Hydraulic pressure is the force exerted by a fluid against a given area such as the inside surface of a container or a pipe. The force is determined by properties of the fluid and the design and dimensions of the container or pipe. In theory, if two containers are linked with a pipe and filled with fluids, force applied against a large container and transmitted via the pipe to a smaller container decreases the force in the smaller container due to the decrease in its size. If pressure is exerted against a container connected to another one of equal size the force stays the same. If the pressure is transmitted to a larger container the force goes up due to the larger size of the area that the pressure is exerted against. In medical sciences, radiologists refer to fluid mechanics and hydraulics in the brain as cranial hydrodynamics.
Cranial hydrodynamics (hydraulics) are driven by cardiorespiratory pressure fluctuations. When the heart contracts it pumps a large quantity of blood into the brain under pressure. It then relaxes, which relieves the pressure. This causes the soft tissues of the brain to rhythmically deform and reform in continuous expansion and contraction cycles. The force cranial hydrodynamics exerts on the different areas of the brain is not uniform. It varies according to the design and dimensions of the different chambers transmitting the force, as well as those that receive it. Thus, a low pressure and force from a smaller chamber can generate greater force when it acts on a larger chamber in the cranial vault.
Poroelasticity is an engineering term used to describe the interaction between rocks and fluids that fill the rock’s pores. External pressue on rocks such as from a large body of water above, pounding waves or large buildings cause rocks to compress which increases pressure on the fluids that fill their pores. Conversely, increased fluid pressure in the pores of rocks such as ice can cause rocks to expand. Over time, poroelastic mechanics can cause rocks and soils to settle, subside, crack and deform. Poroelasticity can be used to describe deformation of the bones of the cranial vault and the deformation of the brain due obstruction of cerebrospinal fluid (CSF) flow.
Caves, caverns and cavities in rocks are often formed by the force of water. In this regard, the skull is a stone structure comprised of many rocks surrounding enclosed caves, caverns, cavities and pores formed in part by cranial hydrodynamics and the force of water. The cranial vault, for example, is a large fluid filled cave. Inside the cranial vault, the tough outer coat of the brain, called dura mater divides the cranial vault into caverns called fossas. It also forms caverns called dural sinuses. The openings in the skull called fissures and foramen are cavities that penetrate the bone stones of the skull. All bones have pores but the bones that cover the cranial vault have special, extra-large pores called diploic spaces located between the inner and outer plates of the skull bones. The dipolic spaces contain valveless veins. The diploic spaces and veins insulate, cool and help to maintain the temperature of the brain.
The bones of the cranial vault are like rocks constantly battered by blood and cerebrospinal fluid (CSF). The high pressure in the arteries is strong enough that it causes deformation leaving their distinct impression on the roof of the skull. Blood pressure in the veins is much lower so they don’t cause as much deformation and imprints on the skull bones. The largest veins of the brain called the dural sinuses, such as the superior sagittal sinus, the transverse and sigmoid sinuses, however, influence the shape of the special joints of the skull called sutures. The imprint they leave however, isn’t the same as the arterial imprints. Instead they leave behind a strange looking zig zag pattern similar to surgical sutures or stitches used in sewing.
Interestingly, the inside surface of the sutures of the skull next to the sinuses shows much smaller deformation, comparatively speaking, than the outside surface. The outer surface of the sutures and skull is instead effected by the much smaller diploic veins. The deformation and impression they leave behind are large and erratic and get progressively larger toward the back and bottom of the skull. I discussed the shape of the sutures in previous posts. They are a reflection of fluid mechanics in the brain. Suffice it to say that their shapes are not like veins but instead suggest lateral strains such as from water that sloshes from side to side in bucket while being carried.
In addition to arteries and veins, cerebrospinal fluid (CSF) can also batter the bones of the skull. Left unchecked, CSF can cause the skull to enlarge in a child with hydrocephalus or Dandy-Walker syndrome. The previous post discussed Dandy-Walker Syndrome in children which is often associated with hydrocephalus and enlargement of the posterior fossa.
