WHAT IS TRIGEMINAL NEURALGIA?
Trigeminal neuralgia is a common cause of facial pain. It is usually an episodic, sharp shooting pain in the face. The trigeminal nerve is the cranial nerve (arising from the brain) which supplies sensation to the face, hence the name ‘trigeminal neuralgia’.
Trigeminal neuralgia may be extremely severe and unremitting.
What causes trigeminal neuralgia?
Trigeminal neuralgia is usually caused by compression of the trigeminal nerve just as it leaves the brainstem by a small artery or, less commonly, a vein. These vessels may be visualized preoperatively with a brain scan looking at blood vessels (magnetic resonance angiography). It is thought that ongoing pulsations from these vessels may be particularly 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 syndrome is unclear.
Trigeminal neuralgia may also occur in the setting of multiple sclerosis, as a result of demyelination (damage to the myelin coating on the outside of the nerve fibres).
How is trigeminal neuralgia treated?
Trigeminal neuralgia is treated pharmacologically (in other words, with medications) initially. Nerve membrane-stabilising agents such as carbamazepine (Tegretol) typically improve the pain, and often provide long-term relief.
In cases of trigeminal neuralgia which are resistant to medications, more invasive treatment approaches 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 employed surgical approaches.
What are the percutaneous techniques used to treat trigeminal neuralgia?
Percutaneous techniques involve the insertion of a needle through the cheek into Meckel’s cave. Meckel’s cave is a small cavern at the base of the skull which houses part of the trigeminal nerve called the trigeminal ganglion.
This is usually done under local anesthesia (the patient is awake), under X-ray control or with the aid of a computerised neurosurgical navigation system (frameless stereotaxy). Once the needle is in the correct place, one of three strategies may then be employed:
- Glycerol may be injected around the ganglion (glycerol rhizolysis), deliberately damaging the nerve via a chemical mechanism.
- A small balloon may be inflated to transiently compress (and damage) the ganglion (balloon rhizotomy).
- Radiofrequency ablation. This involves controlled heating of the ganglion using a radiofrequency electrode (radiofrequency rhizolysis).
The benefit of most percutaneous procedures relies upon the production of a degree of facial numbness. This “trade-off” against pain relief must be understood and accepted by the patient beforehand.
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.
The benefits of percutaneous strategies include a relatively low morbidity, but the small risk (<1%) of stroke and anesthesia dolorosa (constant, unremitting and extremely difficult-to-treat facial pain) must be considered, as well as the extremely small chance of death.
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 refers to an operation at the base of the brain 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
A window of bone behind the ear is removed, and the trigeminal nerve is approached by gently retracting (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 high, over 70-80% in many series. The risk of stroke or mortality is higher than for the percutaneous techniques (<2%), but the incidence of facial numbness is lower.
What about radiosurgery?
Stereotactic radiosurgery may also be used to treat trigeminal neuralgia. This technique avoids the requirement 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 onset of beneficial effect 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.