There are 3 main types of trigeminal neuralgia:

• classical trigeminal neuralgia – which is caused by pressure on the trigeminal nerve; this nerve provides sensation to the face
• secondary trigeminal neuralgia – which is caused by another medical condition or problem
• idiopathic trigeminal neuralgia – this term is used when the cause is unknown

Classical trigeminal neuralgia is caused by pressure on the trigeminal nerve close to where it enters the brain stem.

The brain stem is the lowest part of the brain that merges with the spinal cord.

In most cases the pressure is caused by an artery or vein squashing (compressing) the trigeminal nerve. These are normal blood vessels that happen to come into contact with the nerve at a particularly sensitive point.

It's not clear why this pressure can cause painful attacks in some people but not others, as not everyone with a compressed trigeminal nerve will experience pain.

It may be that, in some people, the pressure on the nerve wears away its protective outer layer (myelin sheath), which may cause pain signals to travel along the nerve. However, this does not fully explain why some people have periods without symptoms (remission), or why pain relief is immediate after a successful operation to move the blood vessels away from the nerve.

Secondary trigeminal neuralgia is the term used when trigeminal neuralgia is caused by another medical condition or problem, including:

• a tumour
• a cyst – a fluid-filled sac
• arteriovenous malformation – an abnormal tangle of arteries and veins
• multiple sclerosis (MS) – a long-term condition that affects the nervous system
• facial injury
• damage caused by surgery including dental surgery