34. Cerebrospinal fluid Flashcards
What is CSF?
> Cerebrospinal fluid (CSF) is the clear colourless fluid that bathes the brain and spinal cord acting as a fluid layer for protection of the central nervous system (CNS).
Where is CSF produced?
What rate
Total volume (% ICV)
How much a day
What happens with a rise in icp
> CSF is produced by the
four choroid plexuses
located in the third, fourth and lateral ventricles.
> Produced at a rate of approximately
0.3 mL per minute.
> Total volume of CSF is approximately
150 mL, which equates to
approximately 10% of intracranial volume.
> 450 mL of CSF is produced per day
and so CSF volume is replaced three
times every 24 hours.
> CSF is derived from plasma filtration and subsequent secretion by the choroid plexuses.
> CSF is one of the three determinants of intracranial pressure (the other two being brain tissue and blood volume).
> In situations of raised intracranial pressure,
CSF production remains relatively constant;
however, CSF absorption increases thereby reducing
total CSF volume (see Chapter 33, ‘Cerebral blood flow’).
Describe the circulation of CSF.
CSF flows from the lateral ventricles
through the foramen of Monro
into
the third ventricle
and from there
via the aqueduct of Sylvius
into the fourth ventricle.
> CSF leaves the ventricular system via the midline foramen of Magendie and lateral foramen of Lushka, entering the subarachnoid space of the brain and spinal cord.
> CSF is absorbed into the dural
venous sinuses via arachnoid villi and
granulations that project into the dural sinuses.
What is the relevance of the
blood–brain barrier (BBB)?
> Plasma constituents do not pass freely
into the CSF.
This phenomenon is known as the BBB.
> Anatomical and physiological
factors involved in maintaining the BBB are:
• Tight junctions and fenestrated
choroidal capillaries within the brain
• Specialised bidirectional transport system for ions, glucose and amino acids.
Describe the normal CSF
composition.
Reference values for CSF are as follows: Protein 0.15–0.45 g/L Osmolality 280–300 mmol/L Sodium 135–145 mmol/L Potassium 2.6–3.0 mmol/L Chloride 115–125 mmol/L Calcium 1.00–1.40 mmol/L Magnesium 1.2–1.5 mmol/L Lactate 1.1–2.4 mmol/L pH 7.28–7.40 Creatinine 50–110 μmol/L Glucose 2.8–4.4 mmol/L Urea 3.0–6.5 mmol/L
A comparison of the composition of CSF and plasma reveals that:
> CSF proteins are ∼1% that of plasma, resulting in reduced buffering capability
> CSF calcium levels are ∼50% that of plasma
> CSF glucose levels are ∼60% that of plasma
> CSF chloride and magnesium levels are higher than plasma.
What investigations can be
performed on a CSF sample?
Important information about CSF can be derived from the following parameters:
> Opening pressure: traditionally measured in cm H2O (normal = 10–15 lying down, 20–30 sitting up).
Elevated in raised intracranial pressure
> Macroscopic appearance, e.g. xanthochromia
> Total and differential cell count
> Bacterial culture and sensitivity
> Protein and glucose
> Analysis of immunoglobulins (detect chronic CNS inflammatory conditions)
> Cytology.
What changes in CSF cell counts
occur with CNS infection?
CSF normally contains a small number of cells
(usually lymphocytes and monocytes)
and the total cell count is less than 5 cells/mm3.
An increase in cell counts suggests
either an infection of the CNS
or a number of
pathological CNS conditions.
The differential cell count provides further
information regarding the
possible cause of the CNS disease.
> Increased neutrophils may indicate
bacterial meningitis.
Other causes of an increased neutrophil count include a cerebral abscess, seizures and CNS haemorrhage.
> Increased lymphocytes may
indicate viral meningitis.
Lymphocyte counts are also elevated in
meningitis due to
TB, syphilis, and fungal and parasitic infections.
Degenerative diseases of the CNS,
such as multiple
sclerosis, will also generate elevated lymphocyte counts.
> ‘Mixed reaction’, an increase in neutrophils, lymphocytes and plasma cells. This is characteristic of TB meningitis, fungal meningitis and chronic bacterial meningitis.
> Increase in plasma cells is a feature of TB meningitis.
> Leukaemic cells indicate
meningeal infiltration by haematological malignancy.
Can biochemical analysis of CSF
be diagnostic?
> CSF total protein:
The CSF normally contains less than 0.45 g/L protein.
Increased levels may be found in:
• Infection
• Blood contamination
• Chronic inflammatory disorders of the CNS
(TB, syphilis, Guillain–Barré)
> CSF electrophoresis:
Electrophoretic separation of CSF proteins and
detection of CSF immunoglobulin.
CSF immunoglobulin can arise from three causes:
• Secondary to an increase in
plasma immunoglobulin,
e.g. multiple myeloma
• Impairment of the blood–brain barrier
• Local synthesis in the CNS, e.g. in multiple sclerosis the increase in CSF immunoglobulin is characterised by an oligoclonal pattern of immunoglobulin synthesis and this can be detected in 90% of patients with MS.
> CSF glucose: Low levels of CSF glucose suggest:
• Infection (local metabolism by white cells)
• Hypoglycaemia (although CSF glucose is of limited diagnostic utility and the plasma glucose concentration must be known in order to interpret the CSF glucose level properly).
> Polymerase chain reaction (PCR) is a
technique to rapidly amplify
a defined region of DNA or RNA.
PCR has been used to detect
the presence of bacterial pathogens
(e.g. syphilis and TB) and viral
pathogens (e.g. HIV) in the CSF.
