Ulnar Tunnel Syndrome (Guyon's Canal)

Ulnar tunnel syndrome (Guyon's canal syndrome) is compression of the ulnar nerve or its branches at the wrist, within the fibro-osseous tunnel formed by the pisiform, the hook of hamate and the overlying soft tissues. It is rare compared with carpal tunnel and cubital tunnel syndromes.

How it presents. Unlike cubital tunnel syndrome there are no elbow symptoms, and the deficit depends on where in the canal the nerve is caught: it may be motor, sensory or mixed. The zone of compression is therefore the organising idea for the whole topic, from examination to imaging to the operation.

What causes it. A ganglion cyst arising from the pisotriquetral joint is the most common cause, followed by the hook of hamate fracture in cyclists and golfers. Ulnar artery thrombosis is also among the common causes.

The canal. Guyon's canal (the ulnar tunnel) is a fibro-osseous tunnel about 4 cm long through which the ulnar nerve enters the hand. Its walls:

• Roof: palmaris brevis, the palmar (volar) carpal ligament, and skin and subcutaneous tissue
• Floor: the pisiform (ulnar), the hook of hamate (radial) and the pisohamate ligament, with the transverse carpal ligament deeper
• Ulnar border: the pisiform and the flexor carpi ulnaris tendon
• Radial border: the hook of hamate and the flexor digitorum superficialis tendons

Contents. The ulnar nerve, the ulnar artery and the ulnar veins. The artery lies radial to the nerve.

The bifurcation. The nerve divides within the canal. The deep motor branch curves around the hook of hamate into the palm; the superficial sensory branch continues distally to supply the hypothenar eminence and the ulnar one and a half digits. Because the division happens inside the canal, compression can catch the trunk, the motor branch alone or the sensory branch alone, and that is what produces the zone-specific presentations.

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What the deep branch supplies. A lesion of the trunk or of the deep branch (Zones 1 and 2) weakens the ulnar intrinsics:

• Interossei, palmar and dorsal: the patient cannot spread or adduct the fingers; it is the palmar interossei that adduct
• Lumbricals 3 and 4: contributes to clawing
• Abductor digiti minimi: hypothenar wasting
• Flexor digiti minimi brevis: weak small finger flexion
• Opponens digiti minimi: weak small finger opposition
• Flexor pollicis brevis (deep head): usually minimal impact

What is spared. Four ulnar-supplied structures take their supply from branches that leave the nerve proximal to the canal, and finding them normal is what places the lesion at the wrist:

• Adductor pollicis: its branch leaves the nerve before Guyon's canal
• Flexor carpi ulnaris: motor branch proximal, in the forearm
• FDP to the ring and small fingers: motor branch proximal, in the forearm
• Dorsal ulnar cutaneous nerve: branches 5-8 cm proximal to the wrist

Shea and McClain (1969). The classification divides Guyon's canal into three zones by the branching of the nerve. The zone determines the clinical presentation, and with it the imaging and the surgical approach.

Zone 2 is the most common, at 50% of cases, because pisotriquetral ganglion cysts preferentially compress the deep motor branch as it curves around the hook of hamate.

By cause. The causes fall into four groups, and the proportions are worth knowing:

• Space-occupying lesions, 60%: ganglion cyst (85-90% of these), lipoma (including spindle cell lipoma), neurofibroma (a rare nerve sheath tumour), ulnar artery aneurysm, and rarely a carpal bone tumour
• Bony or traumatic, 30%: hook of hamate fracture, pisotriquetral arthritis, distal radius malunion
• Vascular, 5%: hypothenar hammer syndrome and ulnar artery thrombosis
• Chronic pressure, 5%: cycling (handlebar palsy), wheelchair use, occupational tool use

Ganglion cysts. They arise from the pisotriquetral joint and preferentially compress the deep motor branch (Zone 2). They may be small and difficult to palpate, so MRI or ultrasound makes the diagnosis.

