4. Conduction block of ulnar motor fibers, recording the ADM. This is evident by comparing CMAP amplitudes and areas with below-elbow to above-elbow stimulations, recording the ADM (Figure C15-1). The former has an amplitude of 5.0 mV and the latter 2.5 mV, which amounts to a 50% amplitude decay without temporal dispersion. The block is confirmed by a decrease (33.5%) in CMAP area (from 11.7 mV ms to 7.78 mV ms). The amplitude and area decay is significant since the distance between the two stimulation sites is short (around 10 cm). Despite the conduction block to ADM, there was no conduction block across the elbow, recording the first dorsal interosseous. This is consistent with selective fascicular involvement (by a demyelinative conduction block) of the ulnar fibers directed to the hypothenar muscles.

5. Focal slowing of ulnar motor conduction velocities (CVs) across the elbow, recording the ADM. This was achieved by comparing values of below the elbow-to-wrist segment (CV = 56 m/s) with those across the elbow segment (CV = 33 m/s). Here, the difference of 23 m/s is significant and consistent with focal slowing of ulnar motor fibers across the elbow. In contrast, when recording the first dorsal interosseous, the CV difference between the same segments (56 − 47 = 9 m/s) is not significant enough to localize the lesion definitely to the elbow region (despite absolute slowing of the across the elbow segment to CV of 47 m/s). This is again compatible with preferential fascicular involvement of fibers directed to the ADM in this patient.

6. Needle EMG, revealing fibrillation potentials of mild to moderate degree in all three ulnar-innervated muscles tested in the hand (the first dorsal interosseous, the abductor digiti minimi, and the adductor pollicis). There was moderate reduction of recruited motor unit action potentials (MUAPs). All MUAPs recorded from the ulnar muscles were increased in duration and amplitude, with a significant increase in polyphasic MUAPs. The forearm muscles revealed a mildly denervated flexor carpi ulnaris (FCU) but a normal ulnar part of the flexor digitorum profundus (FDP). In contrast, all C8/T1-innervated muscles via the median (abductor pollicis brevis and flexor pollicis longus) and radial (extensor indicis) nerves were normal, excluding a lower brachial plexopathy (lower trunk or medial cord).

Figure C15-1 Motor conduction studies of the right ulnar nerve, recording the abductor digiti minimi in this patient (A), compared with an age-matched control (B). The sensitivity (vertical scale) is 2 mV/division in both. Responses 1 are at the wrist, 2 below the elbow, and 3 above the elbow. Note that there is (1) partial conduction block across the elbow, evidenced by the significant decrease in amplitude (50%) and area (35%); (2) focal slowing, as easily recognized by the prominent delay in proximal latency of the third response, compared with the second (despite a shorter distance across the elbow compared with forearm), and resulting in a 33 m/s velocity across the elbow compared to a 56 m/s velocity in the forearm; and (3) slight motor fiber axonal loss, based on the low-amplitude distal compound motor action potential (Response 1A = 6.5 mV) compared with normal controls (Response 1B = 8.5 mV).

Case 15: Nerve Conduction Studies

Case 15: Needle EMG

In summary, this patient has a slowly progressive right ulnar mononeuropathy across the elbow, with signs of segmental demyelination (confirmed by conduction block and focal slowing) and axonal loss (evidenced by low distal/ulnar CMAPs and absent/low ulnar SNAPs); there is also ongoing denervation (fibrillation potentials) and reinnervation (demonstrated by long-duration, high-amplitude, and polyphasic MUAPs). In view of the patient’s history (remote elbow fracture), this case is an example of tardy ulnar palsy.

DISCUSSION

Applied Anatomy

The ulnar nerve sensory and motor fibers are derived from the spinal nerves C8 and T1. Before arising from the plexus in the proximal axilla, the ulnar nerve fibers pass through the lower trunk and the medial cord of the brachial plexus (Figure C15-2). In the axilla and proximal arm, the ulnar nerve is closely related to the radial and median nerves and the brachial artery. Around the midarm, the ulnar nerve pierces the intermuscular septum and lies in close contact with the medial head of the triceps and humerus. The ulnar nerve develops no branches in the arm.

