Aetiology

The majority of patients with TN have focal compression of the trigeminal nerve root at the root entry zone by an aberrant vascular loop (typically the superior cerebellar artery). However, many patients have similar compression on the asymptomatic, contralateral side, and asymptomatic patients without TN may demonstrate similar vascular compression. To attribute TN to neurovascular compression, the aberrant vessel should induce anatomical changes in the trigeminal nerve root, e.g. distortion or atrophy.[2][4]​​​ Reports of trigeminal nerve compression by true vascular malformations (aneurysms or arteriovenous malformations) exist but are rare.​​​[7]​​[8] Posterior fossa tumours can also produce symptoms mimicking TN.​​[9]

TN is 20 times more prevalent in people with multiple sclerosis (MS), compared with the general population.[10]​ People with MS who have TN usually demonstrate demyelinating plaques in the pons that encompass the root entry zone of the trigeminal nerve.​[11]​​​[12]​​ However, one study suggests that MRI-positive root entry zone abnormalities, which are more prevalent than previously thought in the MS population, often occur in the absence of trigeminal symptoms.​​[13]​​

Rare cases of TN have been reported with brainstem infarcts and amyloid or calcium deposition along the trigeminal sensory pathway.[14]​​

Pathophysiology

Focal demyelination and the resultant conduction aberrations (ephaptic transmission) are thought to represent the pathophysiological mechanism of the neuropathic pain of trigeminal neuralgia (TN).[3]​ Pathological examinations of post-mortem patients with TN secondary to an aberrant vessel loop demonstrate focal, chronic myelin loss at the root entry zone in close proximity to the region of vascular compression. Pathological specimens have minimal, if any, evidence of ongoing inflammation. Foci of demyelination encompassing the root entry zone have been described in pathological specimens from patients with MS and TN.[15][16]

Classification

Trigeminal neuralgia is classified as classical, secondary, or idiopathic, depending on the underlying cause:​[1]​​[2][3][4]​​​​

  • Classical TN (75% of cases) is caused by intracranial vascular compression of the trigeminal nerve root

  • Secondary TN (15% of cases) is caused by another pathological process, e.g., multiple sclerosis or tumour.

  • Idiopathic TN (10% of cases) is diagnosed when no lesion or disease that could cause trigeminal neuralgia is found.

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