Anatomy

The ulnar nerve is essentially derived from the C8 and T1 roots (Figure 19–1), although some anatomic dissections have also demonstrated a minor component from C7. Accordingly, nearly all ulnar fibers travel through the lower trunk of the brachial plexus and then continue into the medial cord. The terminal extension of the medial cord becomes the ulnar nerve. The medial brachial and medial antebrachial cutaneous sensory nerves and a large contribution to the median nerve are derived from the medial cord as well. As the ulnar nerve descends through the medial arm, it does so without giving off any muscular branches. The ulnar nerve pierces the medial intermuscular septum in the mid-arm and then passes through the arcade of Struthers, which is composed of deep fascia, muscle fibers from the medial head of the triceps, and the internal brachial ligament. The ulnar nerve then travels medially and distally toward the elbow.

FIGURE 19–1 Ulnar nerve anatomy.

The ulnar nerve, along with the medial brachial and medial antebrachial cutaneous nerves, is derived from the medial cord of the brachial plexus. Inset: Cutaneous distributions of the ulnar, medial antebrachial, and medial brachial cutaneous nerves.

(Reprinted with permission from Haymaker, W., Woodhall, B., 1953. Peripheral nerve injuries. WB Saunders, Philadelphia.)

At the elbow, the nerve enters the ulnar groove formed between the medial epicondyle and the olecranon process. Slightly distal to the groove in the proximal forearm, the ulnar nerve travels under the tendinous arch of the two heads of the flexor carpi ulnaris (FCU) muscle, known as the humeral–ulnar aponeurosis (HUA) or cubital tunnel. Muscular branches to the FCU and the medial division (fourth and fifth digits) of the flexor digitorum profundus (FDP) are then given off.

The nerve then descends through the medial forearm, giving off no further muscular branches until after the wrist. Five to eight centimeters proximal to the wrist, the dorsal ulnar cutaneous sensory branch exits to supply sensation to the dorsal medial hand and the dorsal fifth and medial fourth digits. At the level of the ulnar styloid, the palmar cutaneous sensory branch originates to supply sensation to the proximal medial palm.

The nerve next enters the medial wrist through Guyon’s canal to supply sensation to the volar fifth and medial fourth digits and muscular innervation to the hypothenar muscles, the palmar and dorsal interossei, the third and fourth lumbricals, and two muscles in the thenar eminence, the adductor pollicis and the deep head of the flexor pollicis brevis.

Detailed Anatomy at the Elbow

As the nerve approaches the ulnar groove, it becomes quite superficial (Figure 19–2). The ulnar nerve normally runs in the groove formed by the medial epicondyle of the humerus and the olecranon process of the ulna. In some individuals, fully flexing the elbow may allow the ulnar nerve to sublux out of the groove medially over the medial epicondyle. In a small number of individuals, a dense fibrotendinous band or an accessory epitrochleoanconeus muscle (or both) may be present between the medial epicondyle and the olecranon process. Just distal to the groove is the HUA (cubital tunnel).

FIGURE 19–2 Detailed ulnar nerve anatomy at the elbow.

Entrapment of the ulnar nerve occurs both at the groove (between the medial epicondyle and the olecranon) or distally at the cubital tunnel.

(Reprinted with permission from Kincaid, J.C., 1988. AAEE minimonograph no. 31: the electrodiagnosis of ulnar neuropathy at the elbow. Muscle Nerve 11, 1005.)

Studies have shown that the distance from the medial epicondyle to the cubital tunnel distally varies between 3 and 20 mm in cadaver dissections and from 0 to 22 mm in surgical specimens. This variation underscores the importance of stimulating the below-elbow site at least 3 cm distal to the elbow, in routine ulnar motor studies, to ensure that the stimulation is distal to the cubital tunnel, a common site of entrapment. In the cubital tunnel, the ulnar nerve then continues under the FCU to exit between the deep fascia separating the FCU and FDP. The location of this exit from the cubital tunnel varies from 3 to 7 cm distal to the ulnar groove, according to cadaver studies. The muscular branch to the FCU usually arises distal to the cubital tunnel in 93–95% of cadaver dissections, and always follows the same course as the main ulnar nerve.

