Median Nerve Compression: It's Not Just Carpal Tunnel Syndrome
Many people have heard of the median nerve because when it is compressed or injured, carpal tunnel syndrome (CTS) occurs. But the median nerve that's involved with carpal tunnel syndrome actually starts way up in the upper arm. It travels all the way down to the wrist and continues on to the hand. Compression or irritation of the median nerve causing pain, numbness, tingling and other symptoms of peripheral neuropathies can occur anywhere along its path.
Carpal tunnel syndrome just happens to be one of the most common median nerve entrapments. It occurs when the nerve is compressed in the wrist. Less often, compression of the median nerve occurs in the forearm. The result can be one of two other median nerve entrapments: pronator syndrome and anterior interosseous nerve syndrome. These peripheral neuropathies are the focus of this review article.
To help us understand these two peripheral neuropathies, the authors provide a mini-anatomy lesson. The pathway of the median nerve from upper arm to hand is described in detail with drawings to illustrate its location along the way.
To appreciate how these syndromes develop, it's helpful to know that midway down the forearm, the median nerve divides to form a branch called the anterior interosseous nerve (AIN). The anterior interosseous nerve has no sensory branch. It only controls movement of the flexor muscles on the inside of the forearm. It's this nerve that makes it possible for you to make the OK sign with your thumb and index finger or hold a piece of paper between the thumb and index finger.
The median nerve divides again at the wrist crease just past the carpal tunnel. Here it forms the palmar cutaneous sensory nerve. This nerve gives the palm of your thumb and first two fingers sensation. The hows and whys of median nerve compression in the forearm aren't fully understood. For example, why do some people develop one type of neuropathy but not the other? Why do some people have either problem when others have no trouble with compression?
One explanation for the anterior interosseous nerve syndrome rests again with the anatomy. There may be differences in the bone, muscle, and tendon structures that can put pressure on the median nerve in various locations.
Nerve entrapment from some type of odd anatomical variation is a common cause of peripheral neuropathies. For example, a larger or wider bony bump along the inside of the elbow might be a factor. If the nerve passes close to this bump and gets trapped between the unusual shaped-bone and the ligament of Struthers, nerve entrapment occurs. The ligament of Struthers is a fibrous band that isn't part of everyone's anatomy.
In the case of pronator syndrome, the median nerve gets the pinch when it passes through two sides of the pronator teres muscle, which helps explain the name. Tumors, bone spurs, or other space-occupying lesions can also contribute to the development of either of these neuropathies.
What can be done about these problems? Well, truly, the first step is to make sure the diagnosis is correct. And that can take some time and a bit of sleuthing along with additional studies. There's a lot of overlap in symptoms among the many neuropathies. Getting to the bottom of the problem and making sure the correct location of compression has been found can be a challenge.
The hand surgeon has a variety of clinical tests at his or her disposal to use when sorting out what's going on. MRIs, nerve conduction studies, and electromyography (EMGs) can help pinpoint the problem. Management depends on the cause. Whenever possible, rest, immobilization, and hand therapy are used to avoid surgery. But sometimes surgery is the only treatment that can change the anatomical features at fault in order to relieve painful symptoms.
The surgeon will perform the least invasive procedure but it may be necessary to release the connective or scar tissue around the nerve. Sometimes, soft tissue structures around the nerve (e.g., tendon, muscle) must also be cut in order to remove the compressive forces exerted by those moving parts. This procedure is called a nerve decompression.
Who should have surgery and when remain two points of debate among hand surgeons. There just isn't enough evidence to provide specific guidelines. Some experts say 12 weeks of hand therapy with no change is the signal to schedule surgery. Others report recovery is possible as much as 12-months after the symptoms started. And there are even observations that patients get better with no treatment of any kind, including conservative (nonoperative) care.
Without treatment, there is a risk that patients with either of these neuropathies may develop permanent paralysis and loss of function. That's not an acceptable outcome. The authors suggest that the final results may be an indication of what was causing the problem in the first place. For example, viruses affecting the nerve can cause symptoms very much like compression. It's possible that the folks who get better over time really had an inflammatory, not a compressive, cause of the problem.
The authors conclude that more studies are needed to determine what type of treatment is best for patients with anterior interosseous or pronator syndrome. The results of these studies could help determine who should have surgery, how soon to do the surgery, and the ideal length of time to try nonsurgical approaches before turning to decompressive surgeries.
Alan C. Dang, MD, and Craig M. Rodner, MD. Unusual Compression Neuropathies of the Forearm, Part II: Median Nerve. In Journal of Hand Surgery. December 2009. Vol. 34A. No. 10. Pp. 1915-1920.