Abstract
Entrapment of the ulnar nerve at the wrist in the piso-hamate hiatus or Guyon canal occurs rarely but is a condition that the clinician should be able to identify via an appropriate history and physical examination in addition to electrodiagnostic evaluation. Although much less common than the typical ulnar neuropathy at the elbow, this condition may be encountered in individuals who are involved in a physical activity that places excessive pressure on the hypothenar area of the hand or, even more rarely, due to a space-occupying lesion. There are different presentations depending at what level of the Guyon canal the compression occurs and which branches are affected with motor and sensory findings varying. In the worst cases, there is severe functional loss due to atrophy and weakness of the intrinsic muscles. Knowledge of the typical anatomy of the ulnar nerve allows the clinician to differentiate this entity from the more common lesion at the elbow, especially when there is sensory sparing of the medial dorsal aspect of the hand in dorsal ulnar cutaneous nerve distribution as this branches proximal to the Guyon canal. Careful electrodiagnostic evaluation via nerve conduction study and electromyography serves to confirm and to classify the condition and space-occupying lesions can be more reliably evaluated via modern imaging technology including magnetic resonance imaging and sonography. Treatment can range from measures to avoid trauma to the hypothenar area to the use of nonsteroidal anti-inflammatory drugs and rehabilitation therapy/static splinting to local injection and surgery in certain cases where compression is due to a space-occupying lesion in the Guyon canal.
Definition
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 piso-hamate hiatus). These are connected by an aponeurosis that forms the ceiling of the 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.
Type I affects the trunk of the ulnar nerve proximally in the 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 the 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). The positioning of the biker’s wrist on the handlebars has been shown via magnetic resonance imaging (MRI) to make a statistical difference in terms of the measured distance between the hook of the hamate and the superficial and deep branches of the ulnar nerve in the Guyon canal, especially with hyperextension and ulnar deviation. This information may be of use in the future in the ergonomic design of handlebars. Entrapment at the Guyon canal has also been associated with prolonged, repetitive occupational use of tools, such as pliers and screwdrivers. 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 the 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.
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.
Of 250 wrists studied by 3 Tesla MRI 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, making this a rare condition.
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 the Guyon canal itself the compression occurs ( 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.
| Muscle | Action |
|---|---|
| Flexor carpi ulnaris | Flexes wrist, ulnarly deviates |
| Flexor digitorum profundus | Flexes distal interphalangeal joint (fourth and fifth) |
| Abductor digiti quinti a | Analogous to dorsal interosseous |
| Flexor digiti quinti a , b | Analogous to dorsal interosseous |
| Opponens digiti quinti a , b | Flexes and supinates fifth metacarpal |
| Volar interossei a | Adduct fingers, weak flexion metacarpophalangeal |
| Dorsal interossei a , b | Abduct fingers, weak flexion metacarpophalangeal |
| Lumbricals (ring and fifth) a , b | Coordinate movement of fingers; extend interphalangeal joints; flex metacarpophalangeal joints |
| Adductor pollicis a | Adducts thumb toward index finger |
| Lumbricals (ring, small) a | Coordinate movement of fingers; extend interphalangeal joints; flex metacarpophalangeal joints |
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 ). In lesions involving the motor branches where the compression is in the proximal aspect of the Guyon canal, there will be weakness and eventually atrophy of the interossei, the adductor pollicis, the fourth and fifth lumbricals, and the deep head of the flexor pollicis brevis. The palmaris brevis, abductor digiti quinti, opponens digiti quinti, and flexor digiti quinti may be involved or spared, depending on the level of the lesion.
