One of the suspected causes of hydrocephalus, associated with neurodegenerative diseases seen in children and adults, is a dissociation of cerebrospinal fluid (CSF) flow between the cranial vault and spinal canal. The term hydrocephalus means water on the brain. Hydrocephalus causes the ventricles to enlarge, which is called ventriculomegaly. The ventricles are chambers in the brain where CSF is produced. Ventriculomegaly can be caused by anything that obstructs the pathways and normal flow of CSF, or it can be caused by inadequate absorption of CSF described below. In children, hydrocephalus is associated with high presssure in the brain called intracranial pressure. The high intracranial pressure, however, may be due to the open joints of the skull in a child, which provide less resistance to CSF pressure. The combination of high intracranial pressure and open joints in the skull causes the head to enlarge.
Normal pressure hydrocephalus (NPH) is a condition seen in adults in which the CSF volume increases and the ventricles enlarge but intracranial pressure remains normal or just slightly elevated. The size of the head, likewise, remains normal due to the closed joints of the skull. More than likely, the closed joints of the adult skull act like a counter-weight on a pressure cooker and limit CSF pressure inside the cranial vault. This will be discussed further in future posts. NPH and ventriculomegaly in adults can be caused by conditions such as traumatic brain injuries, subarachnoid hemorrhage (bleeding), prior intracranial surgery, and meningitis (inflammation of the protective coats called meninges). Most cases of NPH, however, are unknown.
The brain scan on the right is of an adult patient with NPH. NPH is typically associated with ventriculomegaly without atrophy (shrinkage) of the brain. Atrophy is seen as a widening of the spaces of the brain called fissures and sulci (dark spaces between the folds in the picture) that separate the gyri and folds of the different lobes. Ventriculomegaly is usually found around the frontal and temporal horns of the lateral ventricles.
In adults, NPH and enlarged lateral ventricles have been associated with Alzheimer’s and Parkinson’s disease, as well as dementia, schizophrenia, bipolar disorder, Parkinson’s Plus, Huntington’s disease and other neurodegenerative conditions for decades. More recently, enlargement of the third ventricle has been associated with multiple sclerosis. The fourth ventricle is sometimes enlarged in a condition called multisystem atrophy (MSA), which is a variant of Parkinson’s disease. MSA will be discussed further below.
In the brain scan above, the black arrows point to the lateral ventricles, which are seen as the large black spaces in the core of the brain. The ventricles are part of the brain. They are surrounded by the lobes, diencephalon (thalamus and hypothalamus) and brainstem and contain CSF. The corpus callosum forms the roof of the lateral ventricles and is seen as the white rim over the black space. The white rim below the lateral ventricles is called the fornix which sits over the roof of the third ventricle. As in this case, NPH often causes the corpus callosum to bow upward, and compress the outer cortex of the brain against the inner surface of the skull. White matter lesions are often present in the periventricular areas that surround the lateral ventricles. Periventricular white matter lesions are also seen in MS.
CSF flows from the lateral ventricles into the third and then the fourth ventricle. After leaving the fourth ventricle CSF flows into the cisterns. The cisterns are not part of the body of the brain called the parenchyma. Instead, they are part of the subarachnoid space that surrounds the brain. The subarachnoid space is a network of tunnels formed in the protective outer coats of the brain called meninges. The cisterns are seen in the brain scan above as the black spaces located around the brainstem and beneath the cerebellum (cauliflower-like structure in the lower rear of the skull). In this particular case, the fourth ventricle, seen in front of the cerebellum, is enlarged as is the space below it which is called the cisterna magna. The white arrow points to the cerebral aqueduct which is also enlarged. The cerebral aqueduct is a canal that connects the third ventricle to the fourth ventricle. The third ventricle is the dark space beneath the lateral ventricles. Some researchers now suggest that the term hydrocephalus should include an increase in CSF volume outside the ventricles as well, such as in the cisterns and the subarachnoid spaces.
CSF from the fourth ventricle drains into the pontine cistern, the cisterna magna and the central canal of the spinal cord. The pontine cistern is the black space in front of the brainstem (the long white structure). The pons is the part of the brainstem that sticks out toward the face like a big round potbelly. The cisterna magna is the black space beneath the cerebellum.
The central canal of the cord is not seen in this image. The connection between the fourth ventricle and the central canal will be covered separately as it relates to a type of hydrocephalus in the cord called hydromyelia or syringomyelia. Sometimes they are simply referred to as a syrinx. For now suffice it to say, they are abnormal cavities in the cord that are probably caused by pressure problems within the cord, similar to hydrocephalus which is due to pressure problems in the brain.
After entering the cisterns, CSF can flow down into the subarachnoid space of the spinal cord or it can flow upward through the subarachnoid space of the brain to the arachnoid granulations at the top of the brain, where the bulk of CSF absorption takes place to eventually leave the central nervous system via the venous system. The arachnoid granulations are one-way valves that connect the subarachnoid space to the superior sagittal sinus. The superior sagittal sinus is part of the venous drainage system of the brain located at the top of the skull.
