Lumbar
puncture is indicated in the investigation of infections (meningitis or
encephalitis), subarachnoid haemorrhage, inflammatory conditions (multiple
sclerosis, sarcoidosis and cerebral lupus) and some neurological malignancies
(carcinomatous meningitis, lymphoma and leukaemia); it is also used to measure
CSF pressure (in idiopathic intracranial hypertension). It is, of course, part
of the procedure of myelography, and can be used in therapeutic procedures,
either to lower CSF pressure or to administer drugs. Lumbar puncture involves
inserting
a
needle between lumbar spinous processes (usually
between
L3 and L4) through the dura and into the CSF under local anaesthetic.
Intracranial pressure can be measured and CSF removed for analysis. CSF is
normally clear and colourless and the tests that are usually performed include
a naked eye examination of the CSF, centrifugation to determine the colour of
the supernatant (yellow, or
xanthochromic, some hours after subarachnoid haemorrhage),
biochemical
analysis (glucose, total protein, and protein electrophoresis to detect
oligoclonal bands), microbiological analysis (polymerase chain reaction (PCR)
for herpes simplex or tuberculosis), immunology (paraneoplastic antibodies) and
cytology (to detect malignant cells). f there is a space-occupying lesion in
the head, lumbar puncture can result in a shift of intracerebral contents downwards,
towards and into the spinal canal.
This
process is known as coning, and is
potentially fatal. Consequently, lumbar puncture is contraindicated if there is
any clinical suggestion of raised intracranial pressure (papilloedema),
depressed level of consciousness, or focal neurological signs suggesting a
cerebral lesion, until imaging of the head (by CT or MRI) has excluded a space
occupying lesion or hydrocephalus. Lumbar puncture is contraindicated in the
presence of thrombocytopenia or disseminated intravascular coagulation, and in
those on warfarin or heparin therapy, unless specific measures are taken to
compensate for the clotting deficit on a temporary basis. Lumbar puncture can
be safely performed in patients on antiplatelet drugs such as aspirin,
clopidogrel and low-dose heparin. About 30% of
lumbar punctures are followed by a headache, which is thought to be due
to reduced CSF pressure. The risk of this can be reduced by using smaller
needles. Other minor complications involve transient radicular pain, and pain
over the lumbar region during the procedure. Infections such as meningitis are
extremely rare following lumbar puncture, provided that
an aseptic technique is used.
