Comparison of Effects of Propofol and Dexmedetomidine on Motor Evoked Potentials in Neurosurgery: A Prospective Randomised Single Blinded Interventional Study
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Date
2019-12
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SCTIMST
Abstract
Intraoperative neurophysiological monitoring (IONM) is often used in various
intracranial and spine procedures to prevent damage to eloquent areas, cranial nerves or
motor or sensory tracts. Motor evoked potential (MEP) monitoring is invariably an
essential tool in the armamentarium of the operating surgeons to avoid injury to the motor
tract in various intracranial and spine surgeries. (1)
Transcranial motor evoked potential (TcMEP) monitoring is stimulation of the motor
cortex through the skull and eliciting compound muscle action potentials (CMAP) from
the peripheral muscles to test the intactness of the motor pyramidal pathway. (1)
TcMEP is being used in surgeries for monitoring and mapping of the motor pathways. It
is used in the mapping of the motor cortex in resection of tumours or arteriovenous
malformations located near the motor cortex or in epilepsy surgeries. It is also used in the
subcortical mapping of corticospinal tract. It is also used in brainstem surgeries and in
Chiari malformation. It is also used in vascular surgeries like carotid endarterectomy,
reconstructive surgeries of the neck, aneurysms of the aortic arch and of
thoracoabdominal aorta or intracerebral aneurysms of middle or anterior cerebral arteries.
It is very commonly used in spinal surgeries for extradural or intradural (extramedullary
or intramedullary) tumour resection, embolization of arteriovenous malformations and in
deformity corrective surgeries like scoliosis and spondylolisthesis. (2)
Intraoperatively, there are many factors other than surgical manipulation that can affect
the quality of the CMAP like temperature, blood pressure, partial pressure of expiredcarbon dioxide, oxygen, etc. These factors need to be optimized for correct interpretation
of the MEPs. (2)
The anaesthetic agents can affect the quality of MEP intraoperatively as they inhibit
synaptic transmission. Muscle relaxants antagonize the transmission of signals across the
neuromuscular junction. Inhalational agents suppress the CMAP and should be used at a
lower minimum alveolar concentration (MAC). Opioids seem to have very little effect on
CMAP. Intravenous anaesthetics suppress MEP lesser than inhalational agents, so total
intravenous anaesthesia (TIVA) or combination of intravenous with minimal inhalational
anaesthetic supplementation is used when MEPs are monitored. (3)
TIVA with propofol and opioid is most commonly used for MEP monitoring. (4) As
propofol gets rapidly metabolised, its sedative effects and effects on MEP can be adjusted
quickly. But MEP can get depressed at high doses required to maintain surgical depth,
hence, adjuvant agents that maintain anaesthetic depth without affecting the MEP are
often required. (5)
Dexmedetomidine is a selective alpha-2 agonist. It causes sedation, analgesia,
sympatholysis and minimal respiratory depression. (6) Its addition to the anaesthetic
regimen can reduce hypnotic requirement, especially propofol. Dexmedetomidine has
invariably been used as an adjuvant to various anaesthetic agents and has been found to
have minimal affect on the MEP when combined with other agents. (7) It has found
widespread acceptance in neuroanaesthesia because of its favourable recovery
characteristics and absence of significant impact on cerebral blood flow and intracranial
pressure.