noun | ba-rom-et-rick presh-shure|
1) the force exerted on an object by the weight of the column of air above it; affected by changes in weather patterns and altitude
2) the uncontrollable force of nature that can drive make blood shoot out of the eyeballs of a PERSON suffering an coping from an ICPSD
see also: weather fronts, cold fronts, warm fronts, troughs, stationary fronts, Weather.com, weatheruderground.com, and the app “Barometer” on your iPhone ; )
The latest evolution in my understanding of how and why the barometer affects Intracranial Pressure after a doctor told me “(medicine) recognized that changes in barometric pressure affected (IPSCD) patients, but we don’t know why”.
Maybe they do and I’m on a fool’s errand, but in the off chance that one of the two people who read this get something useful from it, here, we go:
Before reading the long part, here are key points in the ongoing question of why changes in barometric pressure are affecting me, and I’m willing to bet apply to just about everyone else:
1) Intact adult skulls don’t expand
2) Our bodies expand and contract in response to changes in atmospheric pressure.
3) Increases in barometric pressure squeeze our bodies, pushing blood into the head
4) Blood vessels in the can head fill with blood if veins don’t drain it off
5) Swelling blood vessels push on brain matter
6) Brain matter expands into spaces filled with cerebrospinal fluid: around the brain and the ventricles within
7) CSF becomes pressurized, and brain stuctures are compressed, causing pain, interfering with functions of the compressed structures (most notably but nowhere near the only function affected is vision), and if not relieved, deformity of the brain
8) In shunted patients, this swelling can cause flow of CSF out of the brain. Over a long term, this can lead to Slit Ventricle Syndrome.
ME AND THE BAROMETER, PART 2
My understanding of why changes in the barometer affect me, and certainly other ICPSD patients, advanced last night after reading this article: “Monro-Kellie 2.0: The Dynamic Vascular and Venous Pathophysiological Components of Intracranial Pressure” by Dr. Mark H. Wilson, a neurosurgeon practicing in London. In coping with my ICPSD I had come to the obvious conclusion that ICPSD conditions (IIH, NPH, PTC) have at their root a failure of a patient’s physiology to maintain a fluid in/fluid out balance in the cranium. Currently, my personal condition is complicated by the fact that my shunt is turned up to its maximum pressure, but I still have symptoms of low ICP. Keeping the right amount of blood flowing to my head has become very important. I had been concentrating on the “fluid in” side of the ICPSD equation using by modifying the method medicine uses to manage ICP in head injuries. I take my blood pressure, calculate the mean arterial pressure (MAP), and multiply it by 0.2. This gives me a number I call the “Cerebral Perfusion Quotient 20”, or CPQ20, representing a very generous portion of cardiac output flowing to my brain; this is simplistic but served a purpose. Tracked over a couple of weeks and referenced with activities, barometric pressure, temperature, and humidity, I was able to determine identify a range where I was relatively comfortable and functional. I have identified certain activities and conditions that aggravate my low ICP, and take certain measures to manage it as best I could. It has helped me to a point to manage my ICP until I can have a shunt revision. I had a passing semblance of control until autumn in Virginia brought with it the wide swings (20+mb every 7-10 days) of barometric pressure normal for the season. The unstable weather has brought with it an instability in my ICPSD.
Barometric pressure changes are known to affect the symptoms of ICPSD patient. In research, I’ve also discovered that it is associated with aggravations of other central nervous system mediated disorders including seizure activity, multiple sclerosis, and dementia. Barometric pressure is literally the pressure that the air around our bodies exerts on every square inch of our skin . Barometric pressure change has always been an issue for me; but understanding why has been an ongoing challenge. I came to an initial part of the answer recently (“Why Barometric Pressure and Weather Affect Intracranial Pressure”). This was so surprisingly simple as to be elusive. In short, changes in barometric pressure cause our bodies to expand and contract; like a balloon in a pressure chamber, our bodies expand with lower pressure and contract with increasing pressure. This expansion/contraction literally changes the volume of our bodies and circulatory system. When the barometer rises, our bodies are squeezed, and blood is pushed into confined areas where air pressure does not directly affect, like the inside of the skull*.
Charting my CPQ20 numbers, I noticed that a drop in barometric pressure also meant a drop in my CPQ20 number. When that number falls below the comfort zone, I develop lightheadedness along with a sensation that my ears were being sucked into my head. My shunt system has several components that change in nature depending on my perception of my ICP. Pliable components of my shunt system, chiefly the domed silicone antechamber in front of the shunt body and the silicone catheter behind my ear became shrunken and limp; the antechamber would become pliable and easy to depress. Another shunt system indicator is the tantalum ball in the antisiphon/assist device behind my ear: at low pressures when I lightly tap the skin over the assist, the little ball rattles freely. These are indicators that there is little to no CSF flowing in the shunt. Contrast this to increases in barometric pressure and CPQ20 numbers above my comfort zone. The antechamber and catheters becomes swollen and very firm, and the tantalum ball doesn’t “rattle”; it might “tick” once, but seemed restrained by the pressure/flow of draining CSF.
I was still confused as to how I could have so much pressure in my shunt if I was “overdrained”, that is, have low ICP. Increases in barometric pressure inevitably lead to this peculiar condition; however, until I read Dr. Wilson’s article, I didn’t understand why. Now I believe I do, and strangely enough, being a dentist has given me a particular insight**.
