Lacertus syndrome is a compression of the median nerve at the elbow, causing hand weakness, fatigue, and forearm pain. We often refer to it as proximal median nerve entrapment. It’s frequently missed — and often mistaken for carpal tunnel syndrome, because both involve the same nerve. So what is lacertus syndrome, and how do we diagnose and treat it?

What is lacertus syndrome? 

The median nerve runs down the arm and into the forearm, passing under a tough fibrous band just below the elbow called the lacertus fibrosus (a sheet of tissue extending from the biceps tendon). When the nerve is compressed at this point, the result is lacertus syndrome — a proximal median nerve entrapment.

It’s helpful to think of it as the elbow-level counterpart of carpal tunnel syndrome: carpal tunnel is compression of the median nerve at the wrist; lacertus syndrome is compression of the same nerve at the elbow.

anatomy of lacertus fibrosis

Why is lacertus fibrosis syndrome so often missed? 

This is the key point. Lacertus fibrosis syndrome is commonly missed and misdiagnosed as carpal tunnel syndrome — studies suggest a large proportion of people labelled with “carpal tunnel” who didn’t fully respond to treatment actually have lacertus syndrome, or have both. Two things make it easy to overlook:

  • The symptoms overlap with carpal tunnel, but the pattern is different — weakness and fatigue rather than numbness
  • The usual nerve tests (EMG/nerve conduction studies) are often normal in lacertus syndrome, so a normal test doesn’t rule it out

So if “carpal tunnel” symptoms persist after carpal tunnel release, the elbow should be examined for lacertus syndrome. We often refer to this syndrome as proximal median nerve entrapment. 

Symptoms of lacertus fibrosis syndrome

The picture of proximal median nerve entrapment is dominated by weakness and fatigue, more than numbness:

  • Loss of grip and pinch strength — a weak key-pinch and tip-pinch
  • Hand fatigue and clumsiness — dropping objects, losing fine motor control
  • Forearm pain, at the front of the elbow/upper forearm
  • Sometimes some tingling or numbness in the median-nerve part of the hand, but this is usually not the main complaint

Repetitive forearm rotation makes it worse, so it’s seen in athletes, manual workers, dentists, and surgeons.

How is lacertus syndrome diagnosed?

The diagnosis is largely clinical, because the tests are often normal. The key findings (the recognised clinical triad) are:

  • Weakness of the three median-nerve muscles below the lacertus — the flexor carpi radialis, flexor pollicis longus (thumb tip), and the flexor to the index finger (FDP-II). Patients aren’t able to form an ‘OK’ sign. 
  • Tenderness and a positive Tinel’s sign (tingling) when pressing over the median nerve at the lacertus fibrosus

Investigations support, rather than make, the diagnosis:

  • Ultrasound — useful for dynamic imaging of the median nerve under the lacertus, although the nerve often looks normal
  • MRI — can show nerve signal changes, or exclude other causes such as pronator teres syndrome
  • Nerve conduction studies — frequently normal, which does not exclude the diagnosis

Is it lacertus fibrosis syndrome or pronator teres syndrome?

They’re closely related and can look similar — both cause forearm pain, median-nerve weakness, and altered hand sensation. The difference is the site of compression: in pronator teres syndrome the nerve is compressed a little lower, between the two heads of the pronator teres muscle, whereas in lacertus syndrome it’s under the lacertus fibrosus just below the elbow. A careful examination localises which is which.

How is lacertus syndrome treated?

Treatment starts conservatively:

  • Activity modification — avoiding repetitive forearm loading and rotation
  • Physiotherapy — improving forearm flexibility and mechanics, and nerve gliding
  • Anti-inflammatory medication — for temporary relief

Where symptoms persist, surgical release of the lacertus fibrosus is effective. It’s a small procedure, increasingly done minimally invasively — sometimes wide-awake under local anaesthetic (WALANT) — with good outcomes.

Frequently asked questions about lacertus syndrome.

How is lacertus syndrome different from carpal tunnel syndrome?

Both are compressions of the median nerve, but at different levels — carpal tunnel at the wrist, lacertus at the elbow. Carpal tunnel is dominated by numbness and tingling; lacertus syndrome by weakness, fatigue, and dropping things. They can coexist, and lacertus is often missed — so persistent symptoms after carpal tunnel release should prompt an elbow assessment.

Why is my nerve test normal if I have lacertus syndrome?

Because nerve conduction studies are frequently normal in lacertus syndrome — the diagnosis is made clinically, from the pattern of weakness, tenderness at the lacertus, and provocative tests. A normal test result is expected and doesn’t rule out the condition, which is one reason it’s so often missed.

Can lacertus syndrome be treated without surgery?

Yes, in many cases — with activity modification, physiotherapy, and sometimes an ultrasound-guided injection. Where these don’t settle it, a lacertus release (often minimally invasive, sometimes performed under local anaesthetic, sometimes awake) is effective, and recovery is usually quick.

What causes lacertus syndrome?

Repetitive forearm rotation and loading are the main drivers, which is why it’s seen in athletes, manual workers, dentists, and surgeons. It can also follow trauma or occur alongside carpal tunnel syndrome. The compression happens where the median nerve passes under the lacertus fibrosus just below the elbow.

Final word from Sport Doctor London about lacertus syndrome

Lacertus syndrome is a proximal median nerve compression at the elbow, and one of the most commonly missed nerve entrapments — frequently mistaken for carpal tunnel syndrome. The clues are hand weakness and fatigue rather than numbness, tenderness at the lacertus, and normal nerve tests. Most improve with conservative treatment, and a minimally invasive release works well for the rest. If “carpal tunnel” symptoms haven’t settled as expected, it’s worth considering lacertus syndrome.

If you have hand weakness, fatigue, 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.

Other related nerve conditions: