CSF & the Ventricular System

Question 1

Where is cerebrospinal fluid found?

Answer 1

• CSF is found within and around the CNS

- it is located within in the ventricular system and around in the subarachnoid space

Question 2

Explain the general layout of the ventricular system.

Answer 2

• within each cerebral hemisphere lies a lateral ventricle
• these communicate into the 3rd ventricle, located between the thalami, via the foramina of Monro (AKA interventricular foramina)
• the 3rd ventricle communicates with the 4th ventricle, located in the brainstem, via a narrow passage called the cerebral aqueduct (in the midbrain)
• 1/2 way through the medulla, the 4th ventricle tapers off into the central canal

Question 3

What are the cavities of each major region of the CNS?

Answer 3

• telencephalon (cerebral hemispheres): lateral ventricles
• diencephalon: 3rd ventricle
• mesencephalon (midbrain): cerebral aqueduct
• rhombencephalon (brainstem below midbrain): 4th ventricle
• spinal cord: central canal

Question 4

Which cell type lines the ventricular system?

Answer 4

• ependymal cells

Question 5

What creates the CSF? How does it do this? Where is CSF produced?

Answer 5

• choroid plexuses produce CSF
• a choroid plexus is a collection of specialized secretory capillary vessels (arising from the choroidal arteries), surrounded by invaginated pia mater (the arteries are in the subarachnoid space) and ependymal cells (lining the ventricle/cavity)
• these ependymal cells actively secrete Na+ (Cl- and H2O will follow) and, to a lesser extent, glucose
• choroid plexuses are found in the lateral, 3rd, and 4th ventricles (most CSF is made in the lateral ventricles)

Question 6

How does CSF enter the subarachnoid space from the ventricular system?

Answer 6

• in the 4th ventricle, CSF enters the central canal, but it also passes through 3 foramina: 2 lateral foramina of Luschka (AKA the lateral apertures) and 1 posterior foramen of Magendie (AKA the medial aperture)
• these 3 foramina allow entry into the SAS

Question 7

The areas where CSF enters the subarachnoid space results in the slight dilation of the area - what are these areas of dilated subarachnoid space called?

Answer 7

• the openings of the foramina of Luschka result in the cerebellopontine cisterns
• the opening of the foramen of Magendie results in the cerebellomedullary cistern (AKA cisterna magna)

Question 8

How far down the spinal column does the CSF flow?

Answer 8

• CSF is present so long as the meninges are present, so CSF fills the SAS down to the S2 level (when the meninges terminate)
• (remember that the actual spinal cord terminates around L1/L2)

Question 9

How is CSF drained from the subarachnoid space?

Answer 9

• it is drained by the dural venous sinuses, the major one being the superior sagittal dural sinus located at the top of the head (where the two layers of the dura mater - periosteal and meningeal - separate to form a triangular space)
• the meningeal layer here has openings from which arachnoid villi (arachnoid granulations) extend through - providing a route for CSF drainage
• from the dural sinuses, veinous blood and CSF drain into the internal jugular veins

Question 10

What happens if the dural venous pressure exceeds the pressure of the CSF/SAS?

Answer 10

• if the dural venous pressure is greater than the pressure of the SAS, then the arachnoid granulations clamp shut
• this means, these granulations are one-way valves; fluid can not flow from the sinus back into the SAS

Question 11

What are the major functions of CSF?

Answer 11

• cushions the brain
• allows the brain to float: (specific gravities are equal) this decreases the weight of the brain from 1400 g to 50 g!
• reservoir to help maintain proper intracranial pressure
• nourishment and waste removal: CSF and interstitial fluid of the CNS communicate freely because pia mater is relatively leaky (unlike the BBB)
• carries hormones around the CNS (via the pineal gland and hypothalamus)

Question 12

What is the normal volume of CSF? How much is produced each day? What’s the normal pressure? What makes up CSF?

Answer 12

• normal volume: 130-150 mL
• 550 mL produced each day (0.5 mL/min); turns over 4-5 times each day
• normal pressure: 60-160 mmH2O (4-12 mmHg)
• components: salt, water, and glucose (the normal CSF glucose concentration is 60-70% that of the blood/plasma)

Question 13

What structures are pierced during a lumbar puncture?

