TN, or ‘tic douloureux’, is most easily recognised in medical practice as “a sudden, usually unilateral, brief stabbing recurrent pain in the distribution of one or more branches of the fifth cranial nerve”.¹ TN may have no apparent cause (idiopathic, essential or classic TN) or be secondary to major neurological disease (symptomatic TN). Symptomatic TN can be related to slowly growing tumours, such as cholesteatomas, meningiomas or neurinomas of the VIII nerve that compress the trigeminal nerve root near the dorsal root entry zone, or multiple sclerosis (MS), which is typically associated with TN.^2–4 Many investigators refute the term ‘idiopathic TN’ because they support the view that, when no lesion affecting the trigeminal system can be demonstrated, TN is due to a vascular compression of the trigeminal nerve root by tortuous or aberrant vessels. Microsurgical interventions in the posterior fossa always find a compressing vessel, most often the superior cerebellar artery. Further support for this view comes from magnetic resonance imaging (MRI) studies reporting a frequent contact between vessels and the trigeminal root.⁵ Microvascular decompression consistently relieves TN pain.⁶ Nevertheless, other investigators do not support the view that a vascular compression is the main factor, as compression of the trigeminal nerve root near the root entry zone is often found (in 7–12% of cases) during standard autopsy of patients with no history of TN, and on MRI TN patients may have bilateral compressions but no bilateral symptoms.^7,8 Several factors possibly contribute to the development of TN. The hypothesis of a multifactorial origin is favoured by several investigators.

As the debate about the possible causes of idiopathic TN is still open, we believe that the terminology and definitions proposed by the International Headache Society (IHS)⁹ are the most appropriate. Classic TN has no apparent cause other than vascular compressions, and symptomatic TN is pain indistinguishable from that of classic TN but is caused by a demonstrable structural lesion other than vascular compressions.⁹ The categorisation of TN into typical and atypical forms is based on symptom constellation and not aetiology, thus typical TN must not be confused with classic TN.⁷

Although trigeminal neuralgia is classified as neuropathic pain because it is a direct consequence of somatosensory system lesion, it has unique features that make it different from other neuropathic pain. TN symptoms are unmistakable, and TN is usually recognisable by patient history alone. Pain distribution is unilateral (bilateral TN sometimes occurs in multiple sclerosis) and follows the sensory distribution of the trigeminal divisions, typically radiating to the maxillary (V2) or mandibular (V3) territories. Ophthalmic (V1) pain is less common and was previously considered indicative of symptomatic TN (see below).

Strongly indicative of a symptomatic form, a focal neuropathy is often a pain in the tongue, which is very rarely affected in classic TN. The pain is never felt in the teeth.10 The right side of the face is involved more frequently than the left, and the disorder is more common in women than in men (3:2). Pain, usually referred to as stabbing or electric-shocklike, is brief and paroxysmal, lasting a few seconds, with no pain between paroxysms. However, there is sometimes an after-pain described as slowly fading away. Pain may be provoked by stimulating cutaneous or mucous trigeminal territories (trigger zones). Gently touching the face, washing, shaving, talking, brushing the teeth, chewing, swallowing or even a slight breeze can trigger the paroxysms. Pain provokes brief muscle spasms of the facial muscles, thus producing the ‘tic’. Especially in the early years of the condition, there can be long pain-free periods; however, these remission periods gradually become shorter and shorter. Classic TN occurs more often in the sixth or seventh decade of life and is stereotyped in each individual.