History

TN has been known since ancient times. Two thousand years ago, Aretaeus of Cappadocia referred to headache with “spasm and distortion of countenance,” and Persian scholar Avicenna described a similar syndrome of facial pain in the 10th century.¹ In the 17th century, Fehr and Schmidt described the syndrome in a eulogy.² John Locke described facial pain of the Countess of Northumberland, wife of the English Ambassador to France, as a “fit of such violent and exquisite torment that it forced her to … cries and shrieks … which extended itself all over the right side of her face and mouth.”¹ In 1756, Nicolas Andre coined the term tic douloureux, and described it as “a cruel and obscure illness, which causes … in the face some violent motions, some hideous grimaces which are an insurmountable obstacle to the reception of food, [and] which put off sleep.”³ The first comprehensive clinical description occurred in 1773 when John Fothergill wrote to the medical society of London about 14 patients with TN, and his descriptions of triggerable lancinating pain are still considered accurate.⁴

Early surgical treatments for TN involved intentional lesioning of the trigeminal nerve. In 1934, Walter Dandy described the retrosigmoid approach to the trigeminal nerve and noted frequent vascular contact in patients with TN. He wrote, “In many instances the nerve is grooved or bent in an angle by the artery. This I believe is the cause of tic douloureux.”⁵ Despite his insight, Dandy did not attempt to decompress the nerve, believing the pathologic process to be irreversible. Microvascular decompression (MVD) was first performed by W. James Gardner in 1959, who described mobilizing a vessel from the trigeminal nerve and placing a piece of Gelfoam between them without any intentional damage to the nerve itself.⁶ The procedure was subsequently refined and popularized by Peter Jannetta, who, with the aid of the operating microscope, performed thousands of MVD operations and demonstrated that long-term relief of pain is possible in most of appropriately selected patients.^7–9

Pathophysiology

The etiology of TN is believed to be related to abnormal conduction within the trigeminal nerve, possibly owing to changes in myelin induced by pulsatile mechanical trauma from an adjacent vessel. At the point just before it enters the brainstem, there is a short segment where nerve axons are still ensheathed in central myelin (produced by oligodendrocytes), but after a few millimeters, there is a transition to peripheral myelin (produced by Schwann cells). The region of this transition is called the Obersteiner-Redlich zone. It is thought that the area of the nerve containing the central form of myelin is especially susceptible to pathologic changes from vascular contact that result in demyelination and altered conduction. Pathologic studies from patients with TN have demonstrated severe damage to myelin as well as axon loss within the nerve adjacent to the site of compression.¹⁰ The resulting conduction abnormality may lead to nerve hyperactivity owing to ectopic impulse discharge, spontaneous and triggered afterdischarge, and cross-excitation among neighboring afferent fibers (ephaptic transmission).^10,11

Separation of the nerve from the offending vessel appears to immediately reverse many of the physiologic changes. In Leandri and colleagues’¹² study of 10 patients undergoing MVD who underwent nerve root and scalp electrode recordings, 7 showed signs of immediate improvement in neurophysiologic parameters after decompression. Others authors have reported improvement in sensory thresholds for touch, pinprick, and temperature sensations after MVD¹³ as well as resolution of asymmetric jaw motion.^14,15 These findings suggest that the changes associated with neurovascular compression are likely to be reversible if the nerve is decompressed, at least in the early stages.

Radiographic and anatomic studies have demonstrated that vascular contact with the trigeminal nerve is common even in asymptomatic individuals but tends to be more severe and more proximal on the nerve ipsilateral to TN symptoms.¹⁶ Patients with TN are more likely to have contralateral arterial compression than asymptomatic people even though bilateral TN is distinctly rare.^16,17 Symptomatic and asymptomatic arterial compression of the trigeminal nerve increases with age because of elongation of cisternal arteries, which explains why it is primarily a disease of older adults.¹⁶

Alternative Treatments

TN symptoms often improve with medications that exert a stabilizing effect on neural conduction such as antiepileptics. Medications that have been successfully used include carbamazepine, phenytoin, valproate, gabapentin, pregabalin, baclofen, and clonazepam. Most patients obtain good pain control initially,^18,19 but the effect tends to diminish over time, and after 10 years about half will no longer respond.²⁰ Because the clinical and pathologic changes associated with TN may be progressive over time, initial failure of pharmacologic therapy may represent an indication to proceed with more aggressive treatment.²¹ Nevertheless, medical therapy is recommended as a first-line treatment for patients with TN because some patients will require no further treatment.

