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1.
A six month old female infant was brought to the outpatient department of The Medical City for
progressive head enlargement. Patient’s head size is almost double of what is normal for an infant. She
also has sunken eyeballs.
a.
What abnormality of the ventricular system is most likely responsible for the patient’s
presentation?
b.
Identify the structures that comprise the ventricular system.
c.
Discuss the production, flow, and absorption process of the cerebrospinal fluid.
Cerebrospinal Fluid
Cerebrospinal fluid is the clear, colorless fluid that occupies the subarachnoid space and the ventricular
system around and inside the brain. In essence, the brain "floats" in it. The CSF occupies the space between the
arachnoid mater (the middle layer of the brain cover, meninges), and the pia mater (the layer of the meninges
closest to the brain). It constitutes the content of all intra-cerebral (inside the brain, cerebrum) ventricles, cisterns,
and sulci (singular sulcus), as well as the central canal of the spinal cord.
It acts as a "cushion" or buffer for the cortex, providing a basic mechanical and immunological protection
to the brain inside the skull. It also plays an important role in the homeostasis and metabolism of the central
nervous system. Its four primary functions are the following:
1.
Buoyancy
The actual mass of the human brain is about 1400 grams; however the net weight of the brain
suspended in the CSF is equivalent to a mass of 25 grams.[7] The brain therefore exists in neutral
buoyancy, which allows the brain to maintain its density without being impaired by its own weight, which
would cut off blood supply and kill neurons in the lower sections without CSF.
2.
Protection
CSF protects the brain tissue from injury when jolted or hit. In certain situations such as auto
accidents or sports injuries, the CSF cannot protect the brain from forced contact with the skull case,
causing hemorrhaging, brain damage, and sometimes death.
3.
Chemical Stability
CSF flows throughout the inner ventricular system in the brain and is absorbed back into the
bloodstream, rinsing the metabolic waste from the central nervous system through the blood-brain
barrier. This allows for homeostatic regulation of the distribution of neuroendocrine factors, to which
slight changes can cause problems or damage to the nervous system. For example, high glycine
concentration disrupts temperature and blood pressure control, and high CSF pH causes dizziness and
syncope.
4.
Prevention of brain ischemia
The prevention of brain ischemia is made by decreasing the amount of CSF in the limited space
inside the skull. This decreases total intracranial pressure and facilitates blood perfusion.
Production
The cerebrospinal fluid (CSF) is produced from arterial blood by the choroid plexuses of the lateral and
fourth ventricles by a combined process of diffusion, pinocytosis and active transfer. A small amount is also
produced by ependymal cells. The choroid plexus consists of tufts of capillaries with thin fenestrated endothelial
cells. These are covered by modified ependymal cells with bulbous microvilli. The total volume of CSF in the adult is
about 140 ml. The volume of the ventricles is about 25 ml. CSF is produced at a rate of 0.35 ml per minute. The
circulation of CSF is aided by the pulsations of the choroid plexus and by the motion of the cilia of ependymal cells.
Flow/Circulation
The CSF is secreted by the choroid plexus. It circulates from the lateral ventricles into the third ventricle
through the interventricular foramina before passing to fourth ventricle through the cerebral aqueduct. The flow is
aided by arterial pulsations of choroid plexuses and cilia on ependymal cells. The fluid from the fourth ventricle
passes through the median aperture and lateral foramina of lateral recess of fourth ventricle to enter
subarachnoid space. Fluid moves through the cerebromedullary cistern, pontine cisterns to flow superiorly through
tentorial notch of tentorium cerebelli to reach inferior surface of cerebrum. Fluid moves over lateral aspect of each
hemisphere via cerebral artery pulsation. Some CSF moves to subarachnoid space around the spinal cord, cauda
equina (further circulation relies on spinal artery pulsation, vertebral movement, and position changes). The CSF
bathes ependymal, pial surfaces of the brain and spinal cord. It penetrates nervous tissue along blood vessels.
Absorption
CSF is absorbed across the arachnoid villi into the venous circulation. The arachnoid villi act as one-way
valves between the subarachnoid space and the dural sinuses to regulate the flow. The arachnoid are grouped into
dural venous sinuses especially the superior sagittal and are grouped together into arachnoid granulations which
increase in size and number with age, eventually calcified.
The rate of absorption correlates with the CSF pressure. Absorption in venous sinuses occurs when the
cerebrospinal fluid pressure exceeds venous pressure in the sinus. Tubules are lined with endothelium allowing
direct flow of the CSF from subarachnoid space into the lumen of venous sinuses. If venous pressure exceeds CSF
pressure, the tips of villi are compressed which closes the tubules and prevents reflux.