CSF & the Ventricular System Flashcards
Where is cerebrospinal fluid found?
- CSF is found within and around the CNS
- it is located within in the ventricular system and around in the subarachnoid space
Explain the general layout of the ventricular system.
- 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
What are the cavities of each major region of the CNS?
- telencephalon (cerebral hemispheres): lateral ventricles
- diencephalon: 3rd ventricle
- mesencephalon (midbrain): cerebral aqueduct
- rhombencephalon (brainstem below midbrain): 4th ventricle
- spinal cord: central canal
Which cell type lines the ventricular system?
- ependymal cells
What creates the CSF? How does it do this? Where is CSF produced?
- 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)
How does CSF enter the subarachnoid space from the ventricular system?
- 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
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?
- 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)
How far down the spinal column does the CSF flow?
- 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)
How is CSF drained from the subarachnoid space?
- 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
What happens if the dural venous pressure exceeds the pressure of the CSF/SAS?
- 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
What are the major functions of CSF?
- 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)
What is the normal volume of CSF? How much is produced each day? What’s the normal pressure? What makes up CSF?
- 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)
What structures are pierced during a lumbar puncture?
- 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)
What is the major contraindication to lumbar puncture? How do we know if a patient has this condition?
- 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)
Normal CSF
- 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)
CSF in Pyogenic Meningitis
- 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)
CSF in Tuberculosis Meningitis
- 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)
CSF in Viral Meningitis
- 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)
CSF in Subarachnoid Hemorrhage
- 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!
What mechanisms can cause hydrocephalus? Which are more common?
- overproduction of CSF (rare)
- obstruction of CSF circulation (more common)
- obstruction of CSF drainage (more common)
Although rare, what can potentially cause CSF overproduction, resulting in hydrocephalus?
- an active tumor of the choroid plexus
- (very rare)
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?
- 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
What can cause CSF flow obstruction within the subarachnoid space?
- TB meningitis can cause fibrosis/scarring of the SAS
What can obstruct CSF drainage?
- 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
What is meant by non-communicating hydrocephalus? By communicating hydrocephalus?
- 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
How do we treat hydrocephalus?
- (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)
What is normal pressure hydrocephalus (NPH)? How do patients classically present?
- 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)
What is hydrocephalus ex vacuo?
- 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)
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?
- (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)
What forms the blood-brain-barrier? Is it the endothelial cells of the capillary vessels, the basement membrane, or the astrocyte’s food processes?
- 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!)
Are there any areas of the brain where the BBB is non-existent?
- 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)
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?
- 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)
What are the parts of the lateral ventricle?
- has an anterior (frontal) horn, body, posterior (occipital) horn, and inferior (temporal) horn
What procedure can you do to temporary lower raised ICP in an emergency setting?
- 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)
CSF of Neonates
- neonatal CSF will have slightly elevated protein levels because of the immature BBB and blood-CSF barrier
What is cerebral aqueduct stenosis?
- 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
What is a major potential complication of raised intracranial pressure? What are the areas where this most commonly occurs?
- 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)