Surgical treatments for TN other than MVD generally involve intentional production of a lesion within the trigeminal nerve. Many commonly performed procedures involve percutaneous access to the nerve through a needle advanced through the face followed by direct creation of a lesion using a radiofrequency generator, glycerol injection, or balloon compression. Stereotactic radiosurgery targeting the nerve root entry zone is also effective, although pain relief is delayed by months, and complete elimination of pain may occur less frequently than with other methods. In general, the success of each of these lesioning techniques requires some degree of facial numbness, with greater numbness associated with a higher rate of pain control but also greater likelihood of complications such as facial dysesthesia and anesthesia dolorosa. Also, all these procedures are associated with a high recurrence rate after a few years as facial sensation returns, suggesting that they work primarily by blocking triggering impulses rather than by treating the cause of the pain itself.

MVD differs from the other treatments in that the primary cause of TN is treated so that long-term pain relief is possible. Also, because there is no intentional damage to the nerve, facial dysesthesia and numbness are rare. Although it is the most invasive and expensive treatment, MVD is associated with the lowest rate of pain recurrence and the highest rate of patient satisfaction among all surgical treatments for TN.²² MVD can also be safely performed after a lesioning procedure and appears to be no less effective so long as there is no evidence of trigeminal neuropathy.²³

Patient Selection and Classification of Facial Pain

MVD is ideal for young healthy patients with TN because no other treatment offers a significant likelihood of long-term pain relief.²⁴ However, advanced age is not by itself a contraindication because there is no difference in complication rate or outcome in elderly patients.^25–27 The operation is in fact technically easier in older patients because cerebellar atrophy leads to less need for retraction and less risk for cerebellar swelling. If life expectancy is very short or general anesthesia cannot be tolerated, a less invasive destructive procedure may be more appropriate.^28,29

A careful history is essential during preoperative evaluation. Patients generally report intense stabbing or electric shock–like sensation, although there may be an overlying constant pain that may be more severe than the stabbing pain. Any distribution within the trigeminal nerve innervation territory may be observed. V2 and V3 branches are more common,⁷ especially radiating out from near the mouth. V1 symptoms are sometimes associated with decreased corneal sensation. The pain is often worse during the day and may be positional with relief when supine, with the affected side up, or during sleep. Trigger points are present in most patients and are activated by light cutaneous stimuli such as wind, eating, talking, and shaving. Often, the triggers lead to guarding of the face and refusal to be touched, wash, apply makeup, shave, or brush the teeth because of fear of an attack. Pain-free intervals lasting weeks to months are common at first but become rare as the syndrome progresses. Initial onset of pain is frequently quite memorable. Many patients undergo dental procedures without relief before a diagnosis is made. If the patient is given antiepileptic medication, pain usually improves dramatically. Physical examination is usually normal, although about one third of patients have some degree of sensory loss in the affected area.

When evaluating a patient for surgery, it is helpful to classify facial pain according to the classification scheme reported by Burchiel.³⁰ Patients with TN type 1 have predominantly shock-like pain, whereas patients with TN type 2 report that at least 50% is constant pain, although there still may be a component of lancinating pain. Pain relief after MVD is more strongly correlated with the lancinating pain component than with any other symptom, so although most patients with either type will have long-term pain control, patients with TN type 1 are more likely to do well than those with TN type 2 (Miller and coworkers, unpublished data). Facial pain diagnoses other than TN are unlikely to improve after MVD. TN with a history of multiple sclerosis (MS) is called symptomatic trigeminal neuralgia (STN). MS is present in 1% to 3% patients with TN, and 2% to 4% of patients with MS have TN, probably due to intrinsic demyelination within the nerve or increased sensitivity to vascular trauma.^31,32 Although MS patients sometimes improve after MVD, the recurrence rate is higher, and long-term elimination of pain is rare,^33,34 so destructive procedures may be more appropriate for these patients.³⁵ Sensory loss with burning pain is a sign of trigeminal neuropathic pain (TNP); if it occurs after a previous destructive procedure, this is called trigeminal deafferentation pain (TDP). Allodynia and dysesthesia with a history of herpes zoster suggest postherpetic neuralgia (PHN). MVD is not a good option for any of these patients. Atypical facial pain (AFP), which refers to pain of psychological onset, requires neuropsychological testing for the diagnosis and is unlikely to improve after MVD.

Pain outside the trigeminal nerve distribution is not TN, although other vascular compression syndromes may be present with throat pain (glossopharyngeal neuralgia) or pain deep in the ear (nervus intermedius neuralgia). Other causes of facial pain include dental disease, orbital disease, sinusitis, cluster headache, temporomandibular joint disease, temporal arteritis, and posttraumatic neuralgias. None of these have the clinical characteristics of TN, nor do they respond to MVD of the trigeminal nerve.

