Palmaris Brevis Anatomy

Origin

• The ulnar border of the palmar aponeurosis.
• The flexor retinaculum (transverse carpal ligament) along its ulnar margin.
• Some fibres arise from the fascia over the pisiform region and from the antebrachial fascia as it continues into the palm.

Insertion

• The dermis of the skin along the ulnar border of the palm, over the hypothenar eminence.
• It has no bony or tendinous insertion at all - the defining feature, and what makes it a cutaneous (dermal) muscle, in the same functional category as platysma.

Form and position

• A thin quadrilateral sheet, typically 2 to 3 cm across, running transversely across the proximal hypothenar eminence, immediately deep to the skin and subcutaneous fat.
• It lies superficial to the ulnar neurovascular bundle and to the hypothenar muscles, and it is the most superficial muscle of the ulnar palm.
• It is frequently thin, occasionally hypoplastic and sometimes absent, and its size varies considerably between individuals.

Relationship to the palmar fascia

• Because it arises from the palmar aponeurosis and inserts into skin, palmaris brevis is part of the skin-fascia anchoring system of the palm, alongside the vertical fascial septa and the natatory and pretendinous bands.
• This is why the ulnar palm has firm, tethered, relatively immobile skin adapted to shear loading during grip - and why an ulnar-sided Dupuytren cord produces puckered, dimpled hypothenar skin.

Nerve supply

• The SUPERFICIAL branch of the ulnar nerve, root values C8 and T1.
• It is the only muscle supplied by the superficial branch. Every other ulnar-innervated intrinsic muscle - the interossei, the ulnar two lumbricals, adductor pollicis, the deep head of flexor pollicis brevis, and the hypothenar group (abductor, flexor and opponens digiti minimi) - is supplied by the deep motor branch.
• The branch to palmaris brevis leaves at or immediately after the bifurcation of the ulnar nerve within Guyon canal, entering the deep surface of the muscle.

Why it is not under voluntary control

• Most people cannot contract palmaris brevis on command. It fires as part of grip and of palmar cupping rather than as an isolated voluntary act. This observation was made explicitly in the original description of palmaris brevis spasm syndrome and matters for two reasons:
  1. Examination: you observe hypothenar dimpling during grip rather than asking the patient to contract the muscle.
  2. Pathophysiology: its involuntary nature is part of the reason a spontaneous spasm syndrome of this muscle exists at all - the patient cannot suppress it.

The zonal anatomy it helps define

Arterial supply

• Direct muscular branches from the ulnar artery as it traverses Guyon canal, and from the superficial palmar arch into which the ulnar artery continues.
• Additional supply from the deep palmar branch of the ulnar artery and from small perforators to the overlying skin.
• The muscle therefore has a segmental, proximally reliable pedicle from the ulnar artery immediately deep to it.

Why the blood supply matters - the turnover flap

• The consistent proximal supply from the ulnar artery allows palmaris brevis to be raised as a turnover (transposition) muscle flap based proximally and reflected radially to cover the ulnar nerve and artery within Guyon canal.
• Indications: coverage of an ulnar artery vein graft after resection for hypothenar hammer syndrome, coverage of a scarred ulnar nerve in revision ulnar tunnel surgery, and interposition of vascularised tissue between the nerve and a bony prominence such as a hook of hamate nonunion.
• Advantages: local, thin, vascularised tissue in the same operative field, with no functional deficit from losing the muscle.
• Caution: the flap is raised directly over the ulnar neurovascular bundle. The nerve and artery must be identified and protected first, and the pedicle preserved proximally.

The artery beneath - the hypothenar hammer segment

• The distal ulnar artery crosses the hook of the hamate immediately deep to palmaris brevis, where it is relatively unprotected against the bone.
• This is precisely the segment injured by repetitive blunt trauma to the hypothenar eminence, producing thrombosis, intimal irregularity or aneurysm - hypothenar hammer syndrome.

Guyon canal - the boundaries

• Roof: the volar carpal ligament and palmaris brevis - the muscle is the distal, muscular component of the roof.
• Floor: the transverse carpal ligament and the pisohamate ligament, with the hypothenar muscles distally.
• Ulnar wall: the pisiform and the flexor carpi ulnaris tendon.
• Radial wall: the hook of the hamate.
• Length: approximately 4 to 4.5 cm.

