Trigeminal Neuralgia: Diagnosis and Management

Introduction

Trigeminal neuralgia is a chronic pain syndrome affecting the trigeminal nerve (cranial nerve V), causing paroxysms of severe, unilateral facial pain that are among the most intense described in medicine - sometimes called the 'suicide disease' in older literature because of the impact of unrelieved attacks on quality of life.¹

It typically affects adults over 50, is more common in women, and is a favourite exam topic because the history is so distinctive that the diagnosis is almost always clinical - and because the drug of choice, carbamazepine, is used almost nowhere else in neurology in quite the same first-line way.

Incidence is roughly 4-13 per 100,000 per year, and the right side is affected more often than the left. The condition is almost always unilateral, and although it may switch sides over the course of a lifetime, simultaneous bilateral pain is distinctly unusual and should raise the question of an underlying secondary cause such as multiple sclerosis.

Aetiology

The great majority of cases are considered idiopathic but are now understood to result from neurovascular compression - most often the trigeminal nerve root being compressed by an adjacent blood vessel, classically the superior cerebellar artery, at its entry to the pons (the root entry zone). This is thought to cause focal demyelination and aberrant, hyperexcitable signalling within the nerve.

The leading explanation for the pain itself is the ignition hypothesis. Focal demyelination at the root entry zone allows electrical activity to spread directly between adjacent axons - ephaptic transmission - rather than being confined to individual fibres. A light touch stimulus carried by large myelinated fibres can therefore cross over and activate pain fibres, producing a burst of synchronised, self-amplifying discharge that stops abruptly when the neurones become refractory.

This mechanism accounts for almost every feature of the history. It explains why innocuous touch triggers agonising pain, why the pain is paroxysmal and electric rather than constant, why attacks stop as suddenly as they start, and why a refractory period follows. It also explains why sodium channel blockers such as carbamazepine are so effective: by stabilising axonal membranes they raise the threshold for exactly this kind of ectopic, synchronised firing.

• Vascular compression - the commonest identifiable cause, usually an arterial loop against the trigeminal root entry zone
• Multiple sclerosis - trigeminal neuralgia can result from a demyelinating plaque within the pons affecting the trigeminal nerve root entry zone; consider particularly in younger patients, or those with bilateral symptoms
• Compressive lesion - a cerebellopontine angle tumour (such as a vestibular schwannoma or meningioma) or other space-occupying lesion compressing the nerve
• Idiopathic - when no cause is found despite imaging

Risk factors

• Increasing age (peak incidence in the 50s-60s)
• Female sex
• Hypertension
• Multiple sclerosis - a much stronger association than in the general population, and a cause to actively consider in younger patients

Clinical features

The history is highly characteristic, and recognising the pattern is the single most important diagnostic skill for this condition.

It is worth asking directly about the impact on daily activities, because patients often do not volunteer it. Many will have stopped brushing the affected side of their mouth, altered how they eat, avoided going outdoors in cold or windy weather, or stopped speaking on the telephone - and these avoidances, rather than the pain score, are frequently the best measure of severity and of response to treatment.

• Sudden, severe, unilateral facial pain, often described as electric shock-like, stabbing, or like a knife
• Paroxysmal - attacks last from a fraction of a second up to around 2 minutes, but can recur in quick succession
• Pain-free intervals between attacks, though a dull ache can persist in some patients between paroxysms in more established disease
• Distribution follows one or more divisions of the trigeminal nerve - most commonly the maxillary (V2) and mandibular (V3) divisions; ophthalmic (V1) involvement alone is less common
• Triggered by light, non-painful stimuli - touching the face, washing, shaving, brushing teeth, chewing, talking, or cold wind, an important and distinctive feature
• Refractory periods after an attack, during which further triggering is difficult
• No sensory loss and no other neurological deficit in typical idiopathic disease - their presence should prompt reconsideration of the diagnosis

Clinical examination

• Facial sensation in all three trigeminal divisions - should be normal in idiopathic disease
• Corneal reflex - should be intact
• Motor function of the trigeminal nerve (muscles of mastication) - should be normal
• Other cranial nerves - should be entirely normal in idiopathic disease; any abnormality raises concern for a secondary cause
• Full neurological examination to look for other signs of multiple sclerosis or a cerebellopontine angle lesion (ataxia, hearing loss, other cranial neuropathies)

Differential diagnosis

• Dental pain - a very common cause of misdiagnosis in the other direction; patients are frequently referred for unnecessary dental extractions before the correct diagnosis is made
• Temporomandibular joint dysfunction - pain related to jaw movement, tenderness over the joint
• Cluster headache and other trigeminal autonomic cephalalgias - longer attacks with prominent autonomic features (lacrimation, ptosis, rhinorrhoea), unlike the brief pure pain of trigeminal neuralgia
• Post-herpetic neuralgia - history of a preceding shingles rash in the same distribution
• Glossopharyngeal neuralgia - similar quality of pain but in the throat, ear and tonsillar distribution, triggered by swallowing
• Giant cell arteritis - in an older patient with jaw claudication, though the pain pattern differs and is usually not paroxysmal in the same way
• Atypical facial pain - a persistent, poorly localised, constant ache, not paroxysmal, and without clear triggers

Investigations

Trigeminal neuralgia is a clinical diagnosis. Imaging is not required in every case but is important in selected patients.