Figure C15-2 Course of the ulnar nerve and its branches. 1. Palmar ulnar cutaneous branch; 2. dorsal ulnar cutaneous branch; 3. terminal cutaneous superficial branch; 4. terminal deep palmar motor branch.

(From Haymaker W, Woodhall B. Peripheral nerve injuries. Philadelphia, PA: WB Saunders, 1953, with permission.)

At the elbow level, and in contrast to most major human peripheral nerves, the ulnar nerve traverses the extensor, rather than the flexor, surface of the elbow joint. This renders the nerve more vulnerable to trauma around the elbow. At the elbow, the ulnar nerve crosses the ulnar groove (also called the condylar or retroepicondylar groove) behind the medial epicondyle and then passes the aponeurotic arch of the FCU to enter the cubital tunnel (Figure C15-3). This tunnel, also called the humeroulnar arcade, Osborne ligament, or arcuate arcade, is formed by the attachment of the muscle to the olecranon and medial epicondyle. Its proximal edge is variable but usually is approximately 1 cm distal to an imaginary line drawn between these two insertional points. With flexion of the elbow, the distance between the olecranon and medial epicondyle increases by approximately 1 cm, which results in tightening of the FCU aponeurosis over the nerve. In addition, the medial elbow ligament bulges, flattening the concave surface of the ulnar groove.

Figure C15-3 View of the medial surface of the elbow, showing the course of the ulnar nerve through the ulnar groove and cubital tunnel.

(From Kincaid JC. The electrodiagnosis of ulnar neuropathy at the elbow. Muscle Nerve 1988;11:1005–1015, with permission.)

In the forearm, the ulnar nerve gives off its first branches. These are the motor branches to the FCU and FDP. These branches arise approximately 10 cm distal to the medial epicondyle. The ulnar nerve continues in the forearm deep to the FCU but superficial to the FDP to become superficial in the distal forearm, lying between the tendons of these two muscles. Two cutaneous sensory branches arise in the forearm, without passing through Guyon canal at the wrist, to innervate the skin in the hand. The first is the palmar ulnar cutaneous branch, which takes off at midforearm and innervates the proximal part of the ulnar border of the palm. The second is the dorsal ulnar cutaneous branch, which arises 6 to 8 cm proximal to the ulnar styloid, winds around the ulna, and innervates the dorsal surfaces of the little finger and half of the ring finger, along with the ulnar side of the dorsum of the hand (Figure C15-4).

Figure C15-4 The cutaneous distribution of the three sensory branches of the ulnar nerve.

(From Stewart JD. The variable clinical manifestations of ulnar neuropathies at the elbow. J Neurol Neurosurg Psychiatry 1987;50:252–258, BMJ Publishing Group.)

At the wrist, the ulnar nerve enters the distal ulnar tunnel (Guyon canal), where it divides into superficial (primarily sensory) and deep palmar (pure motor) branches (Figure C15-5). The superficial branch innervates the palmaris brevis muscle and the palmar aspects of digit V and half of digit IV. The deep branch innervates the hypothenar muscles, including the ADM, and travels through the palm to the dorsal and palmar interossei, the third and fourth lumbricals, the adductor pollicis, and a portion of the flexor pollicis brevis.

Figure C15-5 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 C15-2), 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.)

The flexor brevis digiti minimi (or quinti), a hypothenar muscle, has two separate attachments, at the hook of the hamate and at the pisiform bone. These musculotendinous attachments form a fibrous arch and create the superficial boundary of the pisohamate hiatus (PHH), an opening through which the deep motor branch of the ulnar nerve passes. The posterior boundary of the PHH, the pisohamate ligament, extends from the pisiform bone to the hook of the hamate (Figure C15-6). The origin of the major motor branch to the ADM is proximal to this hiatus in the majority of hands.

Figure C15-6 The pisohamate hiatus (PHH) in the distal portion of Guyon canal. U. = Ulnar; F.C.U. = flexor carpi ulnaris; ABD.D.Q. = abductor digiti quinti (or minimi); F.B.D.Q. = flexor brevis digiti quinti (or minimi); Opp.D.Q. = opponens digiti quinti.

(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.)