Cavities such as cysts and syrinxes that may or may not contain cerebrospinal fluid (CSF) are sometimes seen in the brain and cord. In some cases these cavities can be silent and cause no signs or symptoms. In other cases they can cause significant even life threatening problems.
On this page we are concerned with the types of cavities that contain CSF. Faulty fluid mechanics can cause cysts and syrinxes that contain CSF to form in the brain and cord. Likewise, these types of CSF filled cavities in the brain and cord can cause faulty fluid mechanics by obstructing CSF pathways and altering its normal flow patterns and velocities.
Cavities sometimes develop in the middle layer of the three protective coverings of the brain and cord, called meninges and named dura, arachnoid and pia mater. If you look at the picture below, toward the bottom on the right hand side, there is a label pointing to a dark line called dura mater. The dura mater is the tough outer covering that surrounds the brain and cord. Above the dura mater is another label denoting the arachnoidea and is the white line surrounding the brain and cord. The arachnoidea is the arachnoid mater, which is the middle layer of the meninges. Still another label above that, points to a black line called the pia mater, and is the inner most layer of the covering of the brain and cord. The subarachnoid space is the stipled area that lies between the arachnoid and pia mater.
Arachnoid cysts are fairly common. They project out from the arachnoid mater covering and subarachnoid space. They are also referred to as leptomeningeal. These cavities tend to occur in certain places in the brain and cord. One of the common areas is the middle fossa of the cranial vault, as well as the Sylvian fissure of the brain which is located in the middle fossa. The Sylvian fissure is a deep fold which separates the frontal, temporal and parietal lobes of the brain.
Other common areas for these cavities are within the area beneath the covering over the posterior fossa called the tentorium cerebelli (the black line over the top of the cerebellum). They are thus referred to as infratentorial. Another area is the cisterna magna located beneath the bottom of the cerebellum and the medulla oblongata of the lower brainstem the stipled area beneath the cerebellum in the sketch above).
Still another common area is the sella turcica. The sella turcica is a small subcompartment in the sphenoid (bat shaped) bone located in the middle portion of the base of the skull. It houses the pituitary gland, which is the hypophysis hanging down beneath the front part of the brain in the sketch above. The suprasellar region is the area immediately above the protective pocket of bone for the pituitary gland. The infrasellar area is within the pocket (sella turcica). These cysts are more commonly seen in the suprasellar area but they can also occur in the infrasellar area.
In empty sella syndrome the pituitary gland appears to be absent or partially absent. In most cases the pituitary seems to simply be smaller in size. It is currently maintained that the infrasellar space consequently gets filled with fluid, similar to a cavity, to fill the empty space due to the undersized pituitary. On the other hand, it is possible that supra and infrasellar cavities can invade and cause chronic compression and subsequent degeneration of the pituitary gland.
If you look at the middle of the brain in the picture above, the arrows, representing CSF start in the lateral ventricles and flow down through the third ventricle, the cerebral aqueduct, and into the fourth ventricle. They then leave the fourth ventricle and enter the stipled area which represents the subarachnoid space.
Enlared ventricles are typically referred to as ventriculomegaly. Enlargement of the fourth ventricle, however, is sometimes classified as a cyst or cystic ventricle. Dandy-Walker syndrome occurs in children and far more frequently in females. Certain cases are caused by undersized outlets from the fourth ventricle called foramem of Lushka and Magendie. In most cases the cause is unknown. Some cases of Dandy-Walker syndrome are associated with ventriculomegaly of the fourth ventricle and is referred to as a cystic fourth ventricle. Others are associated with cystic-like enlargement of the large cistern that sits beneath the brainstem and cerebellum, called the cisterna magna.
Congenital versus Traumatic Causes of Cavities
It is currently maintained that most people are born with arachnoid cysts. Some are caused by genetic weakness in the tissues or design flaws in the subarachnoid space. I suspect that still others may occur due to intrauterine positions of the fetus during pregnancy that can potentially increase pressure in critical areas of the brain and cord.
Some cavities can occur during traumatic delivery and some are acquired later in life from trauma. Cavities occur because the meninges (mentioned above) of the brain and cord are separated by fluids which are lymph-like and allow the membranes to slide. In traumatic events the three layers of the meninges slide and move against one another at different speeds due to their individual tissue make up. The sliding strain creates shear forces and stress. When the tissues exceed a their particular strain threshold it can cause tears in the meninges. The weak point in the wall of the meninges can cause subsequent cavities to form.
