Modalities and Surgical Applications

NCS provides multi-modality intraoperative monitoring—aligned with the standard of care—to support optimal outcomes in both routine and complex surgical procedures.

Multi-Modality Monitoring

Using a diversity of proprietary or third-party platforms, NCS employs technology to facilitate the physiological assessment of neural structure integrity and to map neural anatomy during complex procedures—even beyond spine.

Monitoring Modalities

Intraoperative EEG

Electroencephalography (EEG) is the measurement of electrical activity produced by the brain, as recorded from electrodes placed on the scalp. In the extraoperative, clinical setting, EEG can be used as a diagnostic tool for sleep disorders, tumors, encephalopathy, epilepsy, coma, brain death, etc. EEG is the basis for recording some evoked potentials, and has several important uses in IONM.

EMG

Electromyography (EMG) is the measurement of electrical activity produced by skeletal muscles. In the extraoperative, clinical setting, EMG is used to diagnose certain types of peripheral nerve dysfunction (e.g., radiculopathy). In the context of IONM, EMG can be utilized for several purposes, including peripheral nerve monitoring, mapping/identification, and estimating proximity to a nerve. There are two methods for recording EMG in surgery: free-running EMG and stimulus-triggered EMG.

VEP

Visual evoked potentials (VEPs) are electrical responses recorded from the brain’s vision center following presentation of visual stimuli to the eyes.

D-WAVES

Direct waves (D-waves) are a special type of MEP in which the motor system is monitored by recording electrical activity directly from the spinal cord itself. As the volley of electrical activity travels along the corticospinal tract en route to the muscles, it can be recorded from a sub/epidural electrode situated on the dorsomedial surface of the spinal cord. This response is called a “D-wave” because it is the result of direct activation of motor fibers in the brain. D-wave recordings are invasive and typically reserved only for tumors of the spine (intradural/intramedullary or intradural/extramedullary), however, they are not as sensitive to anesthesia, and can be monitored with minimal patient movement.

Motor Mapping

Motor mapping is a special form of DCMEP in which a handheld probe is used to identify very specific regions (~1mm) of the brain that are critical for purposeful movement. The goal is to identify and preserve these regions during brain surgery.

Language Mapping

During awake craniotomy, electrical stimulation is applied to the brain in order to identify and preserve language function. The patient is assessed intraoperatively with a battery of functional speech and language tests selected based on the location of the tumor. Regions typically identified include Broca’s area, Wernicke’s area, and the arcuate fasciculus, which bridges the two.

H-REFLEX

Hoffman reflex (H-reflex) is a monosynaptic response that is recorded from muscle after electrical activation of an afferent nerve (e.g., gastrocnemius CMAP following posterior tibial nerve stimulation). The H-reflex has been used to monitor spinal cord, nerve root, plexus, and peripheral nerve function, and has been used for research as a measure of the level of spinal cord excitability.

TOF

The train-of-four (TOF) is used to assess neuromuscular transmission when neuromuscular blocking agents (NMBA) are given to block musculoskeletal activity. By assessing the depth of neuromuscular blockade, peripheral nerve stimulation can ensure proper medication dosing and thus decrease the incidence of side effects.

TCD

Transcranial doppler (TCD) is a non-invasive, painless ultrasound technique that uses high-frequency sound waves to measure the rate and direction of blood flow inside vessels. The test examines and records the speed (velocity) of the blood flow in cerebral arteries to facilitate the diagnosis of a wide range of conditions affecting the brain.

ECoG

Electrocorticography (ECoG) is similar to EEG in that it measures electrical activity from the cerebral cortex. The critical difference is that ECoG uses electrodes placed directly on the exposed surface of the brain. In the context of IONM, ECoG is used in surgery to identify epileptogenic tissue and to rule out epileptic activity. Subdural grids and depth electrodes can be surgically implanted to evaluate a patient’s brain activity in the epilepsy monitoring unit over a period of time ranging from days to weeks. IONM is not typically used to guide the placement of these chronic recording electrodes.

SSEP

Somatosensory evoked potentials (SSEPs) are electrical responses recorded from the nervous system following electrical stimulation of a peripheral nerve. This activity can be recorded with electrodes positioned along that pathway.

BAER

Brainstem auditory evoked potentials (BAERs) is a series of electrical responses recorded from the auditory pathway between the ear and the upper brainstem following presentation of specific sounds to the ear. BAERs have a variety of uses in the diagnostic setting. In the context of IONM, BAERs are used to evaluate the eighth cranial nerve, the ascending auditory pathway, and the blood flow to the auditory brainstem and cochlea.

TcMEP

Transcranial motor evoked potential (tcMEP) involves electrical stimulation of the motor pathway using subdermal needle electrodes positioned in the scalp above the primary motor cortex. This technique is most frequently used to monitor motor pathways during surgery involving the brainstem, spine, and peripheral nerves.

DCMEP

Direct cortical motor evoked potentials (DCMEPs) involve electrical stimulation of the motor pathway using electrodes placed directly on the surface of the brain in the region of the primary motor cortex. This technique is most frequently used to monitor motor pathways during supratentorial brain surgery when it is most critical to limit the spread of current to the upper layers of cortex in an effort to bracket the site of surgical risk.

Sensory Mapping

When the eloquent cortex is at risk for injury during tumor resection surgery, identification of the central sulcus with the SSEP phase reversal technique (sensory mapping) is the first step toward preserving neurologic function. A grid of recording electrodes is placed directly on the brain, and SSEPs are recorded from regions both anterior and posterior to the central sulcus. SSEP waveforms invert when recordings are made anterior to the central sulcus, called phase reversal.