CSF Physiology and Cerebral Blood Flow Keith R

CSF Physiology and Cerebral Blood Flow Keith R. Lodhia, MD, MS Department of Neurosurgery University of Michigan 12/20/03

CSF Functions provide mechanical protection q maintain a stable extracellular environment for the brain q Remove some waste products q nutrition q Convey messages? (hormones/releasing factors/neurotransmitters) q

Brain Fluid “Balance”

CSF Production n 70 % CSF produced in choroid plexuses of lateral, third and fourth ventricles produced at rate of 500 cc/day or approximately 20 cc/hour (0. 3 -0. 5 cc/kg/hr) eliminated by being absorbed into the arachnoid villi --> dural sinus --> jugular system n The secretion of fluid by the choroid plexus depends on the active Na+-transport across the cells into the CSF. The electrical gradient pulls along Cl-, and both ions drag water by osmosis. The CSF has lower [K+], [glucose], and much lower [protein] than blood plasma, and higher concentrations of Na+ and Cl-. The production of CSF in the choroid plexuses is an active secretory process, and not directly dependent on the arterial blood pressure.

CSF Production n n Other sources of CSF production from capillary ultrafiltrate (Virchow-Robin spaces) Additionally some produced from metabolic H 2 O production

CSF Production n Virchow. Robin spaces

CSF PRODUCTION- Choroid Plexus n CSF is produced by choroid plexus and secreted at ciliated cuboidal epithelial cell surfaces of the microvilli into the ventricles

CSF PRODUCTION- Choroid Plexus

Ependymal Cell Membrane Transport

CSF Production n CSF secretion involves the transport of ions ( Na+, Cl¯ and HCO 3¯) across the epithelium from blood to CSF Basolateral H 20, Na+, HCO 3¯, Cl¯ Apical Secretion can occur because of the polarized distribution of specific ion transporters in the apical or basolateral membrane of the epithelial cells.

CSF Production n n 5 -HT 2 C receptors– from 5 HT subfamily. {e. g 1) SSRI’s block 5 -HT 1 A receptor presynaptic uptake of 5 HT 2) antimigraine “triptans” stimulate vasoconstriction- agonists mediating 5 HT 1 B/1 D receptors 3) ondansetron/granisetron are 5 -HT 3 receptor antagonists - antinaseau effects} 5 -HT 2 C receptors found in high concentration in choroid plexus

CSF Production n n ANP receptors found in choroid plexus ANP decreases CSF production Choroid plexus epithelial cells express receptors for atrial natriuretic peptide that when stimulated increase c. GMP levels and inhibit cerebral spinal fluid production Aquaporin-AQP 1 channels are thought to be involved in the production of cerebral spinal fluid

CSF Constituency n n CSF volume: 25 cc ventricular, 25 cc intracranial subarachnoid space, and 100 cc in spinal subarachnoid spaces β 2 transferrin

CSF Constituency- β 2 transferrin n PROTEIN ELECTROPHORESISon cellulose/PAGE/filter etc Transferrin is an iron binding protein used to shuttle iron stores to cells- marker of severe malnutrition. Elevations in: hypothyroidism, biliary cirrhosis, nephrosis, chronic iron deficient anemia, and some cases of diabetes CSF shows increased β 2 peak c/w mucous. Therefore useful in evaluating potential CSF rhinorrhea

CSF Circulation n lateral ventricles--> foramen of Monro third ventricle --> aqueduct of Sylvius --> fourth ventricle --> foramina of Magendie and Luschka --> subarachnoid space over brain and spinal cord --> reabsorption into venous sinus blood via arachnoid granulations

CSF Circulation

Lundberg Waves n Lundberg has described 3 wave patterns ICP waves (A, B, and C waves). A waves are pathological. There is a rapid rise in ICP up to 50 -100 mm Hg followed by a variable period during which the ICP remains elevated followed by a rapid fall to the baseline and when they persist for longer periods, they are called 'plateau' waves which are pathological. 'Truncated' or atypical ones, that do not exceed an elevation of 50 mm Hg, are early indicators of neurological deterioration. B & C waves are related to respiration and 'Traube-Hering-Mayer' waves (rhythmical variations in blood pressure) respectively and are part of normal physiology with little clinical significance. Lundberg A- waves

