11.2 Ulnar Motor, Sensory, Dorsal Ulnar Cutaneous, and Medial Antebrachial Studies

11.2 Ulnar Motor, Sensory, Dorsal Ulnar Cutaneous, and Medial Antebrachial Studies

Protocol Pearl: Record routine ulnar motor from abductor digiti minimi (ADM). Add first dorsal interosseous (FDI) when you need the deep palmar motor branch or a fascicular question. Stimulate at the wrist, below elbow, and above elbow with the elbow flexed about 70–90°. Record ulnar sensory from digit 5. Dorsal ulnar cutaneous (DUC) branches proximal to Guyon's canal, so it is typically spared in a Guyon lesion. Medial cutaneous nerve of the forearm (MAC/MABC) is a lower-trunk / medial-cord sensory study, not an ulnar-at-elbow study.

Independent OpenExamPrep teaching for outline topic VI.A.1.b stays on montage, segments, and pitfalls. Distances below are common laboratory teaching distances, not unpublished AAET-required distances. Across-elbow interpretation rules (how many meters per second, how much amplitude drop) belong with lesion localization, not with building the tray.

Ulnar motor: ADM, and when to add FDI

Place G1 over the ADM motor point on the ulnar border of the hypothemar eminence (midpoint of ADM, between the fifth MCP and the pisiform). Place G2 distal on the fifth digit, typically over the MCP or, in some labs, the DIP of digit 5. Ground on the dorsum of the hand. This is the routine hypothenar CMAP.

FDI is the extra recording when the question is fascicular ulnar neuropathy or the deep palmar motor branch after Guyon's canal. G1 sits in the first dorsal web space over FDI; G2 is on the MCP of the thumb or index. At Guyon's canal the ulnar nerve divides into a superficial sensory ramus (palmar digit 5, ulnar digit 4, palmaris brevis) and a deep motor ramus. The deep ramus innervates hypothenar muscles first (including ADM), then curves around the hook of the hamate to the interossei, adductor pollicis, and deep head of flexor pollicis brevis. A lesion of the deep branch distal to the hypothenar takeoff can drop the FDI CMAP while sparing ADM. If you only record ADM, you miss that pattern.

Ulnar motor stimulation sites and the across-elbow segment

Elbow position is part of the protocol, not a comfort afterthought. Stimulate and measure with the elbow flexed about 70–90°. In full extension the ulnar nerve sits slack in the retrocondylar groove, so a straight skin measurement is shorter than the true nerve length and across-elbow velocity looks falsely slow. Flexion takes up that slack so the tape matches the nerve. Do not wrap the tape in a wide arc around a hyperflexed, swollen elbow, which overestimates distance and can make velocity look falsely fast. Keep the same flexed position for BE and AE so the segment is internally consistent.

Cathode is distal (toward the recording muscle) at each site. Bring each site to supramaximal. Mark the cathode locations before you measure; moving the probe after you already stored a latency is how invented velocities are born.

Ulnar sensory to digit 5

Ulnar palmar digital sensory is recorded from digit 5. Antidromic rings: G1 proximal ring on the little finger, G2 distal ring, cathode at the volar-ulnar wrist over the ulnar nerve. Common teaching distances are 11–14 cm; many labs use 14 cm so the digit segment is comparable to median digit 2 or 3. Orthodromic reverses rings and wrist discs.

These palmar digital fibers travel through Guyon's canal with the superficial ramus. They are therefore vulnerable in Guyon lesions (especially zone 1 or the superficial/zone 3 sensory branch). They do not substitute for DUC.

Dorsal ulnar cutaneous (DUC)

DUC leaves the ulnar nerve in the distal forearm, typically 5–8 cm proximal to the ulnar styloid, winds dorsally between ulna and flexor carpi ulnaris, and supplies the dorsal ulnar hand (dorsum of digit 5 and the ulnar dorsum of digit 4). It does not pass through Guyon's canal.

Localization pearl: DUC is typically spared in Guyon canal lesions and affected when the ulnar lesion is at the elbow or more proximal (if those dorsal cutaneous axons are involved). A Guyon lesion can drop the digit-5 palmar SNAP and selected motor branches while DUC remains recordable. An across-elbow ulnar neuropathy can take DUC down with the rest of the distal ulnar sensory tree.

DUC montage

DUC SNAPs are small. Forearm edema, a high-riding ulnar styloid, and shock artifact are the usual failures. Compare sides before you call DUC absent.

Palmar cutaneous ulnar branch

The palmar cutaneous branch of the ulnar nerve arises in the distal forearm and supplies proximal hypothenar skin. Like median PCB, it generally does not travel through Guyon's canal. It is a listed cutaneous option on upper-extremity protocols, not a replacement for digit-5 or DUC. Do not mix up three different ulnar sensory territories: palmar digital (digit 5, through Guyon), DUC (dorsal hand, proximal to Guyon), and palmar cutaneous ulnar (proximal hypothenar skin, also typically outside Guyon).

Medial cutaneous nerve of the forearm (MAC / MABC)

Medial antebrachial cutaneous (also written MAC or MABC) is the sensory nerve of the medial forearm. Anatomically it is a lower-trunk / medial-cord nerve (C8–T1), not a branch of the ulnar nerve at the elbow. That is why it is on the ulnar-side tray even though you are not stimulating the ulnar nerve.

Localization pearl: MAC SNAP is spared in a lesion confined to the ulnar nerve at the elbow. MAC SNAP falls when the lesion is postganglionic lower trunk or medial cord (and in some medial-arm trauma that hits MAC directly). Root-level C8–T1 lesions are preganglionic, so MAC SNAP is typically preserved in radiculopathy—another reason not to treat MAC as a “C8 sensory study” in the SNAP sense.

MAC / MABC montage

MAC is a small SNAP. Medial-arm stimulation is uncomfortable if you over-current; volume can pull in ulnar or median. Side-to-side amplitude is more useful than a single “normal table” number.

Segment table: putting ulnar studies on one map

Wrist pitfall: co-stimulation of median

At the wrist the median nerve is only about a centimeter or two radial to a misplaced ulnar cathode. If you park the probe too far toward FCR, or you keep raising current after the ADM CMAP has already plateaued, you co-stimulate median.

Watch for: a sudden jump in CMAP amplitude with a change in waveform shape; thenar contraction when you thought you were on ulnar; thumb flexion/abduction mixed into the hypothemar twitch; an initial positivity that was not there at lower current. The trap is worse when recording FDI, because FDI sits next to thenar muscles and median-innervated lumbricals. Confirm the cathode is just radial to FCU, not in the mid-wrist crease over median, and stop at true supramaximal.

Other ulnar motor pitfalls: Martin-Gruber anastomosis can make the BE (or elbow) ulnar CMAP smaller than the wrist CMAP because MGA fibers travel with median at the elbow and only join ulnar in the forearm—this mimics forearm conduction block. Volume from a high BE stimulus can look like a recovered AE CMAP. Stimulating in the groove at a “below-elbow” site that is still at the compression point yields a small BE response and a confusing map. Cold elbows slow the across-elbow segment. Write down flexion angle, distances, and temperature with the traces.