Trigeminal neuralgia

Overview

Trigeminal neuralgia also known as tic douloureux, is a painful and greatly debilitating affliction of the face. Symptoms have been described as excruciating, stabbing, electrical, shock-like, lancinating, or convulsive. Pain classically localizes to one side of the face although bilateral cases have been described. Symptoms typically affect the cheek and jaw and less commonly the forehead. Pain may be accentuated by eating, and light tactile stimuli such as a facing a breeze, applying makeup, brushing teeth, or talking. Pain generally occurs in discrete and brief attacks followed by a variable period without any symptoms whatsoever. In other more chronic cases, the duration between attacks changes and a lower level of dull pain may persist between paroxysms. Although this condition was recognized in antiquity, it was not clearly described until the 17th century by the renowned physician and philosopher, John Locke. Despite this long history of however, trigeminal neuralgia is often misdiagnosed and treatment delayed.

Demographics

There are 14,000 new cases trigeminal neuralgia diagnosed in the United States every year with about 150,000 people currently living with the condition. Trigeminal neuralgia tends to affect women more than men and people over the age of 50 although it has been described in younger people and children. There can be clustering of the condition in families and risk factors include multiple sclerosis and hypertension.

Anatomy

The trigeminal nerve is the fifth of the 12 cranial nerves. Cranial nerves arise from the undersurface of the brain and mediate a variety of elemental functions ranging from vision and the sense of smell to swallowing and the modification of heart rate in times of stress. The nerve originates in a subdivision of the brainstem called the pons and travels through the liquid filled intracranial cavity to form the Gasserian or Semilunar ganglion. This structure houses the cell bodies of the sensory portion of nerve and is situated in a niche on the floor of the skull called Meckel's cave. The nerve then divides into 3 main bundles as it exits the base of the skull to enter the face. The first or 'ophthalmic' division is responsible for transmitting sensory information of the eye, upper eyelid and forehead. The second 'maxillary' branch receives sensory information from the lower eyelid, cheek, nostril, upper lip and gum. The third and final branch called the 'mandibular' delivers sensation of the jaw, lower lip and gum as well as fibers innervating the muscles that allow for chewing.

Pathophysiology - The Source of Pain

The symptoms of trigeminal neuralgia are felt to result from a dysfunction or "short-circuit" of the electrical conductivity of the trigeminal nerve. This injury develops from a chronic compression of the nerve by an otherwise normal artery or vein. With each heartbeat, the offending vessel distorts the apposed nerve causing for a subtle but progressive injury (Figure 1). Pathologically, this injury is characterized by a loss of the protective fatty insulation of the nerve fibers thereby resulting in hypersensitivity of the nerve.
In a separate proportion of patients presenting with typical trigeminal neuralgia symptoms, nerve dysfunction is the result of compression or infiltration by a neoplastic, inflammatory or infectious process of the brain or skull-base. This 'secondary' trigeminal neuralgia should be identified early in the patient's clinical course to allow for an expedited treatment of the underlying condition. Alternatively, the adjective 'idiopathic' is used to describe trigeminal neuralgia in circumstances where there is no identifiable tumor or infection causing symptoms.

Diagnosis

The diagnosis of trigeminal neuralgia is established by the history provided by the patient. In most cases, pain develops spontaneously although facial trauma and dental surgery are often associated with onset of symptoms. A patient's initial experience of pain is a memorable event and is charachterized by a sudden and cataclysmic shock of pain. Attacks occur therefter at variable rates. Localization of pain in the cheek and lower jaw are classic and these symptoms may be mistaken for a dental condition. Pain is often reproducible by touching a defined region in the face or 'trigger point'. Symptoms are episodic often lasting only seconds; periods of pain-free quiescence may last days or weeks. The frequency of attacks, however varies greatly from several within an hour to only once a day. At times, the symptoms become unbearable causing facial twitching. Patients in whom pain the great proportion of symptoms are episodic are diagnosed with 'typical' trigeminal neuralgia and generally respond well to medical and surgical treatments.

A subgroup of patients demonstrate symptoms that are described as burning or throbbing and are constant without the lightning-like attacks defined by typical trigeminal neuralgia and without pain free periods. At times, the character of pain in individuals with long standing typical trigeminal neuralgia evolves to this more indolent pattern of discomfort. Others present with this pattern of pain de novo. This subgroup of patients has been classified as atypical trigeminal neuralgia and classically respond less well to treatment.

MR imaging remains an essential component of patient evaluation first and most importantly to distinguish between idiopathic and secondary trigeminal neuralgia. Additionally, imaging may demonstrate vascular structures and their relation to the course of the trigeminal nerve (Figure 2).

Differential Diagnosis - Similar but Distinct Conditions

Other conditions that may be confused with trigeminal neuralgia include glossopharygeal neuralgia, geniculate neuralgia, cluster headaches, dental abscess, temporomandibular joint disorders (TMJ) and hemifacial spasm. A careful review of the patient's description of the character and localization of symptoms will help to differentiate between these entities.

Medical Treatment

Medical management of trigeminal neuralgia is very effective and a beneficial response to these treatments further supports the clinical diagnosis. Most medications available had initially been developed for the management of seizure disorders and were used for trigeminal neuralgia as a secondary application. These compounds dampen the electrical conductivity of the nerve thereby reducing the excitability of fibers mediating pain. Medications most commonly administered include Tegretol (Carbamazepine), Dilantin (Phenytoin), Neurontin (Gabapentin), Trileptal (Oxycarbazepine) among others. Although medical treatment may be successful for years and with limited side effects, there are limits to treatment. Patients must be routinely followed for kidney, liver and bone marrow function. Additionally, higher doses and/or multi-drug therapy may be required to treat symptoms that recur over time.

Surgical Management

Surgical management of trigeminal neuralgia should be reserved for patients that have not responded lastingly or substantially to medical therapies. As indicated earlier, patients that had benefitted from medicine are more likely to realize results from surgical procedures. Surgical management may be classified into two distinct categories. Historically, the earliest operations focused on injuring or sectioning the nerve in order to diminish the appreciation of pain. These techniques form the basis of a substantial number of operations offered today including peripheral neurectomy, percutaneous rhizotomy and Gamma Knife radiosurgery. Alternatively, microvascular decompression, a surgery developed in the late 1960's focuses on repositioning the artery or vein from the distorted and injured trigeminal nerve via a direct open surgical approach.

Figure 1
This intra-operative view demonstrates a the distortion of the trigeminal nerve (CNV) by a petrosal vein (Petro V). The origin of the nerve from the brainstem is seen (Pons).

Figure 2
This axial view shows a vascular structure (Petrosal V.) distorting the right fifth cranial nerve (Trigeminal) in a patient with right sided trigeminal neuralgia.