Contents and their relationship to the muscle

• Immediately deep to palmaris brevis: the ulnar nerve and the ulnar artery, with the artery lying RADIAL to the nerve.
• The nerve bifurcates within the canal into the superficial and deep branches.
• The deep motor branch then curves dorsally and radially around the hook of the hamate, passing between the origins of abductor digiti minimi and flexor digiti minimi brevis and beneath the fibrous arch of the hypothenar muscles - a discrete second compression point.
• Deep to the neurovascular structures: abductor digiti minimi, flexor digiti minimi brevis and opponens digiti minimi.

Surface anatomy

• The pisiform is the palpable landmark at the proximal hypothenar eminence; the hook of the hamate lies approximately 1 to 2 cm distal and radial to it and is palpable with deep pressure - a manoeuvre uncomfortable even in a normal hand.
• Palmaris brevis lies in the skin between and just distal to these two landmarks, running transversely.
• Kaplan cardinal line, drawn from the apex of the first web space parallel to the proximal palmar crease, crosses the hypothenar region and helps localise the deep branch and the superficial palmar arch.

Relationship to the median side

• The transverse carpal ligament forms the floor of Guyon canal and the roof of the carpal tunnel - the two canals share this structure. Dividing the transverse carpal ligament during a carpal tunnel release therefore decompresses the floor of Guyon canal, which is why many zone 1 ulnar symptoms resolve after a simple carpal tunnel release.

Palmaris brevis spasm syndrome

A rare, benign and highly characteristic condition of localised muscular hyperactivity confined to this single muscle.

Presentation.

• Spontaneous, irregular, tonic contractions of the hypothenar eminence, producing visible dimpling that comes and goes at rest. Patients often present alarmed by the appearance rather than by symptoms.
• The original series described five men with exactly this picture. A separate series described five patients who developed the syndrome after prolonged use of a computer mouse and keyboard - the occupational form.
• Symptoms are usually painless, though paraesthesiae in the ulnar digits are described in the occupational form.
• The examination is otherwise normal - no wasting, no weakness, no sensory loss, no positive Tinel.

Investigation.

• EMG is the diagnostic test: it shows spontaneous high-frequency discharges of normal motor units and myokymia arising from palmaris brevis, without evidence of neuropathy or denervation.
• The localising test is an ulnar nerve block at the wrist - and the two case series disagree about its result. In the occupational series (Liguori 2003, five patients) the discharges did NOT disappear after the block, placing the generator distal to it, within the terminal branch. In the earlier report (Loron 1985, one patient) the contractions were abolished by local anaesthetic infiltration of the ulnar nerve, placing the generator at or proximal to the block. Both are cited on this page and both cannot be generally true, so treat the block as a test that localises the generator in the patient in front of you, not as a fact about the disease. A spasm abolished by a wrist-level block has a different implied site from one that persists.
• Exclude a structural cause: ultrasound or MRI of Guyon canal for a ganglion, a hook of hamate nonunion or an ulnar artery aneurysm, and consider cervical imaging, since a root compression was the probable origin in one of the originally reported patients.

Mechanism.

• The favoured explanation is ephaptic transmission - abnormal cross-excitation between axons - possibly secondary to transient and repeated stretching of the superficial branch of the ulnar nerve.
• The mechanism follows wherever the block localises it, which is why the disagreement above matters. Loron's contractions were abolished by local anaesthetic infiltration of the ulnar nerve, and the clinical and electromyographic picture was likened to hemifacial spasm - the analogy that frames this as focal hyperexcitability rather than a compressive neuropathy. Note that this abolition points to a generator at or proximal to the block, the opposite direction from the occupational series, and neither report has been reconciled against the other.
• The fact that palmaris brevis is not under voluntary control means the patient cannot suppress the contraction, which is part of why the phenomenon is so visible.

Management.

