Case 9

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Case 9

HISTORY AND PHYSICAL EXAMINATION

Right hand weakness and atrophy, without numbness or pain, developed over several months in a 38-year-old man. A prior electromyography (EMG) suggested cervical radiculopathy. The patient then had a cervical magnetic resonance imaging (MRI) scan, a cervical myelogram/computed tomography, MRI of the right brachial plexus, and MRI and magnetic resonance angiography of the brain. All were normal. His hand weakness worsened. He was suspected to have early motor neuron disease and was referred to the EMG laboratory.

On examination, he had right hand “ulnar” clawing, positive Froment sign with severe weakness, and atrophy of all interossei. The strength and bulk of the thenar and hypothenar muscles were normal. No other weakness was detected and deep tendon reflexes were normal. The patient complained of mild, deep, boring pain in the palm near the hypothenar eminence and had some tenderness over the hypothenar eminence to deep pressure.

Electrodiagnostic (EDX) examination was performed.

Please now review the Nerve Conduction Studies and Needle EMG tables.

EDX FINDINGS AND INTERPRETATION OF DATA

Pertinent EDX findings include:

1. Absent ulnar compound muscle action potential (CMAP), recording first dorsal interosseus (DI) in the setting of borderline-low ulnar CMAP, recording abductor digiti minimi (ADM; 5 mV on the affected side versus 9 mV on normal side). The latter had a borderline distal latency (3.1 ms) and normal proximal conduction velocities in the forearm and across the elbow (55 ms and 54 ms, respectively).

2. Normal ulnar and dorsal ulnar sensory nerve action potentials (SNAPs).

3. Normal median motor conduction study (CMAP amplitude, distal latency, and conduction velocity).

4. Prominent active denervation and loss of motor unit action potentials (MUAPs) in all dorsal interossei and the adductor pollicis in the setting of minimal neurogenic changes in the ADM.

5. Normal median and radial C8/T1-innervated muscles (abductor pollicis brevis, flexor pollicis longus, and extensor indicis).

This is consistent with ulnar mononeuropathy at the wrist, affecting the motor branch exclusively, distal to the main branch to the hypothenar muscles, but proximal to the branch to the fourth dorsal interosseus (i.e., at the pisohamate hiatus [PHH]). The normal ulnar sensory study rules out a proximal ulnar nerve, or a lower brachial plexus lesion. This case is not due to C8/T1 radiculopathy is because the median CMAP is preserved, and there is no denervation seen in other C8/T1-innervated muscles.

Case 9: Nerve Conduction Studies

Case 9: Needle EMG

DISCUSSION

Applied Anatomy

The ulnar nerve gives off the dorsal ulnar cutaneous branch 6 to 8 cm proximal to the ulnar styloid, to innervate the skin over the ulnar side of the dorsum of the hand and the dorsal aspect of digit V and half of digit IV. Then, the ulnar nerve enters Guyon canal (distal ulnar tunnel) at the level of the distal wrist crease where it divides into superficial (primarily sensory) and deep (pure motor) palmar branches. The deep branch enters the hand through the pisohamate hiatus while the superficial branch travels subcutaneously passing over the hypothenar muscles.

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

Compression at the pisohamate hiatus (distal to Guyon canal)

– Associated with occupational or recreational risks (e.g., use of hand tools, bicycling, etc.)

– Associated with acute closed injury (e.g., fall on hand, carpal fracture)

– Idiopathic (entrapment)

Carpal bone fracture

Other space-occupying lesions (neuroma, lipoma, cyst, calcification, false aneurysm, giant cell tumor)

Imaging should be considered in patients with ulnar neuropathy at the wrist who have a history of progressive worsening and who do not have clearly identified occupational or recreational risk factors. MRI imaging of the wrist is helpful in identifying structural lesions within Guyon canal, including ganglion cyst and nerve sheath tumor.

Decisions regarding the management of these lesions are made on the basis of their etiologies, presentations, and clinical courses. In patients with fractures, ganglia, or mass lesions, surgical intervention is necessary. However, in patients with ulnar neuropathy at the wrist who do not harbor an obvious mass or fracture and have a predominantly demyelinating lesion, careful review of the history for any occupational or recreational trauma should be initiated; if found, sources of the trauma should be eliminated. Then, the patient should be followed clinically and by serial EDX studies. If recovery is not evident, surgical exploration of Guyon canal extending into the PHH should be done. The prognosis for patients with this disorder is usually good after surgical decompression because the lesion is distal and reinnervation to the target hand muscles is efficient.

