Trigeminal Neuralgia: A Closer Look at This Enigmatic and Debilitating Disease
Trigeminal neuralgia (TN), also known as tic douloureux, is a relatively common neuropathic disorder that is characterized by sudden severe episodes of lancinating or electric pain along the distribution of cranial nerve V (CNV). With an annual incidence of approximately 4.7 per 100,000 people, TN remains one of the most frequent causes of facial pain worldwide.1 Despite its prevalence, there is not a precise description of its etiology or pathogenesis. Occurring slightly more often in women, TN appears more commonly after the fifth decade of life, although some cases have been reported in childhood. With regards to risk factors, hypertension seems to have a tenuous association with TN. People suffering from this condition have been known to acquire anxiety disorders due to the continual fear of an episode; depression has also been documented in a fair number of people who persistently suffer from TN. These devastating effects as well as the intense periods of pain that trademark this disorder have allowed it to earn the title of the “suicide disease.”
Cranial Nerve V
The trigeminal nerve is composed of three divisions that innervate distinct regions of the face: the ophthalmic nerve (V1), the maxillary nerve (V2), and the mandibular nerve (V3). Each division originates from the trigeminal ganglion, or gasserian ganglion, which emerges on the lateral portion of the pons bilaterally.2 After departing from the pons, each division disperses to their particular facial dermatomes (Figure 1).
The ophthalmic nerve (V1) gives off three main branches (the frontal nerve, nasociliary nerve, and lacrimal nerve), which innervate areas of the face that extend from the vertex of the scalp to the apex of the nose. The frontal nerve is considered the largest branch of the ophthalmic nerve and it separates into the supra-orbital and supra-trochlear nerves, which supply a large portion of the scalp and anteromedial forehead, respectively.3 The nasociliary nerve branches off into the infra-trochlear nerve as well as the anterior and posterior ethmoidal nerves. The anterior ethmoidal nerve is notable because its terminal branch, the external nasal nerve, innervates the nasal vestibule. The smallest branch of the ophthalmic nerve is the lacrimal nerve, whose distribution includes the lateral aspect of the eyelids. The lacrimal nerve also participates in the parasympathetic innervation to the lacrimal glands.3
The maxillary nerve (V2) is a purely sensory nerve that emerges from the cranium through the foramen rotundum to innervate the maxillary region of the face and the skin along the zygomatic arch.4 The maxillary nerve also carries parasympathetic fibers from cranial nerve seven to innervate the mucous glands of the palate, maxillary sinus, and nasal cavity. The major branches of the maxillary nerve include the infra-orbital, zygomaticofacial, and zygomaticotemporal nerves.
The mandibular nerve (V3) is the only branch of the trifurcation of cranial nerve five that serves both motor and sensory functions. It breaks off into four major branches including the auriculotemporal, buccal, lingual, and inferior alveolar nerves. The auriculotemporal nerve is unique in that it is in close proximity to the middle meningeal artery as it courses to the posterior temporal region to innervate the auricle.3 The terminal branch of the inferior alveolar nerve is the mental nerve, which supplies the skin of the lower lip and chin. Analogous to the V1 and V2, the mandibular nerve also relays parasympathetic fibers from cranial nerve seven, which supply the submandibular and sublingual glands. Additionally, the mandibular nerve carries secretomotor fibers from cranial nerve nine to innervate the parotid gland. The motor fibers of the mandibular nerve are essentially responsible for innervating the muscles of mastication including the temporalis, masseter, medial and lateral pterygoids, and the anterior belly of the digastric muscle. Thus, proper functioning of the mandibular nerve is essential for movement of the mouth and chewing.
Although the trigeminal nerve allows for the perception of both touch and pain, these facial sensations are relayed to the sensory cortex through two distinct pathways. Touch fibers from the trigeminal nerve synapse on the main trigeminal nucleus located in the pons. These fibers are then carried along the trigeminal lemniscus to the ventral posteromedial nucleus of the thalamus, which in turn runs to the sensory cortex.5 Pain fibers differ in that they synapse onto the spinal trigeminal nucleus, located primarily in the medulla, before using the trigeminothalamic tract to ascend into the thalamus and eventually the primary somatosensory cortex.2
Classification: Classic vs Secondary
The International Headache Society has separated TN into two categories: classic and secondary. While both categories present with similar symptoms, they differ in respect to their causality. Classic TN includes neuralgia that is idiopathic or caused by compression of the trigeminal nerve from a nearby blood vessel. Secondary TN, on the other hand, accounts for cases triggered by other structural abnormalities. Research has demonstrated that the classic form is accountable for roughly 80% of the cases of TN.6 The vessel that is most commonly associated with classic TN is the superior cerebellar artery, although other vessels, including the anterior inferior cerebellar artery, have been documented.7 The relationship between TN and neurovascular compression was examined by Hamlyn in a study of 46 patients. Of the 46 cases, 91% had a vessel that was compressing the trigeminal nerve.8
As noted, several systemic disorders have been attributed to the development of secondary TN, including multiple sclerosis and sarcoidosis. Other common triggers for secondary TN are tumors and skull base abnormalities particularly in the posterior fossa. Tumors ranging from vestibular schwannomas to meningiomas have been known to cause TN. It should be noted that patients with secondary TN are typically younger than those with classic TN.9 This age disparity could possibly be attributed to the underlying etiologies associated with the secondary form (Table 1).