Page 298 - Medicine and Surgery

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294 Chapter 7: Nervous system

Myotonic discharges can be seen, e.g. in myotonic Contraindications

dytrophy, and myopathic changes may occur with any Infection at the site of LP.
primary muscle disorder. However, a normal EMG Suspected intracranial mass lesion – focal neurology
does not exclude a muscle disorder. or depressed GCS should lead to a CT brain scan prior
to LP.
Suspected raised intracranial pressure or papil-

Nerve conduction studies
loedema before CT evaluation.
Surface electrodes or occasionally needles are used both Suspected spinal cord compression.
for stimulation and recording: Bleedingdisordersshouldbecorrectedfirst(including
Motor and sensory conduction velocities (slow in de- 9

a platelet count of <40 × 10 /L, anti-coagulant drugs
myelinating disorders).
such as heparin or warfarin).
Actionpotentialsize–inaxonalneuropathies,thecon- Congenital lumbosacral lesions such as meningomye-

ductionvelocityandlatenciesarenormal,butthetotal
locele, because the cord may be tethered or low.
action potential is reduced in amplitude.
Fwave latencies and velocities are also useful – these

Procedure
are like an ‘echo’ which occurs as a nerve that is stim-
Aftergiving consent, the patient is positioned on their
ulated peripherally, the action potential travels up to
left side on a firm surface, with the back at the edge of
the spinal cord, and back down – it allows the eval-
the couch. The knees are drawn up as far as possible and
uation of brachial and lumbosacral plexus and nerve
the neck flexed, to open up the spinous processes of the
roots.
lumbar vertebrae.
Repetitive stimulation is useful in the diagnosis of

TheaimistoinserttheneedlebetweenL3–L4orL4–L5
neuromuscular junction disorders – see myasthenia
in adults (below the level of the spinal cord). L4 normally
gravis, Eaton–Lambert syndrome.
lies at the level of the iliac crests. The area is cleaned and
infiltrated with lidocaine.
The lumbar puncture needle is inserted in the midline
Lumbar puncture with its stylet in place aiming slightly towards the um-
bilicus. The needle is advanced slowly ∼4–5 cm, and a
This is the procedure by which cerebrospinal fluid (CSF)
slight give is often felt as it penetrates the dura mater. The
is aspirated by an approach between the lumbar verte-
styletiswithdrawnandifnoCSFappears,itisre-inserted
brae.
and the needle advanced slightly – this is repeated until
CSF appears. If the needle encounters firm resistance, it
Indications should be withdrawn and another approach tried.
When any of the following are suspected: Sometimes the patient will feel a pain radiating into
Infection (meningitis, encephalitis, fungal infections the leg or back – this is due to the needle touching a

or neurosyphilis). root – if it persists, the needle will have to withdrawn
Multiple sclerosis. and a slightly different angle attempted.

Subarachnoid haemorrhage (with a normal CT head). Once CSF appears, the CSF pressure can be measured

Guillain–Barr´ e syndrome. by attaching a manometer (normal 6–15 cm H 2 O). CSF

Meningeal carcinomatosis (malignant meningitis) in- iscollectedinthreesteriletubes(sentformicroscopyand

cluding lymphoma. culture, protein and cytology) and an additional sample
CSF pressure measurement is useful – particularly in the is sent for glucose measurement. A simultaneous blood
diagnosis of idiopathic (benign) intracranial hyperten- sample for glucose should be sent. Oligoclonal bands are
sion, where CSF removal may be a therapeutic manoeu- identified using paired CSF and serum samples.
vre.
Lumbar puncture (LP) is also required for intrathecal Complications
administration of contrast media (in myelography), and The most common complication is headache, which
drugs such as antimicrobials and chemotherapy. may be treated by lying flat and adequate hydration.