Some cavities are formed in the cord and are called syrinxes. The word syrinx comes from the Greek word tube. If you look at the picture above you will see a canal going down through the center of the cord. The label points to it on the left near the bottom of the canal, it is the canalis centralis, which means central canal of the cord. Syrinxes typically start and are found in the central canal but they can spread outwards and affect other parts of the cord.
Syrinxes are typically expansions in the central canal of the cord, which is connected to the lowest ventricle in the brain, the fourth ventricle. These syrinxes are also referred to as syringomyelia. Similar to cysts, syrinxes are sometimes connected to and sometimes more isolated from CSF pathways, which can trap fluids.
Sometimes these cavities communicate with the normal CSF pathways. In the case of subarachnoid cysts the fluid can flow more freely between the cavity and subarachnoid space. In other cases the cavity is more isolated by connective tissues so that fluids that manage to find there way into the space, find it hard to get out.
Cavities and Skull Deformation
Cysts and syrinxes are sometimes associated with significant pressure that causes fluids to eject somewhat forcefully when penetrated surgically. The force from the hydraulic nature of cysts and syrinxes can consequently compress nearby tissues. The turbulant flow and subsequent pounding they create can also erode nearby tissues like waves beating on rocky cliffs on a shoreline. Turbulant flow can similarly erode bone.
In fact, over the course of a lifetime the normal pulsatile (beating) flow of CSF is strong enough to leave impressions that look like little dents on the inside roof of the cranial vault. Cavities can increase local CSF pressure and velocity. In this regard, certain types of cavities that occur during development, such as Dandy-Walker, are strong enough to change the normal shape of the bones of the cranial vault. In Dandy-Walker syndrome it can cause the normally straight edge of the clivus to bend into a concave shape. Dandy-Walker cysts can also cause thinning of the bones of the occiput in the back of the skull. Lastly, cavities and other forms of blockage of CSF flow can cause enlargment of the foramen magnum in the base of the skull, as well as other foramen. It seems logical then that if these cavities are strong enough to erode bone away they most likely can also compress and damage the soft tissues of the brain and cord.
Sometimes defects can occur in the skull or in the posterior arch (rear wall) of the vertebra of the spine during development. The posterior arch of the spinal segment typically covers and protects the cord. A defective opening in the posterior arch is called a spina bifida, which means that the left and right sides of the posterior arch of the spine failed to unite and instead remained split in the back in two pieces. The opening (located at the area of the spinous process in the picture below) caused by the split allows the protective covering of the brain or cord, called meninges, to poke through the protective walls of the spinal canal.
Pressure from faulty CSF and fluid mechanics in the brain and cord can also either cause, or complicate, a defect in the skull or spine. In this regard, Chiari malformations (which impact these mechanics) of the brain in the posterior fossa are sometimes associated with cyst-like problems in the brainstem and cord as well.
For example Chiari malformations can cause an opening defect in the rear of the skull. If a portion of the brainstem or cerebellum pokes through the defect it is called an encephalocele. Opening defects further down in the spine can, likewise, cause the contents of the spinal canal to poke through. If the meninges of the cord and CSF pathways poke through the defect it is called a meningocele. If part of the cord also pokes through the spine along with it, it is called a myelomeningocele.
Ventriculomegaly and Cystic Cisterns in AD, PD and MS
These types of cavities speak volumes about CSF and the hydraulic stress it causes in the brain and cord. What’s more, faulty fluid mechanics and the hydraulic stress may be one of the root causes of Alzheimer’s, Parkinson’s and multiple sclerosis, as well as other neurodegenerative diseases of the brain and cord. Ventriculomegaly (enlargement of a ventricle) has been associated with all three of these conditions. Two variants of Parkinson’s disease are also associated with cystic conditions within the ventricles and cisterns. Mild to moderately dilated cystic cisterns may be more common than currently realized. Cystic cisterns can increase local CSF pressure acting on the brainstem. In most cases, the cause of these cavities is unknown. Phase contrast cine upright MRI is starting to provide more clues.