Answer 13

• skin, subcutaneous fascia, spinal ligaments (supraspinous, interspinous, and ligamentum flavum), epidural space (mainly fat), dura mater, subdural space, arachnoid mater
• you now have access to the SAS
• do NOT pierce the pia mater (it is very closely associated with the underlying structures and nerves)

Question 14

What is the major contraindication to lumbar puncture? How do we know if a patient has this condition?

Answer 14

• increased intracranial pressure (doing a LP on these patients may cause severe herniation of the brain through the foramen magnum and death)
• 5 signs of raised ICP: unexplained headache upon waking up, papilloedema (edema causes blurred edges of the optic disc on fundoscopy), projectile vomiting (compression/stimulation of vagus nerve), progressively increasing BP (when ICP > arterial flow, ischemia of the brain will occur; this results in a large sympathetic response to try and re-perfuse the brain), and progressively decreasing heart rate (compression/stimulation of vagus nerve)

Question 15

Normal CSF

Answer 15

• clear
• few lymphocytes (less than 5 per mL3)
• no RBCs
• no neutrophils
• glucose 60-70% of plasma
• very low protein (0.4 g/L vs. plasma of 70-80 g/L)

Question 16

CSF in Pyogenic Meningitis

Answer 16

• yellow, cloudy
• few lymphocytes
• no RBCs
• MANY neutrophils (it’s an acute infection!)
• LOW glucose (less than 50% of plasma, bacteria use it!)
• RAISED protein (choroid plexuses become more leaky with inflammation)

Question 17

CSF in Tuberculosis Meningitis

Answer 17

• cloudy with fibrin webs (fibrin leaks in because very leaky!)
• MANY lymphocytes (it’s a chronic infection!)
• no RBCs
• few neutrophils
• LOW glucose (less than 50% of plasma, organism uses it!)
• RAISED protein (more so than pyogenic; choroid plexuses become more leaky with inflammation)

Question 18

CSF in Viral Meningitis

Answer 18

• clear
• MANY lymphocytes (it’s a viral infection!)
• no RBCs
• few neutrophils
• normal glucose (60-70% of plasma)
• slightly RAISED protein (less than pyogenic, shouldn’t exceed 1 g/L; choroid plexuses become more leaky with inflammation)

Question 19

CSF in Subarachnoid Hemorrhage

Answer 19

• RBCs will be present
• in addition, there will be xanthochromia (a yellow-ish colored fluid at the top of the collection tube); this is due to the presence of bilirubin!

Question 20

What mechanisms can cause hydrocephalus? Which are more common?

Answer 20

• overproduction of CSF (rare)
• obstruction of CSF circulation (more common)
• obstruction of CSF drainage (more common)

Question 21

Although rare, what can potentially cause CSF overproduction, resulting in hydrocephalus?

Answer 21

• an active tumor of the choroid plexus

- (very rare)

Question 22

What can cause a unilateral enlargement of one lateral ventricle? Enlargement of both lateral and the 3rd ventricles? Enlargement of both lateral, 3rd, and cerebral aqueduct?

Answer 22

• unilateral lateral ventricle: one foramen of Monro is blocked (from a cyst or tumor)
• 3rd and both lateral: congenital stenosis of cerebral aqueduct (these infants will have massively swollen skulls, as they are still pliable)
• aqueduct, 3rd, both lateral: cerebellar tumor blocking foramen of Leschke and/or Magendie

Question 23

What can cause CSF flow obstruction within the subarachnoid space?

Answer 23

• TB meningitis can cause fibrosis/scarring of the SAS

Question 24

What can obstruct CSF drainage?

Answer 24

• arachnoid granulations can get clogged up with excessive cells, inflammatory infiltrates, RBCs, etc.
• rarely, the internal jugular vein can be obstructed, which would also cause dural sinus pressure to exceed the pressure in the SAS, stopping CSF drainage

Question 25

What is meant by non-communicating hydrocephalus? By communicating hydrocephalus?

Answer 25

• non-communicating hydrocephalus: when the CSF’s flow to the SAS is obstructed
• communicating hydrocephalus: when the CSF obstruction occurs after entry into the SAS

Question 26

How do we treat hydrocephalus?

