Furr and Reed Chapter 2: Cerebrospinal Fluid and the Blood-Brain Barrier

What is the blood CSF barrier made up of?

Choroid plexus and other tiny regions of the ventricles

What determines the composition of brain interstitial fluid?

Active transport of substance through the BBB

What determines the composition of the CSF?

Secretory processes through choroid plexus epithelia

What composes the BBB?

Capillary endothelial cells, basal lamina, pericytes, astroglia, and perivascular macrophages

What composes the blood CSF barrier?

Capillary endothelium, loose connective tissue, basal lamina, and ependymal cells

What portions of the CNS don’t have a BBB?

Portions of the hypothalamus, area postrema, and subfornical and subcommissural regions

How is glucose transported across the BBB?

By facilitated diffusion through the glucose transporter-1 (GLUT-1)

How can the integrity of the BBB be evaluated clinically?

Using the albumin quotient

Iatrogenic blood contamination of the CSF samples, intrathecal hemorrhage, or inflammation/traumatic disruption of the BBB will result in an elevated AQ

Normal Albumin Quotient

<2.1 for mature horses

1.8 ± 0.2 for foals

Immunologic Function of the Blood Brain Barrier

• Inflammatory cytokines and endotoxin have been demonstrated to influence BBB endothelial cells by upregulating their expression of adhesion and MHC molecules

  • Leads to increased BBB permeability, formation of vasogenic edema, leukocyte extravasation, and vascular thrombosis

• Pericytes contribute to microvascular reactivity, as well as being phagocytic, and express various adhesion molecules and MHC II receptors

• Astrocytes interact with the endothelial cells and enhance their barrier function

Formation of CSF

Actively secreted by ependymal cells and choroid plexus

• Majority is produced by the choroid plexus in the lateral ventricles

• 30-40% may be produced by the ependymal lining of the ventricles, the leptomeninges, and brain and spinal cord blood vessels

What is CSF production proportional to?

• Production is directly proportional to the transport of sodium via a Na-K ATPase in the brush border of the choroidal epithelium and is independent of vascular hydrostatic pressure

What can alter the rate of CSF production?

• Rate of CSF production can be altered by a variety of compounds

  • Carbonic anhydrate and Na-K ATPase inhibitors and hyperosmolality decrease production rate

  • Cholera toxin and adrenergic stimulation increase CSF production rate

  • Osmotic agents and hypertonic solutions such as mannitol and DMSO decrease CSF production in other species, effects unknown in horses

Where does CSF go after formation?

• Following formation, CSF flows into the third and fourth ventricles and exits caudally through foramina in the fourth ventricle to enter the subarachnoid space

• Pulsation of blood in the choroid plexus forces the CSF in a cranial to caudal flow

• CSF is absorbed by collections of arachnoid villi in the dural sinuses or cerebral veins

  • When CSF pressure exceeds venous pressure, these villi act as a one-way ball valve forcing CSF flow to the venous sinus

What are the three physical components of the CNS that interact to generate intracranial pressure (ICP)?

Brain

Blood vascular component

CSF

What are the two major factors that determine ICP?

Arterial pressure

Intracranial venous pressure

Monro-Kellie Doctrine

Monro-Kellie doctrine - given that the total cranial volume is fixed, an increase in the volume of one component must be compensated for by a decrease in the volume of at least one of the other components, or an increase in pressure must result

Queckenstedt’s Test for Spinal Occlusion

• Jugular occlusion should lead to an increase in the CSF pressure measured at the lumbar space, if this does not occur then spinal subarachonid blockage must exist

  • Queckenstedt's maneuver can be used as an aid during spinal fluid collection

What is ICP in awake standing horses?

2 ± 4 mmHg

What are lumbosacral CSF pressures highly correlated to?

Lumbosacral CSF pressures are highly correlated to lateral ventricle CSF pressure but specific values not reported

Does xylazine cause a change in CSF pressure?

No

Does hypercapnia increase CSF pressure?