Preoperative Imaging

All patients should undergo magnetic resonance imaging (MRI) before MVD to verify that there is no intracranial mass lesion that may be responsible for the pain, such as a tumor, cyst, aneurysm, or arteriovenous malformation. MRI also can identify evidence of demyelinating disease or inflammatory changes and may reveal anatomic characteristics that would make MVD technically problematic, such as an ectopic basilar artery or bony abnormalities. High-resolution MRI using steady-state precession images can be used to identify neurovascular compression preoperatively with a high degree of accuracy, and fusion with magnetic resonance angiography followed by three-dimensional reconstruction can be used to simulate the intraoperative view for surgical planning (Fig. 163-1).³⁶

FIGURE 163-1 Preoperative magnetic resonance imaging to identify arterial compression of the trigeminal nerve before microvascular decompression. Steady-state precession imaging demonstrates contact of a vessel with the nerve (A), and magnetic resonance angiography reveals that the vessel is an artery (B). Fusion and three-dimensional reconstruction of these sequences produce an image (C) that corresponds well with intraoperative findings (D).

(Adapted from Miller J, Acar F, Hamilton B, et al. Preoperative visualization of neurovascular anatomy in trigeminal neuralgia. J Neurosurg. 2008;108:477, 2008.)

Operative Technique

Standard neuroanesthetic techniques are used, with chemical paralysis and controlled ventilation to prevent motion in the field. Use of diuresis or spinal drainage for brain relaxation is generally not necessary because release of cerebrospinal fluid (CSF) from the trigeminal cistern will relax the cerebellum enough to allow adequate retraction. Intraoperative monitoring is helpful to prevent injury to the brainstem and cranial nerves. Brainstem auditory evoked potentials are very sensitive to stretch-induced injury to the eighth cranial nerve, and a delay over baseline of greater than 20% or shift in interpeak latency of more than 1.5 to 2 milliseconds requires loosening of cerebellar retraction until signals normalize. Facial nerve monitoring may also be used but is less helpful.

A cranial fixation device such as the three-pin Mayfield head holder is applied before positioning. There are several options for patient positioning for MVD. The simplest option is to place the patient in a flat supine position with the head rotated and flexed to the opposite side. Ideally, no shoulder roll should be used so that the ipsilateral shoulder does not obscure the operative approach. This position requires a flexible neck and may not work for obese or short-necked patients. Other options include the lateral decubitus or three-quarter prone position with the shoulder taped caudally and neck flexed, ensuring the chin is at least two fingerbreadths from the sternum. Regardless of the position chosen, it is generally best to place the vertex parallel to the floor so that the seventh and eighth cranial nerves are inferior relative to the trigeminal nerve, simplifying the approach.³⁷ All pressure points are padded, and an axillary roll is used if necessary. The sitting position can also be used, although it is associated with complications such as air embolism and subdural hematoma.

Preoperative MRI is used to evaluate the anatomy of the sinuses, cerebellum, and any anomalous veins. A small posterior fossa or Chiari malformation may necessitate alteration of surgical technique. The position of the transverse and sigmoid sinuses may be estimated from bony landmarks. The transverse sinus generally runs along a line connecting the inion to the external auditory meatus, parallel and posterior to the zygomatic arch. The sigmoid sinus runs along the digastric groove posterior to the mastoid eminence. Alternatively, frameless stereotaxy may be used to accurately define the position of the sinuses. Hair is clipped from the surgical site, and the scalp is sterilized and then infiltrated with local anesthetic with epinephrine to reduce bleeding. The linear incision runs longitudinally two fingerbreadths behind the ear (5 mm behind the hairline) and extends 3 to 5 cm, with one fourth of the incision above the iniomeatal line. Monopolar electrocautery is used to dissect and clear muscle and soft tissue until the bone of the mastoid eminence and digastric groove is seen. The occipital artery is often encountered in the muscular tissue and sacrificed. A large mastoid emissary vein is usually seen at this point, which represents an important landmark because it overlies the junction of the transverse and sigmoid sinuses. The bony exposure is extended to the curving portion of the suboccipital bone at the floor of the posterior fossa. The wound is held open with a self-retaining retractor or with sutures. A dural graft is generally not necessary, but if desired, the periosteum along the superior nuchal line may be harvested, or fascia may be collected if a C-shaped incision was used.