Tasty Morsels of Critical Care 098 | Meningitis and Encephalitis

Welcome back to the tasty morsels of critical care podcast.

Today we cover some of the material from Oh’s manual chapter 54 on meningitis and encephalitis. This is a broad enough topic so the post will be correspondingly broad and somewhat superficial.

There are a number of presentations to consider this in and you will undoubtedly commence treatment on a lot of patients who do not end up not having an acute CNS infection. And that is OK. Until you get CSF you really just can’t be sure and if it’s on the differential then in general it’s best to cover it appropriately pending appropriate investigations.

Well, what are those “number or presentations” that I’ve just mentioned. There’s a whole set of pneumonocal or meningococcal meningitis that moves through the hospital at ward level care never needing us with our tubes and machines. For us in critical care we’ll typically only be getting involved in the unconscious or the seizing patient. The history of headache and fever etc is all very useful but is often not available in the context of the patients we’re seeing. So what we’re left with is unconsciousness and seizures. Thankfully in that very common cohort there is often an easy alternate diagnosis staring you in the face like the pCO2 of 13 or the large bleed on the head CT. But in the patients for whom the diagnosis remains uncertain after the initial pieces of data then we have to consider CNS infection.

While we have so far lumped meningits and encephalitis into one presentation, at this stage it’s probably worth separating them out by specific cause as that can be a helpful way to talk about risk factors and presentations

First off meningitis. Most common will be bacterial, strep pneumo at top of list with meningococcal outbreaks still remaining a thing despite relatively widespread vaccination. Both are rapid and nasty, though thankfully both respond beautifully to appropriate antibiotics as long as you get them early. CSF will make the diagnosis but a +ve urinary penumococcal antigen in urine might make me consider meningitis a little stronger in the obtunded pneumoina patient if the presentation fits. In the alcoholic and elderly population listeria is a significant consideration.

Encephalitis is a much trickier beast with a much more subtle presentation. Obtundation rather than florid coma may catch you out. Seizures may be focal or generalised. Fever is less obvious. CSF is less obvious. HSV is the number one diagnosis we’re worried about because the outcomes are poor and we actually have a treatment that might work in the form of aciclovir. There are much more indolent tricky to diagnose things like autoimmune encephalitidies that are in the encephalitis bucket but they are not really a 2am consideration in general.

CSF as mentioned is essential. LP remains a great and safe test and too often deferred for concerns over bleeding risk or hassle factor. The British society of Haematology guidelines have a recommendation of platelets >40 and INR <1.5 as cut off for doing an LP. The guidelines suggest antiplatelets including DAPT should not preclude LP if urgent. The phrase “if urgent” is of course carrying a lot of weight in that sentence and can be interpreted in a variety of ways as you might imagine

Once you’ve got CSF what patterns should you look for? This is a classic question for exams and focuses on neutrophils vs lymphocytes and what each might predict in combination with different levels of protein and glucose on CSF. Classic bacterial meningitis is high neutrophils with a low glucose. Viral tends to have lower overall white count with more lymphocytes. A raised CSF lactate seems to be a useful test in predicting or excluding bacteria in the CSF but I have not seen it done routinely. Gram stain is +ve in ~50% of bacterial meningitis but this rate of positivity massively increases in the exam scenario where the presence of gram positive bacilli allows follow up “guess the bug” type questions. (it’s lis