Hook of hamate fracture. It occurs in cyclists, golfers and baseball players, from a direct blow to the hypothenar eminence. Nonunion is common, and the fibrous callus compresses the nerve.

Pisotriquetral arthritis. It follows chronic wrist trauma, and osteophytes compress the nerve.

Vascular lesions. Hypothenar hammer syndrome (see Complications and Special Situations) is thrombosis of the ulnar artery from chronic repetitive trauma such as hammering; it may compress the nerve or cause ischaemia, and Allen's test is abnormal. The ulnar artery may also thrombose from an aneurysm or atherosclerosis.

Chronic pressure and the idiopathic case. Handlebar pressure in cyclists, wheelchair use and tool use put chronic pressure on the hypothenar eminence. When no cause is found the syndrome is idiopathic, a diagnosis of exclusion, and it often responds to conservative treatment.

History. The questions that localise and explain the lesion:

• Onset, acute or gradual
• Hand dominance and occupation
• Trauma: a fall on the hand, a hook of hamate fracture
• Cycling, tool use or wheelchair use
• A palpable mass in the hypothenar region
• Weakness or numbness, which points to the zone

Inspection. Look for hypothenar wasting (Zones 1 and 2), interosseous wasting on the dorsum of the hand, a claw deformity of the ring and small fingers, and a visible or palpable mass.

Palpation. Tenderness over the hook of hamate suggests a fracture. A ganglion may be palpable, but may be subtle. Test for Tinel's sign over Guyon's canal, and perform Allen's test for ulnar artery patency.

Motor testing. Test interosseous function by finger spread and adduction (Wartenberg's sign), and hypothenar strength by abduction and opposition of the small finger. Froment's sign should be negative, and flexor carpi ulnaris and FDP to the ring and small fingers should be normal; these are the findings that localise the lesion to the wrist.

Sensory testing. Test the hypothenar eminence (superficial branch) and the palmar surface of the ulnar one and a half digits (the small finger and the ulnar side of the ring finger). Dorsal ulnar cutaneous sensation should be normal.

Localising the lesion. The question in every ulnar neuropathy is the level: wrist, forearm or elbow. A Guyon's lesion spares everything supplied above the wrist, has no elbow signs, and can produce a pure motor or a pure sensory deficit depending on the zone. Its common causes are a ganglion or a hamate fracture, so imaging is often diagnostic; the common causes of cubital tunnel syndrome are chronic pressure and anatomical. Sensory loss in cubital tunnel syndrome covers the hypothenar area and the ulnar 1.5 digits.

Three patterns in practice. Three illustrative cases, one for each zone.

• The cyclist with a pure motor deficit. A 35-year-old cyclist with a 3-month history of weak grip and difficulty with fine motor tasks has interosseous wasting and Wartenberg's sign, normal sensation in the hand and no Froment's sign. This is Zone 2 compression, likely a ganglion, and MRI shows a pisotriquetral ganglion.
• The baseball catcher with hypothenar pain. A 22-year-old catcher has hypothenar pain after a foul tip, tenderness over the hook of hamate, interosseous weakness and hypothenar numbness. This is a hook of hamate fracture compressing the nerve in Zone 1, and CT shows the fracture with displacement.
• The carpenter with pure sensory loss. A 50-year-old carpenter has hypothenar numbness without weakness and an abnormal Allen's test with no ulnar artery flow. This is Zone 3, from ulnar artery thrombosis, and Doppler ultrasound shows the occlusion.

The work-up has two jobs: confirm the level of the neuropathy, and find a treatable cause.

Radiographs. PA, lateral and oblique views of the wrist, with a carpal tunnel view, which is the best view of the hook of hamate; a supinated oblique is an alternative hamate view. Look for a hook of hamate fracture or nonunion, pisotriquetral arthritis and carpal bone masses. Standard views have a sensitivity of only 50% for hamate fracture.

CT is the gold standard for hook of hamate fracture. Thin slices through the carpus identify fracture, nonunion and displacement, and can show bony impingement on the nerve. It is indicated for a suspected hamate fracture with negative radiographs, for planning excision, and to assess pisotriquetral arthritis.

