Meninges, Ventricles, and CSF

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3/7/12 1pm

Meninges Intro

The brain and spinal cord are surrounded by the skull and vertebral column. The membranous coverings of the central nervous system, the meninges, are located internal to the skull and vertebral column. The meninges 1) protect the underlying brain and spinal cord, 2) serve as a support framework for important arteries, veins, and sinuses, and 3) enclose a fluid-filled cavity (the subarachnoid space) which is vital to normal function of the brain and spinal cord. The meninges are derived from neural crest cells and mesoderm, and are composed primarily of fibroblasts and collagen fibers. The three layers of meninges are:
Dura mater
Arachnoid mater
Pia mater

Dura Mater

1. Two layers
a. Periosteal - outer layer
-stops at foramen magnum
b. Meningeal - inner layer
-forms sheath for a short distance as each cranial nerve passes through a
foramina in the skull
-also continues inferiorly to cover the spinal cord
2. Dural reflections
a. Falx cerebri - separates cerebral cortex into left and right hemispheres
-contains superior and inferior sagittal sinuses
b. Tentorium cerebelli - separates cerebral hemispheres from cerebellum
-Tentorial notch anteriorly (Excess pressure--midbrain pressed against to cause neurologic problems. Rostral border to tentorium cerebelli)
-contains straight sinus
c. Falx cerebelli - separates the cerebellar hemispheres
-contains occipital sinus along its posterior edge
d. Diaphragma sella - covers the sella turcica
-encircles the stalk of the pituitary
3. Dural spaces
a. Epidural space - The periosteal layer of the dura is normally adherent to skull but can become separated under pathological conditions, e.g., trauma to the head causing damage to a meningeal artery resulting in an epidural hematoma. The increased intracranial pressure results in progressive headache, confusion, a "lucid interval," lethargy, then loss of consciousness. Classic lens shaped appearance
b. Subdural space - contains veins and lymphatics
- trauma resulting in tearing of veins near where they enter dural venous sinuses results in a subdural hematoma in which the accumulating blood forces apart the meningeal layer of the dura from the arachnoid mater (Slower pressure build up in diffuse space. Not isolated location. Will eventually lead to compression. Not as much of an immediate emergency. Does not flow into sulci)

Arachnoid mater & Pia Mater

B. Arachnoid mater - thin layer of cells that is loosely attached to the dura by
occasional cell junctions
-separated from the underlying pia mater by the subarachnoid space
-connected to pia via fine fibroblasts called arachnoid trabeculae
1. Subarachnoid space
a. contains cerebrospinal fluid (CSF), major vessels, roots of spinal & cranial nerves
-CSF produced by the choroid plexus within the ventricles, flows into
the subarachnoid space, and exits into venous sinuses through arachnoid villi
-collections of arachnoid villa called arachnoid granulations, are most
common in superior sagittal sinus
b. rupture of vessels - subarachnoid hemorrhage
-commonly from ruptured aneurysm. Blood in sulci
-described as a sudden headache ("worst headache of my life"), often
accompanied by neck stiffness, nausea/vomiting, loss of consciousness
-blood can be detected in the CSF via spinal tap but usually diagnosed
by a CT scan
Contralateral side--brain is suspended and upon being hit on one end, it pushes the brain into the other side. White infiltrates parenchyma. Conflicting signs & symptoms: visual on one side and motor problems on other side. Can have both--especially in closed head injury
2. Subarachnoid cisterns -
enlarged areas within the subarachnoid
space containing CSF
C. Pia mater (from neural crest cells)
1. Flattened cells which closely adhere to
the brain surface
2. Follow contours of brain and spinal cord,
dipping into sulci and fissures
3. Forms denticulate ligaments and filum
terminale internum in spinal cord

Meningeal Blood Supply

The middle meningeal artery is the primary artery serving the dura mater. There are others as well, but this is the most important clinically. It is a branch of the maxillary artery and enters the skull through the foramen spinosum where it then divides into anterior and posterior branches. This is a clinically important artery as blows to the side of the head can fracture the thin bones in the area of the pterion and rupture branches of the middle meningeal artery leading to formation of an epidural hematoma. Veins of the dura typically accompany the arteries, with the middle meningeal veins draining into the pterygoid plexus of veins.

