Intraoperative Neurophysiological Monitoring : A Review of Techniques Used for Brain Tumor Surgery in Children

Abstract

Intraoperative monitoring (IOM) utilizes electrophysiological techniques as a surrogate test and evaluation of nervous function while a patient is under general anesthesia. They are increasingly used for procedures, both surgical and endovascular, to avoid injury during an operation, examine neurological tissue to guide the surgery, or to test electrophysiological function to allow for more complete resection or corrections. The application of IOM during pediatric brain tumor resections encompasses a unique set of technical issues. First, obtaining stable and reliable responses in children of different ages requires detailed understanding of normal ageadjusted brain-spine development. Neurophysiology, anatomy, and anthropometry of children are different from those of adults. Second, monitoring of the brain may include risk to eloquent functions and cranial nerve functions that are difficult with the usual neurophysiological techniques. Third, interpretation of signal change requires unique sets of normative values specific for children of that age. Fourth, tumor resection involves multiple considerations including defining tumor type, size, location, pathophysiology that might require maximal removal of lesion or minimal intervention. IOM techniques can be divided into monitoring and mapping. Mapping involves identification of specific neural structures to avoid or minimize injury. Monitoring is continuous acquisition of neural signals to determine the integrity of the full longitudinal path of the neural system of interest. Motor evoked potentials and somatosensory evoked potentials are representative methodologies for monitoring. Free-running electromyography is also used to monitor irritation or damage to the motor nerves in the lower motor neuron level : cranial nerves, roots, and peripheral nerves. For the surgery of infratentorial tumors, in addition to free-running electromyography of the bulbar muscles, brainstem auditory evoked potentials or corticobulbar motor evoked potentials could be combined to prevent injury of the cranial nerves or nucleus. IOM for cerebral tumors can adopt direct cortical stimulation or direct subcortical stimulation to map the corticospinal pathways in the vicinity of lesion. IOM is a diagnostic as well as interventional tool for neurosurgery. To prove clinical evidence of it is not simple. Randomized controlled prospective studies may not be possible due to ethical reasons. However, prospective longitudinal studies confirming prognostic value of IOM are available. Furthermore, oncological outcome has also been shown to be superior in some brain tumors, with IOM. New methodologies of IOM are being developed and clinically applied. This review establishes a composite view of techniques used today, noting differences between adult and pediatric monitoring.

Keywords: Brain neoplasm; Child; Monitoring, Intraoperative; Neurosurgery.

Fig. 1.

Generation of motor evoked potentials at different levels of brain. Depending on the intensity of stimulation and the montage of electrode, motor evoked potentials are generated at different levels of brain. Superficial white mater just beneath the motor cortex, deep white matter of internal capsule, and pyramidal decussation are known to be major sites to be excited.

Fig. 2.

Median sensory evoked potential (SEP) phase reversal. A : A strip electrode is placed perpendicularly across the approximate central sulcus. B : After stimulation of the contralateral median nerve at the wrist, median SEPs are recorded on the strip electrode; between the third and fourth electrode, SEP phase is reversed, which indicate physiological central sulcus.

Fig. 3.

Neuro-navigation for brain tumor surgery. Neuro-navigation system provides 3-D image position guidance of anatomical location. However, it cannot re- flect anatomical shifts during operation from positioning, tissue removal, or edema and it does not provide neurophysiological or functional information.

Fig. 4.

A case of direct cortical stimulation. To map the motor cortex in proximity to a lesion, direct cortical stimulation is applied.

Fig. 5.

Stimulation techniques for motor evoked potentials. Schematic illustration shows multi-pulse train stimulation (A) and single-pulse stimulation (Penfield technique) (B). Multi-pulse train stimulation requires lower intensity of stimulation and poses less risk of seizure than single-pulse stimulation.

Fig. 6.

Setting for visual evoked potential. Visual evoked potential can monitor the visual pathway. A : Light stimulation is applied via goggles. B : Needle electrodes are placed at the scalp of the occipital cortex and record visual evoked potentials.

Fig. 7.

Cranial nerve monitoring for infratentorial tumor surgery. For intraoperative monitoring of infratentorial tumor surgery, multi-modal monitoring should be combined including monitoring for the cranial nerves at risk. In the figure, motor evoked potentials (MEPs) of cranial nerve V, VII, X, XI, and XII are being recorded. Red boxes indicate disappearance of left laryngeal and left facial muscle MEPs. Of note, single pulse stimulation is immediately followed by multi-pulse stimulation, to rule out current spread in corticobulbar MEP.