Radial Tunnel and Posterior Interosseous Syndrome
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Direct answer
Radial tunnel syndrome is compression of the radial nerve's deep branch in the proximal forearm, usually at the arcade of Frohse, producing lateral elbow pain without weakness. Posterior interosseous nerve (PIN) syndrome is the motor version of the same entrapment, with weakness of finger extension and a finger drop. Wrist extension is preserved in PIN palsy, which is the key difference from a radial nerve palsy at the spiral groove.
What you must remember
- Radial tunnel: the space in front of the radiocapitellar joint, containing the deep branch of the radial nerve as it passes towards the supinator.
- Arcade of Frohse: a fibrous arch at the proximal edge of supinator, the commonest entrapment site for the PIN.
- Radial tunnel pain: dull pain over the lateral proximal forearm and elbow, without weakness or sensory loss, often mistaken for lateral epicondylitis.
- Middle finger test: resisted extension of the middle finger, with the elbow extended, reproduces pain over the proximal dorsal forearm in radial tunnel syndrome.
- PIN syndrome: a pure motor lesion with weakness of finger and thumb extension, causing a finger drop while the wrist deviates to the radial side on extension.
- Sparing: extensor carpi radialis longus and brevis are supplied before the PIN, so wrist extension is preserved (radial deviation, as extensor carpi ulnaris is paralysed), and there is no sensory loss.
- Causes: fibrous bands, lipoma, rheumatoid synovitis and radial head fractures or dislocation, the last a classic cause in Monteggia injuries.
Comparing radial tunnel syndrome and PIN syndrome
Both conditions involve the same nerve in the same region, and the presence or absence of weakness separates them. The table below sets out the features that examiners test most often.
| Feature | Radial tunnel | PIN syndrome |
|---|---|---|
| Weakness | Absent | Finger extension weak |
| Finger drop | Absent | Present |
| Wrist extension | Normal | Preserved, with radial deviation |
| Sensory loss | Absent | Absent |
| Main symptom | Lateral forearm pain | Motor loss, sometimes pain |
The essential question is whether the patient can extend the fingers. A patient with pain and full power has radial tunnel syndrome, while a patient with finger drop has a motor lesion and needs nerve assessment and early reconstructive decisions.
In the clinic, the examiner asks the patient to extend the fingers at the metacarpophalangeal joints with the wrist held straight, then tests wrist extension with the fingers relaxed. Preserved wrist extension with a radial shift confirms that the lesion lies beyond the branches to the radial wrist extensors.
Because the PIN is a purely motor nerve, sensation over the dorsum of the hand is normal in both conditions. This absence of sensory loss is an important clue that the radial sensory branch, which is superficial, is not involved.
Where the exam tries to catch you
Radial nerve questions are usually decided by reading the difference between pain and weakness, and by knowing which muscles are spared.
- Trap: calling a finger drop radial tunnel syndrome. Correct: finger drop is a motor deficit, so it indicates PIN syndrome, while radial tunnel syndrome causes pain without weakness.
- Trap: expecting wrist drop in PIN palsy. Correct: the radial wrist extensors are spared because they are supplied before the PIN, so the wrist extends and deviates to the radial side.
- Trap: looking for sensory loss in PIN palsy. Correct: the PIN is purely motor, so sensation is normal.
Frequently asked questions
What is the arcade of Frohse?
The arcade of Frohse is a fibrous arch at the proximal edge of the supinator muscle. It is the commonest site of entrapment of the posterior interosseous nerve. Compression at this site causes radial tunnel pain or PIN palsy, depending on the severity of nerve involvement.
Why is wrist extension preserved in posterior interosseous nerve palsy?
Wrist extension is preserved in posterior interosseous nerve palsy because the extensor carpi radialis longus and brevis are supplied before the nerve divides. Their branches leave the radial nerve above the posterior interosseous nerve. The wrist therefore extends but deviates to the radial side.
How does radial tunnel syndrome differ from lateral epicondylitis?
Radial tunnel syndrome causes pain over the proximal lateral forearm, often with a positive middle finger test, while lateral epicondylitis causes pain at the lateral epicondyle with grip-related pain. Radial tunnel syndrome has no weakness or sensory loss. Clinical testing and sometimes injection help differentiate the two.
Which clinical tests help diagnose radial tunnel syndrome?
The middle finger test helps diagnose radial tunnel syndrome, as resisted extension of the middle finger with the elbow extended reproduces pain. Resisted supination with the elbow extended may also reproduce pain. Nerve conduction studies are often normal in radial tunnel syndrome, so the diagnosis rests mainly on clinical findings.