Meningitis: infection of the meninges with brudzinski's sign: painw ith flexion of the neck.

Ventricles

The ventricles are cavities in the brain that contain CerebroSpinal Fluid (CSF). The ventricles are derived from the neural canal. As the brain develops and enlarges, the neural canal is pulled along with it. The spaces in each brain vesicle form the ventricle of that part of the brain. Structures within the ventricles produce the CSF and it circulates throughout the brain and CNS to allow the CNS to perform its various functions.

Lateral Ventricles

1. Paired structures, one located in each cerebral hemisphere
2. Floor of these two ventricles are lined with choroid plexus which produce CSF
3. Parts of the lateral ventricles
a. Frontal horns
1) roof and anterior walls - corpus callosum
2) lateral walls and floor - head of caudate
3) medial wall - septum pellucidum
b. Body - from foramen of Monro to splenium of corpus callosum
1) roof - corpus callosum
2) medial walls - posterior portion of septum pellucidum
3) floor - fornix, choroid plexus, parts of thalamus and caudate
c. Atrium (trigone) - at junction of body, occipital and temporal horns; a lot of choroid plexus
d. Occipital horn (small) - extends into the occipital lobe
e. Temporal horn
1) roof and lateral wall - tail of caudate, optic radiations
2) medial portion and part of floor - hippocampus
3) anterior or termination - amygdala
4. CSF exits from lateral ventricles to third ventricle via foramen of Monro (2)

Third Ventricle

1. Located in diencephalon between the paired thalamus
2. Roof contains choroid plexus
3. Anatomical boundaries
a. roof - fornix medially
b. floor - hypothalamus
c. anterior - lamina terminalis and anterior commissure
d. laterally - thalamus and hypothalamus
4. Recesses - preoptic, infundibular, suprapineal, pineal
5. CSF exits third ventricle into fourth ventricle via cerebral aqueduct (aqueduct of Sylvius)
a. Narrowest passageway in ventricular system - ~1.5mm
b. Often site of blockage

Fourth Ventricle

1. Located between cerebellum and brain stem
2. Boundaries
a. Ventral and floor - pons and medulla
b. Lateral and dorsal - cerebellum
c. Roof - superior medullary velum, inferior medullary velum
3. Inferior medullary velum (part of roof) lined with choroid plexus
4. Four outlets for CSF to enter subarachnoid space
a. Foramina of Luschka (2) - lateral ventricular foramina
b. Foramen of Magendie (1) - medial ventricular foramen
c. Central canal - into spinal cord

Subarachnoid Cisterns

1. Description
a. Cisterns occur in places where arachnoid spans wide or deep depressions
resulting in enlargements of the subarachnoid space
b. Creates enlarged spaces where CSF can accumulate
2. Cisterns ventral to brainstem
a. Chiasmatic cistern (#5 on picture)
b. Interpeduncular cistern (#1 on picture)
c. Pontine cistern (#4 on picture)
3. Cisterns dorsal to brainstem
a. superior cerebellar (quadrigeminal) cistern
(#2 on picture)
b. cerebellomedullary cistern (cisterna magna)
(#3 on picture)
2. Lumbar cistern

CSF

A. Description, Purpose, and Composition
CSF is a clear, colorless fluid that fills subarachnoid space, cisterns, and ventricles. It's pH is slightly lower than the pH of blood (7.33 vs. 7.36-7.44). It contains very few white blood cells (less than 3 lymphocytes/cc). CSF also has lower glucose and much lower protein concentration, but a higher NaCl concentration than plasma. The purpose of CSF is to preserve homeostasis in the central nervous system. It also cushions the brain; providing buoyancy, protection from impact, and weight reduction. Waste products produced by the brain are collected and removed through the CSF which also provides a source of distribution for neuroactive substances.
1. Production
a. Site of production
1) Choroid plexus (70%) - extensive capillary network supplies cells of choroid; CSF from choroid plexus secreted into ventricles
2) Other (30%) - metabolic water production (water as a product of various chemical reactions)
b. Volume
1. At any given time, total volume of CFS is about 100-150 mls in ventricles, cisterns, and subarachnoid space
2. Turnover rate is high, total volume produced daily averages 500 mls (~21 ml/hr)
3. Blood-CSF barrier (BBB) - substances in blood cannot access CSF. Tight junctions between capillary endothelial cells prevent substances (e.g. macromolecules) from passively diffusing from blood to CSF.