A- waves/Plateau Waves § § Steep rises and abrupt falls in ICP, peaking at 50 -100 mm Hg, that last 5 - 20 minutes (also known as plateau waves). May signify intracranial vasomotor decompensation. May or may not be associated with clinical deterioration. Pathogenesis related to dilation of resistance vessels, increased intracranial blood volume, decreased flow, and increased pressure. “Loss of Autoregulation”

CSF Absorption n CSF is reabsorbed into the blood of the venous sinuses via the arachnoidal villi. The absorption here is directly related to the CSF pressure in the cranial cavity. Lymphatics/cribiform plate Transependymal flow

Route and Absorption of CSF n n Arachnoid villi are microscopic one-way valves (modified pia and arachnoid) that penetrate the meningeal dural layer that line the sinuses; hence, arachnoid villi reside within the sinuses (especially the superior sagittal sinus). Clumps of arachnoid villi = arachnoid granulations = macroscopic.

Arachnoid Villus

Route and Absorption of CSF n Hydrostatic pressure in subarachnoid space > pressure in dural sinuses n n Typical hydrostatic values of CSF are 150 mm H 2 O (11 mm Hg) in subarachnoid space vs. about 70 mm H 2 O (5 mm Hg) in dural sinuses. Arach. villi are one-way valves that open when the hydrostatic pressure of CSF in the subarachnoid space is about 1. 5 mm Hg greater than venous hydrostatic pressure in the dural sinuses (i. e. , passive process).

Drugs affecting Rate of CSF Production n Drugs Carbonic anhydrase inhibitors (acetozolamide/Diamox) n Cardiac glycosides (digoxin) inhibit ATPase pump, thereby reducing CSF formation in a dose-dependent manner. n Steroids- Effects on CSF formation are inconsistent. n Future- Aq. P inhibitors? , 5 -HT 2 C receptor inh ? n

CSF Pharmacology cont. n n Carbonic Anhydrase CO 2 + H 2 O <=H 2 Co 3=> HCO 3 - + H+ Inhibition of CAII decreases production of CSF by 60 % by decreasing bicarbonate formation in choroid plexus Acute Mountain Sickness- an aside. CO 2 + H 2 O <=> HCO 3 - + H+ VENTRICLE

Acute Mountain Sickness-AMS n n n AMS symptoms (HA fatigue somnolence etc) represent the effect of early cerebral edema with increased intracranial pressure a loss of cerebral autoregulation mechanisms leading to vasogenic edema (also migrainous-like), or an osmotic swelling of the brain cells (cytotoxic edema). Hypoventilation appears to contribute to development of AMS. A brisk increase in ventilation on ascent to altitude is associated with a lower incidence of AMS

Acute Mountain Sickness-AMS n n n Prophylaxis: slow ascent, Diamox, Rx: ASA or tylenol for mild HA Acute therapy for High Altitude Cerebral Edema (severe form of AMS): decadron, but descent to a lower altitude is still the most reliable treatment

CSF Pathology n n In cases of subarachnoid hemorrhage or traumatic spinal fluid taps, approximately 1 WBC is added to every 700 RBCs (literature range, 1 WBC/500 -1, 000 RBCs). This disagreement in values makes formulas (Fisher ratio etc) unreliable that attempt to differentiate traumatic tap artifact from true WBC increase. Also, the presence of subarachnoid blood itself may sometimes cause meningeal irritation, producing a mild to moderate increase in PMNs after several hours that occasionally may be greater than 500 WBCs/ mm 3. Xanthochromia begins in > 4 hours (literature range, 248 hours) due to hemoglobin pigment from lysed RBCs.

