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Summary Table

Feature	Detail
Risk Factors	Female sex, age > 50, history of multiple sclerosis, posterior fossa tumor, hypertension (associated with arterial ectasia)
Etiology	Neurovascular compression of the trigeminal nerve root entry zone, most commonly by the superior cerebellar artery
Patient Presentation	Recurrent episodes of sudden, severe, unilateral, lancinating (electric shock-like) facial pain lasting seconds to < 2 minutes, often in the cheek or jaw distribution
Classic Physical Exam	Normal neurological examination between attacks; pain triggered by light touch to the face, chewing, talking, brushing teeth, or cold wind; the presence of a "trigger zone" on the affected side
Key Diagnostic Results	Clinical diagnosis; MRI brain to exclude secondary causes (MS plaques, cerebellopontine angle tumor); MRA may show vascular loop compressing CN V
Management	First-line: Carbamazepine. Second-line: Oxcarbazepine. Refractory: Microvascular decompression (MVD) surgery
Keywords	"Electric shock-like," "lancinating," "triggered by chewing/brushing teeth," "lasts seconds," "V2/V3 distribution," "trigger zone," "normal neuro exam"

1. Pathophysiology

Trigeminal neuralgia arises from demyelination of the trigeminal nerve root at its entry zone into the pons. In the classical form, the culprit is neurovascular compression, most frequently by the superior cerebellar artery (and less commonly the anterior inferior cerebellar artery or a venous structure). Chronic pulsatile contact between the vessel and the nerve damages the myelin sheath at the transition zone between central (oligodendrocyte-derived) and peripheral (Schwann cell-derived) myelin, which is the most vulnerable segment of the nerve.

This focal demyelination leads to a phenomenon called ephaptic transmission, which is abnormal cross-talk between adjacent nerve fibers. Light touch fibers (A-beta) that are normally innocuous begin to activate nociceptive (A-delta) pain fibers through short-circuit connections at the site of myelin damage. This is why a gentle stimulus such as a breeze on the cheek or light touch during tooth brushing can provoke excruciating pain. The pain is paroxysmal because the ectopic discharges fire in bursts, producing the characteristic "electric shock" quality that lasts only seconds and then abruptly stops.

In secondary trigeminal neuralgia, the demyelination is caused by a structural lesion rather than a vessel. Multiple sclerosis produces a demyelinating plaque at the root entry zone of CN V in the pons, while a cerebellopontine angle tumor (such as a meningioma, schwannoma, or epidermoid cyst) compresses the nerve externally. This distinction is critical because secondary causes tend to affect younger patients (< 40 years old), may involve the V1 (ophthalmic) division more frequently, and can produce sensory deficits between attacks, which classical trigeminal neuralgia does not.

The reason carbamazepine works as first-line treatment ties directly back to this mechanism: it stabilizes sodium channels in demyelinated axons, reducing the capacity for ectopic firing and ephaptic cross-excitation.

2. Classification and Clinical Manifestation

Type	Mechanism	Key Features
Classical	Neurovascular compression (most often superior cerebellar artery)	Age > 50; MRI/MRA shows vascular contact; no neurological deficit
Secondary	Structural lesion (MS plaque, CPA tumor)	Age < 40; may have sensory loss, bilateral involvement, or V1 distribution; MRI shows the causative lesion
Idiopathic	No identifiable cause on imaging	Clinically identical to classical form but MRI/MRA shows no vascular compression or structural lesion

By Division

CN V Division	Territory	Frequency	Trigger Examples
V1 (Ophthalmic)	Forehead, upper eyelid, bridge of nose	Rare (~4%); think secondary cause if isolated V1	Wind on forehead
V2 (Maxillary)	Cheek, upper lip, upper teeth, nasal ala	Most common (~35% alone, ~35% combined V2+V3)	Brushing upper teeth, touching cheek, smiling
V3 (Mandibular)	Lower lip, chin, lower teeth, jaw	Common (~30% alone)	Chewing, talking, shaving
V2 + V3	Cheek through jaw	Very common	Any oral/facial stimulus

Pain Characteristics

Feature	Description
Quality	Lancinating, electric shock-like, stabbing
Duration	Each paroxysm lasts seconds to < 2 minutes
Frequency	Can occur dozens to hundreds of times per day in clusters
Laterality	Strictly unilateral (bilateral involvement strongly suggests MS)
Trigger zone	A small area on the face that reliably provokes an attack when touched
Refractory period	Brief pain-free interval after each paroxysm during which a new attack cannot be triggered
Between attacks	Completely normal neurological exam (in classical form)

