11.1 Median Motor, Sensory, AIN, and Palmar Cutaneous Studies

11.1 Median Motor, Sensory, AIN, and Palmar Cutaneous Studies

Protocol Pearl: Record median motor from abductor pollicis brevis (APB) with G1 on the APB motor point and G2 distal on the thumb. Stimulate at the wrist and the elbow. Record median sensory from digit 2 or digit 3 (antidromic ring electrodes are the usual montage; orthodromic digital or palmar setups reverse the path). The anterior interosseous nerve (AIN) is motor-only—there is no cutaneous SNAP. The palmar cutaneous branch does not travel through the carpal tunnel, which is the localization pearl that separates a tunnel lesion from a more proximal median lesion.

Upper-extremity nerve conduction is the large practical block of the AAET R.NCS.T. outline (VI.A.1). Independent OpenExamPrep teaching in this section is protocol-specific: where G1 and G2 go, where the cathode goes, which distances labs commonly teach, and which mistakes wreck the trace. This is not the carpal-tunnel diagnostic-criteria chapter. Palmar mixed studies appear here only to explain why that montage exists.

Distances in the tables are common laboratory teaching distances (for example 8 cm wrist-to-APB motor and 14 cm antidromic digit). They are not unpublished AAET-required distances. Use your laboratory's measured technique and that laboratory's own reference data.

Median motor: APB recording

The routine median motor study records a compound muscle action potential (CMAP) from APB, the thenar muscle supplied by the recurrent motor branch after the median nerve leaves the carpal tunnel.

Place G1 (active) over the APB motor point—the muscle belly. A practical landmark is the midpoint of a line from the first metacarpophalangeal (MCP) joint to the midpoint of the distal wrist crease, or the most prominent APB bulge when the patient abducts the thumb. Place G2 (reference) distal: over the APB tendon at the thumb MCP joint, or slightly more distal on the thumb. Distal G2 keeps a belly–tendon montage. A proximal G2 can invert or distort the CMAP. Place ground between the stimulator and G1, usually on the dorsum of the hand.

Keep the thumb slightly abducted so APB is easy to palpate, but do not stretch the thenar skin so far that G1 slides onto adductor pollicis (ulnar-innervated). Palpate the muscle twitch under G1 as you bring up the stimulus; if the thumb adducts more than it abducts, G1 is too ulnar.

Median motor stimulation sites

Cathode faces the recording electrodes; anode sits 2–3 cm proximal along the nerve. Raise current until the CMAP is supramaximal (no further amplitude growth), then add a small extra increment. Do not keep climbing at the wrist: current spread can recruit ulnar-innervated thenar muscles (adductor pollicis, deep head of flexor pollicis brevis) and inflate amplitude.

Median motor pitfalls

• Ulnar thenar volume: G1 too ulnar records a hybrid thenar CMAP. Thenar twitch that includes thumb adduction is a warning.
• Martin-Gruber anastomosis (MGA): median-to-ulnar communicating fibers in the forearm. Elbow stimulation may look larger than wrist stimulation, or an initial positivity may appear, because some fibers that will innervate ulnar-innervated hand muscles are still traveling with median at the elbow. Do not call that pattern reverse conduction block.
• Cathode–anode reversal: stimulating with the anode distal falsely lengthens distal latency.
• Tape error: a flexed, redundant wrist shortens the measured 8 cm and spuriously shortens latency. Neutral wrist, tape on the nerve path.
• Temperature: a cold thenar eminence lengthens distal latency. Warm the hand before you call a distal latency abnormal.

Median sensory: digits 2 or 3, antidromic and orthodromic

Routine median sensory studies record a sensory nerve action potential (SNAP) from digital cutaneous fibers that do travel through the carpal tunnel.

Antidromic (most common): stimulate at the volar wrist over median and record with ring electrodes on digit 2 (index) or digit 3 (middle). Digit 3 is typically a purely median digit; digit 2 is equally standard. G1 is the proximal ring (proximal phalanx or PIP). G2 is 3–4 cm distal (DIP or distal phalanx). A common teaching distance is 14 cm from the wrist cathode to G1. Ground on the dorsum of the hand.

Orthodromic: reverse the path. Stimulate the digit with rings; record over the median nerve at the wrist. Amplitude is often smaller because the wrist discs sit farther from digital skin, but shock artifact can be easier to manage in some hands. The same 14 cm distance can be used so digit segments stay comparable.