CSF Pathology Patterns of Cerebrospinal Fluid Abnormality: Cell Type and Glucose Level n POLYMORPHONUCLEAR: LOW GLUCOSE n n n Acute bacterial meningitis POLYMORPHONUCLEAR: LOW OR NORMAL GLUCOSE n Some cases of early phase acute bacterial meningitis n Primary amoebic (Naegleria species) meningoencephalitis n Early phase Leptospira meningitis POLYMORPHONUCLEAR: NORMAL GLUCOSE n Brain abscess n Early phase coxsackievirus and echovirus meningitis n CNS syphilis (some patients) n Acute bacterial meningitis with IV glucose therapy n Listeria (about 20% of cases)

n n LYMPHOCYTIC: LOW GLUCOSE n Tuberculosis meningitis n Cryptococcal (Torula) meningitis n Mumps meningoencephalitis (some cases) n Meningeal carcinomatosis (some cases) n Meningeal sarcoidosis (some cases) n Listeria (about 15% of cases) LYMPHOCYTIC: NORMAL GLUCOSE n Viral meningitis n Viral encephalitis n Postinfectious encephalitis n Lead encephalopathy n CNS syphilis (majority of patients) n Brain tumor (occasionally) n Leptospira meningitis (after the early phase) n Listeria (about 15% of cases)

Cerebral Blood Flow (CBF) n n n CBF = CBV/t 750 m. L/minute, which is 15% of the cardiac output The normal cerebral blood flow is 4550 ml/100 g/min, ranging from 20 ml 100 g 1 min-1 in white matter to 70 ml 100 g 1 min-1 in grey matter. Highest in neurohypophysis

CBF n n When CBF falls to less than 1023 ml/100 g/min, physiological electrical function of the cell begins to fail- “ischemic penumbra”. Below 8 ml/100 g/min irreversible cell death- ionic membrane transport failure

Cerebral Perfusion Pressure (CPP) MAP-ICP=CPP n normal CPP is between 50 -150 mm. Hg n <50 mm. Hg --> ischemia n >150 mm. Hg --> hyperemia

Autoregulation n CBF is maintained at a constant level in normal brain in the face of the usual fluctuations in blood pressure by the process of autoregulation. It is a poorly understood local vascular mechanism. Normally autoregulation maintains a constant blood flow between CPP 50 mm. Hg and 150 mm. Hg. Poiseuille’s law- flow through a rigid vessel: Q = ΔPπr 4/8 Lη §

Autoregulation n n Dysregulation can occur in pathologic states In traumatised or ischaemic brain, or following vasodilator agents (volatile agents and sodium nitroprusside) CBF may become blood pressure dependent. Thus as arterial pressure rises so CBF will rise causing an increase in cerebral volume. Similarly as pressure falls so CBF will also fall, reducing ICP, but also inducing an uncontrolled reduction in CBF.

Autoregulation pressure/myogenic autoregulation arterioles dilate or constrict in response to changes in BP n n n and ICP in order to maintain a constant CBF “myogenic theory”- vascular smooth muscle within cerebral arterioles intrinsically contract to stretch thereby regulating pressure NO- limited role overall, but if completely abolish NO production then loss of autoregulation; with CBF being completely BP-dependent

Metabolic Autoregulation arterioles dilate in response to potent chemicals that are by-products of metabolism such as lactic acid, carbon dioxide and pyruvic acid CO 2 is a potent vasodilator increased CO 2/decreased BP --> vasodilation decreased CO 2/increased BP -->vasoconstriction n

Neurogenic Autoregulation n Autonomic- sympathetic adrenergic receptors seen in cortical layers IV and V. Β 1, β 2, and ą2 (“dilators”), and ą1 (“constrictor”) receptors Overall sympathetic system plays minor role unlike in non-cerebral vascular beds.

Neurogenic Autoregulation- cont n n n 5 -HT- potent “constrictor, ” antagonized by NO Neuropeptide Y- “vasoconstriction”, in assoc with NO and sympathetic system Vasoactive intestinal polypeptide (VIP) and peptide histidine isoleucine (PHI)- “vasodilators” Substance P, neurokinin A, calcitonin generelated peptide histamine H 2 -”vasodilatory” esp. substance P CCK, neurotensin, somatostatin, vasopressin, endorphin

Neurogenic Autoregulation-cont § Autonomic system and neurochemical control of CBF in general is a minor control § Overall, pressure and metabolic autoregulation most important

Increasing CBF-Hyperemia n Low arterial oxygen tension has profound effects on cerebral blood flow. When it falls below 50 mm. Hg (6. 7 k. Pa), there is a rapid increase in CBF and arterial blood volume

CBF and CO 2 n Carbon dioxide causes cerebral vasodilation. As the arterial tension of CO 2 rises, CBV and CBF increases and when it is reduced vasoconstriction is induced.

