Trigeminal neuralgia and cauda equina syndrome

• Trigeminal neuralgia (TN) is a disorder of the fifth cranial nerve that produces brief, electric-shock-like paroxysms of facial pain, usually in the maxillary or mandibular division, triggered by light touch or chewing.
• Carbamazepine or oxcarbazepine is first-line medical treatment for TN; surgery (microvascular decompression in classical TN) is reserved for drug failure or intolerance. ^[1]
• Cauda equina syndrome (CES) is compression of the lumbosacral nerve roots producing saddle anaesthesia and bladder or bowel dysfunction; it is a neurosurgical emergency.
• Every patient with suspected CES needs an urgent MRI of the lumbar spine and prompt surgical decompression, because delay risks permanent bladder, bowel and sexual dysfunction.
• Stroke is the dangerous mimic of acute focal neurological loss; neither TN nor CES is a stroke, but a sudden deficit always needs stroke pathways excluded first.
• High-yield peripheral nerves (trigeminal nerve, facial nerve, median, ulnar, radial, sciatic, common peroneal, femoral) are the standard motor and sensory map you take to the bedside.

By the end of this page you should be able to:

1. Define trigeminal neuralgia and cauda equina syndrome, and classify each.
2. Recognise the cardinal clinical features of TN and the red flags of CES.
3. Explain the mechanism of TN pain and of CES compression.
4. Choose and interpret the first investigations (brain MRI for TN, lumbar spine MRI for CES) and key cut-offs.
5. Outline first-line treatment of TN with drug, dose, route and monitoring.
6. Describe the urgent management of CES and the evidence on timing of decompression.
7. Distinguish TN and CES from similar conditions, including stroke, in a table.
8. Counsel a patient briefly about prognosis and complications.

This topic deliberately pairs two conditions that a medical student must never confuse with anything else. Trigeminal neuralgia is the most common cranial neuralgia and is managed medically first, with one classic first-line drug. Cauda equina syndrome is the archetypal spinal emergency and is managed surgically first, with one classic first investigation. Both are disorders of peripheral neural structures and both are common sources of medicolegal delay.

The page also covers the high-yield peripheral nerve map and stroke, because the relevant source groups them together. Stroke is the most common dangerous mimic of acute focal neurological loss; it is included here so you can keep it in mind at the bedside rather than after the fact.

The International Classification of Headache Disorders, third edition (ICHD-3) criteria used in the standard practical description are: unilateral facial pain in trigeminal distribution with no radiation beyond it; lasting from a fraction of a second to 2 minutes; severe; electric-shock-like, shooting, stabbing or sharp; precipitated by innocuous stimuli within the affected distribution; and not better accounted for by another diagnosis. ^[1]

Classification separates three types by cause. Classical (idiopathic) groups are combined as primary TN by the European Academy of Neurology (EAN) guideline, because the same medical treatment applies. ^{[2] [6] [7]}

• Classical TN, the most common at about 75% of cases, is diagnosed when there is trigeminal neurovascular compression with morphological changes (distortion, indentation or atrophy) on the symptomatic side, shown on MRI with appropriate trigeminal sequences or at surgery. Simple contact without morphological change is not sufficient, because it is common in healthy people. ^[1]
• Secondary TN, about 15% of cases, is due to an identifiable neurological disease other than neurovascular compression, such as a cerebellopontine angle tumour, arteriovenous malformation or multiple sclerosis; about 2% of people with multiple sclerosis have TN-like symptoms, although estimates in multiple sclerosis range from about 1% to 6.3% depending on the series. ^{[1] [7]}
• Idiopathic TN, about 10% of cases, has no apparent cause on imaging. ^[1]

Primary (classical plus idiopathic) and secondary TN are further split into purely paroxysmal pain versus concomitant continuous or near-continuous pain, because concomitant continuous pain responds less well to treatment. ^[1]

Historically the literature used many definitions. Two tiers are in common clinical use. ^[4]

• CES-I (incomplete): altered urinary sensation, loss of desire to void, poor urinary stream or needing to strain to micturate.
• CES-R (retention): painless urinary retention with overflow incontinence, where the bladder is no longer under volitional control.

Earlier classifications such as Fraser and Gleave-Macfarlane also split complete from incomplete disease. In practice most guidelines treat any combination of red flags as an emergency. ^[4]

The source page groups trigeminal neuralgia and cauda equina syndrome with a review of peripheral nerves that a clinician should know by heart. The table is a working map rather than an exhaustive list.

Stroke and transient ischaemic attack produce a focal neurological deficit. They do not produce the shock-like, triggerable paroxysms of TN or the saddle anaesthesia and bladder dysfunction of CES. A sudden neurological deficit still goes through stroke pathways first, because stroke is the time-critical differential. See also "How it differs from similar conditions (Table)".

