so contraction occurs reflexively during grip or fifth digit abduction 3

Palpation

• Palpate the hypothenar eminence during fifth digit abduction to feel the thin muscle contract beneath the skin 1
• The muscle can be distinguished from the underlying hypothenar muscles (abductor digiti minimi, flexor digiti minimi brevis, opponens digiti minimi) by its superficial location and transverse fiber orientation 2

Functional Assessment

• Observe for spontaneous, irregular, tonic contractions of the hypothenar skin, which may indicate palmaris brevis spasm syndrome 3, 4
• These involuntary contractions appear as visible rippling or twitching of the skin over the hypothenar eminence without voluntary effort 3
• The contractions are typically high-frequency and involve normal motor units without evidence of neuropathy 3

Clinical Significance

The palmaris brevis examination is particularly useful for:

• Diagnosing superficial ulnar nerve lesions that spare the deep motor branch, as seen in cyclist palsy or crutch palsy, where sensory impairment occurs with preserved deep motor function 1
• Identifying palmaris brevis spasm syndrome, characterized by spontaneous muscle hyperactivity that may be triggered by repetitive hand use (such as pipetting or computer mouse use) or compression of the superficial ulnar nerve branch 3, 4
• Differentiating superficial from deep ulnar nerve pathology, since the palmaris brevis is the only muscle supplied by the superficial branch 1

Common Pitfalls

• Do not confuse palmaris brevis contraction with movement of the underlying hypothenar muscles, which produce fifth digit motion rather than skin wrinkling 1
• The muscle's predominance of type I (slow-twitch) fibers (72.2%) indicates it functions as a fatigue-resistant structure that protects the ulnar neurovascular bundle during prolonged gripping, rather than producing forceful movements 5
• Avoid injecting botulinum toxin (used to treat palmaris brevis spasm) deeper than 7 mm or along the lateral aspect of the muscle, as this risks injury to the underlying ulnar artery 2