MRI is best for soft-tissue masses: a ganglion cyst (high T2 signal), ulnar artery aneurysm or thrombosis, a nerve sheath tumour or a lipoma. Its sensitivity for ganglion cyst is greater than 90%. Use a dedicated wrist coil, T1, T2 and fat-suppressed sequences, and coronal and axial planes through Guyon's canal.

Ultrasound gives a dynamic, real-time view of the nerve and any mass, with Doppler for the ulnar artery, and can guide aspiration of a ganglion. It involves no radiation, is quick and inexpensive, and is good for ganglion cysts and vascular lesions, but it is operator-dependent.

Nerve conduction studies. Motor studies stimulate the ulnar nerve at the wrist and record from the hypothenar eminence (ADM), comparing the wrist-to-hypothenar latency with the normal side. Distal latency is prolonged in Guyon's compression, and amplitude may be reduced if severe. Sensory studies of the superficial branch are abnormal in Zones 1 and 3 and normal in Zone 2, and the dorsal ulnar cutaneous nerve should be normal, which localises the lesion to the wrist.

EMG shows denervation in the ulnar intrinsics in Zones 1 and 2, with first dorsal interosseous and ADM the most sensitive muscles. Adductor pollicis should be normal.

Vascular tests. Allen's test assesses ulnar artery patency, and angiography is used if hypothenar hammer syndrome is suspected.

Algorithm

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Who is treated conservatively. Non-operative treatment suits:

• Mild symptoms of short duration (less than 3 months)
• No muscle wasting
• Idiopathic compression, with no mass lesion on imaging
• An occupational cause with modifiable activity

The conservative programme.

1. Activity modification: avoid repetitive hypothenar pressure, with padded gloves for cyclists, ergonomic tool handles and wheelchair cushion padding
2. Splinting: a wrist splint in neutral, as a night splint for 6-8 weeks, to reduce pressure in Guyon's canal; the evidence is limited but the risk is low
3. NSAIDs: a short course of 2-4 weeks for their anti-inflammatory effect, with minimal evidence in nerve compression
4. Therapy: nerve-gliding exercises, then strengthening as symptoms improve; there is no evidence of benefit, but it is commonly prescribed

What it achieves. 30-40% improve with conservative treatment. Results are best in idiopathic and occupational cases and poor when a ganglion or other structural lesion is present.

Surgical indications. Any of these moves the patient to decompression:

• A ganglion cyst or other mass lesion on imaging
• Progressive motor weakness
• Muscle atrophy
• No improvement after 3 months of conservative treatment
• Acute onset with a severe deficit
• Hook of hamate fracture nonunion

The hook of hamate fracture. Fractures are typed as I (tip avulsion), II (base fracture) or III (comminuted). Nonunion is common because the hook has a poor blood supply.

• Acute (less than 4 weeks), non-displaced: a trial of immobilisation for 4-6 weeks in a short arm cast with an ulnar gutter. Success is only 30-50%
• Acute, displaced: primary excision is recommended. Union is rare even with prolonged immobilisation, and early excision prevents nerve compression
• Chronic nonunion or delayed presentation: excision of the hook is the treatment of choice if symptomatic; an asymptomatic, incidental nonunion can be observed

Set-up. Supine with the arm on a hand table, an upper-arm tourniquet and the hand supinated. Loupe magnification is recommended.

Incision. Longitudinal, along the ulnar border of the hypothenar eminence, starting 1 cm proximal to the wrist crease and extending 3-4 cm distally into the palm, radial to the FCU tendon and ulnar to the thenar crease.

Decompression, proximal to distal.

