Intraoperative Considerations in the Pediatric Patient

Gloria M. Galloway, MD, FAAN

CNS MYELINATION

The value of neurophysiological intraoperative monitoring (NIOM) in reducing the risk of injury to neural structures has been well documented in the literature (1,2). However, performing NIOM in young children and infants requires special considerations as compared to those of adults. Central nervous system (CNS) myelination is not complete until approximately age 3 years, and in this age group, there is also significant sensitivity to the effects of volatile anesthetic agents.

MULTIMODALITY MONITORING

Somatosensory Evoked Potentials

SSEPs are elicited by electrical stimulation to a mixed peripheral nerve usually and recorded over the dorsal root entry zone, brainstem, and sensory cortical areas. SSEPs correspond to the posterior column medial lemniscus spinal system. This system has synaptic connections in the ventral posterior lateral (VPL) nuclei of the thalamus. From the VPL, fibers traverse the internal capsule to the primary sensory cortex at the postcentral gyrus. Upper extremity representation is along the lateral cortex and representation of the lower extremities is along the medial and parasagittal area. The effect of anesthesia on NIOM is particularly apparent with SSEPs, in which amplitude is reduced and latency is prolonged as an effect. This is most prominently seen with inhalational agents and the effects are most pronounced for potentials generated from the cortex (cortical potentials), with potentials arising from brainstem generators (brainstem potentials) being more resistant to the effects of anesthesia. These effects are also more pronounced in children and the very young. Therefore, attention in the operating room (OR) to the choice and dosing of anesthetic agents is important in order to have successful NIOM in this age group.

Transcranial Electric Motor Stimulation

Brainstem Auditory Evoked Responses

Brainstem auditory evoked responses (BAERs) involve an acoustic stimulus and recording of several waveforms (I–V are typically used in clinical practice) recorded from the scalp. Each waveform has several generators, but for clinical purposes, each is predominately attributed to a specific anatomic source allowing for reasonable interpretation of findings. Because myelination is not complete at a younger age, these waveforms do not reach adult values until approximately age 3 years. Intraoperatively, this may not be significant as long as the waveform is recordable; then subsequent recording will be compared to that baseline value. In cases of Arnold Chiari 2, malformation repair with decompression brainstem auditory monitoring may be useful (6), and in some neurophysiology reports, the use of the blink reflex is considered useful for monitoring as well (7).

Cortical Stimulation

In clinical settings in which epilepsy surgery and brain mapping is needed, higher-intensity stimulation protocols than those in adults will likely be needed in pediatric patients in order to identify the eloquent cortex. For the same reasons outlined earlier when using TcES on a young patient, cortical stimulation requires higher stimulation parameters due to lack of complete myelination of cortical tracts in this age group.

EMG Monitoring

Selective Dorsal Rhizotomy

More commonly in pediatric patients, a procedure referred to as selective dorsal rhizotomy (SDR) is utilized in cases of spasticity nonresponsive to medical management and often in the case of spastic cerebral palsy. At spinal levels L2 to S2, the surgeon divides each dorsal root into individual rootlets followed by selective stimulation of each rootlet in sequence. Neurophysiologic monitoring is performed so that as each rootlet is stimulated, the response can be graded as to the degree of abnormality seen both clinically and by EMG recording. Depending on the degree of abnormality, the surgeon will selectively sever dorsal rootlets in order to decrease facilitatory afferent input. SDR partially removes sensory input so the degree of motor tone is reduced but the patient is not left hypotonic.

Pedicle Screw Placement

Another situation in which EMG monitoring is utilized in pediatric patients involves scoliosis repair typically involving spinal fusion along with pedicle screw placement. In this situation, EMG is recorded from muscles innervated at the levels in which the pedicle screws are placed. The presence of an EMG response or compound muscle action potential (CMAP) after stimulation of the pedicle screw indicates that a breach within the pedicle wall is present and readjustment of the pedicle screw is needed. An intact pedicle wall should not allow the production of a CMAP after stimulation of the screw.

EMG recording from muscles of the lower limbs and anal sphincter may be useful in cases of tethered cord repair and can be used along with SEP recording of the lower limbs.

Tumor Resection

Additionally, EMG monitoring with free-run EMG recording or after nerve stimulation may be used in cases in which cranial or peripheral nerves are at risk of injury such as with tumor resection. In these cases, avoidance of neuromuscular blockade and anesthetic considerations as in other situations in a pediatric patient are needed. In these situations, recording of EMG activity indicates to the surgical team that their surgical interventions are in close proximity to or risking the integrity of neural tissue and that adjusting surgical exploration or resection away from that area would avoid neural injury.

