Trigeminal nerve

21 Trigeminal nerve

Trigeminal Nerve

The trigeminal nerve has a very large sensory territory which includes the skin of the face, the oronasal mucous membranes and the teeth, the dura mater, and major intracranial blood vessels. The nerve is also both motor and sensory to the muscles of mastication. The motor root lies medial to the large sensory root at the site of attachment to the pons (Figure 17.16). The trigeminal (Gasserian) ganglion, near the apex of the petrous temporal bone, gives rise to the sensory root and consists of unipolar neurons.

Details of the distribution of the ophthalmic, maxillary, and mandibular divisions are available in gross anatomy textbooks. Accurate appreciation of their respective territories on the face is essential if trigeminal neuralgia is to be distinguished from other sources of facial pain (Clinical Panel 21.1).

Clinical Panel 21.1 Trigeminal neuralgia

Trigeminal neuralgia is an important condition occurring in middle age or later, characterized by attacks of excruciating pain in the territory of one or more divisions of the trigeminal nerve (usually II and/or III). The patient (who is usually more than 60 years old) is able to map out the affected division(s) accurately. Because it must be distinguished from many other causes of facial pain, the clinician should be able to mark out a trigeminal sensory map (Figure CP 21.1.1). Attacks are triggered by everyday sensory stimuli, e.g. brushing teeth, shaving, and chewing, and the tendency of patients to wince at the onset of attacks accounts for the French term tic doloureux.

Episodes of paroxysmal facial pain occurring in young adults should raise a suspicion of multiple sclerosis as the cause. Postmortem histology in such cases has revealed demyelination of the sensory root of the trigeminal nerve where it enters the pons. Demyelination of large sensory fibers receiving tactile signals from skin or mucous membranes in trigeminal territory may cause their exposed axons to come into direct contact with unmyelinated axons serving pain receptors. Animal experiments have shown that this type of contact can initiate ephaptic transmission of action potentials between them. It is now widely accepted that the most frequent etiology in later years is vascular compression, usually by a ‘sagging’ posterior cerebral artery in transit around the brainstem. The trigeminal CNS/PNS transition zone (Ch. 6) is several millimeters lateral to the entry zone into the pons, and postmortem histology has provided evidence of the demyelinating effect of chronic pulsatile compression.

Antiepileptic drugs that exert a blocking effect on sodium and/or calcium channels (e.g. carbamazepine) may suffice to keep ephapsis at bay. Surgery is indicated for those who fail to respond.

A procedure which can be performed under local anesthesia is electrocoagulation of the affected division, through a needle electrode inserted through the foramen rotundum or ovale from below. The intention is to heat the nerve sufficiently to destroy only the finest fibers, in which case analgesia is produced but touch (including the corneal reflex) is preserved.

The final option is to decompress the afflicted nerve root through an intracranial approach whereby neighboring vessels are lifted away from it.

A surgical procedure of historic interest is medullary tractotomy, whereby the spinal root was sectioned through the dorsolateral surface of the medulla. In successful cases, pain and temperature sensitivity was lost from the face but touch (mediated by the pontine nucleus) was preserved. This procedure was abandoned owing to a high mortality rate associated with compromise of underlying respiratory and cardiovascular centers.

Jun 10, 2016 | Posted by in NEUROLOGY | Comments Off on Trigeminal nerve
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