• Reassurance is the mainstay. The condition is benign and often self-limiting, and the diagnosis itself relieves considerable anxiety.
• Activity modification in the occupational form - altering mouse and keyboard ergonomics and avoiding sustained hypothenar pressure.
• Botulinum toxin injection and, in refractory cases, surgical excision of the muscle have been reported; excision is acceptable precisely because the muscle is functionally expendable.
• Do not decompress Guyon canal for this condition in the absence of a structural lesion - there is nothing to decompress, and an unnecessary operation over the ulnar bundle carries real risk.

Hypothenar hammer syndrome

• Mechanism: repetitive blunt trauma, or a single severe blow, to the hypothenar eminence drives the distal ulnar artery against the hook of the hamate, immediately deep to palmaris brevis. The result is intimal damage, thrombosis, corkscrew irregularity or aneurysm formation, with distal embolisation to the digital arteries.
• Who gets it: mechanics, carpenters and labourers who use the palm as a hammer; and a long list of sports - baseball, badminton, handball, football, frisbee, softball, karate, weight-lifting, hockey and golf have all been implicated in reported cases.
• Presentation: cold intolerance, pain, and ischaemic changes in the ulnar digits, sometimes with ulceration. The colour and temperature change is more diffuse than classical Raynaud phenomenon and characteristically lacks the triphasic colour change - a discriminator worth stating.
• The differential that matters: it must be distinguished from hand-arm vibration syndrome and from connective tissue disease. The occupational, unilateral, ulnar-distribution pattern points to a mechanical arterial cause, which is potentially curable, whereas vasospastic disease is not.
• Diagnosis: an abnormal Allen test raises the suspicion; Doppler ultrasound or arteriography confirms it, showing thrombosis, a corkscrew configuration of the distal ulnar artery, or an aneurysm, with digital arterial filling defects from emboli.

The palmaris brevis contribution

• Anomalous accessory fibres of palmaris brevis have been reported forming a sling around the ulnar artery at the level of the hook of the hamate. In a reported case in an amateur golfer, MRI demonstrated exactly this, and contraction of the anomalous muscle belly was proposed to have compressed the artery, slowed arterial flow and promoted thrombosis on top of the direct pressure injury.
• Practical consequence: at exploration for hypothenar hammer syndrome, look for anomalous muscle fibres crossing the artery and release them; do not assume the only pathology is within the vessel wall.

Treatment principles

• Non-operative: cessation of the causative activity, smoking cessation, cold avoidance, calcium channel blockade and antiplatelet therapy for mild disease without critical ischaemia.
• Operative: resection of the diseased arterial segment with reversed vein graft interposition is the standard where there is embolisation, aneurysm or critical ischaemia. Simple ligation is an alternative where the arch is complete and the hand is well perfused by the radial system - which must be demonstrated, not assumed.
• Coverage: a palmaris brevis turnover flap provides vascularised local cover over the vein graft, separating it from the skin and from the hook of the hamate.
• Concurrent nerve decompression: the ulnar nerve within Guyon canal is commonly compressed by the aneurysm or by the surrounding inflammation and should be formally released at the same operation.

Ulnar tunnel syndrome - what palmaris brevis tells you

• The zonal classification, derived from a canal 4 to 4.5 cm long divided by the internal topography of the nerve, remains the framework:
  • Zone 1 (proximal to the bifurcation) produced all the reviewed cases of combined motor and sensory deficit.
  • Zone 2 (deep motor branch) produced intrinsic paralysis, with hypothenar involvement determined by the exact level.
  • Zone 3 (superficial branch) produced sensory deficits only.
  • Combined motor and sensory loss most often followed compression from deep to the nerve; pure sensory deficits followed compression superficial to it.
• Palmaris brevis is the marker of superficial branch integrity and therefore adds a further level of resolution within this scheme.

Causes of ulnar tunnel syndrome to have on the list

• Ganglion arising from the pisotriquetral or hamatocapitate joint - the commonest cause.
• Hook of hamate fracture or nonunion, particularly in racquet, club and bat sports.
• Ulnar artery thrombosis or aneurysm - hypothenar hammer syndrome.
• Anomalous muscles, including an accessory abductor digiti minimi, an anomalous palmaris brevis and a palmaris profundus.
• External pressure - prolonged cycling (handlebar palsy), crutch use, or occupational tool pressure.
• Space-occupying lesions including lipoma and giant cell tumour of the tendon sheath.
• Systemic causes - inflammatory arthropathy and amyloid.

