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Welcome back to the tasty morsels of critical care podcast. A meandering monologue through critical care fellowship exam preparation.
GBS is a clinically important diagnosis for both the emergency department and the ICU. Its rareish but common enough that you will at least suspect it often enough in the ED and many ICUs will have one long stay ICU patient with GBS every few years and a short stay one more often. It has the right mix of just enough clinical findings to diagnose from clinical exam but also has some novel test characteristics and some lovely management options and discussion points making it all very examinable.
The pathophys I was taught was that of molecular mimicry. Some foreign antigen is introduced and has a structure similar enough to the wiring insulation wrapping our nerves that the antibody produced binds not only the foreign antigen but also the native nervous tissue. This leaves the myelin and axons suspect to the full rigors of an immune system crackdown akin to the US Marshall’s office on a man hunt for Richard Kimble
Some of these pathogenic antibodies even have complicated named acronyms. Neutralising such antibodies with something like IVIG or washing it out of the blood with plasmapheresis forms the basis of treatment of the disease. The antigen associated with GBS is classically c jejuni (which probably carries a worse prognosis than other precipitants) but flu, HIV and mycoplasma are also on the list as alternatives. Vaccination was always on my list of precipitants but this seems somewhat in doubt. Given how common vaccinations are and how rare GBS is its tricky to separate out causation from background rate. Indeed the risk of GBS from the flu itself is well in excess of any potential rate with the flu vaccine.
The breakdown of the blood-nerve barrier at the point the peripheral nerves enter and leave the spinal canal allows blood plasma proteins to enter the CSF which is the rationale for the high protein on LP.
To make what was already a complex disease more complicated it is sub divided into 4 subtypes.
- AIDP (acute inflammatory demyleniating polyneuropathy)
- MFV (miller fisher variant)
- AMAN (acute motor axonal neuropathy)
- AMSAN (acute motor and sensory axonal neuropathy)
(the last two being significant for being an axonopathy as opposed to demyelination)
Now that you have heard of them you can reproduce them for exams and then forget them for clinical work as AIDP is by far most common and we treat it largely the same even if the prognosis varies from the other types.
Most of the diagnosis here is clinical with not only the history but real life bonafide examination findings actually being a useful part of the diagnosis. In the ICU the diagnosis is usually fairly easy as they come with a label of possible GBS in the referral. The ED diagnosis is much more of a challenge as they present often before the textbook findings are there. Pain in the legs with an odd or supported gait has been a common presentation in my limited experience. The weakness in the legs may be subtle at this point and often dismissed by idiots like myself as secondary to pain. The real challenge in the ED lies in locating a tendon hammer, a somewhat mythical device in the ED. The much talked about and rarely seen Miller-Fisher variant has more dominant cranial nerves findings
So once we have identified ascending weakness and demonstrated reduced reflexes the next test we reach for is typically the LP. As mentioned above, look for a high protein and normal cell count. This finding is termed albuminocytological dissociation which was a key part in the discovery and labelling of the illness in the early 20th century. The finding is present in a deeply underwhelming 66% of those with GBS so it’s practically of no use if there’s an entirely normal LP as your clinical findings will trump everything and they may well still have GBS. That being said I’m sure neurology will find lots of random antibodies to send on the CSF so it’s still worth sending. The presence of protein increases over the first couple of weeks so a repeat LP might well be useful.
The twitchy electrode needle type tests do have specific findings depending on the type of GBS you’re dealing with but I struggle to see any relevance to what we do in critical care
Next time we’ll cover management.