Anatomic consideration of the trigeminal nerve
Cranial nerves are the nerves that emerge directly from the brain including the brainstem (the posterior part of the brain continuous with the spinal cord which consists of medulla oblongata, pons, and midbrain), in contrast to spinal nerves (which emerge from segments of the spinal cord) (1).Trigeminal nerve is the largest of the cranial nerves (2)(3).The trigeminal (fifth cranial) nerve supplies sensation to the skin of the face and anterior half of the head (2)(3). The motor part innervates the muscles involved in chewing (including masseters and pterygoids) as well as the tensor tympani of the middle ear (hearing especially for high-pitched tones) (2)(3).Three large areas of the face can be mapped out to indicate the peripheral nerve fields associated with the three divisions of the trigeminal nerve. The fields curve upwards apparently because the facial skin moves upwards with growth of the brain and skull (1).
Fig.1. Cranial nerves.Image adapted from (4).
Fig.2. The trigeminal nerve and its branches and sensory distribution on the face. The three major sensory divisions of the trigeminal nerve consist of the ophthalmic, maxillary, and mandibular nerves.(Adapted from Waxman SG: Clinical Neuroanatomy, 26th ed. http://www.accessmedicine.com. Copyright © The McGraw-Hill Companies, Inc. All rights reserved.)
Pathophysiology of Trigeminal Neuralgia
Symptoms result from generation of action potentials in pain-sensitive afferent fibers of the trigeminal nerve root (2)(3). Compression or other pathology in the nerve leads to demyelination of large myelinated fibers that do not themselves carry pain sensation but become hyperexcitable and electrically coupled with smaller unmyelinated or poorly myelinated pain fibers in close proximity; this explains why tactile stimuli, conveyed via the large myelinated fibers, can stimulate paroxysms of pai n(2)(3)(5). The source of trigeminal neuralgia in most patients, is believed to be the compression of the trigeminal nerve root by a blood vessel, most often the superior cerebellar artery or on occasion a tortuous vein (2)(3)(5).
Clinical manifestations of Trigeminal Neuralgia
Trigeminal neuralgia is a common disorder of middle age and later life, characterized by excruciating paroxysms of pain in the lips, gums, cheek, or chin in the distribution of the ophthalmic division of the fifth nerve(very rarely) (2)(3)(5)(7). The pain seldom lasts more than a few seconds or a minute or two but may be so intense that the patient winces involuntarily, hence the term tic (3)(5). The paroxysms tend to recur, both day and night, for several weeks or months at a time, spontaneously or with movements of certain areas involved in speaking, chewing, or smiling (2)(3)(5). Another characteristic feature is the presence of trigger zones, typically on the face, lips, or tongue, that provoke attacks. Patients may report that tactile stimuli—e.g., washing the face, brushing the teeth, or exposure to a draft of air—generate excruciating pain (4)(5).
Most cases of trigeminal neuralgia are without obvious cause (idiopathic), in contrast to symptomatic trigeminal neuralgia, in which paroxysmal facial pain is because of involvement of the fifth nerve by some other disease: multiple sclerosis (may be bilateral), aneurysm of the basilar artery; or tumor (acoustic or trigeminal schwannoma, meningioma, epidermoid) in the cerebellopontine angle(5). Each of the forms of symptomatic trigeminal neuralgia may give rise only to pain in the distribution of the trigeminal nerve, or it may produce a loss of sensation as well(5).
Fig.3.Manifestation of Trigeminal Neuralgia. Image adapted from (8).
Management of Trigeminal Neuralgia
Carbamazepine (600-1200 mg/daily) is effective in 50-75% of patients reducing the severity of attacks(3)(6)(7).Dizziness, imbalance, sedation and rare cases of agranulocytosis are the most important side effects(3)(6). Oxcarbazepine, lamotrigine and gabapentin are also used(3)(6). If drug treatment fails, a number of surgical options are available and in general even more effective than drug treatment.currently the most used method is microvascular decompression to relieve the pressure on the trigeminal nerve(3)(6)(7).Gamma knife radiosurgery is also used and results in complete pain relief with a low risk of persistent facial numbness(3). Another less often used method is radiofrequency thermal rhizotomy which creates a heat lesion of the trigeminal ganglion or nerve, with a common postoperative partial numbness of the face(3).
COPYRIGHT: This article is the property of We Speak Science, a non-profit institution co-founded by Dr. Detina Zalli and Dr. Argita Zalli (Imperial College London). The article is written by Rina Mehmeti, University of Prishtina, Kosovo.
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