The Fourth Ventricle and CSF Backjets
The fourth ventricle is the last and lowest of the four ventricles in the brain. The ventricles are shown in red in the picture below. They are chambers located in the middle of the brain where cerebrospinal fluid (CSF) is produced.
CSF comes from blood that has been pushed and pulled through an extra-fine filter. The result is mostly water mixed with some salt, sugar and proteins, as well as trace amounts of other substances and sometimes a very small amount of white blood cells, if any at all. There are no red blood cells. The role of CSF is to protect the brain from the hard bones of the cranial vault. It also causes the brain to float inside the vault, which protects the brain and blood vessels from compression. More importantly, it prevents the cerebellum and brainstem from sinking into the foramen magnum, which is called a Chiari 1 malformation.
In addition to its protective role, CSF and its pathways are a key part of the circulatory and eliminative system of the brain. It removes metabolic waste products, as well as neurotransmitters and endocrine hormones that are produced and used by the brain. It also removes drugs that enter the brain from outside sources.
The top two ventricles are the left and right lateral ventricles. They are the largest ventricles located in the upper portion of the brain. The lateral ventricles have large projections called cornu because they look like horns. The upper horns are called the anterior cornu. The projection in the back is the posterior cornu and the one on the bottom is the inferior cornu. The structure between the anterior and inferior cornu (horns) of the lateral ventricles is the third ventricle, which is located inside the midbrain. It's role in Parkinson's disease and dysautonomia is discussed elsewhere on this site. The fourth and last ventricle is the lowest one below the lateral and third ventricles. It is located in front of the cerebellum and behind the brainstem in the posterior fossa.
CSF typically flows out of the fourth ventricle and into the cisterns and central canal of the cord. Obstruction to CSF flow out of the cranial vault can cause inversion flows. It can also cause CSF to back up in cisterns and ventricles. The first cistern to feel the effect of increased CSF volume is the cisterna magna. The first ventricle to be affected by CSF backups and backjets is the fourth ventricle. Backups and backjets (inversion flows) of CSF in either the cisterns or ventricles can increase CSF volume and pressure acting on the outside and inside of the cerebellum.
I have discussed the consequences of increased CSF volume in the cisterns elsewhere on this site. This page is about inversion flows resulting in increased CSF volume and pressure in the fourth ventricle. Increased CSF volume and pressure in the fourth ventricle can cause nystagmus, vertigo and balance disorders.
This is interesting because nystagmus, vertigo and loss of balance are common complaints in patients with neurodegenerative diseases such as Alzheimer's, Parkinson's and multiple sclerosis and may be signs of CSF backjets into the ventricles. What's more, patients with Parkinson's get stiffness in the spinal muscles and stooped posture as a result of the stiffness.
The cisterns are part of the subarachnoid spaces of the brain which surround the brain with CSF. They are large expansions in the subarachnoid spaces that are strategically located to protect the brain from contact with the hard bones of the cranial vault.
The lowest and largest cistern is called the cisterna magna. The cisterna magna sits below the cerebellum and medulla of the brainstem in the posterior fossa of the cranial vault. In addition to protecting the brain from compression against the bones of the cranial vault, the cisterna magna, as well as the other cisterns allow the brain to float, which reduces its weight. The flotation of the brain also prevents it from sinking into the foramen magnum.
A condition called a Chiari 1 type malformation occurs when the tonsils, which are the lowest part of the cerebellum, sink into the foramen magnum. In certain cases, depending on the design of the individual, displacement of the cerebellar tonsils into the foramen magnum can block blood and CSF flow. It can also cause changes in the normal direction and flow of CSF resulting in backjets and turbulance in the cisterns and ventricles among other things.
CSF enters the fourth ventricle by way of the cerebral aqueduct, which is also called the aqueduct of Sylvius. Under normal conditions, CSF leaves the fourth ventricle through four different outlets. One is centrally located in the lower portion and is called the foramen Magendie. Two are located on the sides of the fourth ventricle. They are called the foramen of Lushka. Lastly, there is an outlet in the floor of the fourth ventricle that connects it to the central canal of the spinal cord. In fact, some anatomists consider the fourth ventricle to be the top portion of the central canal located in the center of the spinal cord. The central canal of the cord continues down through the length of the spinal cord to the end of the cord. The end of the cord is called the conus medullaris. There is a dilation in the end of the central canal which is called the ventricle terminalis.
