Cerebrospinal and other body fluids

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Cerebrospinal and other body fluids

Cerebrospinal fluid

Cerebrospinal fluid (CSF) is produced by the choroid plexuses, partly by ultrafiltration and partly by secretion, and fills and circulates through the ventricles and spinal cord. Compared with plasma, it has less protein, and the concentrations of protein-bound components like bilirubin are similarly reduced. Its electrolyte composition is similar to but distinct from plasma (more chloride, less potassium and calcium). Infection or the presence of blood in the CSF alters its composition. This provides the basis for biochemical analysis of CSF in the diagnosis of subarachnoid haemorrhage (SAH) and meningitis.

Lumbar puncture

Lumbar puncture (LP) is the procedure performed in order to obtain a specimen of CSF. If signs of raised intracranial pressure such as hypertension, bradycardia and papilloedema are present then an LP should not be performed.

When collecting CSF in suspected infection, e.g. meningitis, microbiological examination takes priority. If subarachnoid haemorrhage is suspected, it may help to collect the CSF as several separate aliquots. These will be equally blood-stained in SAH, but progressively less so if the blood in the CSF results from damage to a blood vessel during the LP procedure (a so-called ‘traumatic tap’).

Subarachnoid haemorrhage

Bleeding into the subarachnoid space most frequently results from rupture of an aneurysm (swelling) in one or more of the arteries located within the space. The patient typically complains of a severe headache of sudden onset (‘thunderclap’), often associated with vomiting and a reduced level of consciousness. The mainstay of diagnosis is imaging by CT or MRI. However, in the presence of a strong clinical suspicion, negative imaging does not rule out a subarachnoid haemorrhage. In these cases, LP should be carried out, unless there are obvious signs of raised intracranial pressure.

Xanthochromia

Xanthochromia simply means yellow discoloration of the CSF. It results from the presence of bilirubin derived from red blood cells (RBCs) that have undergone in vivo lysis. In vitro lysis of RBCs, e.g. a traumatic LP, produces only oxyhaemoglobin, and not bilirubin. It can be detected by visual inspection, but this is unreliable, and where possible scanning spectrophotometry should be used instead. This involves measuring the absorbance of the CSF specimen across a range of wavelengths; the blood pigments have characteristic absorbance peaks (Fig 65.1).

Fig 65.1 Absorbance spectrum of CSF in subarachnoid haemorrhage.

Meningitis

Meningitis refers to inflammation of the meninges which line the central nervous system (CNS). Bacterial meningitis presents acutely and is a medical emergency. CSF biochemistry tends to reflect the nature of the infective organism (Table 65.1

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