Electrodiagnosis

An important purpose of the EDX study is to localize the ulnar nerve lesion to the wrist or to the elbow and to differentiate ulnar mononeuropathy from a lower brachial plexopathy affecting the lower trunk or medial cord, and from a C8/T1 radiculopathy. In addition to the routine NCSs done that include ulnar sensory NCS recording little finger and ulnar motor NCS recording ADM, several other strategic EDX studies are extremely important to assure a correct and precise diagnosis of the ulnar nerve lesion. These include the following:

1. Ulnar motor nerve conduction study recording first DI. This study is essential in the accurate diagnosis of ulnar nerve lesions at the wrist due to the fact that many lesions, such as those at the PHH or in the palm, spares the hypothenar muscles and result in normal ulnar NCS recording ADM. In most cases, the ulnar CMAP recording first DI is low in amplitude stimulating at the wrist, and the distal latency is either borderline or slightly delayed. Slowing of the distal latency should also interpreted cautiously since it may be slowed in any axon-loss ulnar nerve lesion, including at the elbow, due to the loss of large fibers. In cases of ulnar neuropathy at the wrist, the addition of more stimulation site(s) distal to Guyon canal, while recording first DI, is technically feasible with minimal interference by shock artifact. This could be achieved using one of both of the following techniques:

• Palm stimulation recording first DI. Adding a single palm stimulation site to the routine wrist, below-elbow and above-elbow sites, often confirm that the lesion is at the wrist by showing conduction block in the majority of patients (Figure C9-3). Even in patients with a seemingly axon-loss ulnar neuropathy and uniformly low CMAP amplitudes stimulating wrist, below elbow and above elbow, palm stimulation often shows that there is a component of segmental demyelination manifesting by a partial conduction block (i.e., more than 20–50+ drop in CMAP amplitude and/or area). This finding is very specific since it localizes the lesion to the wrist.

• Short segment stimulation across the wrist recording first DI. During ulnar motor NCS, inclusion of the unaffected segments in distal latency calculation may dilute the effect of focal slowing at the injured site and decreases the sensitivity of the test. Segmental stimulation in short increments of 1 cm across the nerve segment helps localize a focal abnormality that might otherwise escape detection. “Inching” the stimulus in short increments of 1 cm along the course of the nerve is very specific and often shows a precise site of compression. However, it is more technically challenging and time consuming than the single palm stimulation. Though this study is technically more demanding and subject to inherent measurement errors (due multiple stimulations in short increments), detecting an abrupt drop in amplitude (>20+) or increase in latency difference (>0.5 ms) compensates for these shortcomings (Figure C9-4).

2. Dorsal ulnar SNAP. This study is useful since an absent or low-amplitude response exclude a lesion at the wrist or hand. The study should be performed bilaterally for comparison purposes since interpreting the result of the dorsal ulnar SNAP may be misleading due to two caveats:

• A fascicular involvement of the ulnar nerve at the elbow may totally or partially spare the dorsal ulnar nerve resulting in normal SNAP.

• There is a considerable overlap, in some individuals, between the territories of the dorsal ulnar cutaneous nerve and that of the superficial radial cutaneous nerve. Hence, this may result in a low-amplitude, or occasionally absent, dorsal ulnar SNAP when the radial sensory nerve dominates the innervation of the dorsum of the hand. In these situations, radial sensory nerve stimulation while recording the ulnar side of the dorsum of the hand will yield a well-defined SNAP.

3. Second lumbrical-interosseous motor distal latencies comparison. This study is most often used in the diagnosis of carpal tunnel syndrome where the median motor distal latency recording second lumbrical muscle is usually prolonged in carpal tunnel syndrome when compared to the ulnar motor distal latency recording second interossei (palmar and dorsal). Since the ulnar second interossei are innervated by the deep palmar motor branch, this test is also useful in identifying focal ulnar slowing at the wrist or palm in ulnar nerve lesions at Guyon canal or deep palmar motor branch lesions at the PHH. A motor distal latency difference greater than 0.5 ms suggests focal slowing across the wrist; the median is relatively slowed compared to the ulnar in carpal tunnel syndrome while the findings are reversed in ulnar nerve lesion at the wrist (Figure C9-5). Obviously, this study is not useful when there is a coexisting carpal tunnel syndrome. Similar to the ulnar motor distal latency stimulating at the wrist and recording the first DI, this study also should be interpreted cautiously since it may be slowed in any axon-loss ulnar nerve lesion, including at the elbow, due to the loss of fast conducting fibers.