TN is uncommon. In the practical review, lifetime prevalence is estimated at 0.16 to 0.3%, with an annual incidence of 4 to 29 per 100 000 person-years. ^[1] It is more common in women, with reported female-to-male ratios varying by series (3 to 2 in the practical guide and 1.5 to 1.7 to 1 in StatPearls), and mean age of onset is 53 to 57 years (range 24 to 93 years in adult series). ^{[1] [7]}

There is no known modifiable risk factor. The main associations are age, female sex and, in secondary TN, an underlying cause such as a posterior fossa lesion or multiple sclerosis. ^[1]

CES is rare. The available guideline comparison gives a crude incidence of 0.3 to 7 per 100 000 depending on region. ^[4] The commonest cause, accounting for 60.9% in the systematic review of surgical series, is lumbar disc herniation; other causes include spinal stenosis, trauma, tumour, infection and postoperative compression. ^[3]

Stroke epidemiology is outside the scope of this page; the relevant clinical point is that both TN and CES are not cerebrovascular events but must not make you forget stroke in a patient with sudden deficit.

• TN: neurovascular compression with morphological change (classical); multiple sclerosis or tumour (secondary); unknown (idiopathic). ^[1]
• CES: anything that narrows the lumbar canal or compresses the roots, most often a large lumbar disc prolapse. ^[3]
• Stroke: vascular risk factors; considered and excluded in acute presentations.

The current hypothesis for classical TN is a vascular compression near the root entry zone, where peripheral Schwann-cell myelin changes to central oligodendrocyte myelin and is vulnerable to demyelination. Focal demyelination and remyelination lower excitability thresholds and allow ephaptic cross-talk, so a light touch signal carried by fast A-beta fibres can activate nociceptive A-delta fibres. This explains the paroxysm and the refractory period that follows. Abnormal expression of voltage-gated sodium channels (Nav 1.3, 1.7 and 1.8) is another proposed mechanism and explains why sodium channel blockers work. ^[1]

Concomitant continuous pain, when present in 14 to 50% of patients, seems to reflect axonal loss and sensitisation of second-order neurones rather than repeated paroxysms alone. ^[1]

Compression of the cauda equina impairs motor, sensory and autonomic fibres. Saddle and perineal sensation and sphincter control are lost because the sacral roots (S2 to S4) are affected; bladder retention rather than simple urgency is the hallmark of more severe compression. Early decompression improves bladder function and overall outcome, although the literature uses heterogeneous start-points and definitions. ^{[3] [4]}

TN is usually right-sided (about 60%) and most often affects the maxillary (V2) and mandibular (V3) divisions; about a quarter of cases have ophthalmic (V1) involvement, often together with V2 or V3, whereas isolated V1 involvement is rare (less than 5%). ^{[1] [11]}

Cardinal features drawn from the standard clinical description: ^[1]

• pain lasts from less than a second up to 2 minutes in 74% of patients, but a minority report longer attacks ^[1]
• up to 70% occasionally have series of paroxysms lasting up to 1 hour ^[1]
• about 40% report more than 10 attacks per day ^[1]
• 91 to 99% have triggerable attacks; common triggers are light touch, talking, chewing, brushing teeth, washing or shaving ^[1]
• spontaneous attacks occur in 68 to 98% of cases ^[1]
• most have a refractory period of seconds to minutes after a triggered attack
• 14 to 50% have concomitant continuous pain
• cranial autonomic symptoms (lacrimation, rhinorrhoea, hypersalivation, flushing) occur in up to 43% and are usually mild ^[1]

Onset is abrupt, remissions last months or years in two-thirds, and the course is chronic in one-third. ^[1]

Examination is usually normal; about 30% have mild hypoaesthesia, and severe attacks can produce ipsilateral facial twitching (tic douloureux). ^[1]

CES is suspected when back pain is combined with any of the red flags below. ^[4]

Bladder post-void residual under 200 mL has a negative predictive value of 97% for CES in one recent series, so it can support triage but never replaces MRI. ^[4]

• TN: episodic, triggerable facial pain with a normal examination between attacks.
• CES: progressive back and leg pain with saddle numbness and sphincter disturbance.
• Stroke: sudden fixed deficit in a vascular territory, not triggerable and not relapsing within minutes.

MRI of the brain with trigeminal sequences is the gold standard investigation to exclude secondary TN and to distinguish classical from idiopathic TN. If MRI is contraindicated, CT of the head, CT cerebral angiography and neurophysiological testing of trigeminal reflexes are used. ^[1] The trigeminal MRI protocol includes high-resolution 3D steady-state or SPACE-type sequences, time-of-flight MR angiography and 3D T1 gadolinium imaging. ^[1]

A typical trigeminal MRI report looks for neurovascular contact with morphological change (distortion, indentation, atrophy); contact alone is common in healthy people and is insufficient for a diagnosis of classical TN. ^[1]

MRI of the lumbar spine is the first-choice investigation for suspected CES; CT myelography is the alternative when MRI is contraindicated or unavailable. Across comparison sources 8 of 10 recommend urgent MRI as the first test. ^[4] Urodynamic studies and post-void residual bladder scanning can support assessment. ^[4]

A compact combined table.