The combined-release point

• The transverse carpal ligament forms the floor of Guyon canal and the roof of the carpal tunnel. Division of that ligament during a carpal tunnel release therefore decompresses the floor of Guyon canal and frequently relieves zone 1 ulnar symptoms without a separate ulnar release - a point examiners like to test.
• A separate formal ulnar tunnel release is indicated where there is a structural lesion within the canal, a zone 2 pattern with deep branch compression at the hypothenar fibrous arch, or persistence of symptoms after carpal tunnel release.

Exposure of Guyon canal

• Incision: longitudinal or gently curved, along the RADIAL border of the flexor carpi ulnaris tendon and the pisiform, crossing the wrist crease obliquely and extending distally into the palm along the hypothenar eminence.
• Landmarks first: identify the pisiform (ulnar wall) and the hook of the hamate (radial wall, 1 to 2 cm distal and radial to the pisiform). Do not begin dissecting until both are located.

Sequence of release

1. Divide the volar carpal ligament - the fibrous part of the canal roof.
2. Divide or reflect palmaris brevis - the muscular part of the roof. This is done without hesitation; the muscle is functionally expendable. If a turnover flap is planned, raise it here on its proximal pedicle rather than dividing it.
3. Identify the ulnar artery, lying RADIAL to the nerve, and the ulnar nerve, and follow the nerve to its bifurcation.
4. Trace the superficial branch distally, noting the branch to palmaris brevis leaving proximally.
5. Follow the deep motor branch as it curves dorsally and radially around the hook of the hamate, and release the fibrous arch of the hypothenar muscles through which it passes - a discrete second compression point and a common site of incomplete release.
6. Identify and address the cause: excise a ganglion, deal with a hook nonunion, resect a thrombosed or aneurysmal arterial segment.

Common technical failures

• Releasing the roof but not the hypothenar fibrous arch, leaving a zone 2 compression untreated.
• Failing to find a cause and decompressing a canal that was never structurally compressed - the specific error to avoid in palmaris brevis spasm syndrome.
• Injuring the deep motor branch during hook excision or ganglion dissection, producing intrinsic paralysis with intact sensation.
• A transverse incision across the wrist crease, producing a tender scar directly over the nerve.

Palmaris brevis turnover flap

A small but genuinely useful local flap in ulnar-sided hand surgery.

Indications.

• Coverage of a reversed vein graft after resection of a diseased ulnar artery segment in hypothenar hammer syndrome.
• Coverage of a scarred ulnar nerve in revision Guyon canal surgery, interposing vascularised tissue between the nerve and the scarred bed.
• Interposition between the nerve and a bony prominence - a hook of hamate nonunion, a prominent pisiform, or the raw bone left after hook excision.
• Augmenting soft tissue cover over the ulnar bundle in a thin or previously operated palm.

Technique.

• Raise the muscle from distal to proximal, dividing its dermal insertion along the ulnar palmar skin.
• Preserve the proximal pedicle from the ulnar artery, which enters the deep surface of the muscle within the canal.
• Identify and protect the ulnar nerve and artery before elevation - they lie directly beneath the muscle being lifted.
• Turn the muscle over radially to lie across the neurovascular structures, and secure it with fine absorbable sutures without tension.

Advantages and limitations.

• Advantages: same field, thin, vascularised, no functional donor deficit, no additional incision.
• Limitations: small surface area - it covers the canal contents and nothing more; the muscle may be hypoplastic or absent; and it cannot be used if the ulnar artery is being ligated proximal to its pedicle.

The alternative in the same region. A hypothenar fat pad flap, raised from the subcutaneous fat of the hypothenar eminence, is the more commonly described adjunct for coverage of a scarred median nerve in revision carpal tunnel surgery. It is a different flap in the same region, the ulnar bundle must be protected during its elevation, and the two should not be confused.

Procedures in which palmaris brevis is encountered