Carpal tunnel syndrome (CTS) is caused by the compression of the median nerve, which runs through a small channel in the wrist on the palm side. Under normal circumstances, there is very little pressure on the median nerve because the carpal tunnel is inflexible. It is surrounded by bone on three sides and a tough ligament on the fourth side.
People with CTS experience numbness, tingling, and pain in the first three fingers of the affected hand (or hands). The pinky finger is usually spared, which often provides a valuable clue in the diagnosis of the condition.
CTS is the most common peripheral nerve compression syndrome, affecting about 2.1 million American adults (Chung 2003; Pritsch 2004). It tends to be more prevalent among women than men. Any activities that involve highly repetitive use of the hands, especially flexion of the fingers, can result in CTS. People at risk include those who use computers, as well as carpenters, grocery checkers, assembly-line workers, meat packers, violinists, pianists, and mechanics. Hobbies such as gardening and needlework can sometimes bring on the symptoms, while sports such as rowing, golf, tennis, downhill skiing, archery, competitive shooting, and rock climbing also place pressure on the hand and wrist joints. In addition, the syndrome can be caused by underlying disorders that affect the carpal tunnel, including arthritis, thyroid problems, gout, and diabetes. Finally, pregnant women are at risk of developing CTS.
The nerve compression associated with CTS is due to fibrous bands of tissue that form inside the carpal tunnel, squeezing the median nerve. Although CTS is linked to repetitive stress, the underlying cause—which would explain why some people suffer from it and others don't—is unknown. Newer research has uncovered some of the chemical changes that occur in response to mechanical injury among people who suffer from CTS. Although CTS is technically a non-inflammatory condition (because there is no systemic inflammation and the immune system is not activated), it is characterized by localized increases in many pro-inflammatory chemicals in the tissue of the carpal tunnel itself.
Researchers have discovered that prostaglandin-2, vascular endothelial growth factor, and interleukin-6 are all elevated in the carpal tunnel tissue of people with CTS. These inflammatory factors act directly on tissue by increasing the ability of fluids and small molecules to cross from the blood into the tissue itself, and they may stimulate the growth of fibroblasts, which are responsible for forming scar tissue (Kuhn 2002). However, levels of interleukin-1, a pro-inflammatory chemical, are the same in people with CTS and people without the condition, which implies that those with CTS do not have a full-blown, systemic inflammatory response. Instead, evidence suggests that the underlying cause of CTS may be an increase in specific local inflammatory factors in response to mechanical stress that causes increased vascular permeability and perfusion (or movement of fluid through the tissues of the carpal tunnel), which leads to the deposition of scar tissue (fibrosis) that characterizes CTS (Bland 2005; Hirata 2004; Freeland 2002; Tucci 1997).
Researchers have also uncovered evidence that the condition may be linked to inherited anatomy in the wrist. People with family members who suffer from CTS in both hands are more likely to develop the condition themselves, suggesting there may be a genetic influence or that familial similarities in the size and anatomy of the wrists may cause a predisposition for CTS (Alford 2004).