Guideline editions relied on are summarised below.

The practical guide (Lambru et al., 2021) lists the principal differentials; only selected ones are restated here to protect the distinctive TN phenotype. ^[1]

• A simple lumbar disc prolapse with radiculopathy: leg pain and numbness, but no saddle anaesthesia or sphincter disturbance.
• Spinal cord compression above the conus (conus medullaris syndrome): more likely to have early bilateral signs and less root pain.
• Peripheral neuropathy and Guillain-Barre syndrome: symmetrical, not restricted to sacral roots, no back pain.
• Spinal infection or metastasis: fever, weight loss or known cancer, MRI still required.

Carbamazepine and oxcarbazepine are the first-line preventive drugs and give meaningful initial pain control in roughly 80 to 90% of patients, depending on the series. ^{[1] [11]} Where both are unavailable, national knowledge summaries such as NICE CKS give the same recommendation. ^[9]

Monitoring during treatment: renal function, serum sodium and liver function tests; in older patients, bone health. Women tolerate these drugs less well than men. Where ethnicity requires, test for HLA-B*1502 before carbamazepine in Han Chinese patients. The practical guide notes that 200 mg carbamazepine is roughly equipotent to 300 mg oxcarbazepine when switching from one to the other; high cross-reactivity exists between the aromatic antiseizure drugs. ^[1]

Second-line or add-on options: lamotrigine (25 to 400 mg daily, titrated slowly because of rash), gabapentin (300 to 3600 mg daily), pregabalin (150 to 600 mg daily), baclofen (15 to 90 mg daily), and botulinum toxin type A (25 to 195 units). Lamotrigine is particularly useful as add-on therapy or when carbamazepine is not tolerated. ^[1]

Opioids are generally ineffective and should be avoided. Topical lidocaine or local anaesthetic injections into trigger zones can give transient relief. Intravenous fosphenytoin (15 mg per kg over 30 minutes) or lidocaine (5 mg per kg over 60 minutes), under cardiac monitoring in a high-dependency setting, can be highly effective but should only be given by teams experienced in their use. ^[1]

Surgery is considered when pain is poorly controlled or drugs are poorly tolerated. In classical TN, microvascular decompression is the operation of first choice, giving pain freedom in 62 to 89% of patients at 3 to 10.9 years, with an annual recurrence risk under 2% at 5 years and under 1% at 10 years. ^[1] In a pooled series of over 5000 patients, mortality was 0.3%. ^[1] Concomitant continuous pain and younger age under 25 years worsen outcomes. ^[1]

If there is no neurovascular contact, or comorbidities make decompression unsafe, ablative procedures are preferred: stereotactic radiosurgery, radiofrequency thermocoagulation, balloon compression and glycerol rhizolysis, each giving 3 to 4 years of relief but with higher complication rates, especially after repeated procedures. ^[1]

Suspected CES is a neurosurgical emergency. The accepted approach, supported by national knowledge summaries and guideline comparisons, is: ^{[4] [8] [10]}

1. Recognise red flags and document them clearly.
2. Alert the neurosurgical or spinal team and arrange urgent lumbar MRI, ideally within hours.
3. If MRI is unavailable, use CT myelography and transfer.
4. Emergency surgical decompression once compression is confirmed.

On timing, the evidence base is inconsistent: systematic reviews show that early decompression improves bladder and overall outcome, but studies use different start-points (symptom onset, presentation, imaging or admission) and different intervals. ^{[3] [4]} No single interval is proven superior and there is ongoing debate; what remains clear is that avoidable delay increases the risk of permanent deficit. Guidelines therefore stress urgent recognition and immediate imaging over watchful waiting. ^{[3] [4]}

Disc herniation is the most common cause. The operation is most often laminectomy or discectomy, with instrumentation when needed. Supportive care includes urinary catheterisation, bowel regimen and nutritional support; postoperative bladder retraining and pelvic floor rehabilitation are important, and sexual dysfunction should be assessed and addressed. ^[3]

A stroke is not a peripheral nerve disorder but it is the differential you must exclude first in sudden focal deficit. Focal weakness, dysphasia or visual field loss are not explained by TN or CES; when present, use local stroke pathways and brain imaging. A peripheral nerve lesion such as TN or a CES pattern cannot mimic a hemispheric deficit.