CSF, circulation

Circulation and Absorption
a. LATERAL ventricles to THIRD ventricle via foramina of Monro (x2)
b. THIRD ventricle to FOURTH ventricle via cerebral aqueduct (Sylvius)
c. FOURTH ventricle to SUBARACHNOID space via
1) Foramen of Magendie (x1) in midline
2) Foramina of Luschka (x2) laterally
3) Central canal
d. SUBARACHNOID space to VENOUS system (superior sagittal sinus, transverse sinus) via arachnoid granulations/villi
e. Arachnoid granulations/villi project into dural sinuses
1) Arachnoid granulations/villi have valves
2) Flow of CSF is unidirectional
3) CSF absorbed into venous circulation

Collection of CSF

1. Why?
a. Look for infections (meningitis) - either bacteria or increased WBC's
b. Confirm diagnosis of multiple sclerosis - observe increased levels of IgG
c. Evidence of blood in subarachnoid space
2. Where ?
a. Spinal cord ends at L1-2 in adults; slightly lower in children
b. In adults, enter at L3-4 or lower (top of iliac crests - L4)
c. In children, enter at L4-5
3. Implications of CSF collection
a. Major constituents of subarachnoid space are CSF and blood vessels
1) If there is a rupture of one of the major vessels, blood will be released into the subarachnoid space
2) Lumbar puncture would reveal RBC's in the CSF
b. Trauma can cause cracks in the cribriform plate. CSF from subarachnoid space can leak into nasal cavity causing patient to have persistent rhinorrhea (runny nose consisting of CSF drainage)
1) following trauma; bacterial infection possible as bacteria enter subarachnoid space
2) can lead to meningitis

Hydrocephalus

1. Obstructive (non-communicating) Hydrocephalus
a) Blocked communication between ventricular system and subarachnoid space causes buildup of intracranial pressure
b) In infants prior to suture closure, blockage results in increased head
circumference because the excess CSF enlarges the ventricles, which in turn push against the brain tissue; with soft skull sutures, the head can become quite enlarged (Congenital hydrocephalus. Better than if bones are fused--compresses brain against skull--causes necrosis)
c) Treatment is shunting (surgical intervention to drain excess CSF from
blocked region) or removal of blockage
d) Most common location is cerebral aqueduct
a) Cerebral aqueduct dilation of 3rd and both lateral ventricles
b) Interventricular foramen dilation of ipsilateral lateral ventricle
2. Communicating hydrocephalus
a) CSF exits ventricular system but gets blocked elsewhere in subarachnoid
space
b) Potential causes; blood or pus in subarachnoid space (trauma, meningitis);
may eventually scar arachnoid granulations further impeding flow of CSF over convexities of brain and altering absorption

Hydrocephalus, Cont'd

3. Normal pressure hydrocephalus
a) Usually found in elderly; cause unknown (prior infection, bleeding?)
b) Pressure within ventricular system is not increased
c) Clinical triad; gait disturbance, urinary incontinence, dementia
d) Shunting is preferred treatment
4. Hydrocephalus ex vacuo
a) Enlargement of ventricles due to severe atrophy of brain tissue
b) Ventricles appear large, but pressure is not increased
5. Pseudotumor cerebri (idiopathic intracranial hypertension)
a) Increased intracranial pressure of unknown etiology
b) Common in females, particularly obese women of childbearing age
or patient with renal failure
c) Symptoms - severe headaches, visual disturbances, papilledema
(enlargement of the optic disk to increased intracranial pressure); however CT and MRI are normal

Shunting: treatment for hydrocephalus. Can get blocked too--remove/replace
Most common sites: cerebral aqueduct (dilation of 3/4 ventricle) or interventricular foramen

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