Etiology

UNE usually occurs as a result of chronic mechanical compression or stretch, either at the groove or at the cubital tunnel. Although rare cases of ulnar neuropathy at the groove are caused by ganglia, tumors, fibrous bands, or accessory muscles, most are caused by external compression and repeated trauma. Elbow fracture, often sustained years earlier, and subsequent arthritic change of the elbow joint may result in so-called tardy ulnar palsy. In addition, chronic minor trauma and compression (including leaning on the elbow) can either exacerbate or cause ulnar neuropathy at the groove. Ulnar neuropathy at the groove also is common in patients who have been immobilized because of surgery or who sustain compression during anesthesia or coma. More controversial is the possibility that repeated subluxation of the ulnar nerve out of the groove (during elbow flexion) also leads to ulnar neuropathy.

Distal to the groove is the cubital tunnel, the other major site of compression of the ulnar nerve in the region of the elbow. Although some use the term cubital tunnel syndrome to refer to all lesions of the ulnar nerve around the elbow, it more properly denotes compression of the ulnar nerve under the HUA. Some individuals have congenitally tight cubital tunnels that predispose them to compression. Repeated and persistent flexion stretches the ulnar nerve and increases the pressure in the cubital tunnel, leading to subsequent ulnar neuropathy.

Clinical

UNE caused by compression at the groove or at the cubital tunnel may present in a similar manner. In contrast to CTS, in which sensory symptoms predominate, motor symptoms are more common in ulnar neuropathy, especially in chronic cases. In some patients, insidious motor loss may occur without sensory symptoms, particularly in those with slowly worsening mechanical compression. Because most of the intrinsic hand muscles are ulnar innervated, weakness of these muscles leads to loss of dexterity and decreased grip and pinch strength. These are often the complaints that bring the patient to medical attention. There may be atrophy of both the hypothenar and thenar eminences (the ulnar-innervated adductor pollicis and deep head of the flexor pollicis brevis are in the thenar eminence). However, thumb abduction is spared (median and radial innervated).

In moderate or advanced cases, examination often shows the classic hand postures that occur with ulnar muscle weakness. The most recognized is the Benediction posture (Figure 19–3). The ring and little fingers are clawed, with the metacarpophalangeal joints hyperextended and the proximal and distal interphalangeal joints flexed (from third and fourth lumbrical weakness), while the fingers and thumb are held slightly abducted (from interossei and adductor pollicis weakness). The Wartenberg’s sign is recognized as a passively abducted little finger due to weakness of the third palmar interosseous muscle (Figure 19–4). The clinical correlate to this sign is that the patient reports getting the little finger caught when trying to put their hand in their pocket. The Froment’s sign occurs when the patient attempts to pinch an object or a piece of paper (Figure 19–5). To compensate for intrinsic ulnar hand weakness, the long flexors to the thumb and index finger (median innervated) are used, creating a flexed thumb and index finger posture.

FIGURE 19–3 Benediction posture.

The deformity results from a combination of finger adduction weakness (interossei) and clawing of digits 4 and 5 (extension at the metacarpophalangeal joints and flexion of the distal and proximal interphalangeal joints, from weakness of the third and fourth lumbricals).

FIGURE 19–4 Wartenberg’s sign.

The sign results from difficulty adducting the fifth digit because of preferential weakness of the third palmar interosseous muscle. In the photo, the patient was asked to hold her fingers together. Note that the patient’s left fifth finger is held abducted.

FIGURE 19–5 Froment’s sign.

Top: Normally to pinch a piece of paper, the finger pads of the thumb and index finger are brought together by the action of the ulnar-innervated adductor pollicis and first dorsal interosseous, respectively. Bottom: In ulnar neuropathy, weakness of these muscles results in a characteristic posture, known as the Froment’s sign. To compensate, the median-innervated flexor pollicis longus and flexor digitorum profundus (digit 2) have to contract, resulting in marked flexion of the interphalangeal joints of the thumb and index finger.

Examination of the patient’s grip often reveals it to be abnormal. Weakness of the ulnar-innervated FDP will result in the inability to flex the joints of the ring and little fingers. This often can be demonstrated just by having the patient make a fist (Figure 19–6). Patients with ulnar neuropathy may not be able to flex the distal fourth and fifth fingers completely when making a grip; in contrast, the median-innervated second and third distal digits flex normally.

FIGURE 19–6 Weakness of ulnar flexor digitorum profundus.

In ulnar neuropathy at the elbow, making a fist may result in the inability to completely flex the distal phalanx of the fourth and fifth digits due to weakness of the flexor digitorum profundus to digits 4 and 5. The median-innervated flexor digitorum profundus to digits 2 and 3 is normal (affected hand shown is the right hand – left side of the photo).