Orthodromic palmar stimulation (small discs in the palm rather than on the digit) is another way to drive median palmar fibers toward a wrist pickup. It is not the same study as palmar mixed (below), which is built as a short wrist-segment mixed recording.

Why palmar mixed studies exist (without turning this into the carpal-tunnel criteria chapter): a digital SNAP includes a long extra-tunnel segment in the finger. That extra length dilutes focal slowing at the wrist. A palmar mixed study stimulates mixed motor and sensory palmar branches in the palm and records at the wrist, so most of the short 8 cm segment crosses the carpal tunnel. That is why the palmar mixed montage sits on the tray when the clinical question is the wrist segment. How those latencies are scored against laboratory cut-offs is the next chapter.

Cool hands slow sensory latencies. Warm skin to about 32–34 °C before you trust a slow SNAP. Sweep speed and averaging matter: SNAPs are microvolt signals.

Anterior interosseous nerve (AIN)

The AIN is the purely motor branch of the median nerve that arises in the proximal forearm, typically dives under the fibrous arch of flexor digitorum superficialis, and innervates flexor pollicis longus (FPL), flexor digitorum profundus to digits 2–3 (FDP 2–3), and pronator quadratus (PQ). It has no cutaneous sensory territory. A patient can have a weak OK pinch (FPL plus FDP 2) and a normal median digital SNAP if the lesion is isolated AIN.

How to set up an AIN motor study

AIN fibers have already left the median trunk in the proximal forearm, so wrist stimulation does not test AIN. Routine APB motor from wrist and elbow can be entirely normal while AIN is out—APB is recurrent motor branch after the tunnel, not AIN.

Why there is no AIN SNAP: AIN may carry muscle afferents, but it has no cutaneous sensory axons. You cannot put ring electrodes on skin and record an AIN SNAP. Median digital SNAPs from digits 1–3 travel with the main median sensory trunk through the carpal tunnel, not with AIN. Normal digital SNAPs plus weak FPL/PQ is the conduction signature that points toward AIN (or an FPL tendon rupture—examine the tendon) rather than a wrist median lesion.

AIN pitfalls: volume from nearby median-innervated forearm muscles; stimulating in the distal forearm and believing you have isolated AIN when you are still on the main median trunk; mistaking a C8–T1 root or lower-trunk problem for AIN. PQ and FPL can fail in those more proximal lesions, but other C8–T1 muscles and the appropriate SNAPs will not look isolated.

Palmar cutaneous branch: anatomy and how it is tested

The palmar cutaneous branch (PCB) of the median nerve typically arises about 5–7 cm proximal to the distal wrist crease, travels between PL and FCR, and crosses superficial to the transverse carpal ligament. It does not enter the carpal tunnel. It supplies skin over the thenar eminence and the proximal radial palm.

Localization pearl: digital median SNAPs (digits 2–3) go through the tunnel. PCB does not. A lesion inside the carpal tunnel can drop digital SNAPs and, when severe, the APB CMAP, while sparing PCB (thenar-eminence skin intact). A lesion proximal to the PCB takeoff (forearm median, ligament of Struthers, plexus) can affect PCB and digital fibers. PCB is a localization tool, not a tunnel-severity score.

Palmar cutaneous montage

Expect a small SNAP. Shock artifact and motor volume from APB are the usual failures—keep intensity just supramaximal for the sensory response, and slide G1 slightly off the APB belly if a motor potential buries the SNAP. Compare sides.

The palmar cutaneous ulnar branch is the ulnar analogue (proximal hypothenar skin; arises in the distal forearm and generally does not traverse Guyon's canal). It is listed with ulnar cutaneous options in the next section. Do not confuse median PCB with dorsal ulnar cutaneous nerve or with a digit-5 SNAP.

Building the median tray

A practical median set is: (1) APB motor at a common 8 cm wrist distance plus elbow, (2) antidromic digit 2 or 3 at a common 14 cm distance, (3) palmar mixed at about 8 cm when the question is the wrist segment, (4) AIN to PQ or FPL when the question is proximal-forearm motor, and (5) PCB when the question is tunnel versus more proximal. Side-to-side comparison beats an isolated number when amplitudes are low but latencies look fair. Document distance, temperature, and exact montage—those three fields are what make another reader trust the traces.