Pronator Syndrome and Anterior Interosseous Nerve Syndrome Info

What you're feeling

These two conditions both come from the median nerve being squeezed high up, around the elbow and the top of the forearm, rather than down at the wrist where it is more famously pinched (carpal tunnel syndrome). They feel quite different from each other.

Pronator syndrome tends to cause a deep, aching, tiring pain in the front of the forearm, often made worse by repeated twisting of the forearm (turning a screwdriver or a key, wringing out a cloth) or by heavy gripping. You may also feel pins and needles or numbness in the thumb, index and middle fingers, and across the palm and the fleshy base of the thumb as well. Unlike carpal tunnel syndrome, it usually does not wake you at night, and the discomfort is felt more in the forearm than in the hand.

Anterior interosseous nerve (AIN) syndrome is different again. It usually causes little or no numbness. Instead it is a weakness problem: the muscles that bend the very tip of your thumb and index finger stop working properly. The classic sign is being unable to make a normal round "OK" sign: when you try to pinch the tip of your thumb to the tip of your index finger, the pinch collapses into a flat or triangular shape. Some people notice a short period of aching in the arm or shoulder first, and then the weakness appears.

What's actually happening

A single large nerve, the median nerve, runs from your neck, down the arm, across the front of the elbow and into the hand. It carries both feeling (for part of the hand) and power (to several muscles).

In pronator syndrome, the nerve gets pressed or irritated as it passes through the crowded space at the elbow and the top of the forearm. It can be squeezed by a tight band of tissue near the elbow crease, or as it threads between or under the muscles that turn your palm down. Because the squeeze is high up (above the wrist), the symptoms include the palm and base of the thumb, which is the main clue that separates it from carpal tunnel syndrome.

In AIN syndrome, the problem affects a deep branch of the median nerve that carries only power, no feeling, which is why there is weakness but no numbness. In many people this is not a simple "pinch" at all but more like an irritation or inflammation of the nerve (sometimes following a viral illness or for no clear reason), which is part of why it so often recovers on its own.

What we can do about it

For both conditions, the first step is almost always non-surgical, and for AIN syndrome in particular that patience is usually rewarded.

Settling things down. This means modifying the activities that aggravate it (easing off the repeated twisting and heavy gripping), sometimes using a splint or brace to rest the arm, and giving the nerve time. Simple pain-relieving measures help with the ache.

Watchful waiting. AIN weakness very often improves by itself over several months, so the usual plan is to watch and wait, frequently for around three to six months, before considering anything more. Many people regain their strength in that window without an operation. Tests such as nerve studies or a scan may be arranged along the way to confirm the diagnosis and track recovery.

Surgery is reserved for the minority who do not improve: pronator syndrome that stays painful despite a fair trial of rest and activity change, or AIN weakness that shows no sign of recovering after a period of observation. The operation releases the tight structures pressing on the nerve so it can recover.

What to expect

Both conditions generally have a good outlook. AIN syndrome in particular has a strong tendency to get better on its own, which is exactly why we don't rush to operate. Recovery is gradual, measured in months rather than weeks, so it takes some patience, and your progress will be reviewed along the way.

When non-surgical treatment isn't enough and an operation is needed, releasing the nerve typically eases the pain of pronator syndrome and gives the best chance of strength returning in AIN syndrome, although nerves recover slowly and the improvement continues to build for many months afterwards.

When to see someone

• Aching forearm pain with numbness or tingling in the thumb, index and middle fingers, especially if you also feel it in the palm and base of the thumb, and it is worse with twisting or gripping.
• Sudden difficulty making a normal "OK" sign, or weakness pinching the tip of the thumb against the index finger.
• Forearm or hand symptoms that don't settle with rest and activity changes, or weakness that is not recovering after a few months.
• A short bout of shoulder or arm aching followed by hand weakness, worth having assessed, as this pattern can point to a nerve problem that benefits from being identified early.