TN is rare, with an incidence of 2 to 5 per 100,000 per year³ and a prevalence of 0.03% to 0.3%.⁴ Patients typically present in their 60s,⁵ although some develop symptoms as early as their late 30s. There is a female preponderance (3:2), and some studies suggest an association with arterial hypertension.³ Case reports have identified familial forms of TN; these appear to be rare, are sometimes bilateral, and may follow an autosomal dominant inheritance pattern.^6,7

Patients typically present with one of two clinical syndromes.⁸ Type 1 TN (“typical TN”) is characterized by a predominance of brief, lancinating, or electrical unilateral pain. Type 1 TN most frequently occurs in the V2 or V3 division, and it does not cross the midline. Triggers may include tactile stimuli or facial movements such as talking or chewing, and there is usually a refractory period during which pain cannot be elicited. The duration of pain is often brief, lasting seconds, but there may be up to several minutes of paroxysms before the pain-free refractory period.⁸ The International Headache Society diagnostic criteria for TN are outlined in Table 104.1.⁹

Type 2 TN (“atypical TN”) is characterized by a predominance of constant pain, often burning or aching.¹⁰ This pain usually exists in addition to the brief, lancinating pain that occurs in type 1 TN. Although not firmly established, some believe that type 2 TN may emerge from long-standing type 1 TN.¹¹

The trigeminal nerve contains motor and sensory fibers that exit the lateral brainstem at the level of the mid-pons. Just distal to the brainstem root entry zone (REZ), the nerve transitions its myelinating cells from central oligodendrocytes to peripheral Schwann cells. This transition zone, which occurs several millimeters from the REZ at about one-third to one-fourth of the distance to Meckel’s cave, is believed to be particularly sensitive to mechanical irritation.

The cisternal segment of the nerve traverses the subarachnoid space, traveling anterolaterally to pierce the dura near the petrous apex to enter Meckel’s cave, a CSF-filled space between two layers of dura over the petrous portion of the temporal bone.¹² The nerve then splits into three divisions: (1) ophthalmic (V1; sensory only), which travels through the superior orbital fissure to enter the orbit; (2) maxillary (V2; sensory only), which enters the pterygopalatine fossa via the foramen rotundum; and (3) mandibular (V3; motor and sensory), which exits the skull through the foramen ovale (FO).

Surgery should be considered for patients with persistent, severe symptoms despite adequate drug therapy or those with intolerable medication side effects. Medication trials at a minimum should include carbamazepine or oxcarbazepine.

An evaluation for surgery should prompt scrutiny of the patient’s diagnosis. There exists a cornucopia of facial pain disorders. TN is a clinical diagnosis, and certain red flags should give the clinician pause: pain that does not respect the midline, age <40 years, absence of any initial response to carbamazepine, and facial numbness, among others.

Surgical options for TN can be divided into two categories: palliative ablative procedures (e.g., percutaneous rhizotomy, stereotactic radiosurgery) and nondestructive MVD. Neuromodulatory techniques are growing in popularity for a select set of facial pain disorders, but there is insufficient evidence or experience to support their use as a first-line surgical option for type 1 or 2 TN.

Regarding procedure selection, Slavin and colleagues²² developed a facial pain surgical algorithm that is concordant with our own experience (Fig. 104.1). MVD is best reserved for those patients with type 1 TN, although some data suggest it may alleviate episodic pain in type 2 TN patients as well.²³ Radiofrequency rhizotomy and other percutaneous variants (glycerol, balloon compression) may be used for both type 2 TN patients and those type 1 TN patients in whom medical comorbidities or personal preferences preclude a retrosigmoid craniotomy or those who have already failed an MVD. Radiosurgery is an option for those patients seeking to avoid an invasive procedure.

