Tasty Morsels of Critical Care 098 | Meningitis and Encephalitis

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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 listeria FYI).

Encephalitis is a lot trickier on CSF, if there’s a few WCC then you end up waiting for a PCR for HSV, though a white count of zero is probably enough to exclude it without the PCR. However if the clinical picture is really worrying a repeat CSF after a number of days is probably the way to go.

We can split management into a few sections. Firstly we have antimicrobials. As always this will vary with geographic region but being able to tie together your antibiotic choices with the likely bugs comes across very well in an exam scenario. In Ireland ceftriaxone is the go to cover for strep penumo and meningococcal disease. Go with the higher BD dosing for improved CSF penetration. Vanc is given routinely, theoretically to cover for resistant strep pneumo, the prevalence of which is really quite low in Ireland but still give it. Listeria in those at risk (pregnant, older, alcohol misuse) requires the use of amoxicillin in addition. In the unconscious patient HSV encephalitis is almost always on the differential and aciclovir is the drug of choice there.

Usual best supportive care, while a little dull and generic is of course critical to a good outcome, with intubation and airway protection immediately relevant in most of the meningitis critical care referrals as they’re either obtunded or seizing.

Steroids were somewhat novel and controversial when i first started training but now seem to be well established, with a cochrane review suggesting benefit in terms of neuro outcomes (primarily hearing) mainly in pneumococcal disease. Of course you won’t know it’s pneumococcal at the time so the recommendation is 0.15mg/kg dexamethasone at the time of or shortly after you give the antibiotics. Conversely if you’re pretty sure this is more of an encephalitis type issue then it’s hard to see why you would give steroids. Indeed a 2026 MCRCT by Solomon et al suggests no benefit to steroids in proven HSV encephalitis

Raised ICP can be a real issue with meningitis but is less common in adults with brains shrunken by age and alcohol. Obtundation and the CT scan are going to be your likely clues to an ICP crisis. While it would seem reasonable to apply some of your basic neuroprotective measures like MAP targets and CO2 control and even CSF diversion in this scenario it’s by no means clear if they change outcomes and indeed without an ICP monitor it would be hard to assess response

For encephalitis the major bug we’re interested in is HSV. It is common and can lead to devastating neuro outcomes untreated. Aciclovir seems to be effective with a substantial fall in mortality since its introduction even in the absence of RCTs. There are of course many other causes of encephalitis with particularly the autoimmune encephalitides being of particular relevance to critical care, however i think beyond the scope of this post.

Reading

Oh Chapter 54