11.3 Radial Motor, Posterior Interosseous, and Superficial Radial Studies
Key Takeaways
- Radial motor is often recorded from extensor indicis proprius, with stimulation in the forearm, at the elbow or spiral groove, and at the axilla or Erb's point when a proximal segment is needed.
- Posterior interosseous nerve is motor to finger and ulnar wrist extensors; it spares triceps, brachioradialis, and the superficial radial SNAP.
- Superficial radial sensory is recorded in the anatomical snuffbox with stimulation along the radius in the distal forearm.
- Spiral-groove radial neuropathy involves brachioradialis and may lower the superficial radial SNAP when sensory axons are lost, but typically spares triceps; PIN spares brachioradialis and the SNAP, while C7 radiculopathy keeps SNAPs and involves other C7 muscles.
- Spiral-groove stimulation is uncomfortable and easily volume-conducts; stop at supramaximal and do not treat an Erb's-point CMAP as a selective radial stimulus.
11.3 Radial Motor, Posterior Interosseous, and Superficial Radial Studies
Protocol Pearl: Record radial motor from extensor indicis proprius (EIP) in the distal dorsal forearm. Stimulate in the forearm (posterior interosseous), at the elbow or spiral groove, and at the axilla or Erb's point when the question is more proximal. Posterior interosseous nerve (PIN) is motor to the extensors and spares triceps, brachioradialis, and the superficial radial SNAP. Superficial radial sensory is recorded in the anatomical snuffbox.
Independent OpenExamPrep teaching for outline topic VI.A.1.c is the radial tray: EIP motor, PIN versus radial proper, snuffbox SNAP, and stimulation pitfalls. Distances are common laboratory teaching distances, not unpublished AAET-required distances.
Radial motor: EIP recording
EIP is a distal PIN-innervated muscle, so an EIP CMAP surveys the long motor path from plexus to posterior interosseous. Place G1 over EIP in the distal dorsal forearm: a common landmark is about two fingerbreadths (roughly 2–4 cm) proximal to the ulnar styloid, just radial to the ulna, over the distal extensor compartment. Place G2 distal—ulnar styloid or the dorsum of the second MCP. Ground between stimulator and G1, often on the dorsal forearm.
EIP lies deep. The surface CMAP is smaller than an ADM or APB CMAP. If G1 sits too proximal you pick up extensor digitorum or extensor carpi ulnaris volume and the waveform looks larger and less specific. Have the patient extend the index finger and palpate the distal tendon/muscle before you tape G1 down.
Radial / PIN stimulation sites
| Site | Cathode placement | Common lab distance / role | Fibers you are driving |
|---|---|---|---|
| Forearm | Dorsal forearm over PIN, commonly 8–10 cm proximal to EIP G1, along the interosseous / extensor path | Distal PIN motor latency to EIP | PIN distal to the takeoff of superficial radial |
| Elbow | Groove between biceps and brachioradialis, or just distal to the spiral groove as the radial nerve enters the anterior compartment | Elbow-to-forearm segment | Radial nerve before or at the PIN/superficial-radial split, depending on exact cathode |
| Spiral groove | Posterolateral mid-arm, in the spiral groove against the humerus | Groove-to-elbow or groove-to-forearm | Radial nerve at the humerus, proximal to BR and ECRL branches |
| Axilla | Axillary neurovascular bundle, radial/posterior | Upper-arm segment | Radial nerve proximal to the groove |
| Erb's point (when indicated) | Supraclavicular fossa | Plexus-to-arm | Not a selective radial stimulus; whole upper plexus can fire |
Cathode faces distally toward EIP. Forearm PIN stimulation is usually the least painful of the set. Spiral-groove stimulation is deep and uncomfortable—warn the patient, use a firm probe, and stop when the EIP CMAP plateaus.
Some labs also record extensor digitorum, anconeus, or brachioradialis as extra radial motor sites. BR recording (G1 over BR belly, stimulate at the spiral groove or elbow) is useful when the question is spiral groove versus PIN, because PIN does not innervate BR.
Posterior interosseous nerve: what PIN is (and is not)
After giving branches to triceps (in the arm) and then to brachioradialis and extensor carpi radialis longus around the elbow, the radial nerve divides into:
- Superficial radial nerve — cutaneous, to the dorsal radial hand and anatomical snuffbox skin.
- PIN (deep motor branch) — dives into supinator (arcade of Frohse is a common entrapment) and innervates the remaining extensors: ECRB, supinator, extensor digitorum, ECU, APL, EPB, EPL, EIP, and extensor digiti minimi.
PIN is motor. It does not supply the superficial radial SNAP. A PIN lesion can produce finger drop and weak thumb extension. Wrist extension may still occur with radial deviation because ECRL is spared (radial proper, before PIN). Triceps is spared. Brachioradialis is spared. Superficial radial SNAP is normal.
If you only stimulate in the forearm over PIN and record EIP, you will see a low EIP CMAP in PIN, but you will not have distinguished PIN from a more proximal radial lesion. Add the snuffbox SNAP and a proximal radial site (and clinically BR and triceps) to place the lesion.