The picture above on the right is of a child’s skull that was effected by hydrocephalus. Notice that the bones that typically make up the base of the skull all around the area where the ear would be are broken into many smaller sections. Those small sections of bone are called wormian or sesamoid bones. They are caused by rapid expansion of the skull, which stretches the bone to its limits. Bone development can’t keep pace and imperfections develop and voids are filled with pieces of bones to patch things over. They are similar to rock fractures caused by ice expansion and other internal forces inside the pores.
CSF is produced from blood and driven by pulsations from the circulatory system and respiration. Although it is extremely low compared to blood pressure, the hydraulic force from the pulsations of cerebrospinal fluid are strong enough to erode and leave impressions in the bones that form the roof over the cranial vault. Physical anthropologists and forensic scientists call the pits in the skull caused by CSF pulsations Pacchionian or arachnoid impressions. They are caused by the arachnoid granulations. The impressions are also known as granular foveolae. If you click on the picture at the top left of the page, you can see a large Pacchionian impression above the letter “e” in the word “bone.” A much smaller round Pacchionian impression can be seen above the larger one. As an aside, this skull also shows hyperostosis, which is associated with excess growth and thicker bones. Hyperostosis is sometimes associated with an increase in intracranial pressure due to the decrease in the inside dimensions and thus capacity of the cranial vault.
The superior sagittal sinus is located at the top of the brain. Several large reservoirs of veins called venous lacunae (lakes) are located on either side of the superior sagittal sinus. Venous blood from the brain enters the venous lacunae. CSF from the subarachnoid spaces flows through the one way valves of the arachnoid granualtions and into the venous lacuna and superior sagittal sinus. Lacuna means lake because its a large venous reservoir. This means that the pressure exerted by the smaller container of CSF against the larger venous lacuna increases the force acting on them. The force it generates is strong enough to put a dent in the roof of the skull. Blood in the venous lacuna then empties into the superior sagittal sinus and travels down through the transverse and sigmoid sinuses and into the internal jugular and the vertebral veins located in the posterior fossa.
The thought first occured to me decades ago when I first saw Pacchionian impressions and skulls with hydrocephalus, that if CSF is powerful enough batter and erode bone, then it certainly must be strong enough to batter and erode the much softer tissues of the brain. Recent evidence from studies being done in Latham, New York, by Dr. Scott Rosa using an upright phase contrast cine MRI by FONAR Corporation continue to confirm my hypothesis. The areas I have seen effected are the front of the posterior fossa called the clivus and the rear of the fossa called the supraocciput. Interestingly, among other things, radiologists look for erosion of the clinoid process of the clivus as a sign of increased intracranial pressure. Another potential sign they look for is an empty sella, which I discussed in previous posts. CSF backjets and turbulent flows can indeed erode bone. Moreover, in addition to bone, chronic CSF backjets and inversion flows can batter and erode the brain by similar hydraulic effects.
Normal pressure hydrocephalus (NPH) is a condition seen in adults and has been associated with Alzheimer’s and Parkinson’s diseases. More recently it has been implicated in multiple sclerosis. In NPH the ventricles, likewise, become enlarged but intracranial (CSF) pressure remains normal or just slightly elevated. In addition, the size of the skull is normal but the brain decreases in size. The decrease is currently blamed on atrophy. It puzzles researchers and engineers how low pressure conditions can cause the ventricles to enlarge. It is my opinion that it may be due to erosion and subsequent atrophy caused by constant battering of the brain by aberrant CSF flow. What’s more, it’s not just the pressure that does the damage in NPH. The force generated by the hydraulic pressures acting on the designs and dimensions of different parts of the brain has to be considered as well. In my next, post I will discuss how CSF can similarly batter the brain and cause it to shrink in size while pressure remains normal or just slightly elevated due to hydraulics and poroelasticity.