Brainstem Rule of Fours

screen-shot-2016-09-15-at-12-52-49-pmWho remembers the brainstem in medical school? You vaguely remember pictures like the one to the right that made you want to adopt a pug? Well, stick with me and let’s learn a system for the brainstem. I promise I won’t use the word Wallenberg.


Clinical Scenario: Called to triage by your nurse for a stroke code. You’re at Kings County so your intern is boss & stroke labs are drawn, FS is “wnl” and EKG is sinus.


You look at the patient and it’s not the classic “face droop opposite the weak side”. You sigh knowing you can’t diagnose ACA or MCA (check arms vs. legs weakness), so what next? Let’s quickly go through a system for these “other strokes”.


We all know the PCA can affect reading (alexia) and facial recognition (prosopagnosia) and sometimes the visual pathway (homonymous hemianopsia), that the Caudate & Putamen cause weakness (paresis) in arms and limbs and that the thalamus has syndromes that affect one side’s face and arms (sensory for VPL and motor as Internal Capsule descends), but what should we do to remember the brainstem?


As you know, the cortex (the “thinky” part) is larger in humans and allows higher functioning, as opposed to the brainstem (the “vitals” part) that regulates things like heart rate, respiratory rate and blood pressure, in all animals (even dumb ones).

At bedside, use these two basic principles to differentiate a cortex and a brainstem lesion.
1 – Brainstem lesions: extension (decerebrate posture) vs. Cortex lesions: flexion (decorticate) (Thanks Red Nucleus!)

2 – Brainstem lesions: eyes look away from lesion vs. Cortex lesions: eyes to look at it (Thanks Reticular Formation!)

Once in the brainstem, neurologists talk about (1) where a lesion is vertically in the brainstem and if a lesion is (2) left or right, or (3) in the middle or the side. So they will say things like a stroke in the “Right Lateral Medulla”. Luckily, this can be easily broken down. Enter the Brainstem Rule’s of Four. (There are four of them)

First, determine the vertical location of the lesion. You’re in luck, the cranial nerves shoot out of the brainstem as it descends. The brainstem has three parts – Midbrain, Pons and Medulla and each contains 4 cranial nerves. Brainstem rules of four! Midbrain gets 1-4, Pons gets 5-8 and Medulla gets 9-12. Just remember your cranial nerves.

For “medial vs. lateral” and “right vs. left” you only have to know two syndromes. Check out the pictures below:


The Medial syndrome is Motor and 4 M’s. Contralateral motor, contralateral medial lemniscus (dorsal column, vibration & proprioception) with Motor CN’s (all of which divide into 12 à 3,4,6,12 and are Medial and Motor) as well as MLF.

The Side syndrome is Sensory and 4 S’s. Ipsilateral Spinocerebellar (ataxia) with ipsilateral Sympathetic (Horner’s: miosis, ptosis, anhidrosis) and ipsilateral facial Sensation (CN V) with contralateral Spinothalamic (pain & temperature).

So, just remember these two syndromes and if you see a patient who has one, just look at which side it is on and which cranial nerve is affected and you’ll be diagnosing “Left Lateral Medullary” strokes in no time! That wasn’t so bad.


Below is a handout that was used in our morning report at King’s County to help our residents better visualize the brainstem, to practice these rules with some examples and to have as a reference for future neurology consults.


Until next time from Brooklyn,

The Original Kings of County


Gates, P. The rule of 4 of the brainstem: a simplified method for understanding brainstem anatomy and brainstem vascular syndromes for the non-neurologist. Internal Medicine Journal 2005; 35: 263-266

Nickson, Chris. “Brainstem Rules of R.” Brainstem Rules of 4. Life in the Fast Lane, n.d. Web.

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