Residency · Residency · Anesthesiology

Neuromonitoring: SSEPs, MEPs, EEG, and EMG

Principles of Intraoperative Neuromonitoring (IONM)

Purpose

Intraoperative neuromonitoring provides real-time assessment of neural pathway integrity during surgery, allowing early detection of injury so that corrective action can be taken before permanent damage occurs. It is critical for spine surgery, intracranial surgery, vascular surgery (carotid and aortic), and thyroid/parathyroid surgery.

General Requirements

Baseline recordings should be obtained after anesthetic induction but before surgical positioning. A consistent anesthetic technique must be maintained throughout the case, avoiding boluses that cause acute signal changes. A communication protocol between the anesthesiologist, surgeon, and neurophysiologist should be established, and clear alert criteria should be defined preoperatively.

Somatosensory Evoked Potentials (SSEPs)

Pathway

SSEPs are generated by stimulating a peripheral nerve -- typically the median nerve at the wrist or the posterior tibial nerve at the ankle. The signal ascends through the dorsal columns of the posterior spinal cord, crosses in the medulla via the medial lemniscus, passes through the thalamus, and arrives at the primary somatosensory cortex.

Recording

Cortical electrodes placed on the scalp, along with subcortical electrodes, record the responses. Because individual signals are small, multiple stimuli are averaged to improve the signal-to-noise ratio, typically requiring 200 to 500 sweeps. The two key parameters are latency (the time to peak) and amplitude (the peak-to-peak voltage).

Alert Criteria

An amplitude decrease greater than 50% from baseline or a latency increase greater than 10% from baseline triggers investigation and surgical notification. Either criterion alone is sufficient to raise an alert.

Clinical Applications

SSEPs are commonly used during spine surgery (scoliosis correction, decompression, and fusion), carotid endarterectomy (to detect cortical ischemia), intracranial aneurysm surgery, and posterior fossa surgery.

Limitations

SSEPs monitor only the dorsal column sensory pathway and do not assess motor tracts. This means that anterior spinal cord ischemia producing a motor deficit can occur without any SSEP changes. This critical limitation led to the development of MEP monitoring. Additionally, the averaging process means SSEP feedback is relatively slow.

Motor Evoked Potentials (MEPs)

Pathway

MEPs are generated by transcranial electrical stimulation (TES) of the motor cortex. The signal descends via the corticospinal (pyramidal) tract in the lateral spinal cord and is recorded from target muscles in the hands and feet using needle electrodes.

Recording

High-intensity, multi-pulse transcranial stimulation delivers a train of 5 to 7 stimuli. The resulting compound muscle action potentials (CMAPs) are recorded from distal muscles. Unlike SSEPs, no averaging is required since a single sweep suffices, providing faster feedback.

Alert Criteria

Complete loss of the MEP signal is the most significant finding and raises immediate concern. An amplitude decrease of 50 to 80% (with thresholds varying by institution) or a threshold increase requiring significantly higher stimulation intensity also warrants attention. In practice, the most common approach is an all-or-nothing criterion: the signal is either present or absent.

Clinical Applications

MEPs are essential during spine surgery as a complement to SSEPs for comprehensive cord monitoring. They are also used during intracranial surgery near the motor cortex or internal capsule and during aortic surgery, particularly descending thoracic and thoracoabdominal aneurysm repairs.

Complications of TES

The most common complication is tongue or lip bite injury, which is prevented by placing a soft bite block. Jaw fracture is rare, as are seizures. Movement during stimulation can occur during critical surgical moments and requires coordination with the surgical team.

<image>Dual-panel diagram showing SSEP and MEP pathways side by side. Left panel: SSEP pathway from peripheral nerve stimulation (posterior tibial nerve) ascending through the dorsal columns, medial lemniscus, thalamus, and primary sensory cortex with recording electrodes on the scalp. Right panel: MEP pathway from transcranial electrical motor cortex stimulation descending through the corticospinal tract in the lateral cord, to the anterior horn cell, peripheral nerve, and target muscle with recording electrodes. Each panel labels the specific spinal cord tracts involved and indicates which surgical injuries each modality detects.</image>

Electroencephalography (EEG)

Intraoperative Uses

EEG is used intraoperatively during carotid endarterectomy to detect hemispheric ischemia during cross-clamping, for depth of anesthesia monitoring through processed EEG devices such as BIS and SedLine, for detection of intraoperative seizures, and for burst suppression monitoring during barbiturate coma.

EEG Changes with Ischemia

The ischemic progression follows a characteristic pattern: loss of fast activity (beta) is followed by loss of alpha, then an increase in theta and delta activity, then burst suppression, and finally electrocerebral silence. These changes appear within 15 to 20 seconds of significant ischemia. During carotid endarterectomy, EEG changes during cross-clamping indicate inadequate collateral flow and may prompt shunt placement.

EEG and Anesthetic Depth

As anesthetic depth increases, the EEG shows progressive slowing of frequency and increasing amplitude. Burst suppression -- periods of high-amplitude activity alternating with isoelectric periods -- indicates deep anesthesia. An isoelectric EEG represents maximal metabolic suppression, as seen during barbiturate coma for neuroprotection.

Electromyography (EMG)

Free-Running EMG

Free-running EMG provides continuous monitoring of muscle activity and detects mechanical irritation of motor nerves during surgery. It offers audible feedback through speakers in the operating room. Applications include monitoring the facial nerve during parotid and acoustic neuroma surgery, the recurrent laryngeal nerve during thyroid surgery, and nerve roots during spine surgery.