1. Incise skin and subcutaneous fat, and identify and protect the palmar cutaneous branch of the ulnar nerve, a small sensory branch
2. Expose palmaris brevis and incise it longitudinally
3. Identify the ulnar artery first: it lies radial to the nerve
4. Identify the ulnar nerve, ulnar to the artery, and follow it distally
5. Zone 1: release the palmar carpal ligament proximal to the wrist crease
6. Identify the bifurcation into motor and sensory branches
7. Zone 2: follow the deep motor branch as it curves around the hook of hamate, and release all fibrous tissue compressing it
8. Zone 3: follow the superficial sensory branch distally and release any constricting tissue

Release the zones as needed, tracing the nerve through each.

Dealing with the cause. The cause is treated at the same operation:

• Ganglion cyst: excise completely, tracing it to its origin in the pisotriquetral joint
• Hook of hamate fracture: excise the hook fragment rather than fixing it
• Ulnar artery thrombosis: ligate or resect the thrombosed segment
• Ulnar artery aneurysm: excision with vascular reconstruction, or ligation
• Pisohamate ligament: release if it compresses the nerve

Closure. Meticulous haemostasis matters beside the ulnar artery. Close the skin with interrupted 4-0 nylon, apply a soft bulky dressing, and, optionally, a wrist splint in neutral for 1-2 weeks.

Hook of hamate excision. The indications are a symptomatic nonunion, a displaced acute fracture, and a chronic fracture with ulnar neuropathy. Through the same incision, identify and protect the ulnar nerve and artery and expose the hook, then remove the fragment with a small osteotome or rongeur. Remove every fragment and smooth the edges with a rasp: a sharp edge left behind risks flexor tendon rupture.

After excision. A soft dressing and early motion, no forceful gripping for 6 weeks, and return to sport at 8-12 weeks. Pain relief is achieved in 90-95%, grip strength returns to 85-90% of normal, and nerve symptoms, if present, resolve in 80-90%.

Ulnar artery injury. The most serious complication, at 1-2%. Prevention is identifying the artery before decompressing the nerve, loupe magnification and no monopolar cautery near the artery. Repair it primarily with 8-0 or 9-0 suture, which may need microvascular expertise; ligation is a last resort because of the risk of hypothenar ischaemia.

Nerve branch injury. The superficial sensory branch is most at risk. Identify every branch before dividing anything, protect the palmar cutaneous branch in the subcutaneous tissue, and dissect the bifurcation carefully. A recognised injury is repaired primarily and a symptomatic neuroma is excised later, but the sensory deficit may be permanent.

Early complications (0-6 weeks).

• Haematoma, 2-3%: a risk beside the ulnar artery. Prevent it with meticulous haemostasis and consider a drain; evacuate it if symptomatic and apply a compressive dressing
• Infection, less than 1%: rare with clean technique. Treat with antibiotics, and washout if severe; a deep infection puts nerve recovery at risk
• Wound dehiscence: the incision lies in a mobile area of the palm. Prevention is careful closure and 1-2 weeks of immobilisation; treat with local wound care, and re-closure may be needed

Late complications (after 6 weeks).

Hypothenar hammer syndrome. The ulnar artery thrombosis or aneurysm it produces may compress the nerve in Zone 1 or 3. The patient presents with cold intolerance, pain and numbness. Doppler ultrasound or angiography makes the diagnosis, and treatment is vascular reconstruction or ligation with nerve decompression.

Ulnar artery aneurysm. A pulsatile mass in the hypothenar eminence and Guyon's canal that compresses the nerve in Zone 1, and may thrombose and cause acute ischaemia. Ultrasound with Doppler makes the diagnosis.

Pisotriquetral arthritis. Degenerative arthritis of the pisotriquetral joint gives chronic wrist pain with an ulnar neuropathy, and radiographs show the arthritis. Treatment is pisiform excision with nerve decompression.

Combined carpal and Guyon's tunnel compression. The "double crush" phenomenon: the median nerve compressed in the carpal tunnel and the ulnar nerve in Guyon's canal, giving combined median and ulnar deficits. Consider it when a patient having carpal tunnel release has hypothenar symptoms; both tunnels are released, and this can be done simultaneously.