“Cerebrovascular Reserve” Ø In functionally activated areas, CBF augmentation exceeds the small increases in oxygen utilization and the concentration of deoxyhemoglobin is relatively low. Thus, this excess supply of oxygen in response to a demand stimulus reflects the cerebral perfusion reserve capacity Ø Cerebrovascular reserve capacity is impaired by risk factors such as hypertension and diabetes, carotid/cerebral vasc. stenosis, and can be an etiologic factor in ischemic stroke

Cerebrovascular Reserve Ø Ø Ø PET, SPECT, Xe-CT, CT-perfusion to assess. Pre/post diamox challenge. acetazolamide challenge and the CO 2 -loading (breath-holding) test raise global CBF (MRI) of T 2 -weighted or Blood oxygenation level –dependent (BOLD)-weighted images correlate ( well with changes in the total amount of oxygenated hemoglobin

Xenon CT perfusion CT BOLD-MRI and singlephoton emission computed tomography (SPECT)

CBF AND CSF- TYING IT TOGETHER

PATHOPHYSIOLOGY CSF/CBF n n 1. the intracranial compartment is a rigid container and consists of three components a. brain-80% of total volume b. blood-10% of total volume c. CSF-10% of total volume

PATHOPHYSIOLOGY CSF/CBF n § § 2. Monro-Kellie Hypothesis to maintain a normal ICP, a change in the volume of one compartment must be offset by a reciprocal change in the volume of another compartment pressure is normally wellcontrolled through alterations in the volume of blood and CSF

Brain P/V curve

P/V CURVE AND COMPLIANCE § Pressure gradients can develop within the brain substance and the compliance or “squishiness” of pathological brain (e. g. tumor) can be different from that of normal brain leading to an altered curve (shift left). §The extent of the change in ICP caused by an alteration in the volume of intracranial contents is determined by the compliance or of the brain. In other words if compliance is low, the brain is stiffer or less "squashable". Therefore, an increase in brain volume will result in a higher rise in intracranial pressure than if the compliance were high.

Blood/Brain-Blood/CSF Barriers n n The blood-brain barrier (BBB) is the specialized system of capillary endothelial cells that protects the brain from harmful substances in the blood stream, while supplying the brain with the required nutrients for proper function. Formed by the nonfenestrated capillaries and to much lesser degree, the astrocytic foot processes —keeps out most macromolecules

Blood/Brain Barrier

Blood-CSF Barrier n n “Tight” junctions at the ependymal level Fenestrated junctions at the choroidal capillaries The choroid plexus is composed of fenestrated capillaries and an epithelial (ependymal) covering, which reverts from "tight" to moderately "open" at the base -–not as strenuous of barrier as blood/brain

Blood/Brain Barrier and Circumventricular organs n The circumventricular organs (CVO) are midline structures bordering the 3 rd and 4 th ventricles. These barrier-deficient areas are recognized as important sites for communicating with the CSF and between the brain and peripheral organs via blood-borne products. CVO's include the pineal gland, median eminence, neurohypophysis, subfornical organ, area postrema, subcommissural organ, organum vasculosum of the lamina terminalis, and the choroid plexus. The intermediate and neural lobes of the pituitary are sometimes included

Causes of an increased ICP n n Conditions Increasing Brain Volume intracranial mass (tumor, hematoma, aneurysm, AVM) cerebral edema CNS infection (abscess, inflammatory process)

Causes of an increased ICP n n Conditions Increasing Blood Volume obstruction of venous outflow hyperemia – decreased p. O 2 - inc. CBF hypercapnea – >p. CO 2 increases vasodilation inc CBV , CBF, and ICP

Causes of an increased ICP n n Conditions Increasing CSF Volume increased production(Ch plexus papilloma) decreased reabsorption of CSF (meningitis, SAH) Obstruction to flow of CSF (e. g. aq stenosis)

THE END