Trigeminal neuralgia

Trigeminal neuralgia or tic douloureux corresponds to a clinical manifestation of sudden severe paroxysms of excruciating pain on one side of the face which usually lasts a few seconds to a few minutes, involving one or more branches of thetrigeminal nerve (CN V). Vascular compression is the most prevalent cause. Other causes include compression due to cerebellopontine angle (CPA) tumours or cysts, perineural tumour spread, and multiple sclerosis.

Epidemiology

The incidence is approximately 4.3 per 100,000 per annum. The vast majority of cases are unilateral, with the right side of the face being affected more commonly (1.5:1); around 3% are bilateral. It peaks around age 60 to 70 years and its prevalence increases with age ^1,4. The maxillary branch (CN V₂) is the most affected and the ophthalmic branch (CN V₁) is the least affected ^ref.

Clinical presentation

It tends to present as attacks of sudden shock-like excruciating pain, which usually lasts a few seconds to about two minutes, most often involving the maxillary branch. The pain is typically triggered by trivial stimuli such as talking, drinking, brushing teeth, shaving, chewing, or touching the face. However, it may also occur spontaneously ¹.

The clinical diagnostic criteria for trigeminal neuralgia are defined by the IASP (International Association for the Study of Pain) and the ICHD/IHS (International Classification of Headache Disorders / International Headache Society) as:

• paroxysmal attacks of pain lasting from a fraction of a second to two minutes, affecting one or more divisions of the trigeminal nerve
• the pain exhibits at least one of the following characteristics:
  • intense, sudden, superficial or stabbing
  • precipitated by trigger factors or trigger areas
• attacks are similar among patients
• no neurological disorder is clinically evident
• not attributed to another disorder, e.g. periapical dental inflammation ¹⁰

Note that if autonomic symptoms are prominent (e.g. lacrimation), the condition more likely represents a trigeminal autonomic cephalgia, such as short-lasting unilateral neuralgiform headache with cranial autonomic symptoms (SUNA) or short-lasting unilateral neuralgiform headache attacks with conjunctival injection and tearing (SUNCT). 

The diagnosis of trigeminal neuralgia is based nearly entirely on history; however, some warning signs should prompt further investigation ⁴:

• difficulty in achieving pain control
• poor response to carbamazepine
• a history of any skin lesions or oral lesions that could lead to a perineural spread
• associated sensory changes, deafness or other ear problems
• when affecting only the ophthalmic division or when bilateral, it is suggestive of benign or malignant lesions or multiple sclerosis
• patients under 40 years
• optic neuritis

Pathology

The most common cause of trigeminal neuralgia is a compressing loop of an artery (most commonly the superior cerebellar artery) or vein (most commonly the transverse pontine vein) compressing the cisternal portion of the trigeminal nerve ².

The site where the nerve exits the brainstem is known as the nerve root entry zone. Typically 3-4 mm distal to this, oligodendrocytes, that supply insulating myelin to the nerve fibres, give way to Schwann cells. This is known as the transition zone and measures approximately 2 mm in length ¹¹. The proximal cisternal portion, especially the transition zone, is far more vulnerable than any other part of the nerve ¹¹. The reason for this, however, is not well understood. Histologically, there is associated demyelination of the compressed nerve in some cases ⁶.

Although rare, posterior fossa tumours can be another cause, most commonly vestibular schwannomas, meningiomas, arachnoid cysts, or epidermoid cysts ⁴. Multiple sclerosis may also cause trigeminal neuralgia and, indeed, its incidence is much higher in multiple sclerosis patients than in the general population.

Radiographic features

The diagnosis of trigeminal neuralgia is based on the patient's history, and an imaging study is usually indicated when there are clinical suggestive signs. Imaging can help diagnose causes such as multiple sclerosis and tumours. CT is limited in evaluating the brainstem and the cisterns. MRI is the imaging modality of choice and should be considered the initial screening procedure in the assessment of patients with trigeminal neuralgia ³.

MRI

Vascular contact deforming the trigeminal nerve is seen in about 15% of cases. As the transition zone between oligodendroglial-derived myelin to Schwann cells cannot currently be identified on imaging, it is important to mention how far anterior to the trigeminal nerve root entry zone the vessel compressing the nerve is located. The majority of transitional zones extend no further than 3-4 mm anterior to the nerve root entry zone, and this has therefore been suggested as a reasonable cut-off between vessels that are likely symptomatic versus those that are more likely incidental ¹¹. It should be noted, however, that occasionally compression of the nerve in the anterior cisternal segment can also be symptomatic and many asymptomatic patients have vessels contacting the proximal nerve. As such, it is not possible to dogmatically state whether a vessel is symptomatic or not ¹¹. 

A dedicated protocol including T2 or T1 volumetric acquisition techniques with thin slices in all three planes should be helpful. However, an evidence-based review did not find evidence to support or refute the usefulness of MRI for this purpose ⁵.

Diffusion weighted imaging can be used to assess the cisternal and brainstem components of the nerve and assess ADC values, fractional anisotropy and mean diffusivity ¹². 

Treatment and prognosis

The initial treatment of trigeminal neuralgia is medical, with carbamazepine and/or gabapentin.

Large surgical series have confirmed that microvascular decompression of the trigeminal nerve root is an efficient and durable treatment for trigeminal neuralgia ⁴.

Other treatment procedures include:

• Gamma Knife radiosurgery focussed at the trigeminal root in the posterior fossa
• rhizotomies: controlled destruction of the trigeminal ganglion or root by various means - thermal, chemical, or mechanical
• trigeminal nerve stimulator: considered "off-label" use (2019) in the USA ⁹