EEG Monitoring

Pediatric AVM

Pediatric Moyamoya Disease

Pediatric Moyamoya disease is a progressive occlusive disease involving the terminal internal carotid artery (ICA) but can be extensive and involve the proximal middle cerebral artery, anterior cerebral artery, and posterior cerebral artery resulting in cerebral ischemia. There may be enlargement of perforating vessels in the basal ganglia in order to compensate for the occlusive disease.

In order to increase cerebral blood flow, revascularization is done most commonly by anastomosing the superior temporal artery to a branch of the middle cerebral artery. During this revascularization procedure, there is significant risk of cerebral ischemia; so multimodality monitoring with EEG and SSEPs can be utilized to assess cortical function during the anastomosis and allow for surgical modifications if needed (8). In the very young patient, EEG patterns vary in regard to rate and amplitude compared to adult patients. In recanalization procedures, the ongoing EEG is compared to the patient’s own baseline intraoperative study for detection of any changes.

Congenital Cardiac Disease

Congenital cardiac disease is a leading cause of mortality in the pediatric population. The incidence of congenital heart defects is 2% to 10% per 1,000 live births. Despite the advancements in technique, congenital heart disease still remains the leading cause of death in all patients with congenital defects and responsible for 10% to 60% rate of neurological compromise during cardiac surgery (9). Cardiopulmonary bypass surgery (CPB) utilizing intraoperative EEG recording along with other measures of functional status have demonstrated usefulness in improving neurological outcomes. EEG is a useful monitoring modality; as it is a sensitive measure of the effects of cerebral blood flow, it can be applied noninvasively and continuously recorded. This appears to be supported by a study by Kimatian and colleagues (10). Their evidence suggested that the addition of NIOM to EEG was associated with a significant change in the intraoperative management of pediatric patients on CPB. Their findings indicated that increases to the use of donor blood were made during the surgery in order to maintain a higher hematocrit level during the bypass period, and this decision was based upon the neurophysiological data. They also noted improvements in postoperative neurologic function with these higher hematocrit levels. Higher hematocrit levels between 25% and 35% have been shown to be associated with improvements in neurological development at 1 year in infants after CPB. The researchers indicated that there were behavioral changes in the surgical team as a result of the NIOM monitoring with a lower tolerance for maintaining a low hematocrit and greater likelihood of using donor blood in these infants during CPB.

Pediatric patients undergoing NIOM require changes to anesthetic technique and in some cases, techniques used for neurophysiologic monitoring as well. Additionally, the clinical scenarios may be particular to their age of onset. In all cases, the goals of NIOM are to reduce the risk of neurological complications and permanent neurological deficit. Selecting the appropriate monitoring modalities, and in many cases multimodality monitoring, is necessary in order to most effectively monitor the neural pathways at risk of injury and prevent neurological compromise.

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   2.  Mittnacht AJ, Rodriguez-Diaz C. Multimodal neuromonitoring in pediatric cardiac anesthesia. Ann Card Anaesth. 2014;17(1):25–32.

   3.  Emerson R. NIOM for spinal deformity surgery: there’s more than one way to skin a cat. J Clin Neurophysiol. 2012;29:149–150.

   4.  Cheng J, Ivan M, Stapleton C, et al. Intraoperative changes in transcranial motor evoked potentials and somatosensory evoked potentials predicting outcome in children with intramedullary spinal cord tumors. J Neurosurg Pediatr. 2014;13:591–599.

   5.  Fulkerson D, Satyan K, Wilder L, et al. Intraoperative monitoring of motor evoked potentials in very young children. J Neurosurg Pediatr. 2011;7(4):331–337.

   6.  Zamel K, Galloway G, Kosnik E, et al. Intraoperative neurophysiologic monitoring in 80 patients with Chiari I malformation: role of duraplasty. J Clin Neurophysiol. 2009;26:70–75.

   7.  Vidmer S, Sergio C, Veronica S, et al. The neurophysiological balance in Chiari type 1 malformation (CM1), tethered cord and related syndromes. Neurol Sci. 2011;32(Suppl 3):S311–S316.

   8.  Lopez J. Neurophysiologic intraoperative monitoring of pediatric cerebrovascular surgery. J Clin Neurophysiol. 2009;26:85–94.

   9.  Jaggers J, Shearer I, Ross M. Cardiopulmonary bypass in infants and children. In: Cardiopulmonary Bypass Principles and Practice. Philadelphia, PA; Lippincott Williams & Wilkins; 2000.

10.   Kimatian S, Saliba K, Soler X, et al. The influence of neurophysiologic monitoring on the management of pediatric cardiopulmonary bypass. ASAIO J. 2008;54:467–469.