Imaging is mandatory in the presurgical evaluation, primarily to rule out pathologies that would be symptomatic of TN yet mandate different treatment, namely, posterior fossa tumors or vascular malformations, multiple sclerosis (MS), and brainstem or thalamic infarctions. The preferred imaging study is a magnetic resonance scan with a thin-cut axial T2-weighted 3D sequence (e.g., constructive interference in steady state [CISS], fast imaging employing steady-state acquisition [FIESTA], or similar) through the brainstem. Such sequences delineate the trigeminal nerve in high resolution. Of particular interest is contact or compression of the cisternal component of the trigeminal nerve by a blood vessel (Fig. 104.2).

Importantly, the presence of a vessel compressing the trigeminal nerve is not an obligate criterion for MVD candidacy, as some TN patients without radiographic neurovascular compression (NVC) are indeed found to have NVC at surgery, and asymptomatic NVC is frequently observed both in healthy controls and on the contralateral (unaffected) side in patients with TN.^24,25 It does appear, however, that imaging features such as nerve displacement and atrophy may be more specific, although less sensitive, for predicting whether radiographic NVC is symptomatic.^26,27

MVD remains the preferred surgical treatment for medically suitable patients with type 1 TN. MVD offers a high success rate, the longest duration of pain relief, and is nondestructive, thus avoiding complications associated with deafferentation. The surgical goal is to identify and mechanically isolate an irritating vessel from the cisternal segment of the trigeminal nerve.

After the induction of general anesthesia, the patient is positioned supine with the head turned away from the affected side and secured in pin fixation, with the neck slightly flexed and elevated. The approximate courses of the transverse and sigmoid sinuses are outlined on the scalp (Fig. 104.3). We place electrodes for auditory brainstem responses to monitor for traction on the eighth cranial nerve. We have found facial nerve monitoring unnecessary.

The surgical field is shaved, prepped, and draped. We open a linear incision two fingerbreadths behind the base of the pinna and extending from the top of the pinna, over the digastric notch, and to the level of the mastoid, pointing toward the ipsilateral foot (see Fig. 104.3). A pericranial graft is harvested from above the nuchal line for later use in the dural closure.
The suboccipital musculature is dissected away, and a burr hole is placed near the junction of the venous sinuses. We use a combination of high-speed drilling and rongeur instruments to create an approximately 3-cm craniotomy, exposing the edges of the transverse and sigmoid sinuses superiorly and laterally, respectively. Mastoid air cells are waxed.

The dura is opened in a stellate manner. A handheld brain retractor is used to depress the cerebellum, and CSF is aspirated from the cisterna magna, slackening the hemisphere. Some surgeons place a lumbar drain at the start of the case or perform a lumbar puncture intraoperatively.

The operating microscope is brought in, and under constant monitoring of the auditory brainstem responses, a fixed retractor is used to displace the cerebellum from the lateral skull base (Fig. 104.4). We identify the complex of the seventh and eighth nerves coursing out to the internal auditory canal. A petrosal vein or venous complex usually obscures the view of the (deeper) trigeminal nerve; this can be coagulated and divided safely.

Arachnoid adhesions are taken down sharply, and the trigeminal nerve is traced to its entry zone in the lateral pons (see Fig. 104.4). The most common offending vessel is the superior cerebellar artery (>80%), which generally contacts the superomedial surface of the nerve. Other vascular offenders may include the anterior inferior cerebellar artery and posterior fossa veins. The segment of the nerve that is most sensitive to mechanical irritation is the transition zone from central to peripheral myelin, which occurs approximately 1 to 3 mm from the brainstem^27,28; sometimes an area of demyelination is visually apparent as a discolored patch on the surface of the nerve.

A straight dissector is used to tease any offending vessel away from the nerve. Teflon pledgets are interposed between the vessel and the nerve, with care taken to ensure the vessel is not kinked in the process.

The cerebellar retractor is removed, and the field is irrigated. We close the dura watertight using a pericranial graft. The duraplasty is coated with a layer of fibrin glue, and the bony defect is corrected with bone cement or a titanium mesh. The soft tissue is closed in layers, culminating in an absorbable stitch on the skin. Patients are monitored in the intensive care unit for one night and usually discharge on the second postoperative day. They are initially maintained on their baseline TN medication regimen, which is then weaned later as an outpatient.