25th June 2023

CN V – Trigeminal nerve

The trigeminal nerve is divided into ophthalmic (V1), maxillary (V2) and mandibular (V3) divisions which convey sensory impulses in the distribution outlined in the image below.

Dermatome distribution of the trigeminal nerve.

Image Source: Wikimedia Commons (2009). Trig innervation, Licensed under a creative commons licence. Available for free at: https://commons.wikimedia.org/wiki/File:Trig_innervation.svg (Accessed on: 11 January 2023).

The ophthalmic V1 branch supplies the scalp and forehead, conjunctiva, cornea, to the nose. The maxillary V2 supplies the lower eyelid, lower nose, maxilla, palate, and maxillary teeth, while the mandibular V3 branch supplies the oral cavity, mandibular teeth, and gums. Of note, all branches have some sensory supply to the commensurate parts of the meninges and sinuses in their distribution. The mandibular (V3) division also carries motor fibres for mastication, supplying the masseter, temporalis muscle, lateral and medial pterygoids, as well as supporting muscles that help with the process of mastication like the tensor tympani, which dampens loud noises including from chewing, and the mylohyoid, anterior belly of the digastric and tensor veli palatini which assist in biting and swallowing.

Hutchinson sign

Hutchinson sign refers to the involvement of the nasociliary nerve, a branch of V1 the ophthalmic nerve which innervates the cornea, eyelid, and lateral dorsum and tip of the nose in herpes zoster infections. It is relevant in the manifestation of herpes zoster ophthalmicus, which can present as periorbital and temporal pain before developing herpes zoster skin lesions in the distribution of the nasociliary nerve and can develop into ocular inflammation with corneal denervation. Care should be taken to differentiate this from Hutchinson’s pupil, which is unrelated and refers to an oculomotor nerve lesion causing an ipsilateral fixed dilated pupil.

Loss of corneal reflex

The corneal blink reflex is a loop between the trigeminal and the facial nerves, where contact with the cornea induces both the blink reflex and tear production. Disorder or disruption of the trigeminal nerve can cause the loss of this reflex, leading to loss of corneal health and damage. This can happen in pontine stroke or progressive supranuclear palsy.

Trigeminal neuralgia

Trigeminal neuralgia is an intense sporadic neuropathic pain confined to and following the distribution of the trigeminal nerve, manifesting with unilateral, severe, shooting or burning episodic pain with each episode lasting seconds to minutes, starting and stopping suddenly. Intervals between each episode are unpredictable and can happen multiple times a day, with episodes occurring over weeks, months to years. It can be triggered by touch (even that of a draught or wind), mastication, changes in temperature, and some types of food, including caffeine-containing, spicy, acidic, or strong flavoured foods.

Trigeminal neuralgia can be primary or secondary. The aetiology of primary trigeminal neuralgia is not clear but is strongly suspected to be related to compression of the trigeminal nerve divisions from contact with normal vascular structures, where the issue is intrinsic to the structure of the nerve with physiological variation. This accounts for 75% of cases and is the most common cause of trigeminal neuralgia. Secondary trigeminal neuralgia arises when the trigeminal nerve is affected by another medical condition, for example, demyelination from multiple sclerosis, invasion from a tumour or abnormal compression from masses like an arteriovenous malformation.

Trigeminal neuralgia needs to be distinguished from trigeminal autonomic cephalalgias such as cluster headaches, paroxysmal hemicrania, short-lasting unilateral neuralgiform headache attacks with conjunctival injection and tearing (SUNCT) and short-lasting unilateral neuralgiform headache attacks with autonomic symptoms (SUNA). The trigeminal autonomic cephalalgias share many features with trigeminal neuralgia, presenting as severe unilateral headache in a trigeminal distribution with ipsilateral cranial autonomic symptoms, such as ptosis, conjunctival tearing, lacrimation, and rhinorrhoea. While there is debate on whether these are distinct entities or a continuum of disorders on the same spectrum, these conditions can be distinguished by attack duration and frequency, triggers, as well as response to treatment and treatment types. Trigeminal neuralgia is characterised by a trigger area, with attacks that tend to be shorter (seconds to a few minutes) but which have a refractory period, with a high variability in frequency in attacks which can happen multiple times in the day, milder to non-existent autonomic symptoms, and with a lack of response to high dose oxygen and triptans which are useful in cluster headaches or lidocaine which is useful in SUNCT.

Treatment is primarily divided into medical and surgical options, with medical options focused on anticonvulsants and nerve modulating analgesic agents. Carbamazepine is the first line treatment for trigeminal neuralgia, with second line treatment being other anticonvulsants including oxcarbazepine which is a structural derivative of carbamazepine (reduced risk of the known adverse effects of agranulocytosis and anaemia of carbamazepine), lamotrigine, gabapentin, and phenytoin. Carbamazepine should be avoided in pregnancy due to the risk of spina bifida and cleft palate, while lamotrigine has the least reported severe effects in pregnancy. Nerve modulating analgesic agents aim to provide symptomatic relief and include amitriptyline and duloxetine.

Surgical treatments are generally reserved for patients who have severe pain with a debilitating impact on quality-of-life refractory to pharmacological treatments. These are categorised as destructive (i.e. lesioning techniques) or non-destructive depending on whether trigeminal nerve function and integrity is intentionally targeted. Non-destructive interventions include microvascular decompression, while destructive interventions like percutaneous radiofrequency rhizotomy, percutaneous balloon compression and stereotactic radiosurgery treat trigeminal neuralgia through the use of electrocoagulation, mechanical compression or ionising radiation respectively to selectively destroy the part of the nerve suspected to cause pain.