Yes, markedly when the PaCO2 increased to 80 mmHg

ICP in Foals over the First 3 Days of Life

Ranged from 5.8-9.5 mmHg

What can cerebral edema be characterized as?

Cellular (cytotoxic)

Vasogenic

Interstitial

What characterizes cellular edema?

• Cellular edema is characterized by swelling of all the cellular elements of the brain (neurons, glia, and endothelial cells) with an associated reduction on the volume of the brain extracellular fluid space

What causes cellular edema?

• Results from a failure of the energy-dependent transmembrane sodium-potassium pumps which allows the accumulation of sodium, chloride, and water in the cell

• Energy failure that causes Na-K pump failure also promotes excessive neuronal depolarization, reduced neurotransmitter reuptake, and increased intracellular calcium concentrations

• End result is neuronal cell death

• Associated with the clinical conditions of hypoxia and ischemia

  • E.g. hypoxic-ischemic encephalopathy of foals

• Water intoxication can lead to cellular edema

What is vasogenic edema characterized by?

• Vasogenic edema is characterized by increased permeability of brain capillary endothelial cells, with extravasation of macromolecules from the vasculature

What causes vasogenic edema?

• Most commonly associated with tumor, trauma, abscess, infarction, lead intoxication, or severe ischemia

• Features of vasogenic and cellular edema often coexist in a particular patient

Interstitial Edema

• Interstitial edema is best observed in obstructive hydrocephalus

  • Results in transependymal movement of CSF with a subsequent accumulation of brain interstitial fluid

Pathogenesis of Vasogenic Cerebral Edema

Increased capillary permeability

ECF Composition in Vasogenic Cerebral Edema

Plasma filtrate with protein

Capillary Permeability with Vasogenic Cerebral Edema

Increased

Clinical Conditions Associated with Vasogenic Cerebral Edema

Trauma, infarct, abscess, hemorrhage

Pathogenesis of Cellular Cerebral Edema

Energy failure and cellular swelling

ECF Composition in Cellular Swelling

Water and sodium

Capillary Permeability in Cellular Cerebral Edema

Normal

Clinical Conditions Associated with Cellular Cerebral Edema

Ischemia, hypoxia hypoosmolarity

Pathogenesis of Hydrocephalic Cerebral Edema

CSF outflow obstruction and increased total brain fluid

ECF Composition in Hydrocephalic Cerebral Edema

Cerebrospinal fluid

Capillary Permeability in Hydrocephalic Cerebral Edema

Normal

When will slight turbidity of CSF be noted?

At a cell count above 400 cells/mm³

Tyndall’s Effect

• Snowy or sparkling appearance when the fluid is observed and mildly agitated in direct sunlight

  • Observed at cell counts below 400 cells/mm3

Turbidity of CSF

• Turbidity scored on a scale from 0-4+

  • 0 normal

  • 4+ so turbid that newsprint cannot be read through the tube

Xanthochromia

•  Yellowish or yellow-orange discoloration

  • Arises due to the presence of bilirubin

  • Most commonly occurs following rupture of red blood cells into the CSF but may occur due to hyperbilirubinemia

  • Takes 1-4h to develop after a hemorrhagic event

  • High total protein (over 150 mg/dL) may cause mild xanthochromia

  • In neonatal foals, CSF is slightly xanthochromic in foals up to 10 days of age

Cell Count in CSF

• In normalcy CSF cell count is very low

  • WBC 0-6 cells/uL for adults and foals

  • Leukocytes are almost totally mononuclear cells

  • Neutrophils and eosinophils are almost never seen in normal horses

  • Due to low concentration of protein, cells within CSF deteriorate rapidly, analysis should occur within 1 hour of collection

    • If analysis will be delayed, the sample should be split and one portion mixed with an equal volume of 40% ethanol until analysis

  • Differential cell counts should be performed and require concentration methods due to the low number of cells in CSF normally

What can you see neutrophilic pleocytosis with?

