Presentation and course
Ulnar nerve lesions can occur at various sites (see Table 1 below), and the clinical manifestations depend in large part on the level of the lesion. The most common lesions occur at the elbow; most of these are in the retroepicondylar groove, although a significant proportion of them occur at the humeroulnar aponeurotic arcade (19; 18; 24). Compression neuropathies may rarely involve the main trunk in the forearm or the dorsal ulnar cutaneous nerve. Many ulnar lesions occur at the base of the palm where the nerve enters the hand through Guyon canal; such lesions may cause either purely motor, purely sensory, or mixed abnormalities. Lesions more distally in the palm itself usually spare all sensory branches, resulting in a pure motor syndrome that sometimes spares the hypothenar muscles.
Table 1. The Sites of Ulnar Nerve Lesions
Site of compression | Etiology |
Erb point | Idiopathic conduction block |
Axilla | External pressure (crutch palsy) |
Upper arm | External pressure, entrapment by medial intramuscular septum (arcade of Struthers) |
Ulnar groove | External pressure flexion, (elbowing), intragroove pressure, eg, repetitive elbow deformity with chronic stretch (true tardy ulnar palsy), osteoarthritic spurs, Epitrochleoanconeus muscle or ligament, proximal humeroulnar aponeurotic arcade, mass (ganglion, sesamoid bone, etc.), idiopathic |
Humeroulnar aponeurotic
Arcade | Entrapment by aponeurosis |
Flexor carpi ulnaris exit | Entrapment by deep flexor pronator, aponeurosis |
Forearm | Entrapment by hypertrophic flexor carpi ulnaris and vascular bundle |
Wrist | External pressure (eg, mass bicyclist palsy), (eg, ganglion) |
Palm | External pressure, entrapment by pisohamate ligament |
Peripheral nerve trunks are not a random, homogeneous collection of fibers. They exhibit a complex, internal fascicular arrangement wherein fibers bound for particular targets travel together in a bundle. The internal location of a particular fascicle at the level of a compressive lesion determines the likelihood of its involvement, especially if the lesion is mild or partial. Unfortunately, a compression syndrome may not behave as a simple transverse lesion with equal involvement of all fibers distally. Selective fascicular vulnerability can result in spotty involvement and a confusing, complex clinical and electrodiagnostic picture. For example, it is not at all rare for the forearm flexor muscles or even the dorsal ulnar cutaneous nerve to be spared, even though the lesion is in the ulnar groove well proximal to the origin of these branches (47; 10; 51).
As an overview, if considering lesions as transverse "sections" (this is often not valid), pathology at different levels would produce the following five types of deficits:
(1) A lesion at the medial intermuscular septum would cause weakness of ulnar hand intrinsics plus forearm flexors, sensory loss on both dorsal and volar aspects of the hand and the fingers including the palm, and perhaps focal tenderness or a Tinel sign several centimeters above the elbow with motor conduction block at the midarm (38).
(2) A lesion in the ulnar groove would produce the same deficits but with the tenderness and Tinel sign in the groove.
(3) A lesion at the humeroulnar aponeurotic arcade might spare some forearm flexor function, depending on the individual anatomy.
(4) A lesion at the flexor carpi ulnaris exit site, or at any point more distal, would spare forearm muscles.
(5) A lesion involving the nerve in the forearm causes a deficit that mirrors its exact placement. If distal to the takeoff of the palmar sensory branch, the sensory loss may spare the palm. If distal to the takeoff of the dorsal ulnar cutaneous nerve, the sensory loss may spare both the palm and the dorsum of the hand, involving only the volar aspect of the fingers.
Ulnar neuropathy at the elbow is discussed in a separate chapter.
Ulnar nerve lesions in the wrist and hand can cause a variety of different clinical findings, depending on precise location. Findings might range from a pure sensory deficit to pure motor syndromes with weakness that may or may not involve the hypothenar muscles. This depends on whether the lesion involves the main trunk, the sensory branch only, or the deep palmar branch at different sites from just at the hypothenar muscles to the lateral palm. Wrist level lesions, if distal to the take-off of the branch to the palmaris brevis, can sometimes be localized by the presence of a palmaris brevis sign (43). This small muscle is useful to examine both clinically and electromyographically (21; 30). As a general rule, "high" ulnar lesions cause less hand clawing than "low" ones. The high lesion may also produce forearm flexor weakness that lessens the deforming pull on the lumbrical muscles.
Compression of the ulnar nerve within the forearm is rare, because the nerve is not confined within anatomical passageways as at the elbow and wrist. Harrelson and Newman reported ulnar compression in the distal forearm associated with a tender, nonpulsatile mass overlying the ulnar aspect of the forearm (27). Surgical exploration revealed that the mass consisted of hypertrophied fibers of the flexor carpi ulnaris extending along the normally tendinous distal third of the muscle; work-induced hypertrophy may have been responsible. Holtzman and colleagues managed a patient with complete conduction block 5 cm proximal to the ulnar styloid, in whom surgery revealed compression and grooving of the ulnar nerve by two fibrovascular bands crossing from the ulnar artery to the distal belly of the flexor carpi ulnaris (28). Campbell reported a similar case wherein a professional golfer developed work-induced flexor carpi ulnaris hypertrophy. The nerve was compressed by a large vascular pedicle coursing from the ulnar artery to supply the excessive muscle tissue (10). The abnormality was localized by a short segment nerve conduction study and confirmed with intraoperative electroneurography. In the latter two cases, complete resolution followed resection of the fibrovascular band.
Of the different lesions of the ulnar nerve near the wrist, the most common and extensively reported is a compression of the deep palmar branch (12). Shea and McClain classified ulnar compression syndromes of the wrist and hand into three types (46):
(1) In type I, the lesion is proximal to or within Guyon canal, involves both the superficial and deep branches, and causes a mixed motor and sensory deficit with weakness involving all the ulnar hand muscles.
(2) In type II, the lesion is within Guyon canal or at the pisohamate hiatus, involves the deep branch, and causes a pure motor deficit with a variable pattern of hypothenar weakness depending on the precise site of compression.
(3) A type III lesion is in Guyon canal or in the palmaris brevis, involves the superficial branch only, and causes a purely sensory deficit.
In the type I and III lesions, sensory loss should spare the dorsum of the hand (innervated by the dorsal ulnar cutaneous branch), as well as the hypothenar eminence, because its innervation is via the palmar cutaneous branch that arises proximal to the wrist.
Wu and colleagues reclassified 55 published cases of ulnar neuropathy at the wrist into five groups according to clinico-anatomic presentations (52). Their type I lesion corresponds to Shea and McClain type I, and their type II corresponds to Shea and McClain type III. Their types III through V all describe pure motor syndromes. Type III is caused by a lesion proximal to the branch to the hypothenar muscles, and causes global ulnar hand weakness. Type IV is caused by a lesion distal to the hypothenar branches, and causes weakness of all muscles except the hypothenars. Type V is caused by a far distal lesion just proximal to the termination of the palmar branch, and causes weakness limited to the first dorsal interosseous and adductor pollicis, but spares the other interossei and the ulnar innervated lumbricalis. The Wu classification system seems more precise and more useful than Shea and McClain system. A Wu type I lesion is the most common, and type V is the most rare.
Prognosis and complications
The prognosis depends on a number of factors, but most prominently on the severity of the neuropathy (11).