Optic nerve sheath diameter: window to the soul?

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

The idea is sound, the optic nerve sheath is an extension of the subarachnoid space – so if ICP goes up it should expand and this ought to be measurable.  So – does it actually work?

Well – yes and no….  there is a pile of studies of various size, measuring various things.  So the better question is – who knows all this stuff?

To that – the answer is: Dr Leanne Hartnett, Emergency Physician and Ultrasound Doc from my old alumni – Fremantle Hospital in W. Australia.

Leanne presented a review of the evidence and techniques involved in ONSD assessment last year at the ACEM meeting here in Broome.  It was one of those talks that left me scratching my head – the data was there, but when you look at studies beside one another – they just don’t make sense.

For example – here is a summary of the measurements from the largest trials.  As you can see the mean in one trial might be completely “abnormal” in another.

Normal range of ONSD with US

 

However – when you compare ONSD to invasive measures of ICP there seems to be great correlation – as below:

ONSD vs ICP

So I managed to catch up with Leanne at the SMACC GOLD conference in March – [ we were both dateless for the gala dinner ]

We got chatting and this podcast is what we decided to do. DOWNLOAD HERE

Here are a few more images to refer to as we go through the numbers and the papers – and try to make sense of it all!

If you are busy and want the punch line:

– more data required… not yet for prime time

– under 5mm = probably OK

– over 6 mm = probably not OK

– And in between 5 and 6 mm = who knows, not us for sure!

Typical image and caliper placement used in many centres
Typical image and caliper placement used in many centres
The caliper placement described by Bauerle in his papers
The caliper placement described by Bauerle in his papers
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