Causes of carpal tunnel syndrome

The symptoms of carpal tunnel syndrome (CTS) are caused by compression (squashing) of the median nerve at the wrist.

The median nerve is responsible for two main functions:

• relaying physical sensations – such as your sense of touch from your hand to your brain
• relaying nerve signals – from your brain to your hand, allowing you to move your thumb

Any pressure on the median nerve can disrupt the nerve signals, affecting your sense of touch and hand movements. The median nerve can become compressed if the tissues inside the tunnel become swollen or the tunnel becomes narrower.

Increased risk

In most cases, it is not known what causes the median nerve to become compressed, although a number of things increase the risk of developing CTS. These include:

• family history
• certain health conditions, such as diabetes and an underactive thyroid gland
• pregnancy
• certain injuries to the wrist
• certain activities

These risk factors are outlined below.

Family history

Research has shown that there may be a genetic link to CTS. This means you may have an increased risk of developing CTS if other members of your family have the condition or have had it in the past.

About one in four people with CTS have a close relative, such as a parent, brother or sister, who also has the condition. It is not fully understood how and why the condition is passed on through families.

Health conditions

Certain health conditions appear to increase your risk of developing CTS. These include:

• type 1 diabetes and type 2 diabetes – a chronic (long-term) condition caused by having too much sugar (glucose) in the blood
• rheumatoid arthritis – a condition where the joints become painful and inflamed as a result of the immune system attacking the body
• gout – a common type of arthritis that causes pain and swelling in one or more joints
• hypothyroidism – an underactive thyroid gland
• obesity – particularly in young people
• oedema – an excess build-up of fluid in the body’s tissue

Less commonly, CTS develops when a person has an abnormal wrist structure, such as an unusually narrow carpal tunnel, or as a result of cysts, growths or swellings in the tendons or blood vessels that pass through the carpal tunnel.

Pregnancy

It is not clear exactly why, but CTS is common during pregnancy. However, many cases resolve after the baby is born. It is not known whether women who have carpal tunnel syndrome during pregnancy are at greater risk of developing the condition in later life.

CTS is also very common in women around the time of the menopause and in women treated with certain breast cancer drugs.

Injuries

CTS can sometimes occur following a hand injury. Injuries such as sprains, fractures and crush injuries can cause swelling, placing pressure on the median nerve.

These types of injuries can also change the natural shape of the bones and ligaments in the hand, leading to increased pressure on the median nerve.

Certain activities

Certain activities may trigger the development of carpal tunnel syndrome. These tend to involve strenuous grip, repetitive wrist flexion and exposure to vibration. Examples include:

• playing a musical instrument
• assembly packing
• work that involves manual labour 
• work with vibrating tools, such as chainsaws

However, further research into the link between work-related hand use and CTS is required to determine whether these types of activities are a definite cause of the condition.

Although much attention is paid to typing as a possible cause of carpal tunnel syndrome, the condition is in fact less common in individuals who type all day than those who carry out more strenuous activities.