Who Needs Intra-operative monitoring?

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.

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:

Unchanged SEP waveforms have on occasions misled surgeons into continuing with surgery, resulting in unforeseen post-operative neurological complications such as severe paraplegia.
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.
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.
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 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 a crucial warning of possible complications.

MEP Monitoring – The Way Forward?

A 1000 patient, 2 centre clinical trial of the Digitimer D185 in the USA, 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 was shown to more reliably indicate whether it was safe for the surgeon to complete a procedure. Evidence from the 5-year study outlined above prompted the FDA to clear the Digitimer D185 MultiPulse Stimulator for marketing and the technique of MEP monitoring is now utilised extensively by cardiovascular and neurosurgeons all over the world.