Who
Needs Intra-operative monitoring?
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Surgical
procedures carried out within or near the spinal column or those involving transient interruption of blood flow to the spinal cord
(e.g repair of thoraco-abdominal aortic aneurysms) are associated
with a risk of neurological impairment ranging from loss of
sensation to complete paraplegia. These deficits can arise
as a result of direct trauma, stretching of nerves or occlusion of
blood flow. Much effort has therefore been made to develop
techniques which allow the health of the spinal cord to be
assessed continuously during these risky surgical procedures.
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Sensory
Evoked Potential (SEP) Monitoring
Surgical teams
currently monitor the status of ascending spinal sensory pathways by
applying stimuli to the patient’s ankle or wrist and observing
the resultant changes in somatosensory evoked potentials (SEP’s)
recorded from the brain. This form of intra-operative
monitoring uses changes in the SEP waveform to alert medical teams of possible
complications and there is no doubt that this technique has
protected many patients from surgically induced neurological
deficits. However, the
technique of SEP monitoring has attracted some criticism,
much of which has been published in peer reviewed journals:
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Unchanged SEP waveforms have
on occasions misled surgeons into
continuing with surgery, resulting in unforeseen post-operative
neurological complications such as severe paraplegia.
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Altered SEP’s have
prompted surgeons to back-off from procedures, only to find that
the patient has suffered no loss in sensory status upon
recovery.
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As SEP’s are generally small in
magnitude, they can be difficult to monitor reliably in some
patients, particularly those presenting with a pre-existing
neuropathology.
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Although
SEP monitoring is used as an indicator of the health of the
spinal cord as a whole, some would argue that for anatomical
reasons, the descending motor fibres may be at greater risk
during surgery. This would suggest that it would be of
tremendous benefit to monitor descending motor fibres
exclusively or in combination with SEP monitoring.
Transcranial
Electrical Motor Evoked Potentials (tceMEPs) & Digitimer Ltd
In
collaboration with leading clinical neurophysiologists, Digitimer
developed the D185 MultiPulse Cortical Stimulator
in order to
provide a more reliable method of minimizing the risk of surgically
induced paraplegia while maximizing the level of
surgical correction that could be safely conducted. This
unique device is used transcranially to electrically stimulate
the brain's motor cortex, resulting in a descending motor evoked
potential (MEP) which is conducted down the spinal cord to upper
and lower limb extremities. The pathways stimulated in
this manner are the same as those used by the brain to trigger
and control voluntary movement. As with SEP monitoring,
any alterations in the MEP waveforms can provide the surgical
staff with crucial warning of possible complications.
MEP
Monitoring - The Way Forward?
A
1000 patient, 2 centre clinical trial of the Digitimer D185 in
the USA, has demonstrated that MEP monitoring during spinal
surgery was (1) more accurate for predicting motor outcome than
the SEP was for predicting sensory outcome; and (2) that useful
motor responses were achievable with a higher probability than
useful sensory responses. Furthermore, in cases where
SEP monitoring alone may have misled the surgeon into aborting or
curtailing a procedure, additional use of MEP monitoring can more
reliably indicate whether it is safe for the surgeon to continue
the procedure to completion.
Evidence
from the 5 year study outlined above has prompted the FDA to clear
the Digitimer D185 for marketing. It is hoped that with this move, the technique of intra-operative MEP monitoring will
be more commonly used by cardiovascular and neurosurgeons
worldwide. Digitimer Ltd now offer the D185 stimulator to
hospitals and neurophysiological monitoring organizations
throughout the world.
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