Guyon canal is formed proximally by the pisiform bone and distally by the hook of the hamate. Its floor is formed by the triquetrum and hamate bones along with the thick transverse carpal ligament, while its roof is composed of a loose connective tissue (Figure C9-1). In the distal portion of Guyon canal lies the pisohamate hiatus (PHH). This aperture is bounded anteriorly by a fibrous arch formed by the two musculotendinous attachments of the flexor brevis digiti minimi (or quinti), a hypothenar muscle, to the hook of hamate and the pisiform bone (Figure C9-2). The posterior boundary of the PHH, is formed by a thick pisohamate ligament which extends from the pisiform bone to the hook of the hamate. The origin of the major motor branch to the ADM is proximal to this hiatus in the majority of hands.

Figure C9-1 Anatomy of the ulnar nerve within Guyon canal at the wrist. 1 = ulnar artery, 2 = superficial branch of the ulnar nerve, 3 = hamulus, 4 = fibrous arch of the hypothenar muscles (see also Figure C9-2), 5 = pisiform, 6 = transverse carpal ligament, 7 = palmaris brevis, 8 = palmar carpal ligament.

Rights were not granted to include this figure in electronic media. Please refer to the printed book.

(From Gross MS, Gelberman RH. The anatomy of the distal ulnar tunnel. Clin Orthop 1985;196:238–247, with permission.)

Figure C9-2 The pisohamate hiatus (PHH) in the distal portion of Guyon canal. ABD.D.Q. = abductor digiti quinti (or minimi), F.B.D.Q. = flexor brevis digiti quinti (or minimi), F.C.U. = flexor carpi ulnaris, Opp.D.Q. = opponens digiti quinti, U. = ulnar.

(Modified from Uriburu IJF, Morchio FJ, Marin JC. Compression syndrome of the deep motor branch of the ulnar nerve [pisohamate hiatus syndrome]. J Bone Joint Surg 1976;58A:145–147, with permission.)

The deep palmar motor branch innervates the hypothenar muscles (the ADM, flexor brevis digiti minimi, and opponens digiti minimi) while in Guyon canal and often gives these muscles an additional branch after it enter the PHH. Then, the deep palmar motor branch travels through the palm and innervates all four dorsal and three palmar interossei, the third and fourth lumbricals, the adductor pollicis, and the deep head of the flexor pollicis brevis. The superficial cutaneous branch innervates the palmaris brevis muscle, as well as the ulnar side of the palm and palmar aspect of digit V and half of digit IV.

Clinical Features

Patients with ulnar neuropathy at the wrist often presents with painless unilateral hand atrophy. These ulnar nerve lesions pose a diagnostic challenge, particularly when the weakness is not associated with sensory loss. It is useful in sorting out the cause of hand weakness or atrophy to distinguish between atrophy of all intrinsic hand muscles from atrophy that is restricted to the thenar or hypothenar muscles. Table C9-1 lists the various causes of wasting and weakness of the hand.

Table C9-1 Causes of Unilateral Atrophy and Weakness of Intrinsic Hand Muscles

The signs and symptoms of distal ulnar lesions vary with the site of compression. In all types of ulnar mononeuropathy at the wrist, sensation over the dorsal medial hand and the dorsal aspect of the fifth and ring fingers, a territory of the dorsal ulnar cutaneous branch, is normal. The terminal branches of the ulnar nerve (deep palmar motor and superficial cutaneous) may be compressed at Guyon canal, but only the deep palmar motor branch is potentially compressed at the PHH.

Several classifications of ulnar mononeuropathy at the wrist have been proposed; most separate the lesions into four types. Table C9-2 lists the common types of ulnar mononeuropathy at the wrist and hand, with their corresponding sites of lesion and clinical presentation. In ulnar neuropathy at the wrist, selective compression of the deep palmar motor branch, with normal superficial sensory branch functions, is the most common type accounting for 39 to 75+ of distal ulnar nerve lesions depending on published series.

Table C9-2 Various Types of Distal Ulnar Mononeuropathy

Lesion Site	Nerve Affected	Clinical Presentation
Proximal Guyon canal	Main trunk of ulnar nerve	Ulnar palmar sensory loss and weakness of all ulnar intrinsic hand muscles
	or	or
	Ulnar cutaneous branch	Ulnar palmar sensory loss only
Distal Guyon canal	Deep palmar branch (proximal to branch to the abductor digiti minimi)	Weakness of all ulnar intrinsic hand muscles (interossei, ulnar lumbricals, and hypothenars) without sensory loss
Pisohamate hiatus	Deep palmar branch (distal to branch to the abductor digiti minimi)	Weakness of ulnar intrinsic hand muscles with sparing of the hypothenar muscles and without sensory loss
Palm (rare)	Deep palmar branch (distal to hypothenars)	Weakness of adductor pollicis, 1st, 2nd, and possibly the 3rd interossei only, usually sparing 4th interossei and without sensory loss

Table C9-3 lists the common causes of ulnar mononeuropathy at the wrist. A ganglion is the most common cause accounting for 28 to 45+ of cases depending on reported series. The second most common cause is entrapment of the motor branch at the pisohamate hiatus, which may be spontaneous or as an occupational or recreational hazard (such as after prolonged bicycling, use of hand tools, etc.). Compression at this anatomic hiatus explains the selective involvement of the deep motor branch at the wrist with complete or relative sparing of the hypothenar muscles and the normal superficial sensory functions.

Table C9-3 Common Causes of Ulnar Mononeuropathy at the Wrist

Ganglion

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