Intracranial compliance is associated with symptoms of orthostatic intolerance in chronic fatigue syndrome, Finkelmeyer et al, 2018

http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0200068

 

For those (like me) who don't know what intracranial compliance is:

https://radiopaedia.org/articles/intracranial-compliance

 

Can anyone understand the physics of this? I cannot see how you measure compliance without a pressure measurement. And the intracranial volume does not change anyway - the cranium is rigid bone and has a fixed volume for all intents and purposes. They seem to be measuring something else but I cannot work out what.

 

According to the paper,

The reference (abstract below) suggests that there is a method of deducing the intracranial pressure based on using CSF velocity in the Navier-Stokes (fluid flow) equation.

 

I suppose I should read through the method papers. But I remain puzzled as to exactly what it is they think they are measuring. I would have thought the elastane of the cranium was largely a matter of how much the medulla and cranial nerves slide in and out of the skull. And I am still unclear what volume changes they are trying to infer pressure changes in response to. Maybe it is the volume change during the cardiac cycle - from arterial systole to diastole. But would this be at all relevant to standing up, when the CSF pressure in the spinal column will change for gravitational reasons etc.

It all sounds very clever but I would like to know how it could explain anything! I am being a bit bear with a sore head here but I am a bit preoccupied with other things this week.

 

Cerebral blood flow varies with heartbeat. The brain accommodates the blood volume changes by varying the pressure of the cerebrospinal fluid.

They are relying on something called the Monro-Kellie hypothesis, which describes the pressure–volume relationship between intracranial pressure, volume of cerebrospinal fluid (CSF), blood, and brain tissue, and cerebral perfusion pressure:

According to the paper,

The maths,

They do discuss the relevance of posture in making the measurements, and conclude that it's unclear as to whether it's significant, and that it needs further investigation.

I wonder if their focus was not so much looking for insight in this case, but maybe for possible avenues for treatment:

 

Well, not quite...

Because of this - which is what I think I said was what I thought - cranial compliance is actually zero in practical terms. So these calculations are not about compliance but about convective flow events. So my worry is whether Finkelmeyer actually understands Monro-Kellie?

I ask these questions because these sorts of basic issues about hydrodynamics were relevant to one of my early research areas - the volume control of joint synovial fluid. The textbooks had all got a false assumption in them. Joint fluid has very odd dynamics such that it comes in to the joint through the same route it goes out. There was an assumption that these routes were separate. Physiologists routinely make simple errors like that. Once I had managed to persuade other pshyiologists that the equations lined up differently we found the system collapsed to a simple elegant answer.

What worries me about bringing in a measure of 'cranial compliance' based on pulse waveform is that I cannot see why it should have anything to do with the much slower changes occurring during standing. I realise that rather than being explanatory it might just be exploratory but I would like t get more of an idea as to why it should be thought to lead anywhere in the longer run.

 

So maybe I've completely missed the point. Given that the cranial volume is fixed, I took ICC to refer to the degree of dynamic adaptation that is possible when the volume of some fluid/associated pressure in the head changes. In essence it seemed to be about a balance between quantities of blood and CSF, taking advantage of the brain being connected to the rest of the body to move in/out fluid as required. I assume 'compliance' in this context really refers to, or derives from, the performance of the brain vasculature. Is that not correct?

Why would the change when going from sitting to standing be slow? According to what I've read, cerebral blood flow fluctuates rapidly in response to fast fluctuations in BP. Sit-to-stand tests have been used to assess cerebral autoregulation, CA, by measuring BP and cerebral blood flow. Why would CBV not be influenced by the consequences of BP changes in those with orthostatic intolerance?

Also, apparently, as you probably already know, the autonomic nervous system is thought to possibly play a role in controlling CA on a beat-to-beat cerebral blood flow basis. I'm not at all familiar with the biology (and too brain fogged to read more) but could this be another potential pathway for autonomic dsyregulation to affect/be related to CA/ICC?

 

This may be entirely off topic, but why do I feel pressure in head and a headache when standing up, while also having that feeling malaise and weakness that comes with low blood pressure?

It is possible to have low and high blood pressure simultaneously?

 

When I said sitting to standing would be slow I was referring to the few seconds it normally takes at minimum for orthostatic intolerance to manifest. The shifts in blood and CSF during the cardiac cycle would occur over about 0.7-1.0 seconds as full cycle and return.

Viscoelastic systems with flow, like brain and its fluids, are likely to show resistances or compliances that are very rate dependent and also very non-linear.

 

No, I think maybe the authors may not quite have got hold of what they are telling us about. But these things are complicated. My experience is that the important thing is not to accept any jargon as bona fide but treat it like making tea - down to earth things moving about. Equations are particularly beguiling because they may look very convincing but may have the teapot upside down.

 

Yes, that's one of the frustrating things about not having the biology/medicine background - I don't have the physical feel for it.