WHAT IS TRIGEMINAL NEURALGIA?
Trigeminal neuralgia is a common cause of facial pain. It is usually experienced as an episodic, sharp shooting pain in the face. Trigeminal neuralgia may be severe and constant.
What causes trigeminal neuralgia?
Trigeminal neuralgia is usually caused by the compression of the trigeminal nerve. The trigeminal nerve is the cranial nerve which supplies sensation to the face, hence the name ‘trigeminal neuralgia’. This compression is usually the result of a small artery or, less commonly, a vein. These vessels may be identified prior to surgery with an MRI brain scan (magnetic resonance angiography). It is thought that ongoing pulsations from these vessels may be responsible for the pain.
In some cases no vascular compression of the nerve is seen, even at surgery. And in those cases the exact cause of the pain is unclear.
Trigeminal neuralgia may also occur in multiple sclerosis. This is usually a result of damage to the myelin coating on the outside of the nerve fibres (demyelination).
How is trigeminal neuralgia treated?
To begin, trigeminal neuralgia is treated with medication. Nerve membrane-stabilising agents such as carbamazepine (Tegretol) are known to improve the pain, and often provide long-term relief.
When trigeminal neuralgia does not respond to pain medications, more invasive treatments may be considered. Percutaneous targeting of the trigeminal ganglion (through a needle in the cheek) and microvascular decompression (through an operation behind the ear) are the most frequently used surgical approaches.
What are the percutaneous techniques used to treat trigeminal neuralgia?
Percutaneous techniques involve the insertion of a needle into Meckel’s cave through the cheek. Meckel’s cave is a small cavern at the base of the skull which contains part of the trigeminal nerve called the trigeminal ganglion.
This procedure is usually done under local anesthesia (the patient is awake), under X-ray control or with the help of a computerised neurosurgical navigation system (frameless stereotaxy). Once the needle is in the correct position, one of three strategies may then be used:
- Glycerol may be injected around the ganglion (glycerol rhizolysis). This deliberately damages the nerve and can produce pain relief.
- A small balloon may be inflated to compress and damage the ganglion nerve (balloon rhizotomy).
- Radiofrequency ablation may be used. This involves controlled heating of the ganglion nerve using a radiofrequency electrode (radiofrequency rhizolysis).
The benefit of most percutaneous procedures relies upon a ‘trade –off’ of facial numbness. This must be understood and accepted by the patient before the procedure.
Percutaneous procedures have a 70-90% success rate, but the incidence of recurrence after 5 years is significant. The procedure may need to be repeated at that time.
Percutaneous strategies have a relatively low chance of morbidity, however there is a small risk (<1%) of stroke and anesthesia dolorosa (constant, unremitting and extremely difficult-to-treat facial pain). These risks must be considered by the patient before surgery.
Percutaneous techniques are often used for:
- Multiple sclerosis-related trigeminal neuralgia
- Patients unfit for major brain surgery
- Patients who do not wish to undergo major brain surgery
What is microvascular decompression?
Microvascular decompression is an operation at the base of the brain. It is used to treat trigeminal neuralgia. ‘Micro’ refers to use of the operating microscope, ‘vascular’ refers to blood vessels, and ‘decompression’ means to relieve pressure.
Microvascular decompression is performed via a posterior fossa craniotomy. The posterior fossa is the compartment at the back of the skull, which houses the part of the brain called the cerebellum, as well as the brainstem. The trigeminal nerve leaves the brainstem in the posterior fossa and runs into Meckel’s cave before passing into the face as several branches
For this procedure, a window of bone behind the ear is removed, and the trigeminal nerve is approached by gently pulling back the cerebellum. The trigeminal nerve is located, and carefully inspected for blood vessels which may be causing the problem. A small piece of teflon is placed between the compressing artery and the nerve. If the offending vessel is a vein, this is coagulated and divided.
The long-term (5-10year) success rate of microvascular decompression is over 80%. The risk of stroke or mortality is higher than for the percutaneous techniques (<2%), but there is a lower incidence of facial numbness.
What about radiosurgery?
Stereotactic radiosurgery may also be used to treat trigeminal neuralgia. This technique avoids the need for surgery in some patients. The long-term results appear satisfactory, and it is reasonable to consider this option in patients who are not suitable for the above surgical techniques, or in those for whom these conventional approaches have failed.
The main disadvantage of stereotactic radiosurgery is the delayed response in reducing facial pain.
What is glossopharyngeal neuralgia?
Glossopharyngeal neuralgia is a similar but much less common condition caused by compression of the glossopharyngeal nerve. It causes pain in the tongue and throat.
The causes and treatment are similar to those for trigeminal neuralgia.
The treatment of choice for glossopharyngeal neuralgia which does not respond to medication is microvascular decompression. This procedure is essentially the same as for trigeminal neuralgia, except that a different nerve is targeted.