Superficial radial sensory: anatomical snuffbox
The superficial radial SNAP is the sensory half of the radial tray and the SNAP that PIN must spare.
| Electrode | Placement |
|---|---|
| G1 | Anatomical snuffbox, over the extensor pollicis longus tendon, between the snuffbox tendons |
| G2 | Distal, typically over the first MCP joint or the thumb |
| Cathode | Distal radial forearm, along the radius, commonly 10 cm proximal to G1 (some labs use 12–14 cm; use one distance and the lab's reference data) |
| Ground | Dorsum of the wrist, between cathode and G1 |
The nerve is superficial. It is easy to over-stimulate and create shock artifact or to slide G1 onto a radial artery pulse and pick up artifact. A common teaching trick is to have the patient slightly ulnar-deviate and extend the thumb so the snuffbox is obvious. Compare sides: a unilaterally small snuffbox SNAP with wrist drop points away from PIN and toward spiral groove (or a more proximal radial / posterior-cord lesion).
Superficial radial can also be injured in isolation (tight watch, handcuffs, venipuncture along the radius) with a low snuffbox SNAP and normal EIP—do not force that pattern into a PIN diagnosis.
Distinguishing spiral groove vs PIN vs C7
This table is the practical product of the radial protocol. Build it from EIP motor, snuffbox SNAP, and which muscles are weak, not from a single painful groove stimulus.
| Finding | Radial neuropathy at spiral groove | Posterior interosseous (PIN) | C7 radiculopathy |
|---|---|---|---|
| Superficial radial SNAP (snuffbox) | May be low or absent with sensory axon loss; can remain normal in a pure demyelinating or partial lesion | Normal (PIN is motor) | Normal (preganglionic root) |
| Brachioradialis | Weak / involved | Spared | Relatively spared (BR is mainly C5–C6) |
| Triceps | Typically spared at the classic spiral groove; weakness points to a more proximal radial / posterior-cord lesion | Spared | Weak (C7) |
| EIP / finger extensors | Weak (PIN fibers still inside radial nerve at the groove) | Weak | May be weak (C7 contribution) |
| Wrist extension | Wrist drop, including ECRL | Often spared ECRL → wrist extends with radial deviation | Variable; not a pure PIN or groove map |
| Other C7 muscles (FCR, triceps reflex, cervical paraspinals) | Not a C7 pattern | Not a C7 pattern | Involved |
| Forearm PIN stimulation to EIP | Response may be present distal to a groove block | Low or absent if the lesion is at arcade of Frohse / distal PIN | Present (root lesion is proximal; SNAP/CMAP patterns follow root rules) |
Spiral groove (“Saturday night” / humeral groove): wrist drop, BR and finger extensors involved. The snuffbox SNAP may fall when sensory axons are lost but can remain normal with a pure demyelinating or partial lesion. Triceps is usually spared in a classic groove lesion; triceps weakness points to a lesion proximal to the groove, such as the axilla or posterior cord.
PIN: finger drop, snuffbox SNAP intact, BR and triceps intact, possible radial-deviation wrist extension via ECRL.
C7: SNAPs intact, triceps and other C7 muscles involved, BR relatively spared, not a pure radial sensory loss in the snuffbox. Needle EMG of paraspinals and non-radial C7 muscles (for example FCR) is how the needle exam finishes that distinction; the conduction tray's job is to show that the snuffbox SNAP is present and that a groove conduction block is not the explanation.
Stimulation pitfalls: pain and volume conduction
Spiral-groove stimulation is uncomfortable. The nerve is against bone, under triceps. Patients withdraw, the probe slips, and you store a submaximal CMAP that looks like conduction block. Warn, stabilize the arm, and confirm supramaximal by a plateau, not by the patient's grimace.
Volume conduction is the second wrecker. High current at the groove or axilla can co-stimulate ulnar, median, or brachial plexus. If G1 is too proximal on the forearm, EIP recording becomes a generic extensor-compartment CMAP. An Erb's-point stimulus can fire the entire upper plexus—do not label that a selective radial CMAP. An initial positivity at EIP often means you are recording volume from a nearby muscle or the stimulator is not on radial.
Other radial pitfalls: anomalous or variant branching of superficial radial; edema filling the snuffbox; a forearm cast window that forces a nonstandard distance; cold limbs slowing distal PIN latency. Measure forearm distance with the tape on the dorsal interosseous path, not around the ulnar styloid in a loop.
Building the radial tray
A practical radial set is: (1) EIP motor from forearm PIN stimulation, (2) a proximal radial site at the elbow or spiral groove when amplitude drops or the question is groove versus PIN, (3) axilla or Erb only when the map still does not close, and (4) superficial radial SNAP in the snuffbox on both sides. Add a BR recording or a careful clinical BR/triceps check when PIN versus groove is the live question. Document distance, temperature, elbow/shoulder position, and that the patient was warned about groove stimulation. That is the protocol; the next interpretive step is matching the table above to the traces you actually stored.
A posterior interosseous neuropathy is distinguished from radial neuropathy at the spiral groove because PIN:
Which pattern best supports C7 radiculopathy rather than radial neuropathy at the spiral groove?
Routine superficial radial sensory recording is placed: