Trigeminal neuralgia (TN) is an extremely painful yet rare orofacial pain condition, and clear understanding of its mechanism and management remains an enigma.¹ Originally known as tic douloureux due to its characteristic facial tics, this disorder may be described as a paroxysmal unilateral facial pain along the distribution of one or more branches of the trigeminal nerve. The maxillary and mandibular branches of the trigeminal nerve are most commonly affected, followed by the ophthalmic branch.² The incidence of TN is 4 per 100,000 individuals, but increases to 20 per 100,000 in those over age 60. It is more common in females, with a 3:2 female:male ratio, and typically presents in individuals aged 50 or older.¹
Trigeminal neuralgia has a variety of etiologies: mass effect from metastases, viral infection, trauma, autoimmune processes, and drug side effects. These processes are considered to lead to impaired trigeminal nerve function, often attributed to demyelination.
Pain associated with TN is described as severe, sharp, electric, and stabbing, is unilateral (less than 4% of cases are bilateral) and usually remains within the anatomic distribution of the affected nerve branch, regardless of remissions and exacerbations.³ TN pain is often triggered by chewing and swallowing, and may even be triggered by a light touch to the face. The pain is often relieved by sleep and may have periods of remission lasting weeks to years.⁴ While symptoms of TN are limited to pain, the quality of life for patients is profoundly worsened, which puts them at a high risk for depression and other psychosocial disorders.⁵˒⁶ As previously noted, patients with TN tend to be asymptomatic when asleep, suggesting a bidirectionality between neuropathic pain and sleep.⁷ As sleep quality improves, pain sensitivity has been shown to decrease.
Differentiating a TN Diagnosis
The International Headache Society (IHS) separates TN into two categories, classic and secondary, described as:
Classic trigeminal neuralgia includes neuralgia that is idiopathic or caused by compression of the trigeminal nerve from a nearby blood vessel. Research has demonstrated that classic TN accounts for approximately 80% of TN cases.⁸
Secondary trigeminal neuralgia accounts for cases triggered by other structural abnormalities, such as tumors and demyelination from multiple sclerosis (MS).⁵
Diagnosis of TN typically begins with taking an accurate medical history and a detailed description of the pain and distribution pattern. Patients often provide a history of sharp, intense facial pain that lasts several seconds, with repeated attacks throughout the day. These attacks may reoccur up to a few hundred times per day and continue for weeks or months.² While the timing and triggers associated with these attacks seem to be unpredictable, patients rarely report pain that interrupts their sleep.
Given the variation in presentation associated with TN, the diagnosis requires a high index of clinical suspicion. Notation should be made of any pattern consistent with the fifth cranial nerve and of exacerbating and/or relieving factors. Additionally, the quality of the pain should be clearly assessed for sensations such as electric shock, burning, or scintillation. Physical exams should focus on common features of pain found with TN: allodynia, hyperalgesia, or triggers. A thorough neurologic exam is often essential to evaluate cranial nerve deficits such as hearing loss, sensory changes, or facial nerve paresis.¹
Magnetic resonance angiography (MRA) is the imaging modality of choice to highlight cranial blood vessels; notches in the blood vessels surrounding the trigeminal root have been reported as an indication that blood vessels are impinging upon the nerve.¹
Facial and Dental Mimics
Since facial and/or tongue movements associated with chewing and swallowing may trigger TN, this disorder is distinct from masticatory pain. Masticatory pain is typically not relieved by a conventional mandibular block anesthetic injection, since the nerves mediating the pain from the joint or the masticatory muscles are not anesthetized.
Neuropathies such as TN may also cause tooth pains of non-dental origin that may be difficult to identify. It has not been unusual to examine a patient with classic TN who has lost some or all of their teeth up to the midline on the side of their pain. In fact, it is not uncommon for patients to undergo several unnecessary dental procedures due to an improper understanding of the pain source. The assessing clinician needs to be aware that characteristics of TN may mimic and contribute to the misdiagnosis of non-odontogenic toothaches. On the other hand, clinicians should be aware that trigeminal neuralgia shares similar symptoms with various pain conditions such as occipital neuralgia and temporal tendinitis, so care must be taken to properly diagnose and treat it effectively.