3. Diagnostic Workup

Test	Role	What to Look For
Clinical history and exam	Best initial diagnostic step	Classic paroxysmal pain profile + normal neurological exam = working diagnosis
MRI brain (with and without contrast)	Rule out secondary causes	MS demyelinating plaques in the pons; CPA mass (schwannoma, meningioma)
MRA or high-resolution MRI (CISS/FIESTA sequence)	Identify neurovascular compression	Vascular loop in contact with the trigeminal nerve root
Trigeminal reflex testing / neurophysiology	Supportive in equivocal cases	Abnormal blink reflex or trigeminal evoked potentials suggest structural pathology

The diagnosis of trigeminal neuralgia is primarily clinical. If the history reveals unilateral, paroxysmal, electric shock-like pain in the V2 or V3 territory, lasting seconds, provoked by innocuous facial stimuli, and the neurological exam between episodes is completely normal, the diagnosis is essentially established at the bedside. No lab test or imaging study is needed to confirm it.

However, the next best step after clinical diagnosis is MRI of the brain with and without gadolinium contrast. The purpose is not to confirm trigeminal neuralgia but to exclude secondary causes. You are looking for two things: (1) a demyelinating plaque in the pontine root entry zone suggesting multiple sclerosis, and (2) a mass lesion in the cerebellopontine angle or Meckel's cave. This step is mandatory for every newly diagnosed patient.

If MRI is normal and surgical treatment is being considered, a dedicated MRA or high-resolution MRI with CISS (constructive interference in steady state) or FIESTA sequences can be ordered to visualize the relationship between the trigeminal nerve and adjacent vessels. Identifying a compressive vascular loop preoperatively helps plan microvascular decompression.

There is no blood test that diagnoses trigeminal neuralgia. The exam will not ask you to order labs for this condition. The critical teaching point is: the best initial test is the clinical history, and the most important imaging study is MRI brain to rule out secondary pathology.

Red flags that should prompt urgent imaging:

Age < 40 at onset
Bilateral symptoms (think MS)
Sensory loss or motor weakness of CN V (jaw deviation)
V1 distribution involvement
Progressive, non-paroxysmal pain
Other cranial nerve deficits

4. Management and Treatment

Setting	Intervention	Details
First-line medical therapy	Carbamazepine	Start 100 mg twice daily; titrate up by 100 mg every 2 days to a target of 200-400 mg twice daily (max 1200 mg/day)
Alternative first-line	Oxcarbazepine	Start 150 mg twice daily; titrate to 300-600 mg twice daily (max 1800 mg/day); better side effect profile
Adjunctive / second-line	Baclofen, lamotrigine, gabapentin	Added when monotherapy provides insufficient relief
Acute rescue	IV phenytoin or IV fosphenytoin	For severe exacerbations (status neuralgicus); rarely tested
Refractory: definitive surgery	Microvascular decompression (MVD)	Gold standard surgical option; highest long-term cure rate (~80% pain-free at 5 years)
Refractory: percutaneous	Balloon compression, glycerol rhizolysis, radiofrequency thermocoagulation	For patients unfit for open surgery; higher recurrence rates
Refractory: radiosurgery	Gamma Knife stereotactic radiosurgery	Non-invasive; onset of pain relief is delayed (weeks to months); useful in elderly or high-surgical-risk patients

Step 1: Start carbamazepine. This is the most tested treatment fact about trigeminal neuralgia. Carbamazepine is the first-line drug and the answer to nearly every management question on this topic. Begin at 100 mg twice daily and titrate upward every 2 to 3 days as tolerated. Most patients respond at 200 to 400 mg twice daily. The mechanism of action (voltage-gated sodium channel blockade) directly addresses the ephaptic transmission at the demyelinated nerve root.

Before and during therapy, monitor for adverse effects: hyponatremia (SIADH), aplastic anemia, agranulocytosis, hepatotoxicity, and Stevens-Johnson syndrome (strongly associated with the HLA-B*1502 allele, which is prevalent in Southeast Asian populations). Obtain a baseline CBC, hepatic function panel, and serum sodium. Check HLA-B*1502 status in patients of Southeast Asian descent before initiating carbamazepine. If the allele is positive, use oxcarbazepine with caution or choose an alternative agent.

Step 2: If carbamazepine fails or is not tolerated, switch to oxcarbazepine. Oxcarbazepine has a similar mechanism and comparable efficacy with a lower incidence of hematologic toxicity and drug interactions. It still carries a risk of hyponatremia.

Step 3: If monotherapy is insufficient, add baclofen (5-10 mg three times daily, titrate to 60-80 mg/day) or lamotrigine as adjunctive therapy.

Step 4: For medically refractory disease, refer for surgery. The gold standard is microvascular decompression (MVD), also known as the Jannetta procedure. The surgeon places a Teflon pad between the offending vessel and the trigeminal nerve root, relieving chronic compression. MVD offers the best long-term outcomes and is preferred in younger, medically fit patients.