In UNE, sensory disturbance, when present, involves the volar and dorsal fifth and medial fourth digits and the medial hand (Figure 19–7). The sensory disturbance does not extend proximally much beyond the wrist crease. Sensory involvement extending into the medial forearm implies a higher lesion in the plexus or nerve roots (i.e., this is the territory of the medial antebrachial cutaneous sensory nerve, which arises directly from the medial cord of the brachial plexus). Another important skin territory to check is the dorsal medial hand. Sensory abnormalities here are important because they indicate that the dorsal ulnar cutaneous sensory nerve territory also is involved. This finding excludes an ulnar neuropathy at the wrist as the dorsal ulnar cutaneous sensory nerve arises proximal to the wrist.

FIGURE 19–7 Sensory loss in ulnar neuropathy.

The ulnar nerve contains three sensory branches: (1) ulnar digital sensory branches supply the volar fifth and medial fourth fingers; (2) palmar cutaneous branch supplies the proximal volar medial hand, arising 1 to 2 cm proximal to the wrist; and (3) dorsal ulnar cutaneous sensory branch arises 5 to 7 cm proximal to the wrist and supplies the dorsal medial hand and the dorsal medial fourth and fifth fingers. Lesions at the elbow may be associated with abnormalities in all three territories; lesions at the wrist never involve the dorsal ulnar territory (3) or the proximal volar ulnar palm (2).

Pain, when present, may localize to the elbow or radiate down to the medial forearm and wrist. Paresthesias may be reproduced by placing the elbow in a flexed position or by applying pressure to the groove behind the medial epicondyle. The ulnar nerve may be palpably enlarged and tender. The nerve may also be palpably taut with decreased mobility, especially in patients with ulnar neuropathy at the cubital tunnel.

Differential Diagnosis

The differential diagnosis in a patient suspected of having UNE (Table 19–1) principally includes C8–T1 radiculopathy, lower trunk or medial cord brachial plexopathy, and ulnar neuropathy at the wrist. Very rare cases of ulnar nerve entrapment in the proximal arm and more distally in the forearm have also been reported.

Table 19–1 Clinical Differentiating Factors in Suspected Ulnar Neuropathy

A cervical radiculopathy at the C8–T1 level, although seen less frequently than radiculopathy at the C6 and C7 root levels (which are more commonly affected in cervical disc disease or spondylosis), may be difficult to differentiate clinically from ulnar neuropathy. Neck pain and radiation into the arm, sensory disturbance extending into the forearm, and weakness involving the median and radially innervated C8–T1 muscles are the major differentiating features. Of course, weakness often is minimal and sensory loss often vague in radiculopathy, making the differentiation between a mild C8–T1 radiculopathy and an ulnar neuropathy demanding, if based on clinical findings alone.

Lower trunk/medial cord brachial plexopathies are uncommon. Entrapment of the lower trunk by a fibrous band or hypertrophied muscle results in neurogenic thoracic outlet syndrome (see Chapter 30). Lower trunk plexopathies may also result from infiltration by neoplasm, prior radiation, or self-limited inflammatory processes (e.g., neuralgic amyotrophy). Like C8–T1 radiculopathy, lower trunk plexopathies may demonstrate weakness of non-ulnar-innervated C8–T1 muscles and sensory disturbance that extends into the medial forearm.

Other than in the region of the elbow, entrapment of the ulnar nerve in the arm or forearm is rare. In the arm proper, entrapment under the arcade of Struthers has been reported. In the forearm, infrequent cases of ulnar neuropathy occur at the exit of the cubital tunnel. The entrapping structure is the deep fascia between the FCU and FDP. Unusual cases of ulnar neuropathy in the distal forearm have also been reported due to a fibrovascular band supplying blood to a hypertrophied FCU muscle. Clinical differentiation of these unusual cases from typical UNE is difficult. They are usually discovered either by careful electrophysiologic examination, at the time of surgery, or at the time of a second surgery after a failed ulnar surgery at the elbow.