• MRI brain (with dedicated thin-slice sequences through the trigeminal nerve root) - indicated for atypical features, younger age at onset, bilateral symptoms, sensory loss or other neurological signs, to look for multiple sclerosis, a compressive tumour, or to demonstrate a vascular loop before considering microvascular decompression
• Dental review - reasonable if there is any uncertainty about a dental cause, but should not delay treatment if the history is classic
• No specific blood test confirms the diagnosis, though inflammatory markers may be checked if giant cell arteritis is a plausible differential in an older patient

Management

Medical treatment

Carbamazepine is first-line and highly effective in the majority of patients, often producing a dramatic response that itself supports the diagnosis.² It is started at a low dose and titrated upward according to response and tolerability.

• Monitor for side effects: drowsiness, ataxia, hyponatraemia, and rare but serious idiosyncratic reactions (Stevens-Johnson syndrome, agranulocytosis)
• HLA-B*15:02 testing is recommended before starting carbamazepine in patients of Han Chinese or Thai ancestry, because of a strongly increased risk of severe cutaneous reactions
• Carbamazepine is a potent enzyme inducer and interacts with many other drugs, including reducing the efficacy of hormonal contraception
• If carbamazepine is not tolerated or is ineffective, alternatives include oxcarbazepine, lamotrigine, gabapentin, or baclofen, usually with specialist input

Refractory or medically unsuitable disease

For patients whose pain is inadequately controlled by medical treatment, who cannot tolerate the drugs, or who wish to avoid long-term medication, several procedural options exist:³

Whichever option is chosen, the counselling should be explicit that these procedures trade pain relief against sensory loss. Microvascular decompression is the only approach that addresses the cause without deliberately damaging the nerve, and so is the only one that reliably preserves normal facial sensation - but it requires a posterior fossa craniotomy with the attendant risks of hearing loss, CSF leak and, rarely, stroke. Ablative procedures are quicker and less invasive but work precisely by injuring the nerve, so some degree of facial numbness is expected rather than a complication.

Patient factors therefore drive the decision as much as the imaging does. A fit patient in their fifties with a clear vascular loop is usually best served by microvascular decompression; a frail patient in their eighties, or someone with trigeminal neuralgia secondary to multiple sclerosis in whom no vascular loop exists to decompress, is generally better suited to radiosurgery or a percutaneous procedure.

• Microvascular decompression - a posterior fossa craniotomy to move the offending vessel away from the trigeminal nerve root, with a pad placed between them. This offers the best chance of durable, long-term pain relief and is preferred in younger, fitter patients with a demonstrated vascular loop on MRI
• Stereotactic radiosurgery (Gamma Knife) - a non-invasive alternative, particularly for patients unfit for open surgery, though the effect takes weeks to develop and recurrence is more common than with microvascular decompression
• Percutaneous ablative procedures (radiofrequency thermocoagulation, glycerol rhizolysis, balloon compression) - less invasive, quicker to perform, but with a higher risk of facial numbness and a greater chance of recurrence than microvascular decompression

Supportive care

• Screen for and address the impact on eating, drinking, oral hygiene and mood - patients may avoid these activities entirely because they trigger attacks
• Involve dietetics if oral intake is significantly reduced
• Psychological support, given the substantial impact of chronic severe pain on quality of life

Complications

• Weight loss and dehydration from avoidance of eating and drinking
• Poor oral hygiene from avoidance of tooth brushing
• Depression and, in severe unremitting cases, suicidal ideation - the historical name 'suicide disease' reflects genuine risk and should be taken seriously
• Facial numbness and dysaesthesia as a complication of ablative procedures
• Recurrence after any procedural treatment, requiring further intervention

Red flags

Idiopathic trigeminal neuralgia has a characteristically normal examination, so any objective neurological abnormality is the main signal that a secondary cause is present. The features below should prompt MRI rather than continued empirical treatment.

Prognosis

Most patients respond well to carbamazepine, at least initially, though the disease often follows a relapsing-remitting course with periods of remission lasting months to years, punctuated by relapses that may need dose escalation or a change in treatment. Efficacy of medical treatment can wane over time. Microvascular decompression offers the best long-term outcomes in appropriately selected patients, with a large majority pain-free at several years, though it carries the risks of any posterior fossa surgery.

The natural history is characteristically relapsing and remitting, and this shapes long-term management. Spontaneous remissions lasting months or occasionally years are common early in the disease, which means a patient who becomes pain-free is not necessarily cured and may relapse. It is reasonable to attempt slow dose reduction after a sustained period of remission, warning the patient that pain may return and that treatment should be restarted promptly rather than endured. Over time remissions tend to shorten and drug requirements to rise, which is often the trigger for considering a procedural option.

A practical point about carbamazepine is that it induces its own metabolism, so serum levels can fall over the first few weeks and a dose that initially worked may appear to fail. Apparent loss of efficacy early in treatment therefore warrants a dose review before concluding the drug has stopped working. Sodium levels should also be monitored, as hyponatraemia is common, particularly in older patients and those on diuretics, and can present as confusion that is easily misattributed to age or to the pain itself.