• A thorough clinical history and basic neurological examination (including saddle sensation) are available anywhere and are the trigger for urgent referral.
• If emergency MRI is unavailable, CT myelography or urgent transfer is the fallback. ^[4]
• Where MRI is limited, strongly suspected CES should still be treated as an emergency and referred, because delayed decompression carries permanent disability.
• For TN, carbamazepine is inexpensive and widely available; if it cannot be tolerated, gabapentin or phenytoin may be used with monitoring.

• Drug adverse effects, including hyponatraemia, sedation, ataxia and skin reactions; leucopenia and thrombocytopenia with carbamazepine. ^[1]
• Stevens-Johnson syndrome and toxic epidermal necrolysis rarely, especially in HLA-B*1502 carriers. ^[1]
• Surgical complications: facial sensory change, CSF leak, hearing loss, cranial nerve palsy; mortality 0.3% with microvascular decompression. ^[1]
• Recurrence after surgery, more likely with concomitant continuous pain. ^[1]

• Permanent bladder, bowel and sexual dysfunction; lower limb weakness and sensory loss.
• Recurrent or persistent low back or radicular pain.
• High medicolegal risk when red flags are missed or diagnosis delayed. ^[4]

TN follows a relapsing-remitting course in two-thirds and a chronic course in one-third; many patients need decades of treatment and about half at a tertiary centre eventually undergo surgery. ^[1] Early drug response is good initially but may wane.

CES prognosis depends on severity at presentation and speed of decompression: early surgery improves the chance of recovery of bladder, bowel and motor function, but complete recovery is not guaranteed, especially when presentation is late. ^{[3] [4]}

• TN has no established primary prevention; early accurate diagnosis prevents unnecessary dental or facial surgery and anticoagulation.
• CES prevention is really screening: educate clinicians to ask about urinary, bowel and sexual function and to test saddle sensation in every patient with acute severe back pain; keep a low threshold for urgent MRI. ^[4]

• Pregnancy: TN has been reported to present or worsen; use drugs with known pregnancy profiles and involve obstetric, pain and neurology teams. Surgical options are reserved.
• Children: TN is rare in children and surgery is usually delayed; CES in children is usually due to tumour, trauma or congenital anomaly rather than disc disease.
• Older adults: drug tolerability is lower, hyponatraemia is more likely, and osteoporosis needs monitoring in long-term carbamazepine or oxcarbazepine use; surgery remains an option when comorbidity allows. ^[1]

Trigeminal neuralgia; cauda equina syndrome; stroke; transient ischaemic attack; lumbar disc prolapse; sciatica; multiple sclerosis; facial pain; peripheral nerve examination.

• Trigeminal neuralgia is a clinical diagnosis: unilateral, severe, electric-shock-like facial pain lasting under 2 minutes and triggered by innocuous stimuli. ^[1]
• Lifetime prevalence about 0.16 to 0.3%, annual incidence 4 to 29 per 100 000, more common in women with reported female-to-male ratios varying by series, mean onset 53 to 57 years. ^{[1] [7]}
• Classical TN (about 75%) implies neurovascular compression with morphological change; secondary TN (about 15%) implies another disease; idiopathic TN (about 10%) has no cause found. ^[1]
• Carbamazepine (start 200 mg, range 200 to 1200 mg daily, titrate 200 mg every 3 days) and oxcarbazepine (start 300 mg, range 300 to 1800 mg daily) are first-line and effective in roughly 80 to 90% initially. ^{[1] [11]}
• Concomitant continuous pain, Stevens-Johnson risk and drug interactions determine monitoring and patient selection. ^[1]
• Acute severe exacerbations use topical lidocaine or IV fosphenytoin 15 mg per kg over 30 minutes or lidocaine 5 mg per kg over 60 minutes; opioids are avoided. ^[1]
• Microvascular decompression is first-line surgery for classical TN with 62 to 89% pain freedom at 3 to 10.9 years and mortality 0.3%. ^[1]
• Cauda equina syndrome is an emergency: back pain plus saddle or perineal sensory loss, bladder, bowel or sexual dysfunction or leg weakness. ^[3], ^[4]
• Red flags for cauda equina syndrome include back pain, saddle or perineal sensory loss, urinary and bowel dysfunction, and leg weakness; guideline sets vary in which they emphasise. ^[4]
• Urgent lumbar MRI is the first test recommended by 80% of sources; post-void residual under 200 mL has a negative predictive value of 97% but never replaces MRI. ^[4]
• Early surgical decompression improves bladder, motor and overall outcomes, although timing studies use heterogeneous start-points and intervals. ^[3], ^[4]
• Crude incidence of CES is 0.3 to 7 per 100 000 and about 61% of surgical cases are due to disc herniation. ^[3], ^[4]
• Stroke must be excluded first in any acute focal neurological deficit; it does not mimic TN or CES pattern for pattern, but missing stroke is catastrophic.
• High-yield peripheral nerves should be mapped mentally and tested methodically at the bedside.