Electrophysiologic Evaluation

Like other mononeuropathies, the goal of nerve conduction studies and electromyography (EMG) is to demonstrate abnormalities that are limited to one nerve, in this case the ulnar nerve. Although in most cases the lesion is at the elbow, entrapment at the wrist, at the medial cord or lower trunk of the brachial plexus, or at the C8–T1 nerve roots can mimic a UNE clinically. Patterns of nerve conduction and EMG abnormalities often can be used to differentiate these possibilities (Table 19–2). If the ulnar nerve lesion is demyelinating, nerve conduction studies may demonstrate conduction velocity slowing, conduction block, or both at the lesion site. Unfortunately, in many cases of UNE, the pathophysiology is that of axonal loss, and nerve conduction studies demonstrate only a non-localizable ulnar neuropathy. The EMG study, if abnormal, then can be used to localize the lesion only at or proximal to the takeoff to the most proximal muscle affected on EMG. Because there are no ulnar-innervated muscles above the elbow, the electrophysiologic impression often is one of an ulnar neuropathy at or proximal to the FCU muscle (the most proximal ulnar-innervated muscle).

Table 19–2 Electromyographic and Nerve Conduction Study Abnormalities Localizing the Lesion Site in Ulnar Neuropathy

Nerve Conduction Studies

The goal of nerve conduction studies in patients with UNE is to demonstrate, when possible, focal demyelination across the elbow (Box 19–1). Focal demyelinating lesions may manifest as slowing of conduction velocity or conduction block between proximal and distal stimulation sites (Figure 19–8). As for focal slowing, one needs to consider how much slowing is abnormal. In general, conduction velocities of more proximal nerve segments are the same as, or more often faster than those of, distal segments. This is due to a combination of (1) larger nerve fiber diameter and less tapering of the nerve more proximally (the reason that conduction velocities are faster in the upper than in the lower extremity) and (2) warmer temperatures in the proximal limb compared to the distal limb. In ulnar motor nerve conduction studies, however, this relationship may not hold true unless the position of the elbow is controlled.

Box 19–1

Recommended Nerve Conduction Study Protocol for Ulnar Neuropathy at the Elbow

Routine studies:

1. Ulnar motor study recording abductor digiti minimi, stimulating wrist, below elbow, and above elbow in the flexed elbow position (note: the optimal site for stimulating at the below-elbow site is 3 cm distal to the medial epicondyle)

2. Median motor study recording abductor pollicis brevis, stimulating wrist and antecubital fossa

3. Median and ulnar F responses

4. Ulnar sensory response, recording digit 5, stimulating wrist

5. Median sensory response, recording digit 2 or 3, stimulating wrist

6. Radial sensory response, recording snuffbox, stimulating lateral forearm

The following patterns may result:

Ulnar neuropathy at the elbow with demyelinating and axonal features:

• Low ulnar SNAP.

• Normal or low-amplitude ulnar CMAP with normal or slightly prolonged distal latency.

• Unequivocal evidence of demyelination at the elbow (conduction block and/or slowing >10–11 m/s across the elbow compared with the forearm segment, in the flexed elbow position).

Ulnar neuropathy at the elbow with pure demyelinating features:

• Normal distal ulnar SNAP and CMAP amplitudes and latencies.

• Unequivocal evidence of demyelination at the elbow (conduction block and/or slowing >10–11 m/s across the elbow compared with the forearm segment, in the flexed elbow position).

Non-localizable ulnar neuropathy (axonal features alone):

• Low ulnar SNAP.

• Normal or low-amplitude CMAP with normal or slightly prolonged distal latency.

• No focal slowing or conduction block across the elbow.

If the ulnar neuropathy is non-localizable, the following studies should be considered:

• Repeat motor studies recording the first dorsal interosseous.

• Inching studies across the elbow.

• Sensory or mixed nerve studies across the elbow.

• Recording the dorsal ulnar cutaneous SNAP (bilateral studies) (remember that the dorsal ulnar cutaneous SNAP can be normal in some patients with ulnar neuropathy across the elbow).

• Recording the medial antebrachial cutaneous SNAP (bilateral studies) if sensory loss extends above the wrist on clinical examination or there is a suggestion of lower brachial plexus lesion by history.

CMAP, compound muscle action potential; SNAP, sensory nerve action potential.

FIGURE 19–8 Focal slowing and conduction block at the elbow.

The ulnar compound muscle action potential amplitude is normal at the wrist and below the elbow. Stimulation above the elbow results in a marked drop in amplitude and focal slowing between the above-elbow and below-elbow sites (40 m/s) compared to the forearm segment (60 m/s). These are the electrophysiologic markers of focal demyelination, which allow for definitive localization of ulnar neuropathy at the elbow.