4. Medial antebrachial SNAP. The medial antebrachial cutaneous nerve is a pure sensory nerve that originates from the medial cord of the brachial plexus and innervates the skin of the medial forearm. In cases of ulnar neuropathies in general, a normal medial antebrachial SNAP is useful in excluding the possibility of a lower brachial plexopathy as a cause of hand weakness and medial hand sensory loss. The medial brachial SNAP serves similar purposes but it is technically more difficult to evoke particularly in elderly and obese patients.

5. Needle EMG of hand intrinsics. In suspected ulnar nerve lesion at the wrist, needle EMG of at least the ADM and first DI should be done to look for disparity between the findings of these two muscles. It is also preferable to needle few other hand intrinsics that are innervated by the deep palmar branch such as the fourth DI, a muscle with a branch that originate immediately after the nerve passes under the PHH, and the adductor pollicis, one of the most distal muscle innervated by the ulnar nerve and its deep palmar branch. In lesions of the deep palmar branch at the PHH, an abrupt change from a normal (or minimally abnormal) ADM to markedly denervated fourth and third interosseous is often evident. The flexor digitorum profundus (ulnar part) and the flexor carpi ulnaris, both forearm muscles innervated by the ulnar nerve are normal in ulnar mononeuropathy at the wrist and useful in confirming the diagnosis. Finally, median and radial C8–T1 muscles (such as abductor pollicis brevis, flexor pollicis longus and extensor indicis proprius) and the lower cervical paraspinals muscles must be sampled to exclude a cervical root or motor neuron lesion.

Figure C9-3 Palm and wrist stimulations recording first dorsal interosseous muscle. This 37-year-old woman developed subacute painless weakness of left hand. Neurological examination reveals severe weakness of all ulnar-innervated muscles in the hand with a positive Froment sign. There was no sensory loss. Tinel signs were negative at the wrist and elbow. Routine EDX studies revealed findings compatible with an axon loss ulnar mononeuropathy at the distal portion of Guyon canal (see Table C9-4). (A) The stimulation points at the palm and wrist (circles) while recording first DI (R). (B) Note that both wrist and palm stimulations result in low ulnar CMAPs, but there is a localizing partial conduction across both stimulation points (>50+ drop in amplitude).

Figure C9-4 Ulnar short segment stimulations across the wrist (“inching”) recording first dorsal interosseous muscle. (A) Nine sites of stimulation (circles) in 1 cm increments along the length of the ulnar nerve while recording first DI (R): 0 level is at the distal crease of the wrist, while negative sites are progressively distal points and positive sites are proximal. (B) Same patient as Figure C9-3. Note that the most distal response, stimulating the palm at point –4, is low (0.9 mV), consistent with axon loss. However, there is also a localizing partial conduction block between points –3 and –2 (arrow), consistent with additional segmental demyelination. This is evidenced by a drop in CMAP amplitude from 0.7 mV at point –3 to 0.2 mV at point –2 (70+ amplitude decay). There is also focal slowing at the same segment (between points –3 and –2) as evidenced by a larger increase in latency between the same two points resulting in a latency difference of 0.9 ms. This is contrasted to 0.1 to 0.2 ms latency differences at all other sites.

Figure C9-5 Second lumbrical-interosseous motor distal latencies comparison in a patient with ulnar mononeuropathy at the wrist. Note that the ulnar motor distal latency recording second interossei (dorsal and palmar) is significantly prolonged when compared to the median motor distal latency recording second lumbrical (0.7 ms difference).

The EDX findings in ulnar mononeuropathy at the wrist parallel the clinical manifestations and vary with the site of the lesion (Table C9-4). In addition to try to localize the lesions accurately within Guyon canal, at the PHH or in the palm, the EDX study play a pivotal role in excluding an ulnar neuropathy at the elbow, a common entrapment neuropathy. Several features on the EDX examination are not consistent with an ulnar neuropathy at the wrist:

1. Low-amplitude or absent dorsal ulnar SNAP, excluding the anatomical variability where the territory of the superficial radial cutaneous nerve overrides that of the dorsal ulnar cutaneous nerve.

2. Focal slowing, conduction block, or differential slowing across the elbow is not compatible with a lesion at the wrist. Note that in a significant number of ulnar nerve lesions at the elbow, especially those resulting in axonal loss, there is no focal slowing, and, if slowing is present, it is diffuse due to the loss of fast conducting fibers, and evident in all segments of the ulnar nerve.

3. Denervation of the flexor carpi ulnaris or the flexor digitorum profundus (ulnar portion) is not compatible with a lesion at the wrist. Note that these muscles are denervated in only a proportion of patients with ulnar mononeuropathy around the elbow.

Table C9-4 Electrodiagnostic Findings in Ulnar Neuropathies