Answer 26

• (remember raised ICP is a major contraindication to lumbar puncture)
• we use a silicon shunt to connect the CNS ventricles with the right atrium (ventriculoatrial shunt) OR to connect the CNS ventricles with the peritoneum (ventriculoperitoneal shunt)

Question 27

What is normal pressure hydrocephalus (NPH)? How do patients classically present?

Answer 27

• NPH occurs when there is an issue with the arachnoid granulations (CSF can’t drain properly), but instead of ICP increasing, the brain matter of the cerebral hemispheres decreases as a compensatory mechanism
• these patients present as “wacky, wobbly, and wet:” they develop dementia, gait abnormality, and urinary incontinence (the areas controlling gait and continence are located right below the superior sagittal dural sinus)

Question 28

What is hydrocephalus ex vacuo?

Answer 28

• this is when ventricles/cavities enlarge, but without causing an increase in ICP
• this occurs when certain areas of the brain degenerate (ex: degeneration of caudate nucleus in Huntington’s results in expansion of the adjacent lateral ventricle)

Question 29

What should you suspect in a patient undergoing lumbar puncture if the measures pressure of the CSF fails to increase when pressure is applied to the jugular vein or when the patient is asked to cough?

Answer 29

• (normally, coughing and applying pressure to jugular vein will cause an increase in the CSF pressure on LP)
• if this increase does not occur, it is called Queckenstedt sign positive
• this means the CSF in the lower half of the spinal tract is not in communication with the upper half (suspect an obstructive tumor in this area)

Question 30

What forms the blood-brain-barrier? Is it the endothelial cells of the capillary vessels, the basement membrane, or the astrocyte’s food processes?

Answer 30

• the BBB is formed by all 3, but mainly by the endothelial cells of the capillary vessels perfusing the CNS; these are pericytes
• these cells have specialized tight junctions not found in other capillaries
• (note that the BBB is quite permeable in newborns, which is why kernicterus can occur!)

Question 31

Are there any areas of the brain where the BBB is non-existent?

Answer 31

• 3 areas of the brain have less tight BBBs
• around the hypothalamus, around the pineal gland, and in the area postrema (near the floor of 4th ventricle)
• this is to allow these structures to measure/monitor hormone levels (hypothalamus and pineal) or to measure noxious chemicals so as to induce nausea and vomiting to get rid of them (area postrema)

Question 32

What forms the blood-CSF barrier in the choroid plexus? What forms the barrier between the CSF and the extracellular/interstitial fluid of the CNS?

Answer 32

• blood-CSF barrier formed by the choroidal epithelial cells’ tight junctions
• there is no barrier between CSF and CNS extracellular fluid; these two fluids are in full communication with each other (technically, the pia mater and ependymal cells are the two barriers, but they are both very permeable)

Question 33

What are the parts of the lateral ventricle?

Answer 33

• has an anterior (frontal) horn, body, posterior (occipital) horn, and inferior (temporal) horn

Question 34

What procedure can you do to temporary lower raised ICP in an emergency setting?

Answer 34

• mechanical hyperventilation
• however, don’t do this for more than 30 seconds, as it can make any cerebral ischemia much worse
• (emergency surgical options include burr holes or a craniotomy)

Question 35

CSF of Neonates

Answer 35

• neonatal CSF will have slightly elevated protein levels because of the immature BBB and blood-CSF barrier

Question 36

What is cerebral aqueduct stenosis?

Answer 36

• a congenital stenosis of the cerebral aqueduct, resulting in excess CSF build-up
• this results in a massively swollen head rather than hydrocephalus because the skull is still pliable in these neonates

Question 37

What is a major potential complication of raised intracranial pressure? What are the areas where this most commonly occurs?

Answer 37

• herniation! (displacement of brain tissue due to a mass effect or raised ICP)
• most common: subfalcine/cingulate hernation (frontal lobe’s cingulate gyrus herniates into the falx cerebri and compresses the anterior cerebral artery)
• tonsillar herniation (cerebellar tonsils herniate through the foramen magnum and compress the brainstem)
• uncal herniation (uncus of temporal lobe herniates into the tentorium and compresses CN III, posterior cerebral artery, and brainstem’s vascular supply)