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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

Comments

  1. 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!

    • Casey Parker says

      Hi James
      I have used it on many students and residents for fun, only a few times in real cases. I do tend to get normal values – around 4.4 – 5 mm.
      I set the depth to 4 – 5 cm and try to get a picture of the globe and the nerve in coronal section – often through the lens or just above it. The studies suggest measuring the nerve sheath 3 mm behind the disc / globe – which correlates with the point where the “shoulder” of the nerve sheath becomes parallel. I do admit, the sheath can be a bit hard to differentiate from the surrounding orbital tissue in this plane. I have added a link to the Sonoguide website which has a good archetypal image with caliper positions marked
      How do I plan to use it? Well – for me it is an extension of clinical exam – eg. the drunk-head-injured chap whom I would usually let sleep it off in ED and review in the morning but not do an early CT head @ 3am – I can do the ONSD measure and if not normal then strongly consider changing the plan – ie. scan early. I do not think it is appropriate to use it to exclude ICP in high risk scenarios, more to improve the diagnostic sensitivity of my own clinical ‘gestalt’
      Casey

  2. Minh Le Cong says

    HI folks

    latest published article on ONSD assessment via optical USS to determine ICP elevation

    Dubourg J et al. Ultrasonography of optic nerve sheath diameter for detection of raised intracranial pressure: A systematic review and meta-analysis. Intensive Care Med 2011 Apr 20; [e-pub ahead of print]. (http://dx.doi.org/10.1007/s00134-011-2224-2)

    worth a read if you are considering trying it out.

    • Casey Parker says

      Thanks Minh
      Have added a link to the post on RICP. Brilliant.
      Seems to confirm the use of ONSD as a measure of ICP – however, need to be cautious, I don’t think it is a “rule out” test, more a tool to improve ones clinical assessment. I think that I will order more CTs on the basis of ONSD US and sleep better.
      Casey

Trackbacks

  1. […] Almost exactly 3 years ago I posted an article looking at the utility of ultrasonographic Optic Nerve Sheath Diameter measurement to assess patients for raised intracranial pressure.  That post is here […]

  2. […] in ICP and that is something that we cannot directly measure. Whilst indirect methods such as ocular USS can be used, it’s clearly not the same. As a result we are often treating raised ICP in the ED […]

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