Tasty morsels of EM 067 – Monro-Kellie 2.0

23 Jun

This is based on Mark Wilson’s paper in the Journal of Cerebral Blood Flow & Metabolism  [Free full text] which i’m sure you all read avidly every month.

COI – I know Mark and had the honour being in the same speaking track as him when he gave a similar talk at SMACC Gold. He was very kind and didn’t point all the things I was wrong about in my talk.

I learned lots from this paper and it turns out there are lots of subtleties to ICP that the classic Monroe-Kellie doctrine doesn’t account for. Level of evidence is as you might expect somewhat low but this is fascinating stuff none the less.

What is Monro-Kellie?

  • Monro (Scotsman with dodgy wig) suggested that skull was a closed box and as soon as you add something to the box either pressure goes up or something gets squeezed out
  • Kelli (Monro’s student) did some autopsy work suggesting Monro was right.

 

click for source

click for source

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Problems with Monro-Kellie (apart from the dodgy wig...)?

  • blood and CSF given the same weight/importance despite the fact that CSF production and flow is tiny and venous flow is huge (equal to arterial inflow at 14% of the cardiac output)
  • it therefore makes sense that a problem with venous outflow or accumulation is likely much more important than CSF.
  • there has been lots of focus on managing the arterial side of things with certain CPP goals that entirely ignore the fact that fiddling with the arterial side when there’s a problem with outflow might miss the point.

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Tell me about these veins then

  • i did a podcast on it once…
  • most people have asymmetric venous drainage and are quite dependent on their dominant side – if this side gets obstructed (maybe by a depressed skull fracture) then that’s probably important
  • they’re formed by little dural folds with no support of their own (unlike the muscular arterial walls) and are hence very dependent on their surroundings. Eg, next time you’re opening a dural sinus in someone sitting upright it’s worth remembering that the dural sinus pressure is probably negative and will cause a massive air embolism. Likewise when you lie someone flat the venous sinus engorges.

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What causes problems with venous drainage or venous hypertension?

causes of venous hypertension wilson

click for source

Mark provides a lovely little classification in the table above

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Some pearls

  • a depressed skull fracture over a venous sinus (esp a dominant one) may cause venous outflow problems worsening ICP. The venous sinus may even thrombose
  • idiopathic intracranial hypertension (that disease of young obese women with papilloedema) is associated with venous sinus stenosis
  • poor head position (slight flexion and rotation) will significantly impair jugular (and hence intracranial) venous drainage. Not to mention the dreaded collars…
  • outside the head, high ventilation pressures transmit to the venous system and indeed even raised abdominal pressure can cause refractory raised ICP (as beautifully described by Tom Scalea
  • even microgravity causes issues with astronauts reporting a syndrome somewhat similar to idiopathic intracranial hypertension during prolonged time in space

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