Neutrophilic pleocytosis occurs in horses with infectious (bacterial or mycotic meningitis, Eastern equine encephalitis, Western equine encephalitis, and Venezuelan equine encephalitis) or inflammatory conditions (trauma or chemical meningitis from hemorrhage or injection of ionic or nonionic contrast agents)

What can you see lymphocytic pleocytosis with?

• Lymphocytic pleocytosis is relatively uncommon in horses with nervous system disease

  • May be seen in horses with CNS lymphoma and viral meningitis, specifically West Nile Virus encephalitis

What can you see eosinophilic pleocytosis with?

Eosinophilic pleocytosis is rare but has been reported in horses with verminous encepahlitis due to Halicephalobus sp.

RBCs in CSF

• "Acceptable" number of RBCs in CSF has decreased and is considered to be a value of 50 RBC/uL

• Has been demonstrated that blood contamination only minimally increases the CSF WBC count and total protein if the RBC count is less than 2000 RBC/uL

Plasma Proteins in the CSF

• Plasma proteins gain access to the CSF primarily by diffusion across the blood-CSF barrier

  • Diffusion determined by the radius of the protein molecule which is directly related to the molecular weight

  • Normal CSF total protein concentration is roughly 1/100th that of blood plasma

• Commonly reported reference range for CSF protein is 50-100 mg/dL for CSF taken from the lumbosacral space

  • Total protein concentration differs depending on the site of collection with samples collected from the LS site having slightly higher concentration than that from the AO site

  • Also reported that ponies have higher CSF total protein than horse breeds

  • CSF total protein concentration of foals is higher than that of normal adult horses

    • Newborn foals (<2 days of age) had a mean CSF total protein of 109.0 mg/dL and decreasing to adult values by 21 days of age

Methods to Assay Total CSF Protein

• Turbidometric

• Spectrophotometric

• Lowry

• Biuret

• Dye binding

• Immunologic methods

What causes increased CSF total protein?

• Increased CSF total protein occur due to increased permeability of the blood-CSF of BBB, increased protein synthesis within the CNS obstruction of CSF flow, or tissue degeneration/necrosis

  • Obstructive diseases result in high protein concentrations due to enhanced resorption of water, as well as protein leakage

    • Complete spinal fluid block is associated with very high CSF protein concentrations - Froin's syndrome

What causes low CSF protein?

Low CSF total protein rare, can occur due to CSF leakage, removal of excessive quantities of CSF, increased ICP, or in cases of water intoxication

AO CSF Collection

• 18 gauge, 3.5 in spinal needle inserted at the point at which a line drawn between the cranial borders of the atlas intersect midline

• Needle directed toward the lower jaw and advanced until dura is penetrated

• Fluid flows without need for aspiration

• Only withdraw 1-2 mL in neonates to minimize the risk of tentorial herniation

• Putting head down to graze appears to increase the degree of neck soreness following an AO puncture

• Described in standing horses

LS CSF Collection

• Can use ultrasound guidance which may be useful in obese horses where the landmarks are difficult to palpate

  • Insertion site is on midline 0.5-1 cm cranial to the point at which the tuber sacrale are most superficial

• Site clipped, prepped, and locally blocked

• Can make a stab incision with #15 blade to minimize dulling of spinal needle

• 6-8 inch, 18 gauge spinal needle inserted and stabilized at the skin surface with one hand and advanced with the other

• Advance straight down with no lateral or cranial-caudal deviation

• Increasing resistance as the needle approaches the space and then decrease as the needle penetrates the ligamentum flavum, felt as a pop

• Penetration of the membranes often accompanied by reaction from the horse

• CSF collected by gentle aspiration

Complications of CSF Collection

• Introduction of infectious agents into the CNS (septic meningitis), aseptic meningitis from hemorrhage, pain and swelling at the site of needle entry, trauma associated with recovery from anesthesia

• Flipping over backwards or falling during LS tap

• Spinal cord penetration (pithing) during AO tap, herniation of the cerebellum, and fractures or worsening of neurologic signs after anesthetic recovery