The cranium is a fixed volume. Intact adult skulls don’t expand and contract. As such, maintaining ICP requires a precise balance of arterial blood flowing into the skull, production and absorption of cerebrospinal fluid, and venous drainage. In calculating my CPQ20, I was looking at blood flowing in. However, as Dr. Wilson points out in his article, CSF is produced and resorbed at low rate of .3-.5ml/minute, give or take. That dynamic is dwarfed by the 700 ml or more of arterial blood that is pumped to the brain every minute. It further occurs to me that the fact that CSF is produced from this arterial blood is important as well, but bottom line: if this blood isn’t drained out fast enough, blood accumulates in the brain.
And that led to this latest epiphany, such as it is in our grand medical cosmos. It’s not an original notion, it turns out, but I still managed to claw my way their leaving a trail of CSF in my wake.
The human brain has been described as “incompressible” in the original Monro-Kellie Doctrine, referred to in Dr. Wilson’s publication, the first effort to explain the dynamics of fluids inside the skull associated with our brains. Brain tissue does have a degree of compressibility: it is seen in cases of hydrocephalus where the brain can be flattened into a thin layer against the skull as excessive CSF pressure. Pressure comes from the network blood vessels in and around the brain. In ICPSDs, blood enters the brain faster than it can be drained away, filling and expanding these vessels. When they expand in the skull, something has to give. In my case, I believe the expanding blood vessel cause the brain itself to expand in segments between the blood vessels. The only space available for the brain to expand “into” is the space occupied by cerebrospinal fluid: the subarachnoid space between the brain and the skull, and the ventricles, hollow cavities in the brain associated with the production, and circulation of cerebrospinal fluid. This pressurizes these spaces and the CSF contained within them. In a patient without an outlet for this fluid, such as a shunt, the end result is pain and damage to structures of the brain such as the optic nerves, the pituitary gland, and others. This is the same process behind hydrocephalus. In shunted patients like myself, it makes for a plausible explanation for why my shunt drains so heavily when the barometer spikes and my CPQ20 is above my comfort zone: a glut of blood is compressing the brain tissue, pushing CSF out of my shunt. When CPQ20 numbers are below the comfort zone, my shunt seems to barely drain owing to a lack of literal “pressure from blood” in the vessels in and around my brain. It explains how I can be living with a shunt that can’t be turned up high enough to maintain an appropriate ICP for me and still have sensations of pressure. And it is definitely related to changes in the barometer, whether weather related or altitude related. I’m writing this the morning after driving to visit my mother; her home is 1000 feet higher in altitude, and according to my iPhone barometer, 30 mb lower in pressure than where I live (interesting fact: one millibar is equivalent to 10mm H20. Average ICP in the adult population is 110mm water; a 30 mb change is the equivalent of 300 mm of water pressure change on my body. Hmmm.). I’ve noticed this phenomenon on other occasions when weather conditions at my home made my symptoms unbearable, but by relocating here, the symptoms usually abate. Barometric pressure is not the only factor I’ve identified: heat and humidity are two others, but will save discussions of those for another time.
ICPSD disorders are very focused on CSF and its management. Dr. Wilson bluntly points out an obvious fact of epic importance: CSF is only one of the two fluids in the skull; blood is the other, and the volume AND FLOW of blood and its dynamics dwarf those of CSF++. Blood volume imbalance is likely one possible explanation for ICPSDs in some patients. Simply put, a failure of the body to balance of blood in/blood out of the cranium leads to pressure on the brain directly as blood vessels fill with blood and expand. Displaced tissues compress the ventricles of the brain and without intervention can lead to damage to the brain, its structures, and the functions of the body those structures control. In my case, pressure forces CSF through my shunt; however, the shunt has a limited drainage rate, and if ICP expels CSF at a rate greater than my shunt system can drain it into my abdomen, in manifests as symptoms of pressure in my head and signs of an engorged shunt system. This explains the paradoxical presence of high pressure symptoms I experience while living with a low ICP. At this point the only management method I have is to lower my barometric pressure, and the only way I can do that is to go to a location at a higher altitude/lower barometric pressure.
This is not the end of this mechanism either. Pressure in the abdomen and chest play a very significant role in not only possibly explaining the development of ICPSDs but their management as well. I’m going to put these related topics into a subcategory under “Topics” entitled “Factors of ICP”. Hopefully readers will find something that sheds light on their own conditions and management. Prayers and comfort to all.
*or the inside of bones/cartilage, giving an explanation of why joints ache when a storm is impending.
**Intact and undamaged teeth are also impervious to the direct effects of air pressure. Teeth also have an internal anatomical space filled with blood vessels and nerve tissue. This tissue can become damaged if it becomes inflamed and begins to swell. The swelling chokes the tiny blood vessels off, thereby causing the tissue to die from what amounts to self-strangulation, the dental equivalent of cerebral edema of the brain.
++Flow is dynamic; volume is fixed. It’s not only how much blood is in the brain, it’s also how fast it flows in and out….
How I felt on the day I received my VP Shunt, when the barometer had hit a near record high for my area, 40 mb above mean. My neurosurgeon told me my CSF literally shot across the room when he was placing the shunt. Contrast this to the day I had my diagnostic lumbar puncture: my opening pressure was only 17cm, but after draining 30 ml of CSF, most of my symptoms and pain disappeared…