Neurophysiological Monitoring

Quick Answer

Intraoperative neurophysiological monitoring (IONM) detects neurological injury during surgery allowing prompt intervention to prevent permanent damage. SSEP (Somatosensory Evoked Potentials): Stimulation of peripheral nerve (median, ulnar, posterior tibial), recorded over spinal cord, brainstem, thalamus, and cortex; assesses dorsal column sensory pathway integrity; reduced amplitude >50% or increased latency >10% indicates injury (cord ischaemia, retractor pressure, hypotension); used for spinal surgery, aortic surgery, craniotomy near sensory cortex. MEP (Motor Evoked Potentials): Transcranial electrical stimulation of motor cortex, recording from peripheral muscles or epidural space; assesses corticospinal tract integrity; critical for motor preservation; sensitive to anaesthetics (avoid volatile agents >0.5 MAC, use TIVA with propofol/remifentanil); complete loss indicates high risk of postoperative paraplegia. BAEP (Brainstem Auditory Evoked Potentials): Auditory click stimuli, recorded from scalp; assesses cochlea to cortex auditory pathway; resistant to anaesthetics; used for acoustic neuroma, brainstem surgery, posterior fossa procedures; wave V loss indicates brainstem compromise. EEG (Electroencephalography): Raw or processed (BIS, entropy); assesses cortical activity; used for carotid endarterectomy (detects ischaemia from clamping—requires shunt if changes), burst suppression (for cerebral protection during circulatory arrest); BIS 40-60 for general anaesthesia, 0 for burst suppression. EMG (Electromyography): Recording from cranial or peripheral muscles; detects nerve irritation (pedicle screw placement near nerve root, cranial nerve manipulation); continuous or triggered; used for thyroid/parathyroid surgery (recurrent laryngeal nerve), posterior fossa (facial, acoustic, trigeminal nerves), lumbar fusion (nerve root monitoring). Cerebral oximetry (rSO2): Near-infrared spectroscopy measuring frontal lobe oxygen saturation; baseline 60-70%; >20% drop from baseline or <50% absolute indicates inadequate cerebral oxygenation; used for cardiac surgery, beach chair position (sitting), carotid surgery; intervention—increase MAP, increase oxygenation, increase CO2, reposition head/shoulders, check cannulation. Anaesthetic considerations: Volatile agents suppress evoked potentials (dose-dependent); TIVA (propofol/remifentanil) preferred for SSEP/MEP monitoring; nitrous oxide suppresses SSEP/MEP; neuromuscular blockade precludes EMG and muscle MEP recording (maintain 1-2 twitches); hypothermia prolongs latencies; hypotension reduces amplitudes; PaCO2 affects SSEP (hypocapnia reduces signal). Interpretation: Baseline established after positioning before incision; continuous monitoring; alerts when significant change; communication between neurophysiologist, surgeon, and anaesthetist critical; reversible causes—hypotension, hypoxia, anaesthetic depth, surgical manipulation. [1-10]