Tennis elbow — extensor tendonitis of the elbow — is a common overuse injury in active people. But not all pain on the outside of the elbow and forearm is tennis elbow. One rare cause of lateral elbow and forearm pain is radial tunnel syndrome, also known as posterior interosseous nerve syndrome. So what is radial tunnel syndrome, and what can you do about it?
What is radial tunnel syndrome?
Radial tunnel syndrome is a compression, or pinching, of the branches of the radial nerve as they pass through the outer side of the elbow. The radial nerve starts high up in the brachial plexus and passes down the upper arm to the elbow. At the outer elbow, it splits into two branches. One supplies the skin of the thumb and the back of the hand. The second — the posterior interosseous nerve — dives deep into the radial tunnel to supply the muscles at the top of the forearm.
It’s this posterior interosseous nerve that becomes compressed in radial tunnel syndrome, which is why the condition is also called posterior interosseous nerve syndrome.
What causes radial tunnel syndrome?
Compression or irritation of the posterior interosseous nerve in the radial tunnel can result from:
- A lump, cyst, or ganglion in the radial tunnel
- Scar tissue in the muscles that make up the radial tunnel
- Osteoarthritis or swelling in the outer elbow joint
- Repeated nerve traction from excessive forearm rotation
That last point explains why certain sports and activities carry a higher risk.
Who gets radial nerve entrapment at the elbow?
The condition is more common in women aged 30 to 50. At-risk sportspeople include tennis players, swimmers, powerlifters, and violinists — all of whom load the forearm through repeated rotation and gripping.
One of the challenges is that the symptoms are often vague and mimic other conditions, so the average time to diagnosis is around two to three years.
Symptoms of radial nerve entrapment at the elbow
People typically experience an aching pain on the outer elbow and the upper forearm. Over time, they find they lose grip strength. This weakness can progress to involve the fingers and thumb, and — in severe cases — a wrist drop develops, where the wrist can’t be held up.
Importantly, sensation in the forearm and wrist is usually normal. This is a useful clue, and part of what separates radial tunnel syndrome from other nerve problems around the elbow.
Radial tunnel syndrome or tennis elbow?
This is the key distinction, because the two are easily confused — and can coexist. Some pointers towards radial tunnel syndrome rather than tennis elbow:
- The most tender point sits a little further down the forearm than the bony point of tennis elbow
- The pain is a deep ache, often aggravated by resisted forearm rotation.
- There may be weakness — loss of grip, and difficulty extending the fingers and thumb — which tennis elbow doesn’t cause
- “Tennis elbow” that hasn’t responded to good treatment is sometimes actually radial tunnel syndrome.
Getting this right matters, because treating one when it’s really the other leads to disappointing results.
How is radial tunnel syndrome diagnosed?
Diagnosis is mainly clinical — based on the pattern of pain and weakness and the location of tenderness. Investigations help confirm it and exclude other causes:
- Nerve conduction studies — to assess the nerve, though they can be normal in milder cases
- Ultrasound — images the nerve and can show a compressing cyst or ganglion.
- MRI — shows the nerve, the surrounding muscles, and any structural cause such as a mass
- A diagnostic injection — local anaesthetic placed around the nerve in the radial tunnel; if it relieves the pain, it helps confirm the diagnosis
How is radial tunnel syndrome treated?
Most cases settle with conservative treatment:
- Activity modification — reducing the repetitive gripping and forearm rotation that aggravate the nerve
- Physiotherapy — nerve-gliding exercises, stretching, and forearm strengthening
- Anti-inflammatories and a splint — a wrist splint rests the forearm muscles and can ease the pain.n
- An ultrasound-guided injection—cortisone around the nerve—can reduce pain and help confirm the diagnosis; nerve hydrodissection is another option, freeing the nerve from surrounding tissue with fluid.
Surgery
In cases that fail simple treatments, we consider decompression surgery of the posterior interosseous nerve. Surgeons perform this surgery either with a small open incision or a keyhole. However, surgical results are not always successful, with only 65% of patients having excellent or good results.
Frequently asked questions about radial nerve entrapment at the elbow?
What’s the difference between radial tunnel syndrome and tennis elbow?
They both cause outer elbow and forearm pain and can occur together. Still, the tender point in radial tunnel syndrome lies farther down the forearm, and the pain is a deep ache that worsens with resisted forearm rotation. Radial tunnel syndrome can also cause weakness — loss of grip and difficulty extending the fingers and thumb — which tennis elbow doesn’t. Persistent “tennis elbow” that hasn’t improved is sometimes actually radial tunnel syndrome.
Why does radial tunnel syndrome take so long to diagnose?
Because the symptoms are often vague and overlap with other elbow problems, particularly tennis elbow — the average time to diagnosis is around two to three years. Being aware that lateral elbow pain isn’t always tennis elbow and looking for the telltale features (weakness, a more distal tender point, pain on resisted rotation) helps reach the diagnosis sooner.
Is the sensation affected in radial tunnel syndrome?
Usually not. Because the posterior interosseous nerve is primarily a motor nerve, sensation in the forearm and wrist is typically normal, and the main problems are pain and, as it progresses, weakness. This helps distinguish it from other nerve problems around the elbow that cause numbness.
Can radial tunnel syndrome be treated without surgery?
Yes, in most cases — with activity modification, physiotherapy, a splint, and sometimes an ultrasound-guided injection or hydrodissection. Surgery (decompression) is reserved for severe or persistent cases, especially where there’s weakness or a structural cause such as a cyst.
How effective is a cortisone injection at relieving pressure on the posterior interosseous nerve?
An injeciton serves two purposes. Firstly, if an injection relieves symptoms, it confirms a diagnosis of radial tunnel syndrome. Secondly, a cortisone injection can reduce the pressure on the nerve and improve symptoms.
How helpful are investigations in diagnosing radial nerve entrapment at the elbow?
Generally, investigations are not great at diagnosing this condition. Sometimes, we see changes in nerve studies that suggest pressure on the radial nerve. Other times, an MRI of the elbow shows swelling and atrophy of the forearm muscles innervated by the radial nerve. However, most of the time, the investigations show nothing. Nonetheless, they exclude other causes of forearm pain and weakness, such as tennis elbow or nerve impingement in the neck.
Final word from Sport Doctor London about radial tunnel syndrome
Radial tunnel syndrome, or posterior interosseous nerve syndrome, is an often-missed cause of outer elbow and forearm pain, easily confused with tennis elbow. The clues are a more distal tender point, pain on resisted forearm rotation, and — as it progresses — weakness with normal sensation. Because it’s frequently mistaken for tennis elbow, the diagnosis is often delayed, so awareness matters. Most people improve with conservative treatment, with surgery reserved for stubborn or severe cases.
If you have persistent outer-elbow or forearm pain, Dr Masci can assess you in London, including ultrasound in clinic. Contact the team here or call +44 (0) 203 488 0350.
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