Differential Slowing: Flexed versus Extended Elbow Conduction Techniques

One of the more complicating factors in ulnar conduction studies is the position of the elbow and its effect on the calculated conduction velocity across the elbow. It has been well established in many studies that the position of the elbow during ulnar conduction studies strongly influences the calculated conduction velocity. Ulnar conduction studies performed in the extended (i.e., straight) elbow position often show artifactual slowing of conduction velocity across the elbow due to underestimation of the true nerve length (Figure 19–9). This is because in the extended elbow position, the ulnar nerve is slack with some redundancy. In normal subjects, this results in ulnar conduction velocities being slower in the across-the-elbow segment than in the segment above or below it, if the study is performed with the elbow in the extended position. Autopsy studies have confirmed that the length of the ulnar nerve across the elbow is measured more accurately with the elbow flexed (i.e., bent).

FIGURE 19–9 Extended versus straight elbow technique and measurement error.

Left: Ulnar conduction studies performed in the extended elbow position often show artifactual slowing of conduction velocity across the elbow due to underestimation of the true nerve length. In the extended elbow position, the ulnar nerve is slack with some redundancy. Right: With the elbow flexed, the true length of the ulnar nerve is more accurately measured, and the resultant conduction velocity is more valid. To prevent this error, the flexed elbow is the preferred position when studying the ulnar nerve.

In several studies of normal controls, the mean differential slowing comparing the across-the-elbow conduction velocity to forearm conduction velocity in the flexed elbow position (90–135 degrees) was 0 m/s, with an upper limit of normal of 10 to 11 m/s. In contrast, in the extended elbow position, mean slowing was 10 to 11 m/s, with an upper limit of normal in the range of 25 to 30 m/s (to reemphasize, in normal controls!). This extent of factitious conduction velocity slowing across the elbow, in the extended elbow position, is poorly appreciated in some EMG laboratories. Some laboratories arbitrarily use a value of 10 m/s differential slowing across the elbow, in the extended elbow position, to localize an ulnar neuropathy to the elbow. However, appreciation of the large range of variability in normal subjects, with the elbow in an extended position, is crucial to avoid erroneously diagnosing UNE in the normal population. An arbitrary cutoff value of 10 m/s differential slowing between the forearm and across elbow segments, in the extended elbow position, will result in many false-positive diagnoses of UNE. A patient with sensory loss in the little finger from a C8 radiculopathy would not be pleased to undergo ulnar nerve surgery simply based on a conduction velocity slowing of 10 m/s across the elbow compared to the forearm segment, if tested in the extended elbow position (as this is a normal finding in the extended elbow position).

Similar considerations apply to the absolute conduction velocity across the elbow in normal controls. The lower limit of normal for absolute conduction velocity across the elbow is 38 m/s in the extended elbow position, but never drops below 49 m/s in the flexed elbow position. Some have found that the absolute conduction velocity across the elbow is a better measure than differential conduction velocity slowing for detecting abnormalities in patients with ulnar neuropathy. Although absolute conduction velocity across the elbow may be considered a sensitive indicator of ulnar neuropathy, it does not localize the lesion. In any patient with significant axonal loss and dropout of the largest conducting fibers, conduction velocity will decrease across all nerve segments. An ulnar conduction velocity across the elbow segment of 40 m/s has little localizing value if the forearm conduction velocity is also 40 m/s.

In studies comparing the relative usefulness of the flexed versus extended elbow position in demonstrating focal slowing across the elbow, in those patients who had localizing electrophysiology, the flexed elbow position has been found to be more sensitive than the extended position. The difference in the yield between the flexed and extended positions likely is related to the greater range and variability found in normal subjects for differential and absolute conduction velocities across the elbow when tested in the extended elbow position, leading to lower cutoff values.

Thus, the flexed elbow position is considered the preferred technique when performing ulnar nerve conduction studies across the elbow. However, the flexed elbow position is more demanding in terms of measuring the curved anatomic course of the ulnar nerve around the elbow. In addition, the flexed position has the drawback of undercalling patients with UNE and subluxable ulnar nerves, which might lead to an overestimation of the true nerve length (see below). Nevertheless, it is far better to undercall UNE in this uncommon patient group, using the flexed technique, than to erroneously diagnose UNE in normal patients, using the extended position, with inappropriate low cutoff values.

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