What’s a normal CSF opening pressure?

Bob has kindly let one or two of us into his Archimedes blog to write about some of the papers we’ve consider for Picket in E&P

This letter in the NEJM (Avery RA, Shah SS, Licht DJ, et al. Reference range for cerebrospinal fluid opening pressure in children. N Engl J Med 2010;363:891-3.) gives us, potentially, a new set of reference ranges for CSF opening pressures in children and young people.  It’s got a bottom line as follows:

a post-hoc analysis of opening pressure percentiles was calculated for the 52 subjects who received minimal or no sedation and were not classified as obese, resulted in a 90th percentile of 25 cm of water

There are a number of things which are of note.

  1. Sedation does not seem to alter the CSF opening pressure.  This is at odds with what we’ve been led to believe previously.
  2. If the 90th centile is 25 cm of water, that’s a pretty big cut off for diagnosing and then treating raised intracranial pressure.  Are the authors seriously suggesting that 10% of all children have raised intracranial pressure?
  3. In their larger population, which includes sedated and obese children the 90th centile is at 28 cm water.  That’s a pretty high pressure, in the context of what I’ve previously understood.
  4. Their way of determining obesity is very strange indeed.  They seem to have taken an adult approach, with the assumption that a certain value of BMI indicates obesity.  However, we know that BMI normal range changes with age, and that in order to compare children over time and between ages, you need to look at BMI SDS – ie a mathematical representation of the centile for the age.
  5. BMI was only available on 131 of the 197 patients.
  6. The other bit they don’t give enough data on – even in the appendix – is the actual discharge diagnosis.  These are a series of diagnostic LPs, and they’ve done their best to exclude what would be reasonably expected to alter intracranial pressure – or specifically raise it.  However, the range discharge diagnoses is quite broad, from “headache” to “undiagnosed white matter disease”.

So, I think this is a flawed paper, and as such it doesn’t reach the threshold of quality we’d apply to Picket it.  However, it does give us some interesting information and food for thought.  Does it change my practice?  I suspect I might be a bit more permissive about slightly raised opening pressures.  In those children with “barn door” raised pressure, this will still be straightforward, but it doesn’t help me in treating children with some symptoms, but with an opening CSF pressure of, say, 27 cm water.  What would I do if I saw this tomorrow?  Probably an n=1 trial of therapy, and see what happened…

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