The central canal and the outlets of the fourth ventricle are small and narrow. This makes them susceptible to obstruction and compression that can cause CSF to back up. Furthermore, the problem with the central canal as an outlet for CSF out of the brain is that the canal often closes with age making it discontinuous and decreasing its capacity and effectiveness as a drainage route. Closure and discontinuity also decreases the flow of CSF out of the central canal. In certain cases, according to one theory, blockage of CSF flow in the central canal combined with fluctuations in CSF pressures due to respiration and heart contractions, can cause hydraulic CSF pressures to erode the canal and form cavities called syrinxes. Since the central canal is not as reliable as a primary outlet, the burden of CSF outflow from the fourth ventricle is through the foramen of Magendie and Lushka and into the basal cisterns of the brain.
The foramen of Magendie and Lushka empty into the lower cisterns called the cisterna magna mentioned above and the pontine and interpeduncular cisterns. CSF is pushed out of the fourth ventricle and into the cisterns during the contraction phase of the heart which drives a large volume of blood into the brain.
The relatively large increase in blood volume in the brain caused by the contraction of the heart must be compensated for by a proportionate amount of venous blood and CSF leaving the cranial vault. Imbalances between the inflow and outflow of blood and CSF can lead to edema (swelling) and hydrocephalus (increased CSF volume) in the brain.
CSF in the cisterns can flow in two directions. One way is upward to the top of the brain where it is absorbed by special valves called arachnoid granulations which connect the CSF in the subarachnoid space to the drainage system of the brain called the dural sinuses. The other direction CSF in the cisterns can take during the contraction phase of the heart is down through the subarachnoid space of cord. In order to flow out of the cranial vault, CSF in the cisterns has to pass through the subarachnoid space of the brainstem and spinal cord located inside the foramen magnum and upper cervical spinal canal.
Chiari malformations are associated with constriction and compression of the subarachnoid space that connects the cisterns of the brain to the subarachnoid space of the cord. Design flaws in the skull, upper cervical spine and brain can also cause problems and obstruct outflow. Injuries and misalignments of the upper cervical spine can, likewise affect CSF flow between the cisterns of the brain and subarachnoid space of the cord.
Typically CSF flows from the point of highest pressure where it is produced in the ventricles, to its lowest point of pressure where it enters the venous dural sinuses of the brain. The difference between its high and low pressure points is called the CSF pressure gradient.
If the CSF pressure gradient is high and CSF flow through the foramen magnum and upper cervical spinal canal are obstructed then CSF will be forced into the cisterns under pressure. Over time chronic obstruction may cause CSF in the cisterns to compress and erode the structures they surround such as the brainstem. If CSF pressure in the ventricles is weak, such as occurs with aging and a decline in heart and circulatory strength, then CSF will not have sufficient pressure to get pushed into the cisterns. This will cause CSF backjets (inversion flows) and backups in the ventricles. As was stated above, the first venricle to feel the effect of CSF backjets is the fourth ventricle.
There is an interesting part of the cerebellum that projects into the fourth ventricle called the flocculus. The flocculi are lobes of the cerebellum. The nodular portion of the flocculonodular lobe of the cerebellum is number 20 in the picture below. The fourth ventricle is number 22. The left and right floccular lobes are connected in the middle by the nodulus. It is one of the oldest portions of the cerebellum that evolved long before the other parts.
The flocculur lobes have a connection to the vestibular nuclei of the eigth cranial nerve, called the accoustic or vestibular choclear nerve. The accoustic nerve is for hearing and balance. They also receive information from the truncal muscles that regulate posture and balance. The flocculus uses the information from the eigth cranial nerve and the neck muscles to effect the eye muscles and movement.
Chronic CSF backjets into the fourth ventricle due to obstruction of outflow through the foramen magnum may effect the flocculonodular lobe of the cerebellum resulting in nystagmus, vertigo and balance disorders. One of the most likely places for obstruction to CSF flow to occur is in the upper cervical spinal canal. The cause can be genetic design problems or acquired through injuries and aging that result in misalignment and loss of structural integrity.
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