Causes / typical injury mechanism: Ulnar tunnel syndrome is most commonly caused by a compressive ganglion cyst. Other compressive lesions include lipoma and ulnar artery thrombosis or aneurysm. Traumatic causes are also common, including hook of hamate fracture, pisiform dislocation, and repetitive microtrauma. Inflammatory arthritis, fibrous bands, congenital bands, palmaris brevis hypertrophy, and idiopathic causes have also been implicated.

Classic history and presentation: The presentation is highly variable depending on the zone of compression and can include a combination of motor symptoms only, sensory symptoms only, or both. History usually includes either a compression lesion such as a ganglion within the ulnar tunnel, or recent trauma such as a hook of hamate fracture (typically in an overuse setting such as handlebar sports and cycling). Patients present with pain and paresthesias in the ulnar 1-1/2 digits and/or weakness in the intrinsics, small and ring finger flexion, and thumb adduction.

Prevalence: Less than 10 cases per 100 000 people per year.

• Age – Most commonly seen in the second to fifth decade of life.
• Sex / gender – More common in males.

Risk factors: Ganglion cyst, lipoma, repetitive trauma (cycling or handlebar sports), hook of hamate fracture, palmaris brevis hypertrophy, congenital bands, fibrous band, inflammatory arthritis, pisiform dislocation, and ulnar artery thrombosis or aneurysm.

Pathophysiology: Ulnar tunnel syndrome is most commonly caused by direct compressive pathology or surrounding trauma that can manifest as mixed motor / sensory symptoms. Compressive pathologies most commonly include ganglion cysts, while traumatic pathologies most commonly involve hook of hamate fracture, pisiform dislocation, or repetitive trauma such as in handlebar sports.

Grade / classification system: Classification is based on location of compression (zones of Guyon's canal).

• Zone 1 is proximal to bifurcation of nerve, caused by ganglia and hook of hamate fractures, and causes mixed motor sensory symptoms.
• Zone 2 surrounds the deep motor branch, caused by ganglia and hook of hamate fractures, and causes motor symptoms only.
• Zone 3 surrounds the superficial sensory branch, caused by ulnar artery thrombosis or aneurysm, and causes sensory symptoms only.