Cerebrospinal fluid (CSF) is a clear, plasma-like fluid that bathes the central nervous system (CNS). It occupies the central spinal canal, the ventricular system, and the subarachnoid space. It is produced mostly by the choroidal plexus of the ventricular system as well as its ependymal lining. The CSF has a physiologic volume of about 150 ml with a daily turn over of about 500 ml. Produced mostly in the lateral ventricles, the CSF passes through the interventricular foramen (of Monro), into the third ventricle. It exits the third ventricle through the cerebral aqueduct (of Sylvius) into the fourth ventricle. It exits the fourth ventricle into the cerebral subarachnoid space through the median aperture of Magendie and the two lateral apertures of Luschka. The CSF continues into the spinal subarachnoid space through the central canal of the spinal cord.
Normal CSF pressure (8 mmHg to 15 mmHg supine and about 20 mmHg erect) is a function of a tightly maintained equilibrium in rates of production and resorption of CSF. The out-pouching of the arachnoid mater called arachnoid granulations are responsible for the resorption of CSF into the dural venous sinuses. Disequilibrium in synthesis and resorption or obstruction of circulation results in CSF accumulation and raised intracranial pressure called hydrocephalus.
The CSF supports the weight of the brain estimated at 1500 gm and suspends it in neutral buoyancy to a net weight of about 25 gm. Hence, the entire brain density is cushioned, protecting it from crushing into the bony cranium.
It protects the brain from damage during head trauma. Otherwise, even minor head bopping will result in severe brain injury.
The biochemical constituents and volume of the CSF play vital cerebral homeostatic roles:
The CSF contains glucose, proteins, lipids, and electrolytes, providing essential CNS nutrition.
The CSF contains immunoglobulins and mononuclear cells.
At the beginning of week 4, before the formation of choroid plexus, the neural tube is found with CSF; this suggests extra-choroidal sources of CSF during embryonic development.
The choroid plexus became visible at around week 6 (about day 40), the arachnoid granulations did not form until the third trimester and up until around 20th month of life.
Congenital hydrocephalus develops during intrauterine life, common developmental defects implicated are aqueductal stenosis and neural tube defects.
The choroid plexus refers to a group of fenestrated blood capillaries located in the ventricular system. The choroid plexus mostly synthesizes CSF.
Arachnoid granulations are responsible for CSF resorption; they drain CSF into the dural venous sinuses.
CSF drains into the lymphatic circulation, through lymph ducts contiguous to the olfactory duct as it passes through the cribriform plate.
This is a condition in which CSF accumulates in the CNS due to a dis-equilibrium between synthesis, circulation, and resorption of CSF.
This may complicate a shunt surgery with re-occurrence of presenting symptoms necessitating a shunt revision. Common causes of shunt failure are infections, blocked shunt, or a displaced shunt.
Lumber Puncture and CSF Analysis
Lumbar puncture is a sterile procedure, done to obtain CSF samples for diagnostic purposes. It involves passing a needle into the subarachnoid space at the levels between L2 and L5 vertebrae. However, most commonly lumbar puncture is performed between L4 and L5. Biochemical, microbiologic and cytologic studies are carried out, some possible pathognomic pictures in comparison to physiologic CSF parameters are as follows:
Also called spinal block, is a method of regional anesthesia. It involves the injection of a local anesthetic agent into the subarachnoid space through a needle at the level between L2 and L5 vertebrae.
Specially formulated drugs administered intrathecally into the CSF to assess the blood-brain barrier. Cancer chemotherapeutic agents like methotrexate are available in intrathecal formulations as well as intrathecal opioids using an intrathecal pump or catheter.
Normal Pressure Hydrocephalus
Increased CSF with concomitant ventriculomegaly, causes this condition; hence, intracranial pressure appears within normal or levels at high normal. However, slightly elevated ICP may be recorded with continuous ICP monitoring. This often affects subjects older than 65 years and presents with the triad of gait abnormalities, urinary incontinence, and dementia. This triad is from where the mnemonic "wobbly, wet, and wacky" comes.