Atrophy or wasting of the thumb (thenar) muscles is the last stage of nerve injury in carpal tunnel syndrome.
Hand muscles bulge underneath the skin of your hand. The most obvious one is on the palm side of the thumb at the thumb base. These are the thenar muscles. Some or all of them may be supplied by the median nerve, the nerve affected by carpal tunnel syndrome.
The median nerve contains sensory and motor nerve fibers – they supply finger sensation and muscle function to the thumb. As more pressure is put on the nerve and carpal tunnel syndrome goes on long enough, the sensory nerve fibers are compressed and damaged first, followed by the motor nerve fibers. That’s why muscle atrophy is a bad sign – the sensory fibers have already been damaged for a long time when you see muscle wasting.
Once the muscles have atrophied, even surgery may not help the nerve recover. Despite this fact, carpal tunnel surgery relieves pain (especially night-time pain) for many people with chronic carpal tunnel syndrome who have muscle atrophy.
To illustrate exactly what I’m talking about, I created this short video on muscle atrophy.
The damaged muscles have a very specific function – opposition. This means lifting the thumb out and away from the palm of the hand. This motion is essential to let you get your hand around large objects, like a 2 liter bottle or to hold onto a large stack of books.
Tendon transfer surgery is the only way of restoring this function after the muscles have been damaged. Many types of tendon transfers have been described to treat this type of muscle atrophy. They involve moving an expendable tendon from one part of the hand and attaching that tendon to the thumb to replace the missing function. Surgeons use a term called opponensplasty to describe this operation. It makes sense, right? It restores that function I mentioned earlier – opposition.