Lumbar puncture
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.
