Strokes & The Rule of 4s || USMLE

when we talk about stroke we need to just divide stroke into one of two categories it can either be a cortical stroke or a brain stem stroke now when I say cortical stroke I'm talking about a stroke to the anterior cerebral artery the medial cerebral artery or the middle cerebral artery or the posterior cerebral artery and all you need to be familiar with is this picture cortical strokes tend to have focal motor symptoms so for example if you stroke out the ACA then you get involvement of the leg if you struck out the MCA you get involvement of the face if you stroke out the PCA you get some type of deficit in either vision or a language if it's the dominant hemisphere so what does that mean let's say that a patient only has motor abnormalities in their left leg that means the right ACA was stroked because the signals cross over and affect the opposite side likewise for example if a patient has only motor weakness on the right side of their face you stroked out the left MCA this is really simple there's really not much to this the difference between the cortical stroke and the brainstem stroke is that if they give you in the clinical vignette that the patient has diffuse or global motor weakness that means they're weak on their entire left side of their body not just the leg not just the face but the entire side of the body it's one of the brainstem strokes but if it's a focal deficit it's one of the cortical strokes and we're affecting one of these arteries here so all you need to know for cortical is the picture I show you here be familiar with ACA MCA and PCA and what it affects if you can get that down you will have no problem with any of the questions they give you on USMLE because they're very basic so let's do a case example to illustrate this a 67 year old male patient presents to the office was a two-hour history of altered Mental Status he was found unresponsive on the couch while watching jeopardy on physical exam you know four out of five motor strength of the left lower extremity 0 out of 5 motor strength of the right lower extremity and 5 out of 5 motor strength in both upper extremities he has intact sensation to all facial muscles and is able to puff out his cheeks swallow and smile his gag reflex is intact there is no tongue or uvula deviation which of the following is the type of stroke experienced by this patient all right why don't you pause the video and take a second to reread that if you need to otherwise let's go ahead and start breaking this down what's important here well they told us that he has normal strength in the upper extremities normal strength in the left lower extremity but completely absent strength in the right lower extremity so right off the bat we know that it's involving the lower extremity now if you remember from the previous page when the leg is involved we've stroked out the anterior cerebral artery now we need to determine which side we stroked the artery on because he has loss of the right lower extremity it's the left anterior cerebral artery and that's the answer again remember the picture that's all you need to know now let's talk about the brainstem strokes because these are notoriously harder but they don't have to be because I'm gonna teach you something today called the rule of fours now the rule of force was actually published by a neurologist so that non neurologists could easily detect brain stem strokes without being a sub specialist of Neurology you can do this in any specialty it's not that hard brain stem strokes use the rules of for the role of force so what are the rule of force well the first part is that we divide the cranial nerves into three sections based on the midbrain pons and medulla in each of those compartments there's four cranial nerves in order from 1 to 12 I'll show you an example of this on the next slide but just bear with me for now now cranial nerves that divide evenly into 12 our midline nerves whereas cranial nerves that do not divide evenly into 12 are lateral so 3 4 6 and 12 are all midline because they divide evenly into 12 but one - we're gonna omit one and two because they're not really in the midbrain they're kind of like floating above the midbrain so we're not even gonna talk about them so 3 4 6 and 12 midline 5 7 8 9 10 and 11 lateral so here's what we're talking about we have the midbrain the pons and the medulla in each of those sections there's four cranial nerves I told you to omit cranial nerves one and two because technically they're not in the midbrain so we start with three three and four both divide evenly into 12 so they're midline cranial nerves in the midbrain in the pons of five six seven and eight which are the four grouped into the pons only six divides evenly into 12 so 6 is the midline cranial nerve 5 7 and 8 are lateral because they do not divide evenly into 12 finally in the medulla we have 9 10 11 and 12 of that group of four the only one that divides evenly into 12 is of course 12 itself now this is the setup you want to keep in your mind for the brainstem strokes now let's continue with the rule of force the next part says that there are four motor syndromes and they are midline this is easy to remember the motor syndromes are midline think about those m's so the medial longitudinal fasciculus which controls motor helps you with lateral gaze and controls motor movement to the eye motor tract of the upper motor neuron is the corticospinal tract that'll be midline the medial lemniscus which is involved in the posterior columns with proprioception and vibration that's gonna be midline and then any of the motor nuclei of the cranial nerves that we already discussed dividing equally into twelve are gonna be midline so what this means is that if a patient has no proprioception or vibration we're gonna immediately think that it's a midline stroke because we know that the medial lemniscus is involved and the MS stick together so it's a midline stroke a medial stroke if the medial longitudinal fasciculus was messed up it would be a medial stroke because M midline medial stroke if the patient had upper motor neuron signs and they could not you know had no innovation on one side and we detected the corticospinal tract as being involved it would be a medial stroke let's continue the next part of the rule of force is the same thing but with sensory send syndromes sensory syndromes are lateral because they're to the side so think about the essence this time spinothalamic tract involvement where you can't sense pain or temperature is going to be lateral spinocerebellar tract where you have problems with rapid alternating movement or are am is going to be lateral sympathetic chain problems like a pseudo Horner syndrome ketosis and hydrosis etc that's going to be lateral and then any of the sensory nuclei that do not divide evenly into 12 are gonna be lateral so here we go case 67 year old male patient presents to the office of the for our history of diffuse motor weakness on his entire left side he's unable to move his eyes laterally towards the left which of the following best categorizes his stroke so we need to break down what's important in this question and then use the rule of force to figure out the answer so first thing here is that he has diffused motor weakness on his entire left side so immediately we know that this has to be a brain stem stroke and it can't simply be a cortical stroke because it's not preferential or focal it's global so we know that it's a brain stem and not focal the next thing we know is that because he has diffused motor weakness on his entire left side that the corticospinal tract is likely involved here that's motor which means it's midline or medial so we already know that this is a medial stroke it's on his left side which means it's a right-sided stroke so already we know that it's a right-sided stroke and it's medial let's continue he's unable to move his eyes laterally towards the left that's cranial nerve six because if you mess up crazy owner of six then you have a cranial nerve 6 palsy you're unable to use the lateral rectus muscle cranial nerve six divides evenly into 12 that means it's medial but we already knew that because the corticospinal tract was involved but what creamy owner of six gave us is that it's in the pons remember this is the picture with the breakdown of the cranial nerves cranial nerve 6 localizes us to the pons that is very important for getting this question right so because we're gonna add up everything that we've solved so far that it's medial right-sided and in the pons we know that it's a right medial pontine stroke you don't have to worry about knowing the name of the stroke for example wallenberg stroke or Wallenberg syndrome just know right medial pontine this will be in parentheses for you on your test you don't have to know the person's name who the stroke was named for

Channel: Dirty Medicine

Views: 450,651

Rating: 4.930449 out of 5

Keywords: USMLE, COMLEX, Step, Boards, Mnemonics, High, Yield, DirtyUSMLE, Stroke, Brainstem, Cortex, Cortical, Medial, Lateral, Midbrain, Pons, Pontine, Medulla, Cranial Nerve

Id: 4BftS-3EIRI

Length: 8min 41sec (521 seconds)

Published: Sat Oct 29 2016