For patients who are elderly, have significant comorbidities, or refuse open craniotomy, percutaneous procedures (radiofrequency thermocoagulation, balloon compression, glycerol rhizolysis) or Gamma Knife radiosurgery are alternatives. These procedures intentionally create a controlled lesion in the trigeminal ganglion or root, trading some degree of facial numbness for pain relief.

Contraindications and cautions:

Carbamazepine is a potent CYP3A4 inducer and reduces the efficacy of oral contraceptives, warfarin, and many other drugs.
In pregnancy, carbamazepine is teratogenic (neural tube defects); lamotrigine or gabapentin may be considered with careful risk-benefit discussion.
In patients with MS-related trigeminal neuralgia, MVD is not appropriate (because there is no vessel to decompress); medical therapy or ablative procedures are used instead.

5. Differential Diagnosis and Distractors

Differential	Why It Is Similar	Key Discriminator
Cluster headache	Severe, unilateral facial/periorbital pain in episodic attacks	Cluster pain lasts 15 to 180 minutes (not seconds); accompanied by autonomic features (tearing, nasal congestion, ptosis, miosis); periorbital, not V2/V3
Temporomandibular joint (TMJ) disorder	Jaw pain worsened by chewing	TMJ pain is dull and aching, not lancinating; associated with clicking, jaw deviation, or limited opening; pain is localized to the preauricular area
Dental abscess / pulpitis	Severe tooth pain triggered by eating	Pain is continuous or throbbing, not paroxysmal shocks; localized to a single tooth; exam shows caries, gingival swelling, or percussion tenderness
Glossopharyngeal neuralgia (CN IX)	Paroxysmal, lancinating pain triggered by swallowing	Pain is felt in the ear, throat, tonsillar fossa, not in the cheek or jaw; triggered by swallowing or coughing, not by touching the face
Postherpetic neuralgia	Unilateral burning/stabbing facial pain in a nerve distribution	History of preceding vesicular rash (herpes zoster); pain is continuous with burning quality, not purely paroxysmal; often in V1 (ophthalmic zoster) with possible scarring
Short-lasting unilateral neuralgiform headache attacks (SUNCT/SUNA)	Very brief unilateral stabbing facial/periorbital pain	Attacks are accompanied by prominent conjunctival injection and tearing; pain is periorbital (V1); does not respond to carbamazepine
Atypical facial pain (persistent idiopathic facial pain)	Chronic unilateral facial pain	Pain is constant, poorly localized, deep, and aching rather than paroxysmal; no trigger zones; does not fit any dermatomal distribution; often associated with psychological comorbidity

6. Traps and High-Yield Pearls

The single most common trap with trigeminal neuralgia questions is failing to recognize the diagnosis because the vignette disguises it as a dental problem. The test-writer will describe a patient who has seen multiple dentists, had unnecessary tooth extractions, and continues to have severe facial pain. The "dental red herring" is the classic distractor. The discriminating feature is the paroxysmal, seconds-long, electric quality of the pain and the presence of a trigger zone that reliably reproduces symptoms with light touch. Dental pathology produces continuous or throbbing pain, not instantaneous shock-like paroxysms.

A second common pitfall is choosing the wrong first-line drug. Gabapentin and pregabalin are reasonable for many neuropathic pain conditions, but for trigeminal neuralgia, the answer is carbamazepine (or oxcarbazepine). If a question asks about the initial pharmacologic treatment for classic paroxysmal facial neuralgia, carbamazepine is almost always the correct choice.

Third, be alert for the young patient with bilateral trigeminal neuralgia. If the vignette describes a patient under 40 with pain on both sides of the face, the diagnosis is not idiopathic trigeminal neuralgia. The answer they are testing is multiple sclerosis as the underlying cause, and the next best step is MRI of the brain looking for demyelinating plaques.

Fourth, watch out for the question that tests the HLA-B*1502 pharmacogenomic screening step before starting carbamazepine, particularly in patients described as being of Southeast Asian, South Asian, or East Asian heritage. This is a high-yield patient safety concept.

Finally, remember that in classical trigeminal neuralgia the neurological exam is entirely normal between attacks. The moment a vignette describes facial numbness, weakness of mastication, diminished corneal reflex, or other cranial nerve deficits, you must suspect a secondary cause (tumor or demyelination) and the correct next step shifts to MRI with contrast rather than empiric carbamazepine.

The core competency being tested is the ability to (1) recognize a classic pain pattern from a well-crafted clinical vignette, (2) select the correct first-line drug, (3) know when imaging is needed to rule out a dangerous secondary etiology, and (4) identify the surgical option for refractory disease.

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