Anatomy and Borders
Guyon's canal (canalis ulnaris) is located in the hand at the wrist level. It is formed by the splitting of the fascia propria, which represents the thickened distal section of the forearm fascia. This splitting occurs near the lateral border of the pisiform bone.
On the ulnar side runs the tendon of the flexor carpi ulnaris muscle (m. flexor carpi ulnaris). It attaches to the pisiform bone, which acts as a sesamoid bone, from which tendinous fibers continue toward the 5th metacarpal bone. In the deeper layer lies the flexor retinaculum (retinaculum musculorum flexorum), the thickest and strongest ligament of the hand.
Contents of Guyon's Canal
The canal contains the ulnar neurovascular bundle. Its components have the following topography:
- Ulnar artery (a. ulnaris) — lies more superficially and laterally.
- Ulnar nerve (n. ulnaris) — lies deeper and medially.
At the wrist level, the ulnar nerve divides into a superficial sensory branch and a deep motor branch.
Clinical Significance: Compression Neuropathy
Compression of the ulnar nerve within Guyon's canal leads to tunnel syndrome. This condition is significantly less common than ulnar nerve entrapment at the elbow, and reliable statistics on its prevalence are lacking.
Causes of Compression:
- External factors: repetitive hand trauma from working with tools or cycling due to compression of the canal area by the handlebars.
- Internal factors: space-occupying lesions in the canal area and ganglia.
Symptoms of Impairment:
- Development of a "claw hand" deformity.
- Sensory disturbances on the 5th digit and the medial aspect of the 4th digit.
- As the disease progresses, weakness develops in muscles innervated by the ulnar nerve: the first dorsal interosseous muscle, other interosseous muscles, and the abductor digiti minimi muscle (m. abductor digiti minimi). Strength in other muscle groups remains preserved. In mild cases, muscle weakness is not typical.
Differential Diagnosis and NCS/EMG
Clinical tests and instrumental methods are used to diagnose the level of the lesion.
- Sensory evaluation: if sensation is impaired in digits 4–5 while sensation in the hypothenar eminence is spared, Guyon's canal is a likely site of compression.
- Hypothenar skin wrinkling test: assesses the function of the palmaris brevis muscle (m. palmaris brevis).
- Excluding motor neuron disease (MND): if a patient presents with weakness and atrophy of the ulnar muscle group without sensory disturbances, this is atypical for neuropathy and requires ruling out motor neuron disease or Hirayama disease.
- Nerve conduction studies (NCS) / Electromyography (EMG): used to identify the type of impairment (axonal vs. demyelinating), assess the severity, and localize the level of the lesion (Guyon's canal, forearm, elbow, or arm). When studying the motor conduction velocity of the ulnar nerve, recording is typically performed from the abductor digiti minimi muscle (m. abductor digiti minimi).