Raised Intracranial pressure – can we pick it with the US?
Working in small hospitals where CT is either not available or hard to get after 16:00 one of the big questions that I find myself asking with a lot of patients is this – “does this guy have raised ICP?”
Common presentations where I find myself wondering: the drunk, head-injured patient; the bad ‘migraine’ headache; teenager with ? meningitis / just a bad flu; cancer patient with ‘something new going on in the brain’…. There are many, I am sure you have experienced this. So what do we do? Get a CT at 1 AM? Do an LP and measure the pressure (pray they don’t have a SOL)? admit them until they sober up? fundoscopy (not reliable in my hands to pick subtle ICP)…
Then along comes this newish idea, the use of Ocular US to look at the optic nerve sheath diameter – it is an extension of the CSF space. Sound hard? Well it is actually really quite simple, even if you are not the greatest sonographer. Basically you plonk a high frequency linear probe on the upper eyelid(closed) and voila – an eyeball is seen,( great for seeing retinal detachments and penetrating FBs also ). At the back of the globe is the optic disc and behind it is the nerve – measure back 3 mm from the globe and that is the spot to measure the sheath diameter – like all things, you need to practice this but it is very non-invasive, you can do it on yourself. Basically if it is < 5.8 mm, ICP is likely < 20mmHg (normal), measure both and average it to be sure.
If you need a Pic to illustrate the view you need – check out the Sonoguide images here
Anyway, you should check out the links below to a small pilot study, there are also a few studies that compared the US to actual measured ICPs in ICU settings and it did pretty well. Can we hang our hats on this – I am not sure, but is a useful tool to add into your “gestalt” when deciding what to do with these patients. Maybe I will sleep a little better! Let me know what you think…
This study from the Annals of Emergency Med
Non-invasive assessment of intracranial pressure using ocular sonography in neurocritical care patients
Just in: Check out the meta analysis of the evidence thus far, published 2011 – suggests it is a useful tool in ED care
I must say I struggle with the optic nerve sheath diameter and remain unconvinced – but perhaps I’m doing it wrong.
Where exactly do you put the calliper markers? There is usually a central hypoechoic area (presumably the nerve), then a slightly more echogenic ring and then another hypoechoic ring (presumably the nerve sheath). Do you use longtudinal or transverse / oblique views of the nerve?
I have measured it in myself and a well colleague – and both had a diameter of 6mm (so raised ICP). I’ve also measured it on several patients with known raised ICP and had mixed results.
Perhaps I should try again!