5.1 Introduction

The trigeminal nerve has two primary physiologic functions, the first being sensation to the face, anterior two-thirds of tongue, and to the dura of the anterior and middle cranial fossa. Sensory modalities include light touch, pain/temperature perception, and proprioception. The second component of the nerve is motor innervation to the muscles of mastication, that is, the masseter, temporalis, medial/lateral pterygoids, mylohyoid, and anterior belly of the digastric. ¹ A multitude of disease pathologies can affect the trigeminal nerve including basilar skull fractures, dental trauma, metastatic tumors, aneurysms, lupus, scleroderma, sarcoidosis, and arterial ectasia. These pathologies affect the peripheral component of cranial nerve V after it has left the brainstem. The gasserian ganglion may be also affected by herpes zoster, as well as by primary and metastatic tumors. ¹ Like trigeminal neuralgia, all these conditions generally present with facial pain, which can be unilateral, paroxysmal, and stabbing in nature. However, unlike classic trigeminal neuralgia, neurologic deficits such as loss of sensation of varying degrees may be present.

The pathophysiology of trigeminal neuralgia is complex and not fully understood. Trigeminal neuralgia may be due to injury or compression of the afferent fibers at the nerve root or due to injury of the trigeminal ganglion in Meckel’s cave. Injury to the trigeminal nerve can lead to over-activation and a low threshold for nerve root firing. The touch sensation A-beta fibers lie in close proximity to the pain C-fibers, leading to cross-activation of these fibers. ²

Overall, a low stimulus is needed to activate the nerve, which may explain why relatively light stimuli can be a trigger for significant facial pain.

5.2 Anatomy

The trigeminal nerve begins in the upper pons, exiting the brainstem as the portio major sensory branch and portio minor motor branch. As the nerve exits the brainstem and travels into the prepontine cistern, it transitions from central myelination by oligodendrocytes to peripheral myelination by Schwann cells, an area of change known as the “transition zone.” ³ Once in the prepontine cistern, the nerve courses toward the skull base into Meckel’s cave, where it forms the gasserian ganglion. At the gasserian ganglion, the nerve diverges into its three peripheral segments: ophthalmic (V1), maxillary (V2), and mandibular (V3) branches. The upper segment V1 exits out of the orbit via the superior orbital fissure, the maxillary or V2 segment exits out of the foramen rotundum after leaving the pterygopalatine fossa, and the mandibular segment V3 exits the cranium via the foramen ovale. The various sensory afferents and the motor efferent of cranial nerve V are carried in separate tracts, with a total of three sensory and one motor nuclei. The nucleus of the spinal trigeminal tract contains three parts known as the pars oralis, pars caudalis, and pars interpolaris. These subnuclei transmit pain and temperature sensation. The principal sensory nucleus integrates tactile sensation and light touch. The final sensory nucleus is the mesencephalic, which transmits proprioception of the face. These nuclei all project to the ventral posteromedial nucleus of the thalamus where they synapse, and project to the primary somatosensory nucleus of the parietal lobe. The motor nuclei innervate muscles of the first brachial arch, which includes muscles of mastication as previously described. ^{1 , 3 , 4 , 5} Critical understanding of trigeminal anatomy is key in localization of the lesion and initiating the appropriate management.

5.3 Clinical Evaluation

Trigeminal neuralgia is a disease entity characterized by sudden, unilateral, severe episodes of stabbing pain in the trigeminal nerve distribution, involving one or more of the peripheral nerve segments, as defined by the International Association for the Study of Pain. ⁶ The initial approach to trigeminal neuralgia begins with a proper history to assure an accurate clinical diagnosis. In order to properly evaluate trigeminal neuralgia, history should include inquiring about pain patterns, location, the presence of triggers, the duration of symptoms, and looking for possible causalgia. Many people suffering from trigeminal neuralgia may become aware of specific triggers that elicit their pain such as light sensation to face, cold air, brushing of teeth, drinking from a straw, stroking the hair on their face, shaving, stress, fatigue, smiling, laughing, or even kissing. It is also important to ask about the conditions that have an increased prevalence and association in developing trigeminal neuralgia, such as multiple sclerosis, stroke, hypertension, and Charcot–Marie–Tooth disease. These conditions may initially present like trigeminal neuralgia or be the causative agent. ^{2 , 4} Physical examination must include a full neurologic examination, with special attention to the head and neck, with high suspicion for secondary etiologies including mass lesions or orofacial conditions.

Classification of trigeminal neuralgia can be made on the basis of history and physical examination and, if needed, using advanced imaging such as MRI. ▶Table 5.1 lists seven different types of trigeminal neuralgia and their distinct features as well as complex grading scheme that is useful in standardizing the diagnosis into specific categories. ⁷ However, a more simplified categorization that can be used by primary care physicians is to organize trigeminal neuralgia into one of two groups: classic trigeminal neuralgia (CTN) and symptomatic trigeminal neuralgia (STN). CTN refers to a typical presentation of paroxysmal pain described as intense, sharp, or stabbing in nature with an associated trigger, lasting no more than a few seconds to 2 minutes. CTN does not involve neurologic deficits on physical examination and may be idiopathic or due to vascular compression with no other underlying etiology. ^{2 , 6 , 7} A clinical picture suggestive of CTN does not require further neuroimaging for confirmation of the disease, but an MRI is still important to obtain to rule out other pathologies. STN, on the other hand, is trigeminal neuralgia secondary to an underlying pathology other than vascular compression that may be causing injury or mass effect on the trigeminal nerve. ^{6 , 8} Atypical presentation includes those with bilateral symptoms, neurologic deficit on examination, or pain lasting more than 2 minutes, all of which should prompt suspicion of STN. A comprehensive medical history and physical examination, along with neuroimaging, can help categorize and diagnose other underlying disease pathologies.