Trigeminal Neuralgia

Take home messages

• Trigeminal neuralgia is the most common facial neuralgia
• Make sure nothing else is going on
• The first line treatment is carbamazepine

Some brief anatomy

What we think happens

Nerves generally work very well indeed, as long as you don't hit them, burn them, cut them, marinade them in sugar or let them get infected.

If a pain nerve gets injured, it often gets hyperexcitable, meaning it will fire off pain signals inappropriately.

Fortunately you also have a central pain desensitisation system, which acts as a volume dial for these pain signals and can help make them less inappropriate.

But if you lose this central system as well, you're in trouble.

The trigeminal nerve has a vulnerable spot at the root entry zone, where the peripheral myelin provided by Schwann cells hands over to that provided by the central astrocytes.

Compression or damage at this point can cause aberrant pain signals to fire in response to even very light touch of the face.

Ephaptic coupling (if you care) is when nerves in very close proximity can actually induce and coordiate action potentials with each other, and we think it's this coupling of Aβ, A-δ and C fibres that gives you the pain response to a light touch.

What happens in trigeminal neuralgia

You get neurovascular compression of the root of the trigeminal nerve.

This happens at the root entry zone, usually by the superior cerebellar artery.

You need to see this compression on MRI to confirm the diagnosis.

You can also get the same thing with the glossopharyngeal nerve - glossopharyngeal neuralgia pain is generally felt in the ear.

Diagnosis

Tell me about carbamazepine

It's the first line treatment, and the only one that really properly works.

• It's a sodium channel blocker
• It works in a frequency-dependent manner
• It stabilises overexcited neural membranes
• It inhibits repetitive firing
• You give 200-1200 mg per day

Oxcarbazepine is an alternative - 300-1800 mg per day

What's not to love?

Well the number needed to harm is also quite high, around 3.5 for minor badness and 25 for major badness.

Badness includes:

• Dizziness
• Rash
• Ataxia
• Drowsiness
• Liver damage

But if it takes away the only pain ever considered worse than childbirth, one might decide it's worth putting up with these.

The other key things to monitor for are:

• Hyponatraemia
• Blood dyscrasias

So they need fairly regular blood tests.

Any other drugs?

Any interventions?

You can inject botox, which can help for a few months, but can unsurprisingly give you motor side effects.

Otherwise there are four classes of surgical intervention, (which definitely do come up in the CRQ):

Long story short - if you're not coping on meds, and you are fit to do so, have the microvascular decompression.

What on earth is secondary trigeminal neuralgia?

It's the symptoms without the neurovascular compression, caused by something else - usually multiple sclerosis, in which case you'd see demyelination in the root entry zone.

Or at least the neuroradiologist would.

The key to differentiating trigeminal neuralgia caused by MS from the classical type is abnormal reflexes.

• Blink reflex
• Evoked potentials

Both very useful for spotting MS as the underlying cause.

Unsurprisingly this doesn't respond nearly as well to carbamazepine etc but Gasserian ganglion blocks can still work, and if an AVM is causing the issue, microvascular decompression is an option.

So what is painful trigeminal neuropathy?

Good question.

A continuous facial pain... in the distribution of one or more branches of the trigeminal nerve... caused by something else

So it's basically exactly the same, but something else is causing it other than anything we've already listed above.

We're as confused as you are.

• It's generally a continuous burning pain rather than a stabbing intermittent attack
• There are sensory deficits including allodynia and cold hyperalgesia
• There are also three types

There's more

Facial pain is an enormous subject, and we've consciously chosen to focus on what tends to be useful for the exam here, rather than a comprehensive review of the topic.

(You can answer the whole CRQ below using the information above.)

If you're keen to dive deeper into the weeds then check out the references below, and the FRCA questions in the members area at the bottom.

A free CRQ from revise FRCA

Our wonderful colleagues over at FRCA revision have kindly let us share some of their hand-crafted CRQs which you should definitely ensure you've seen before the exam.

Tap the image below for the trigeminal neuralgia question

Useful Tweets and Resources

Cranial nerve lesions DDx

Differential diagnosis of cranial nerve lesions includes central and peripheral causes. Causes vary according to which cranial nerve is affected, and whether multiple cranial nerves are involved. See also

Life in the Fast Lane • LITFLChris Nickson

References and Further Reading

CKS is only available in the UK

The NICE Clinical Knowledge Summary (CKS) site is only available to users in the UK, Crown Dependencies and British Overseas Territories

NICE

Lesions of the trigeminal nerve (CN V)

The fifth cranial nerve is routinely tested in the unconscious ICU patient as the sensory component of the corneal reflex.In the conscious patient, one may also test sensation in the major territories (forehead, cheek, chin) as well as the power of the temporalis and masseter. In the unconscious or uncooperative hot case patient, the presence of a bite block or Guedel airway jammed into the oral cavity alongside the endotracheal tube alerts the keen-eyed candidate of the possibility that the muscles of mastication are very much innervated, and capable of forecful chewing actions.

Deranged PhysiologyAlex Yartsev

Examination of the cranial nerves

Though previously this page held a massive table of cranial nerve information “to simplify revision”, it has been re-dedicated to the clinical examination of the cranial nerves. It therefore presents a summary of the steps taken to perform a thorough cranial nerve exam, and has greatest bearing for the hot cases of the CICM fellowship. In the SAQs one also frequently finds gaze palsies and cranial nerve signs. Given the college’s tendency to ask about cranial nerve signs, familiarity with this material seems essential - its easy marks, and rapidly earned. Not to mention the enhancement of one’s clinical abilities, and whatnot.

Deranged PhysiologyAlex Yartsev

Primary FRCA Toolkit

While this subject is largely the remit of the Final FRCA examination, up to 20% of the exam can cover Primary material, so don't get caught out!

Members receive 60% discount off the FRCA Primary Toolkit. If you have previously purchased a toolkit at full price, please email anaestheasier@gmail.com for a retrospective discount.

Discount is applied as 6 months free membership - please don't hesitate to email Anaestheasier@gmail.com if you have any questions!

Just a quick reminder that all information posted on Anaestheasier.com is for educational purposes only, and it does not constitute medical or clinical advice.