12.3 Radial Neuropathy (Saturday Night Palsy) and Related UE Mononeuropathies
Key Takeaways
- Saturday night palsy is radial compression at the spiral groove: brachioradialis is involved, triceps is often relatively spared, and EIP CMAP may drop across the groove.
- Superficial radial SNAP may be low when the groove lesion is axonal and may remain normal in pure neurapraxia because the recording is distal to the block.
- Posterior interosseous neuropathy is motor only: superficial radial SNAP spared, brachioradialis spared, finger extensors weak, and wrist extension often radially deviated.
- C7 radiculopathy spares SNAPs and often involves triceps plus pronator teres and flexor carpi radialis and possibly paraspinals; posterior cord adds deltoid and axillary involvement to a radial pattern.
- Pronator teres syndrome and anterior interosseous neuropathy are median-forearm localizations; AIN spares the median SNAP and APB, so it is not carpal tunnel.
VI.B.5.d on the outline is radial nerve (Saturday Night Palsy). The same sitting is where you must not confuse a spiral-groove radial lesion with posterior interosseous neuropathy (PIN), C7 radiculopathy, or a posterior cord lesion — and where you must not label every median abnormality as carpal tunnel when pronator teres or anterior interosseous (AIN) patterns fit better. Electrode sites for radial motor to extensor indicis proprius (EIP) and superficial radial SNAP were the previous chapter; this section is where the drop is and which neighbors must be spared.
Saturday night palsy: spiral groove
Compression of the radial nerve against the humerus in the spiral (radial) groove — sleeping with the arm over a chair back, “honeymoon” compression, or, more proximally, poorly placed crutches — produces acute wrist drop and finger drop. Sensory complaint in the anatomical snuffbox / dorsal first web is common but not required.
Triceps branches leave at or proximal to the spiral groove. In a classic groove lesion, elbow extension and the triceps reflex are spared or only mildly involved; the exact picture depends on how proximal the compression sits. Brachioradialis (BR) is supplied by the radial nerve after the groove and before the PIN takeoff, so BR is typically weak in Saturday night palsy and strong in PIN. Wrist extensors and finger extensors, including EIP, are weak.
Radial NCS interpretation
A standard motor montage records EIP (or another distal radial/PIN muscle). Stimulate in the forearm (distal PIN), at the elbow, and in the upper arm above the spiral groove. Conduction block or focal slowing across the spiral groove with a relatively preserved distal EIP CMAP is the neurapraxic Saturday-night signature — analogous to ulnar block at the elbow: the hand is dropped, but distal axons may still conduct.
If the lesion has axonal loss, distal EIP CMAP amplitude falls (compare with the other side), and needle EMG of BR, EIP, and extensor digitorum shows fibrillations after an appropriate delay. Superficial radial SNAP (snuffbox recording) may be low when postganglionic sensory axons are lost at or proximal to the superficial radial takeoff at the elbow — fibers that were already in the radial nerve at the spiral groove. In pure demyelinating neurapraxia, the SNAP may remain normal because the recording is distal to the block and axons are intact. A normal SNAP therefore does not exclude spiral-groove palsy; a low SNAP argues for axonal involvement of those sensory fibers (or a more proximal radial or posterior-cord lesion) once temperature and side-to-side baselines are considered.
Triceps EMG that is frankly abnormal pushes the lesion toward the axilla or posterior cord (or a very proximal groove lesion). Deltoid must be checked before you stop at “radial nerve.” Isolated superficial radial sensory loss at the wrist (Wartenberg / cheiralgia paresthetica, tight watch band, or forearm trauma) drops the SNAP with normal EIP CMAP, BR, and PIN muscles — a SNAP-only radial problem, not Saturday night palsy.
PIN: motor only, SNAP spared, brachioradialis spared
The radial nerve divides near the elbow into the superficial radial (sensory) and PIN (motor). PIN dives under the arcade of Frohse / supinator. PIN neuropathy (radial tunnel, ganglion, Monteggia injury, idiopathic) produces finger and thumb extension weakness. Wrist extension may persist with radial deviation because extensor carpi radialis longus (ECRL) is usually innervated before the PIN and remains strong, while extensor carpi ulnaris is PIN-innervated and weak.
Critical contrasts with spiral-groove radial neuropathy:
- Superficial radial SNAP is spared (the lesion is distal to the sensory branch).
- Brachioradialis is spared.
- Triceps is spared.
- Sensory symptoms in the snuffbox are absent.
- Motor conduction change, if present, is across the elbow/supinator segment, not the spiral groove.
If you see wrist drop, a low superficial radial SNAP, and a weak BR, it is not an isolated PIN.
Case: Saturday night versus PIN
A 34-year-old wakes after a wedding with complete right wrist and finger drop. BR is weak; triceps is strong. Superficial radial SNAP is 18 µV (left 22 µV). EIP CMAP is 5.5 mV with forearm and elbow stimulation but 1.2 mV stimulating above the spiral groove, without a large duration increase. Deltoid is strong; pronator teres and APB are normal.
