Nervus Intermedius Neuralgia

Nervus intermedius neuralgia (geniculate neuralgia) is a very rare cranial neuralgia of brief, severe, stabbing pain deep in the ear canal, in the territory of the sensory branch of the facial nerve. Attacks last seconds to minutes and may be triggered from a zone in the posterior wall of the ear canal.

Fewer than a few hundred cases are reported, and the incidence is probably under 0.1 in every 100,000 people each year. It occurs in adults, with a mean onset of about 50 years, and affects women slightly more.

Causes. Most cases follow neurovascular compression of the nervus intermedius at the root entry zone, often by the anterior inferior cerebellar artery. Secondary causes are herpes zoster (Ramsay Hunt syndrome), cerebellopontine angle tumours, multiple sclerosis and surgery.

Course. Pain may be episodic or become continuous. Some patients also have lacrimation, salivation, altered taste or tinnitus.

Pathophysiology.

Focal demyelination and ephaptic transmission at the point of vascular contact cause paroxysmal discharge in somatic and visceral afferent fibres of the nervus intermedius.

Overlap with the glossopharyngeal and trigeminal territories explains the variable spread of pain.

• Paroxysmal stabbing or burning pain deep in the ear, lasting seconds to minutes, sometimes with a dull background ache.
• Trigger zone in the posterior wall of the external auditory canal, around the ear, or on the face; triggers include talking, chewing and touch.
• Lacrimation, salivation, taste change, facial pain or vertigo in some.
• Normal otoscopy and hearing; normal facial movement unless there is a secondary cause.

Red flags.

• Vesicles in the ear or palate, facial weakness or hearing loss (herpes zoster oticus).
• Hearing loss, vertigo or facial numbness (cerebellopontine angle tumour).
• Constant pain, discharge or a mass in the canal.

• Clinical diagnosis after careful exclusion of ear disease.
• Otoscopy, audiometry and examination of the cranial nerves, mouth and neck.
• MRI of the brain and internal auditory canals, with high resolution heavily T2-weighted sequences and MR angiography, to show neurovascular contact and exclude tumour and demyelination.
• Blood tests and varicella zoster serology or PCR when zoster is possible.
• Anaesthetic block of the trigger zone may relieve the pain and support the diagnosis.

Differential diagnosis. Glossopharyngeal neuralgia, trigeminal neuralgia, herpes zoster oticus and postherpetic neuralgia, otitis and mastoiditis, temporomandibular disorder, tumours of the cerebellopontine angle, referred dental pain and cervical disorders.

• Carbamazepine, first line: 100 mg orally twice daily, increased by 100 mg every 2 days to a usual 400 to 800 mg daily in 2 to 3 divided doses. Maximum 1600 mg daily. Monitor sodium, full blood count and liver function at baseline and during titration; check HLA-B*1502 before starting in people of at risk Asian ancestry; stop at the first rash. Avoid in atrioventricular block, bone marrow depression and porphyria; reduce dose in hepatic impairment and older adults; it lowers the effect of oral contraceptives and is teratogenic (folic acid 5 mg daily before conception).
• Gabapentin 300 mg orally at night on day 1, 300 mg twice daily on day 2, then 300 mg three times daily on day 3, increased by 300 mg every 3 to 7 days to 900 to 2400 mg daily in 3 divided doses. Maximum 3600 mg daily. Monitor sedation, dizziness, peripheral oedema, mood and misuse. Adjust for renal function: creatinine clearance 50 to 79 mL/min, 600 to 1800 mg daily; 30 to 49 mL/min, 300 to 900 mg daily; 15 to 29 mL/min, 150 to 600 mg daily; under 15 mL/min, 150 to 300 mg daily. Avoid in pregnancy unless benefit outweighs risk.
• Pregabalin as an alternative: 75 mg orally twice daily, increased after 1 week to 150 mg twice daily, then to a maximum of 300 mg twice daily (600 mg daily). Monitor sedation, weight gain, oedema, mood and misuse. Adjust for renal function: creatinine clearance 30 to 59 mL/min, 75 to 300 mg daily; 15 to 29 mL/min, 25 to 150 mg daily; under 15 mL/min, 25 to 75 mg daily. Avoid in pregnancy unless benefit outweighs risk.
• Treat herpes zoster oticus early with antiviral and corticosteroid treatment through specialist advice.
• Microvascular decompression, with or without sectioning of the nervus intermedius, for drug resistant classical disease at a specialist centre; sectioning may leave dry eye or taste change.
• Local anaesthetic blocks of the trigger zone as a bridge.

Monitoring. Review at 2 to 4 weeks, then every 3 months; check sodium and blood count on carbamazepine.

Referral and escalation. Neurology and otolaryngology for diagnosis; neurosurgery for drug resistant pain.

Special groups. Older adults: start low and titrate slowly. Pregnancy: avoid carbamazepine and gabapentin where possible and plan with specialists.