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Intraoperative Monitoring (IOM)

Intraoperative monitoring is used anytime neurological structures are at risk during surgery.  

The surgeon determines if intraoperative monitoring is medically necessary and a request for monitoring is made.  Monitoring begins on the day of surgery in the preoperative holding area.  A certified and credentialed technologist assess the patient prior to surgery through an interview and neurological exam depending on the type of surgery and neurological structures at risk.  The modalities and the method of intraoperative monitoring is determined by the surgeon, anesthesiologist, and the technologist.  

Once the patient is asleep, the technologist apples stimulating and recording electrodes to the patient.  The patient is electrically grounded, then the electrodes are plugged into a neurophysiological monitoring machine.  Baseline data is collected prior to the beginning of the procedure.  The data is securely sent to an interpreting physician.  The remote physician provides interpretive oversight from baseline to closure of the surgery.

The surgeon is informed if any significant changes in the data is identified.  In addition to the continuous monitoring, the technologist performs other neurophysiological tests during the surgery such as pedicle screw stimulation and electrical stimulation for nerve mapping.  Intraoperative monitoring data collection usually ends following closure of the procedure.  Intraoperative monitoring is completed with a postoperative assessment of the patient.


Intraoperative monitoring is commonly used in spinal
decompressions and fusions.

Somatosensory Evoked Potentials (SSEP)

Somatosensory evoked potentials are commonly used to monitor the sensory pathways from the ankles and wrists, into the spinal cord, and up to the somatosensory cortex in the brain.  Compression and distraction of nerves and spinal cord tissue causes changes in the conduction time and amplitude of the recordings.  These changes are identified by the technologist and reported to the surgeon.

Somatosensory evoked potentials are also used for monitoring spinal and cortical perfusion, and for somatosensory cortex mapping

SSEPs are particularly useful intraoperatively because it is a sensitive and non-evasive way to monitor changes in neurophysiology.

The data shows that both cortical and sub-cortical recordings have increased significantly in latency and significantly decreased in amplitude from baseline.

An increase in latency, or conduction time, up the spinal cord indicates risk of sensory deficit.  A control recording made in the peripheral pathway validates the change has occurred in the spinal cord.*


Motor Evoked Potentials (MEP)
Motor evoked potential monitoring is an important useful tool during spine surgery and cranial surgeries because a deficit to the motor pathway has such an impact on the quality of life.  The motor cortex is electrically stimulated and the signal is monitored down the spinal cord into the distal parts of the arms and legs.

Motor evoked potentials are particularly useful intraoperatively because they provide instant information without the need for signal averaging.  Monitoring the motor pathways is useful for providing information about the central nervous system, as well as the peripheral nervous system.

The image to the right shows several instances with loss of motor response during which the surgeon tighten cervical wires for a congenital deformity correction.  Each instance is accompanied by a report to the surgeon.








In addition to bilateral upper and lower extremity monitoring, IOM uses time-lapse stacks to monitor trend changes.*
Elelectromyography (EMG)

Electromyography is used to monitor spinal nerve roots, cranial nerves, and peripheral nerves. Spontaneous EMGs provide continuous and instant information.  Mechanical changes to a nerve causes a deflection in the recording.

Electromyography is also used intraoperatively with electrical stimulation.  A common use for triggered EMGs is during pedicle screw placement.  By electrically stimulating a pedicle screw placed during spinal fusion surgery, the proximity of the screw to nerve tissue is quantified by the amount of current it takes to elicit a response.  Triggered EMGs are also useful for the identification of neuronal anatomy that has been distorted by tumor or cancer cells, such as in Ear, Nose, and Throat surgery.

The recording on the left shows continuous trigeminal nerve EMG activity during a mastoidectomy.  Other significant deflections include high amplitude spikes and burst activity.

Continuous trigeminal nerve EMG activity during a mastoidectomy.*


Brainstem Auditory Evoked Potentials (BAEP)
Monitoring of the auditory nerve during surgeries such as cerebellopontine angle surgery is important because BAEPs monitor the patient's hearing as well as brainstem functions such as balance and walking.

The auditory nerve is typically stimulated by alternating clicks through earphones.  The response is recorded at the ipsilateral and contralateral ears.  Each waveform in the recording corresponds with anatomy in the auditory pathway.  Waves I-V are identified by the technologist and are monitored for changes in absolute and inter-wave latencies.

In the recording to the right, the black trace is the actively recording trace, the red trace is the last averaged recording, and the blue trace is the patient's intraoperative baseline.

A typical brainstem auditory evoked potential recording is an averaged signal from the ear to a cortical reference point.  Both ipsilateral and contralateral recordings are used to identify Waves I-V.*
Electroencephalography (EEG)









Electroencephalography is used to measure cortical activity.  Recording electrodes are arranged in the scalp using a bipolar or referential montages that record a trace with power and frequency characteristics.  Changes in power or frequency of an electroencephalograph such as in carotid endarterectomy surgery monitoring can identify and localize an cortical ishemic event.

A computerized spectral array in the image to the left is commonly used to intraoperatively monitor EEGs.  A change in the spectral edge would indicate a slowing or EEG frequency with a particular montage or montages. 

Electroencephalography is also used to detect burst-suppression in surgeries that require neuronal protection.  A slowing of metabolism is induced and intraoperative EEGs are used to detect the resulting change in neurophysiology indicating when the medication has taken effect.

Typical intraoperative EEG recordings are monitored as live traces as well as processed digital data.*


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* All neurophysiological data collected by Ervin Caballes, CNIM