The MRI above on the left is of a patient with a condition called multisystem atrophy. Multisystem atrophy is a variant of Parkinson’s disease, which also includes olivopontocerebellar atrophy. It is also known as Shy-Drager Syndrome. Among other things, multisystem atrophy is associated with dysautonomia and orthostatic hypotension in which blood pressure drops when changing position from sitting to standing. Dysautonomia is a malfunction of the autonomic nervous system that controls automatic internal operations such as heartbeat, circulation, respiration, temperature regulation, bladder and bowel control and more. In addition to Parkinson’s and multisystem atrophy, signs and symptoms of dysautonomia are common in Alzheimer’s disease and multiple sclerosis.
As can be seen in the brain scan above, in addition to enlargement of the fourth ventricle, multisystem atrophy is also associated with enlargement of the cisterns that surround, cushion and support the brainstem and cerebellum. In this regard, it looks similar to the MRI in the case with NPH at the top of the page. The difference is, the lateral ventricles aren’t enlarged. On the other hand, NPH is more typically associated with enlarged lateral ventricles but not enlarged cisterns. In this case the front of the pons (potbelly) appears slightly compressed due to the enlargement of the pontine cistern.
The brain scan on the right is of a child with a condition called Dandy-Walker Syndrome, which looks somewhat similar to the multisystem atrophy scan above. Here the fourth ventricle (the dark space in front of the cerebellum) and the prepontine cistern (the dark space in front of the cord) aren’t enlarged as above. Only the cisterna magna below the cerebellum is enlarged, which is called mega cisterna magna. Some cases of Dandy-Walker syndrome have enlarged ventricles, hydrocephalus and a mega cisterna magna. According to the classic definition, a mega cisterna magna alone, without ventriculomegaly, isn’t classified as hydrocephalus.
The enlarged cisterns and fourth ventricle seen in MSA (Parkinson’s) in adults, and in Dandy-Walker Syndrome in children are usually attributed to atrophy of the brainstem and cerebellum, which are surrounded by cisterns. Likewise, ventriculomegaly seen in neurodegenerative diseases such as Alzheimer’s, Parkinson’s and multiple sclerosis is typically attributed to atrophy of periventricular structures that surround the ventricles. Ventriculomegaly seen in NPH, on the other hand, is typically attributed to enlargement of the ventricles without atrophy of the surrounding tissues and structures. The problem is, the ventricles don’t always return to normal size when excess CSF volume is surgically decreased with a shunt. This led researchers to suggest that, the sustained enlargement of the ventricles may be due to permanent damage to the surrounding periventricular structures as a result of the NPH.
There are two primary theories regarding the origin of NPH. The first theory has to do with obstruction of CSF flow or blockage of resorption into the venous drainage system of the brain discussed above. The other theory is that NPH is due to atrophy of surrounding structures, such as the periventricular white matter that gets strained and tensioned to the point of breaking, causing the walls of the ventricles to weaken and enlarge. Enlargement of the ventricles also stretches, strains and compresses surrounding blood vessels that can decrease blood flow and cause ischemic tissue damage. In either case, the subsequent weakness in the walls of the ventricles and surrounding structures cause the ventricles to enlarge. NPH, in turn, increases tension, tangential and shear stresses in the brain, as well as compression loads caused by expansion and contraction of the brain with each beat of the heart that forces a relatively large volume of fluid into the mostly closed container of the cranial vault.
There is a reason why these different types of ventriculomegaly (hydrocephalus) and mega cisterns seen in children and adults look remarkably similar. It is because they are all related to faulty CSF flow in the brain. Whether it shows up as ventriculomegaly or mega cisterns depends on the location of the obstruction or malabsorption problem. Aside from internal problems in CSF pathways of the brain inside the cranial vault, researchers now suspect that one of the causes of hydrocephalus in children and adults is a dissociation of CSF flow between the cranial vault and spinal canal. Dissociation simply means that the normal flow between the two compartments is disrupted, and as a result they react independantly. Among other things, this can cause potentially destructive abnormal increases in CSF pressure waves in the brain and cord. Dissociation of CSF flow between the two compartments may similarly play a role in hydromyelia and other conditions of the cord.
Upright posture requires proper CSF flow between the cranial and spinal compartments in order to maintain the correct volume, pressure, protection and bouyancy of the brain. Excess CSF volume in the ventricles, cisterns and subarachnoid spaces can cause destructive tension (stretch) and compression loads in the brain. An insufficient volume reduces bouyancy, which causes the brain to sink and make contact with the base of the skull. The connection between the cranial compartment and spinal compartment is in the upper cervical spine. Malformations, injuries and misalignments of the upper cervical spine can cause a dissociation of CSF flow between the cranial and spinal compartment. Further below in the lower spine, spondylosis (degeneration), scoliosis (abnormal curvature) and stenosis (narrowing) alter the design of the spinal canal and thus affect blood and CSF flow. The changes in the design of the spinal canal can, likewise, result in a dissociation of CSF flow between the cranial vault and spinal canal. Thus far, the studies on dissociation of CSF flow between the cranial vault and spinal canal have all been done using supine MRI. Future research will need to be done using upright MRI. Upright MRI will most likely reveal much more about the destructive consequences of dissociation of CSF flow between the cranial vault and spinal canal.
The areas hardest hit by faulty fluid mechanics in the cranial vault are: 1) the periventricular structures that surround the ventricles, 2) the bottom surfaces of the brain, brainstem and cerebellum located above the base of the skull that are surrounded and supported by the cisterns, and 3) the cortex of the brain closest to the inner surface of the skull. The particular areas that are affected are determined by the cause and the consequences of the faulty CSF flow.
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