Triggered EMG (Stimulated)

In triggered EMG, the surgeon directly stimulates a nerve or surgical field to confirm nerve identity and function. During pedicle screw placement, a stimulation threshold below 10 mA suggests pedicle breach with nerve proximity. During parotid surgery, facial nerve stimulation confirms nerve location before tissue transection.

Key Points

EMG monitoring requires no neuromuscular blockade, or at most minimal partial blockade with 1 to 2 twitches on train-of-four. This is a critical anesthetic consideration that must be coordinated with the surgical team. Spontaneous EMG activity such as bursts and trains during surgery indicates nerve irritation.

<image>Intraoperative neuromonitoring setup illustration showing a patient during spine surgery with all four modalities simultaneously. SSEP electrodes are on the wrists and ankles (stimulating) and scalp (recording). MEP stimulating electrodes are on the scalp with recording needles in hand and foot muscles. EEG leads are on the scalp. EMG needle electrodes are in paravertebral muscles. The neuromonitoring technician's screen shows real-time tracings for each modality with normal waveforms. A communication flowchart between the neurophysiologist, anesthesiologist, and surgeon is shown for when alert criteria are triggered.</image>

Anesthetic Effects on Neuromonitoring

SSEPs

Volatile agents cause a dose-dependent amplitude decrease and latency increase; keeping concentrations below 0.5 to 1 MAC allows reliable monitoring. Nitrous oxide reduces SSEP amplitude and is generally avoided during SSEP monitoring. Propofol causes mild amplitude reduction but is acceptable for monitoring. Opioids have minimal effect on SSEPs and are an ideal component of an IONM anesthetic. Muscle relaxants have no direct effect on SSEPs since the recorded signals are cortical and subcortical.

MEPs

MEPs are the modality most sensitive to volatile agents. Even low concentrations of 0.5 MAC significantly reduce MEP amplitude, which is why TIVA is required for reliable MEP monitoring. Nitrous oxide significantly reduces MEP amplitude and should be avoided. Propofol produces a dose-dependent reduction but is compatible at standard TIVA doses. Opioids have minimal effect, making remifentanil ideal. Muscle relaxants directly suppress CMAPs and must be avoided or used only minimally (maintaining 1 to 2 twitches on train-of-four for EMG, with no paralysis at all for MEPs). Ketamine may augment MEP signals because its NMDA antagonism enhances corticospinal excitability.

EEG

All anesthetic agents affect the EEG in a dose-dependent fashion. Maintaining a stable anesthetic depth is essential for consistent EEG interpretation. Boluses of any agent can cause transient EEG changes that may be confused with ischemia.

Anesthetic Effects on Neuromonitoring Summary

EMG

Muscle relaxants are the primary concern for EMG monitoring, as paralysis abolishes EMG signals. If relaxation is needed for intubation, complete recovery must occur before monitoring begins. The typical approach is to use a short-acting agent such as succinylcholine for intubation and then avoid further relaxants, or to use sugammadex reversal before monitoring starts.

TIVA Protocol for Neuromonitoring

Standard Approach

The standard TIVA protocol for neuromonitoring uses propofol at 75 to 150 mcg/kg/min (titrated to maintain adequate MEP signals) and remifentanil at 0.05 to 0.2 mcg/kg/min, with no volatile agents and no nitrous oxide. No muscle relaxants should be used, or at most a single dose for intubation with full recovery before the baseline is obtained. A ketamine bolus of 0.5 mg/kg or a low-dose infusion may enhance MEP signals when they are marginal. Dexmedetomidine at a low dose of 0.2 to 0.5 mcg/kg/hr may be added and is generally compatible with monitoring.

Troubleshooting Signal Changes

When signal changes occur, the systematic approach begins with ruling out technical issues such as electrode displacement or electrical interference, then ruling out anesthetic causes such as a recent bolus or depth change, followed by ruling out physiologic causes including hypotension, hypothermia, and anemia. If all of these are excluded, the surgeon should be alerted to a potential surgical injury. The surgeon may then adjust retractors, remove instrumentation, or assess perfusion.

Clinical Pearls

TIVA with propofol and remifentanil is mandatory for reliable MEP monitoring; even 0.5 MAC of sevoflurane can abolish MEP signals. SSEPs monitor dorsal columns only; a pure motor deficit (anterior cord syndrome) can occur with preserved SSEPs -- this is why combined SSEP + MEP monitoring is standard for spine surgery. Coordinate with the surgical team regarding neuromuscular blockade: muscle relaxants must be avoided when EMG or MEP monitoring is planned. When a neuromonitoring alert occurs, the anesthesiologist should immediately check: blood pressure, PaCO2, temperature, hematocrit, and recent drug administration before attributing the change to a surgical cause. A soft bite block should always be placed before MEP stimulation to prevent tongue and lip bite injuries. Maintain hemodynamic stability throughout the case; hypotension is a common non-surgical cause of SSEP and MEP signal degradation.

References

• Sloan TB, Heyer EJ. Anesthesia for intraoperative neurophysiologic monitoring of the spinal cord. Journal of Clinical Neurophysiology. 2002;19(5):430-443.
• MacDonald DB, Skinner S, Shils J, Yingling C. Intraoperative motor evoked potential monitoring -- a position statement by the American Society of Neurophysiological Monitoring. Clinical Neurophysiology. 2013;124(12):2291-2316.
• Patel AJ, Ode KM, Engquist EN, et al. Effects of anesthetic agents on monitoring of somatosensory and motor evoked potentials. International Anesthesiology Clinics. 2015;53(1):55-71.
• Lotto ML, Banoub M, Schubert A. Effects of anesthetic agents and physiologic changes on intraoperative motor evoked potentials. Journal of Neurosurgical Anesthesiology. 2004;16(1):32-42.

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