Thread by @IsaacLamb01

Learning brainstem #neuroanatomy would be nearly impossible without the rule of 4s. I'm going to pass this along for anyone trying to learn #neurology, as this might be the single most powerful mnemonic I know of.

I present to you, The Rule of 4s: a #MedEd #tweetorial

(1/10)

4 cranial nerves per brainstem level
- Midbrain: 1-4
- Pons: 5-8
- Medulla: 9-12

* CNs 1 and 11 do not connect with the brainstem
* CN5 enters in the pons, but has nuclei at all three levels

(2/10)

4 lateral (side) structures, all with an "S":
- Spinocerebellar pathway
- Spinothalamic pathway
- Sensory nucleus of CN5
- Sympathetic pathway

(3/10)

4 midline structures, all with an "M":
- Motor pathway (CST)
- Medial lemniscus
- Medial longitudinal fasciculus
- Motor nuclei and nerves (CN 3, 4, 6, 12)* (4 of these)

* CN7 is a motor nerve but does a weird wraparound thing and exits laterally.

(4/10)

Bonus: rule of 12
Cranial nerves that are factors of 12 (3, 4, 6, 12) are midline. The only exception is CN2, which is weird for other reasons too, so this shouldn't be too hard to remember.

(5/10)

Sample case 1: sudden onset L body hemiparesthesia, R facial paresthesia, R Horner syndrome, hoarseness/dysphonia, vertigo with falling to the right.

Now you could just memorize this presentation as R lateral medullary syndrome, or you could apply the rule of 4:

(6/10)

Crossed signs (L body vs R face) points to brainstem. Brainstem signs are usually ipsilateral, localizing to R side.

Sympathetic involvement (Horner syndrome) plus sensory changes (hemiparesthesia of face) suggests lateral involvement (S = side).

(7/10)

Hoarseness suggests CN9/nucleus ambiguus involvement, localizing to medulla. Bonus, 9 is not a factor of 12, again pointing toward lateral.

Voilà, we've diagnosed a right lateral medullary stroke without having to memorize a list of syndromes.

(8/10)

Case 2: sudden onset R hemiplegia with L CN3 palsy.

Crossed signs point to the brainstem, left side. Motor sx suggest a midline lesion, while CN3 localizes to midbrain.

Likely culprit: left medial midbrain (Weber syndrome), at least for standardized testing purposes

(9/10)

Mnemonics like these help you work smarter, not harder. I hope others can put the rule of 4s to good use!

(10/10)