A spasm is a a sudden, involuntary contraction of a muscle or a group of muscles. Hemifacial means half of the face. The muscles of the face are all controlled by the facial nerve. There is a facial nerve for each side of the face. It starts deep inside your brain and makes its way past many structures to reach the face. The facial nerve carries signals from the brain to make your facial muscles contract or relax. For example, the facial nerve makes you blink, twitch your nose and pout your lips. If something presses on the nerve somewhere along its course, it can affect how the signals are carried to the face. This may cause muscles to twitch (or contract), or to go into spasm, when you don't want them to.
How common is it ?
Hemifacial spasm is a rare condition. In the UK there are thought to be about 4,000 people with hemifacial spasm. It affects men and women, although women tend to be affected slightly more often than men. Symptoms usually start in middle age.
What causes it ?
The cause of hemifacial spasm is not fully understood. At the moment, doctors believe the main cause is pressure on the facial nerve from a structure or abnormality within the brain. The most common finding is a blood vessel at the base of the brain, pressing on the nerve. There are other, rare causes too such as infections or strokes. Sometimes there is no obvious cause and doctors may then call it idiopathic hemifacial spasm. Idiopathic means 'of unknown cause'.
Is it inherited ?
Because the causes of hemifacial spasm are not inherited, it is unlikely that you will pass the condition on to your