Entrapment neuropathy of the ulnar nerve can be encountered at the wrist in a canal formed by the pisiform and the hamate and its hook (the pisohamate hiatus). These are connected by an aponeurosis that forms the ceiling of Guyon canal (Fig. 39.1). This canal generally contains the ulnar nerve and the ulnar artery and vein. The following three types of lesions can be encountered [1].

f39-01-9781455775774 — FIGURE 39.1 Distal ulnar tunnel (Guyon canal) showing the three zones of entrapment. Lesions in zone 1 give motor and sensory symptoms, lesions in zone 2 cause motor deficits, and lesions in zone 3 create sensory deficits.

Type I affects the trunk of the ulnar nerve proximally in Guyon canal and involves both the motor and sensory fibers. This is the most commonly seen lesion.

Type II affects only the deep (motor) branch distally in Guyon canal and may spare the abductor digiti quinti, depending on the location of its branching. A further classification is type IIa (still pure motor), in which all the hypothenar muscles are spared because of a lesion distal to their neurologic branching.

Type III affects only the superficial branch of the ulnar nerve, which provides sensation to the volar aspect of the fourth and fifth fingers and the hypothenar eminence. There is sparing of all motor function, although the palmaris brevis is affected in some cases. This is the least common lesion encountered.

This injury is commonly seen in bicycle riders and people who use a cane improperly because they place excessive weight on the proximal hypothenar area at the canal of Guyon and therefore are predisposed to distal ulnar nerve traumatic injury, especially affecting the deep ulnar motor branch (type II) [2,3]. Entrapment at Guyon canal has also been associated with prolonged, repetitive occupational use of tools, such as pliers and screwdrivers [2]. With the advancement of endoscopic carpal tunnel release during the past few years, there have been reports of both adverse and favorable consequences to the ulnar nerve at Guyon canal, which is very close anatomically. There have been inadvertent injuries to the ulnar nerve as well as documented decompression and improvement of nerve conduction [4,5].

Other rare causes have been reported in the literature. These include fracture of the hook of the hamate, ganglion cyst formation, tortuous or thrombosed ulnar artery aneurysm (hypothenar hammer syndrome), osteoarthritis or osteochondromatosis of the pisotriquetral joint, anomalous variation of abductor digiti minimi, schwannomas, aberrant fibrous band, and idiopathic [6–9].

Of 250 wrists studied by 3 T magnetic resonance imaging assessment, it was noted that anatomy of the Guyon canal was normal in 168 (67.2%) wrists; 73 (29.2%) wrists presented with anatomic variations, and 9 (3.6%) wrists had derangements with clinical and radiologic features attributed to Guyon canal syndrome [10].

Symptoms

Signs and symptoms can vary greatly and depend on which part of the ulnar nerve and its terminal branches are affected and where along Guyon canal itself (Table 39.1). It is of great importance to be able to differentiate entrapment of the ulnar nerve at the wrist from entrapment at the elbow, which occurs far more commonly. The two clinical findings that confirm the diagnosis of Guyon canal entrapment instead of ulnar entrapment at the elbow are (1) sparing of the dorsal ulnar cutaneous sensory distribution in the hand and (2) sparing of function of the flexor carpi ulnaris and the two medial heads of the flexor digitorum profundus (Figs. 39.2 and 39.3). Otherwise, the symptoms in both conditions are generally similar and may include hand intrinsic muscle weakness and atrophy, numbness in the fourth and fifth fingers, hand pain, and sometimes severely decreased function.

Table 39.1

Volar Forearm and Hand: Ulnar Nerve

Muscle	Action
Flexor carpi ulnaris	Flexes wrist, ulnarly deviates
Flexor digitorum profundus	Flexes distal interphalangeal joint (fourth and fifth)
Abductor digiti quinti*	Analogous to dorsal interosseous
Flexor digiti quinti*†	Analogous to dorsal interosseous
Opponens digiti quinti*†	Flexes and supinates fifth metacarpal
Volar interossei*	Adduct fingers, weak flexion metacarpophalangeal
Dorsal interossei*†	Abduct fingers, weak flexion metacarpophalangeal
Lumbricals (ring and fifth)*†	Coordinate movement of fingers; extend interphalangeal joints; flex metacarpophalangeal joints
Adductor pollicis*	Adducts thumb toward index finger
Lumbricals (ring, small)*	Coordinate movement of fingers; extend interphalangeal joints; flex metacarpophalangeal joints

* Hand intrinsic muscles.

^† Hypothenar mass.

f39-02a-9781455775774 — FIGURE 39.2 A, The flexor carpi ulnaris functions as a wrist flexor and an ulnar deviator. B, It can be tested by the patient’s forcefully flexing (arrow) and ulnarly deviating the wrist. The clinician palpates the tendon while the patient performs this maneuver. (From Concannon MD: Common Hand Problems in Primary Care. Philadelphia, Hanley & Belfus, 1999.)

f39-02b-9781455775774 — FIGURE 39.2 A, The flexor carpi ulnaris functions as a wrist flexor and an ulnar deviator. B, It can be tested by the patient’s forcefully flexing (arrow) and ulnarly deviating the wrist. The clinician palpates the tendon while the patient performs this maneuver. (From Concannon MD: Common Hand Problems in Primary Care. Philadelphia, Hanley & Belfus, 1999.)

f39-03a-9781455775774 — FIGURE 39.3 A, Flexor digitorum profundi (arrows). B, These tendons can be tested by the patient’s flexing the distal phalanx while the clinician blocks the middle phalanx from flexing. (From Concannon MD: Common Hand Problems in Primary Care. Philadelphia, Hanley & Belfus, 1999.)

f39-03b-9781455775774 — FIGURE 39.3 A, Flexor digitorum profundi (arrows). B, These tendons can be tested by the patient’s flexing the distal phalanx while the clinician blocks the middle phalanx from flexing. (From Concannon MD: Common Hand Problems in Primary Care. Philadelphia, Hanley & Belfus, 1999.)

Physical Examination

Careful examination of the hand and a thorough knowledge of the anatomy of motor and sensory distribution of ulnar nerve branches are required to determine the location of the lesion. Except for the five muscles innervated by the median nerve (abductor pollicis brevis, opponens pollicis, flexor pollicis brevis superficial head, and first two lumbricals), the ulnar nerve supplies every other intrinsic muscle in the hand. Classically, there is notable atrophy of the first web space due to denervation of the first dorsal interosseous muscle (Fig. 39.4

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