Standard Treatment Options
Medical management with anticonvulsant medications is considered first-line for trigeminal neuralgia due to the sodium channel blocking mechanisms of these medications.⁹ Anticonvulsants, it is argued, work due to the fact that, in areas of nociceptor damage, neurons generate a higher concentration of sodium channels. Such an increase in sodium channels cause increased depolarization of the affected fibers and result in increases of spontaneous pain.⁴
While the anticonvulsant phenytoin has historically been used to treat TN, newer agents for medical management exist with fewer side effects. In 2008, the American Academy of Neurology (AAN) and the European Federation of Neurological Societies (EFNS) Task Force suggested that carbamazepine or oxcarbazepine (medications used traditionally for bipolar disorder, neuralgia, and seizure) be used as first-line options. Second-line agents recommended include baclofen (a centrally-acting muscle relaxer) or lamotrigine (a medication for seizure and bipolar disorder).⁸ Additional treatment modalities may include gabapentin, pregabalin, topiramate, valproate, and lamictal (anti-seizure and/or bipolar medications).¹⁰ Lastly, the use of botulinum toxin has shown some promise in small trials.¹¹
Approximately 60 to 70% of patients with TN experience some level of control with medication.¹ Even so, some patients may experience breakthrough pain while on multiple medications at maximum dosage.⁹ Opiates are often used to manage breakthrough TN pain; their use is limited, however, due to a need for high doses, at which point the patient may experience significant sedation and other adverse effects, including the potential for addiction.¹
Pharmacologically, refractory cases may progress to surgical intervention at the trigeminal nerve.⁹ A rhizotomy may be performed in such cases to destroy the pain fibers (A delta and C) in the nerve root, or a gangliolysis may be done to destroy such pain fibers in the trigeminal ganglion. These procedures are primarily a temporary solution as the fibers often regenerate after 1 to 2 years.
A more long-term solution is the utilization of stereotactic radiosurgery (Gamma Knife) to scar the trigeminal root and remove blood vessels that may compress the nerve.⁴ With a lack of evidence for surgical procedures, several sources recommend Gasserian ganglion percutaneous techniques, Gamma Knife (Elekta), or microvascular decompression (microvascular decompression is considered to provide the longest duration of relief).¹²
Another treatment option in the maxillofacial field involves intraoral injections of sodium channel blockers adjacent to the suspected involved portions of the trigeminal nerve. In keeping with this model, both the pain and triggering of TN sensitivity that occurs in the receptor area of the affected nerve may be accurately tested. This effect may be considered diagnostic, especially in clinical cases where differentiating from masticatory pain may be difficult.¹³
Finally, treatment options are generally available across the osteopathic, chiropractic, acupuncture, and functional medicine disciplines.
Discussion
The severity of pain associated with TN and its devastating psychological effects make prompt diagnosis and treatment essential. Without a proper understanding of its clinical features and medical course, TN may go undiagnosed for years. With a proper diagnosis and early treatment, however, patients with this neuropathic disorder may achieve pain relief, and resume a healthy life.
The neuropathic disorder has been associated with demyelination of axons in the Gasserian ganglion and the dorsal root that is thought to stem from pulsations from a vessel or artery abutting the trigeminal root.⁸ One theory attributes the sensitive nature of this condition to ephapsis; therefore, when afferent impulses generated distally propagate through demyelinated areas, the action potential could theoretically spread to other nerves. For instance, a light touch could cause sharp pain when an action potential traveling down an A beta fiber (measures light touch) could jump (ephapse) onto an adjacent demyelinated A delta fiber (signals sharp pain).⁴
Conclusion
While the etiology of trigeminal neuralgia varies and is often considered secondary to vascular compression of the trigeminal nerve leading to axonal demyelination, it may also occur as a result of structural abnormalities. Once correctly diagnosed with other pathologies ruled out, treatment may entail anticonvulsant medications such as carbamazepine and oxcarbazepine. Most patients with TN are well-controlled by these first-line medications, but refractory cases or breakthrough pain may be treated with a surgical procedure to destroy pain fibers or remove the compression on the trigeminal nerve. Current procedures include a rhizotomy, Gamma Knife, microvascular decompression, and Gasserian ganglion percutaneous technique.