Revision surgery. Revision is considered for persistent or recurrent symptoms after 6 months or more, an incomplete first decompression, a recurrent ganglion, or the nerve caught in scar tissue.

• Before operating: repeat NCS to confirm persistent compression, MRI for a recurrent mass or nerve scarring, and exclude a proximal lesion (cubital tunnel)
• At operation: an extended incision with careful dissection through scar, complete decompression of all three zones, neurolysis if the nerve is encased in scar, excision of a recurrent ganglion to its joint origin, and nerve wrapping (fat, vein) if the scarring is extensive
• Salvage: tendon transfers for a permanent motor deficit, nerve grafting for a nerve defect, neuroma excision for intractable pain

What revision achieves. It is less predictable than a primary release, at 60-70% success, so set realistic expectations. If a second revision is being considered, consider an alternative diagnosis.

Rehabilitation.

• Weeks 0-2, protection: bulky soft dressing, finger range of motion immediately, gentle wrist mobilisation, sutures out at 10-14 days
• Weeks 2-6, active motion: remove the dressing or splint, active wrist and finger range of motion, light activities of daily living, no forceful gripping
• Weeks 6-12, strengthening: progressive grip strengthening, return to light-duty work, therapy for stiffness if needed, and sport-specific training begins
• Months 3-6, full recovery: return to full activities, sport-specific training, and a final assessment of nerve recovery

Return to activity. Office work at 2 weeks, light manual work at 4-6 weeks, heavy manual work at 8-12 weeks, and cycling or golf at 8-12 weeks after hamate excision.

Monitoring. Review clinically at 2, 6 and 12 weeks, assessing motor recovery by intrinsic strength; sensory recovery typically precedes motor. Obtain a final NCS at 6 months if symptoms persist.

Decompression succeeds in 90%. How completely the hand recovers depends on the factors below.

Sensory recovery. Sensation recovers faster than motor function, over 3-6 months. Protective sensation returns first, at 1-2 months; dysaesthesias may persist for 3-6 months; and numbness resolves in 95%, by 6 months.

Motor recovery. Improvement begins at 2-4 months and reaches its maximum at 6-12 months. Intrinsic strength improves but may not return to baseline if there was chronic or severe atrophy. Clawing resolves in 90%, and grip strength improves by 30-50%.

Global Epidemiology

Ulnar tunnel syndrome is an uncommon, well-described entity that is substantially rarer than both carpal tunnel and cubital tunnel syndrome, and it is frequently mis-diagnosed because its symptoms overlap with carpal tunnel syndrome, cubital tunnel syndrome, thoracic outlet syndrome, C8-T1 radiculopathy and peripheral neuropathy (Chen & Tsai, J Hand Surg Am 2014, DOI; Bachoura & Jacoby, Orthop Clin North Am 2012, DOI). The hook of hamate fracture - a classic structural cause - is itself uncommon with an unclear true prevalence and is often diagnosed late (Tian & Goldfarb, Hand Clin 2021, DOI). Vascular causes such as hypothenar hammer syndrome arise in people who repeatedly use the hypothenar eminence as a hammer (Schröttle et al, Vasa 2015, DOI).

Practice Variation & Global Notes

• Diagnostic work-up is broadly consistent worldwide: clinical localisation, nerve conduction studies/EMG, then targeted imaging (MRI/ultrasound for soft-tissue masses, CT for the hook of hamate). The dorsal ulnar cutaneous nerve and Froment's sign are universally used to separate wrist-level from elbow-level lesions.
• Surgical thresholds vary by structural cause and patient demand: an identified ganglion, hook of hamate fracture, or vascular lesion lowers the threshold for surgery, whereas idiopathic mild cases are managed conservatively first.
• Hook of hamate management converges on excision across regions for symptomatic fractures and nonunions, particularly in athletes (golf, racquet sports, baseball) and cyclists.
• Vascular causes require Allen's test and duplex ultrasound first-line with angiography as reference standard; smoking cessation is a universal adjunct.

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