Tethered Cord Syndrome

Posterior tibial SSEPs have been shown in some studies to be a sensitive indicator of declining neurophysiologic status and a more sensitive diagnostic tool than the clinical testing of sensation in patients with tethered spinal cord post myelomeningocele repair (163-166). In addition, improvement of the evoked potentials after untethering has been documented (163,164,166). In the author's experience, the spine response is often caudally displaced in myelomeningocele. Absent or reduced amplitude lumbar spine potentials or prolonged lumbar spine or scalp latencies with tibial nerve stimulation in the setting of normal median somatosensory evoked potentials (normal spine latencies and amplitudes with median nerve stimulation, normal cervical to brain central conduction time, and normal median scalp latencies) have been suggested to be indicators of electrophysiologic impairment due to tethered cord syndrome.

In the most comprehensive study to date, 90 children were followed with serial peroneal SSEPs after a repair of their spinal dysraphic lesions with the objective of evaluating whether SSEPs were a useful way of monitoring these children to facilitate early detection of clinically significant retethering. Three hundred and nine studies were performed on these children yielding a mean of 3.4 studies per patient. The median time between SSEP studies was 13 months. A clinical examination was performed at the time each SSEP was done. There was a false-positive rate of 71% and a false-negative rate of 43%. It was concluded that serial SSEPs do not correlate well with clinical status and are not a useful modality for monitoring patients at risk for retethering (167). The author has followed a large population of children with myelomeningocele for decades and similarly has not found mixed nerve SSEPs to be useful in the evaluation of secondary tethered spinal cord after myelomeningocele repair.

Intraoperative Spinal Monitoring

There are many reports detailing the usefulness of intraoperative SSEP monitoring during scoliosis surgery (168-172), as well as during other surgical procedures of the spine. The limitation of SSEPs is the fact that they only monitor afferent pathways in the dorsal columns. Over the past decade, intraoperative spinal monitoring has evolved to include monitoring of the motor pathways. The corticospinal tracts are now being routinely monitored intraoperatively using transcranial electrical stimulation of the motor cortex (172), with motor evoked potentials recorded from either peripheral motor axons or as a CMAP from innervated muscles. Transcranial electrical MEPs to monitor the corticospinal motor tracts are now used routinely in addition to SSEPs for the detection of emerging SCI during surgery to correct spine deformity or resect intramedullary tumors (63-65).

Brachial Plexus Injury

The dermatomal SSEP can be a useful supplement to the assessment of the child with a brachial plexus injury (173). The child needs to be awake during the study. The C5 and C6 dermatomal SSEPs are generally most useful in the author's experience. The C5 dermatome is stimulated over the lateral proximal shoulder using a proximal disk as the cathode and a distal disk as the anode.

Tibial somatosensory-evoked potentials (SSEPs) obtained in the pediatric intensive care unit. Channels 1-4 are responses with left tibial stimulation, and channels 5-8 are responses with right tibial stimulation. Channels 1 and 5 are scalp responses (C2

FIGURE 6.17 Tibial somatosensory-evoked potentials (SSEPs) obtained in the pediatric intensive care unit. Channels 1-4 are responses with left tibial stimulation, and channels 5-8 are responses with right tibial stimulation. Channels 1 and 5 are scalp responses (C2' to Fz); channels 2-7 are spine responses (L2 spine referenced to flank); and channels 4 and 8 are peripheral responses obtained at the popliteal fossa. (A) Normal tibial SSEP study. (B) Abnormal tibial SSEPs in a child with left hemispheric brain injury. Peripheral and lumbar spine (L2 and T12 level) responses are normal bilaterally. The scalp response is normal with left tibial nerve stimulation (channel 1), but absent with right tibial nerve stimulation (Channel 5). (C) Abnormal tibial SSEPs bilaterally in an awake 4-year-old with low cervical spinal cord injury without radiographic abnormality. Peripheral (channels 4 and 8) and L2 spine (channels 2 and 7) responses are normal. Scalp responses (channel 2 and 5) are absent as a result of the low cervical spinal cord injury.

Intraoperative SSEPs with direct stimulation of exposed nerves may demonstrate incomplete injuries of upper cervical roots, a proximal stump of the ruptured C5 root with functional central continuity (thus, potentially suitable for grafting), or complete root avulsion. Preoperative diagnostic SSEPs, while a useful adjunct to conventional electrodiagnosis, do not enable one to discriminate incomplete cervical root avulsion from intact roots (174).

Demyelinating Diseases

Both SSEPs and brainstem auditory evoked potentials have been reported to be abnormal in children with, or carriers of, leukodystrophy (175,176). Peripheral and/ or central abnormalities have been documented in metachromatic leukodystrophy Pelizaeus-Merzbacher disease, Krabbe disease, adrenoleulodystrophy, Canavan disease, Alexander disease, and multiple sulfatase deficiency (177).

Pediatric multiple sclerosis (MS), while relatively rare, does occur in preadolescents and adolescents (178). MRI has been shown to be slightly more sensitive than multimodal evoked potentials in confirming the clinical diagnosis of childhood MS (179). However, in suspected or probable MS, both SSEPs and visual evoked potentials may contribute to the determination of clinical diagnosis because of their capacity to demonstrate asymptomatic involvement in central somatosensory and central optic nerve pathways (180).

Acute transverse myelitis often results in severe myelopathy due to inflammation and demyelination. SSEPs have been shown to be abnormal in this condition and may provide prognostic information regarding the ultimate outcome (181).

The extent and location of nerve involvement in demyelinating peripheral neuropathies have been evaluated with SSEPs; however, SSEPs do not usually provide necessary additional information to standard nerve conductions. Hereditary motor-sensory neuropathy type I shows impaired peripheral conduction in both proximal and distal nerve segments with normal central conduction (182). AIDP patients have been shown to exhibit prolonged posterior tibial peripheral SSEP latencies in addition to prolonged or absent median F-waves. However, posterior tibial F-wave latencies and median nerve SSEPs were less sensitive studies for the detection of demyelination in AIDP (183). SEP can detect an abnormality and thus support the clinical diagnosis of GBS in the acute stage when the results of more conventional tests are inconclusive (184).

CONCLUSION

Pediatric EDSs are a useful diagnostic tool that aids in the localization of abnormalities within the lower motor neuron and often provide helpful prognostic information. EDSs have been less and less utilized in the diagnosis of many myopathic disorders and anterior horn cell diseases due to the importance of molecular genetic studies and/or muscle biopsy for the determination of disease subtypes. However, there remains a use for EMG and NCSs in many focal and generalized lower motor neuron conditions. For children suspected of having hereditary neuropathies, with no family member possessing genetic confirmation, a directed NCS may guide the acquisition of more specific and less costly molecular genetic studies. In other conditions such as GBS and focal neuropathic conditions, EDSs remain critical for diagnostic confirmation.

Practical suggestions relating to the pediatric electrodiagnostic evaluation have been provided. Study results must be interpreted in light of developmental and maturational issues affecting both clinical findings and electrophysiologic processes. A skilled electrodiagnostic evaluation utilizes careful strategic planning to provide the most important diagnostic information needed in an expeditious manner, with the least distress possible to the child and parent. Ongoing electrodiagnostic experience with the pediatric population provides increasing diagnostic acumen regarding pediatric lower motor neuron disease processes and sufficient technical skills to provide the referring physician with accurate diagnostic information.

 
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