Interpretation: radial neuropathy at the spiral groove with conduction block (Saturday night palsy), little axonal loss yet (SNAP and distal CMAP relatively preserved). Not PIN (BR is weak; the block is at the groove). Not C7 (BR is largely C5–C6; neck, pronator teres, and flexor carpi radialis are quiet). Not posterior cord (deltoid is spared).
Pronator teres and AIN: median forearm, not CTS
Lock in two median forearm patterns so you do not stamp every median abnormality as carpal tunnel.
Pronator teres (proximal median) syndrome compresses the median nerve near the two heads of pronator teres, the lacertus fibrosus, or the sublimis arcade. Sensory fibers to digits 1–3 and often the palmar cutaneous branch (thenar eminence) can be involved — unlike CTS, which typically spares palmar-cutaneous skin. Median SNAP may be low or slow in the forearm, but palmar mixed median-versus-ulnar differences at 8 cm can be unimpressive if the wrist segment is not the site of slowing. EMG may involve flexor carpi radialis, flexor pollicis longus, and APB; pronator teres itself may be spared if its branch left before the compression. Forearm median slowing with normal wrist comparative studies is a proximal-median clue, not “failed carpal tunnel NCS.”
Anterior interosseous neuropathy (AIN) is motor only: flexor pollicis longus, FDP to digits 2–3, and pronator quadratus. The patient cannot make a firm OK sign (FPL plus FDP II). Median SNAP is normal. APB is normal. Palmar mixed studies are normal. Calling this carpal tunnel because “the median nerve is involved” is a classic exam error. AIN can follow forearm fracture, excessive exercise, or neuralgic amyotrophy; the localization remains the forearm motor branch, not the wrist.
Localization table: spiral groove versus PIN versus C7 versus posterior cord
| Feature | Spiral-groove radial (Saturday night) | PIN | C7 radiculopathy | Posterior cord |
|---|---|---|---|---|
| Triceps | Often spared or less involved | Spared | Often weak; reflex may fall | Involved |
| Brachioradialis | Involved | Spared | Usually spared (C5–C6) | Involved |
| EIP / finger extensors | Involved | Involved | Variable C7 contribution | Involved |
| Wrist extension | Weak | Often radial-deviated (ECRL spared) | Variable | Weak |
| Superficial radial SNAP | Low if axonal; may be normal if neurapraxia | Normal | Normal (preganglionic) | Low if axonal |
| Deltoid / axillary SNAP | Normal | Normal | Usually normal | Abnormal |
| Cervical paraspinal EMG | Normal | Normal | May be abnormal | Normal |
| Pronator teres / FCR | Normal | Normal | Often involved | Normal |
| Snuffbox sensation | May be numb | Not numb in PIN distribution | Middle finger / neck more typical | Radial plus lateral shoulder |
Case: C7 that fooled a wrist-drop label
A 50-year-old has finger-extension weakness and triceps aching after a cervical flare. Superficial radial SNAP is normal (as is median digit-3 SNAP). EIP CMAP is mildly low without a spiral-groove block. EMG: triceps, flexor carpi radialis, and pronator teres show fibrillations; BR is normal; cervical paraspinals show fibrillations; deltoid is normal.
Interpretation: C7 radiculopathy, not Saturday night palsy. SNAPs spared, BR spared, pronator teres and flexor carpi radialis involved, paraspinals involved. A posterior cord lesion would threaten deltoid and often the radial SNAP together. Independent OpenExamPrep teaching: wrist drop is a phenotype, not a nerve name.
Case: posterior cord
Crutch palsy that is too proximal, or a poorly placed humeral plate, can hit the posterior cord: radial plus axillary (deltoid, teres minor) plus thoracodorsal (latissimus). Triceps is reliably weak. Superficial radial SNAP may fall. If deltoid is weak and an axillary SNAP is low beside a radial pattern, do not stop the study at EIP and call it Saturday night palsy — the lesion is proximal to the radial nerve proper.
Case: AIN mislabeled as severe CTS
A 38-year-old cannot flex the thumb interphalangeal joint or the index distal interphalangeal joint after forearm exertion. Palmar mixed median-versus-ulnar comparison is normal. Median SNAP is normal. APB CMAP and EMG are normal. FPL and pronator quadratus show fibrillations. That is AIN, not axonal carpal tunnel. Thenar strength and wrist comparative studies keep the median nerve at the wrist off the hook.
Name the site of conduction failure (spiral groove, PIN, C7 root, posterior cord, median forearm, wrist, elbow), the pathophysiology (block versus axonal loss), and the structures that must be spared for that site to be true. That is the entire upper-extremity mononeuropathy exam skill this chapter is built to teach.
The typical Saturday-night radial pattern at the spiral groove is:
Which set distinguishes posterior interosseous neuropathy from spiral-groove radial neuropathy?
Palmar mixed median-versus-ulnar comparison is normal, APB CMAP is normal, median SNAP is normal, and the patient cannot flex the thumb interphalangeal joint or the index distal interphalangeal joint. The localization is: