Intraoperative neurophysiological monitoring during microvascular decompression of cranial nerves

33.2.1

Trigeminal neuralgia

TGN is a chronic clinical condition characterized by recurrent episodes of sudden intense, stabbing, electric shock-like pain in the distribution of one or more branches of the fifth cranial nerve. The distribution of pain most often involves the V2 and/or V3 divisions of the trigeminal nerve. Episodes of pain can be triggered by any touch or stimulation of the face such as chewing, shaving, application of make-up, or even a cold breeze . There are two main types of TGN, typical and atypical. Typical TGN results in the classic stabbing pain syndrome that lasts for seconds to a few minutes whereas atypical TGN is characterized by a constant burning pain that is far less severe. The term classic TGN encompasses cases related to vascular compression, while secondary TGN is for those cases caused by an underlying disease such as multiple sclerosis or a tumor along the trigeminal nerve .

Women are more commonly affected than men, and it is estimated that 4 to 13 per 100,000 people develop TGN a year . TGN usually manifests itself in peoples greater than 50 years old but has been reported to occur at any age .

33.2.2

Hemifacial spasm

HSF is a condition that manifests as intermittent twitching of the facial muscles, usually beginning in the orbicularis oculi. The disease usually takes a relentless course and over a variable period of time progresses down the face to involve all of the major muscles on the ipsilateral side. This condition can even involve the frontalis, platysma, and stapedius muscles. Stapedius muscle involvement is unusual and is reported as an odd clicking sound by the patient. Fatigue and stressful activities may exacerbate symptoms. Although the intermittent spasm of the facial muscles is involuntary, voluntary movements, such as talking and smiling, can trigger contractions. The paroxysms of spasm may become severe enough that the patient develops periods of tonic contractures—“tonus phenomena” that can be associated with paresis of the facial muscles. Although cosmetic deformity is the most obvious concern for most patients, HFS can and does become a functional disability. Repetitive closure of the eyelids, mainly when it is associated with tonus phenomena, can impair vision and prohibit the pursuit of both occupational and leisure activities. Most notable limitations reported by patients involve driving, reading, and being comfortable in social situations. As a result, we have used these three parameters as outcome measures to follow patients undergoing MVD for HFS .

The incidence is 0.78 per 100,000 persons; there is a slight predilection for women (1.9:1), and the left side of the face is often more affected than the right. The incidence rates of HFS are highest in those between the ages of 50 and 79 years . The differential diagnosis of HSF includes other conditions that must be entertained prior to consideration for surgery. For example, postparalytic synkinesis can mimic HSF and occurs after facial nerve trauma or Bell’s palsy and is unresponsive to MVD. This synkinesis is diagnosed by obtaining the relevant history and is usually triggered by facial movement of a muscle and does not occur at rest. Other conditions that can be confused with HSF include blepharospasm, facial tics, facial myokymia, and Meige syndrome.

33.2.3

Glossopharyngeal neuralgia

GPN is characterized by paroxysmal pain in areas innervated by the IX cranial nerve and also can involve the territories of the auricular and pharyngeal branch of the vagus nerve and so encompass the ear, tonsillar fossa, base of the tongue, or beneath the angle of the jaw. The pain is similar to the TGN in its quality, its timing, and its ability to be triggered by various stimuli such as swallowing, chewing, coughing, yawning, talking or specific tastes . In about 20% of the cases the pain may be accompanied by syncope as a result of bradycardia/hypotension or even asystole presumably because impulses from glossopharyngeal into the tractus solitarius would stimulate the dorsal motor nucleus of the vagus nerve .

The GPN is rare with an incidence estimated at 0.2–0.7/100,000 in general population. There are idiopathic and secondary forms of GPN. Besides vascular compression of cranial nerve IX, secondary causes include demyelinating lesions, carotid aneurysms, peritonsillar abscess, oropharyngeal malignancies, or skull base lesions.

33.2.4

Others syndromes related to neurovascular compression (NVC)

Seldom, neurovascular compression can involve other cranial nerves such as the vestibulocochlear causing vertigo and tinnitus and the accessory nerve resulting in spasmodic torticollis . The selective involvement of the intermedium nerve induces the called geniculate neuralgia (GN) characterized by brief recurrent attacks of severe pain in the depth of the auditory canal .