Circulation and outflow of CSF
CSF is constantly secreted by the choroid plexus located in the lateral, III, and IV ventricles. The circulation process flows from the sites of production to the pathways of elimination:
- From the lateral ventricles, the fluid passes through the interventricular foramen into the III ventricle.
- Further, through the cerebral aqueduct (the narrowest and most vulnerable segment), it flows into the IV ventricle.
- Through the median aperture of the IV ventricle, the CSF enters the subarachnoid space, distributing among the cerebral cisterns (chiasmatic, interpeduncular, interhemispheric, cerebellopontine, etc.).
- CSF bathes the brain and travels along the spinal cord.
Resorption (absorption) The main outflow of CSF into venous blood occurs intracranially through arachnoid granulations (pacchionian granulations) located in the superior sagittal sinus and diploic veins area. An accessory pathway is absorption in the exit zones of cranial and spinal nerves, as well as through the ependyma and capillaries of the spinal meninges.
CSF changes in pathologies
CSF analysis helps differentiate inflammatory and degenerative diseases of the nervous system.
- Neurosyphilis: clear or turbid fluid, mononuclear pleocytosis, elevated immunoglobulin levels, positive Pandy test (+/-). A specific marker is a positive TPHA test.
- Multiple sclerosis: CSF is clear and colorless. Up to 40 cells per 1 µL. Albumin quotient < 20 × 10⁻³. A key sign is the detection of oligoclonal bands by isoelectric focusing.
- Lyme disease (acute borreliosis) and fungal meningitis: clear CSF, significant pleocytosis (hundreds of mononuclear cells), elevated immunoglobulins. In the former case, specific antibodies are detected; in the latter, the pathogen is identified via staining or culture methods.
- Guillain-Barré syndrome (polyradiculoneuropathy): characterized by "albuminocytologic dissociation" (high albumin quotient up to 50 × 10⁻³ with mild pleocytosis).
Pathogenesis and types of hydrocephalus
Hydrocephalus is a condition caused by an imbalance between CSF production and resorption. The pathological process begins with an increase in CSF pressure, its transudation through the ependyma, and periventricular infiltration. This causes displacement and atrophy of the white matter (gray matter initially remains intact), focal ischemia, myelin destruction, and ultimately, irreversible gliosis. Changes are reversible only with a rapid decrease in pressure.
Pathogenetic classification:
- Obstructive (non-communicating): occurs due to a physical block (tumor, infarction, hemorrhage, aqueductal stenosis). CSF outflow into the subarachnoid space ceases.
- Aresorptive (communicating): occurs due to impaired function of arachnoid granulations (following meningitis or subarachnoid hemorrhage). CSF flows freely from the ventricles but is not absorbed.
- Hypersecretory: a rare form occurring with tumors of the choroid plexus (e.g., papillomas) when excess CSF is produced.
- Post-traumatic: the result of brain injuries.
A distinct category is normal pressure hydrocephalus, characterized by episodic pressure increases. Its classic triad includes dementia, urinary incontinence, and gait apraxia. Hydrocephalus should not be confused with hydrocephalus ex vacuo, which is secondary enlargement of the CSF spaces due to brain tissue atrophy in degenerative diseases (Alzheimer's disease, Pick's disease).
Briefly: answers to topical questions
Through which openings does CSF exit the IV ventricle into the subarachnoid space?
Cerebrospinal fluid exits the IV ventricle into the subarachnoid space through three openings. These include:
- Unpaired median aperture (foramen of Magendie), an outlet located along the midline.
- Paired lateral apertures (apertura lateralis, foramina of Luschka), located in the region of the lateral recesses of the IV ventricle.
These structures ensure communication between the ventricular system and the subarachnoid space of the brain and spinal cord.
What subarachnoid cisterns of the brain are distinguished?
Several subarachnoid cisterns are distinguished, representing enlargements of the subarachnoid space. The basal cisterns of the brain include:
- Chiasmatic cistern, located at the base of the brain.
- Interpeduncular cistern, also classified as basal.
Other distinguished cisterns:
- Interhemispheric cistern
- Transverse cistern
- Ambient cistern
- Cistern of the vermis
- Cerebellopontine cistern
- Cerebellomedullary cistern (cisterna magna, or great cistern — cisterna magna), located near the median aperture of the IV ventricle.
What are the normal levels of protein, glucose, and cell count in cerebrospinal fluid?
Normally, cerebrospinal fluid has strictly defined laboratory values for protein, glucose, and cellular composition.
- Protein content does not exceed 0.45 g/L (or 15–45 mg/dL).
- Glucose content ranges from 2.2 to 3.9 mmol/L.
- Cell count (cytosis) is no more than 5 cells per 1 µL, with lymphocytes predominantly found in the cellular composition.
These parameters are evaluated during CSF analysis obtained via lumbar puncture.
At what spinal level is a lumbar puncture performed to collect CSF in adults?
Lumbar puncture for cerebrospinal fluid collection is performed at the level of the lower lumbar vertebrae. The puncture is carried out directly between the spinous processes of the lower lumbar vertebrae. During the procedure, the patient lies in a lateral recumbent position with legs flexed and drawn toward the abdomen. The skin at the puncture site is pre-treated with an antiseptic, and local anesthesia is applied.
Class and exam questions
What is the difference between non-communicating and communicating hydrocephalus?
In obstructive hydrocephalus, CSF cannot exit the ventricles due to a block in the outflow pathways. In communicating (aresorptive) hydrocephalus, there are no physical obstacles to exit, but the process of fluid absorption into the venous system is impaired.
Where is the narrowest part of the ventricular system located?
This is the cerebral aqueduct (aqueduct of Sylvius), connecting the III and IV ventricles. It is most commonly subject to stenosis.
What is hydrocephalus ex vacuo?
This is secondary compensatory ventricular enlargement occurring not due to disturbed CSF circulation, but as a consequence of brain tissue atrophy and volume loss (e.g., in Alzheimer's disease).
How is neural tissue affected in hydrocephalus?
The periventricular white matter is primarily affected due to CSF transudation and elevated pressure. The gray matter is spared in the early stages.
What else to study on this topic
- Detailed anatomy of the choroid plexus and ventricles
- Subarachnoid cisterns of the brain base
- Method for evaluating oligoclonal bands in multiple sclerosis
- Clinical presentation and diagnosis of normal pressure hydrocephalus
- Compensated and decompensated active hydrocephalus
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