CLINICAL APPLICATIONS OF SSEPs IN CHILDREN
Brain Injury in SSEPs
Abnormalities of median SSEPs can be predictive of poor prognosis in the situation of brain injury due to head trauma or hypoxia. A loss of bilateral SSEP scalp waveforms, as shown in Figure 6.16B, portends a poor prognosis in comatose children (145-149). Asymmetric scalp responses in a comatose child may be associated with the development of motor abnormalities such as hemiparesis because of the proximity of the sensory cortex to the motor cortex (see Figure 6.17B). A recent study compared the predictive powers of clinical examination (pupillary responses, motor responses, and Glasgow Coma Scale [GCS]), EEG, and computed tomography (CT) to that of somatosensory evoked potentials (SEPs) in a systematic review. SEPs appear to be the best single overall predictor of outcome (116). Posterior tibial nerve SSEPs performed
FIGURE 6.16 Median nerve somatosensory-evoked potentials (SSEPs) obtained in the pediatric intensive care unit. Channels 1-4 are responses with left median nerve stimulation, and channels 5-8 are responses with right median stimulation. Channels 1 and 5 are scalp responses (C4' and C3' referenced to Fz); channels 2 and 6 are brain (C4' and C3' referenced to linked mastoids); channels 3 and 7 are lower cervical spine responses (C7 spine referenced to Fz); channels 4 and 8 are peripheral responses obtained at the axillae. (A) Normal median SSEP responses obtained from a child with an epidural hematoma who was paralyzed with vecuronium for intracranial pressure control. There is no evidence of myelopathy. The child later recovered with minimal sequelae. (B) Abnormal median SSEP responses in a comatose child with severe brain injury and C1-C2 vertebral injuries. Note the bilaterally abnormal scalp reponses. Brainstem, C7 spine, and peripheral responses show no evidence of a spinal cord injury affecting posterior column pathways.
on neonates at high risk of future neurodevelopmental impairment have demonstrated a highly significant relationship between bilaterally abnormal posterior tibial nerve SSEPs and the presence of cerebral palsy at 3 years of age (150). Normal posterior tibial nerve SSEPs were associated with a normal outcome in 24 of 25 infants. In this study, posterior tibial nerve SSEPs were more predictive than cranial ultrasound. Another study of 43 children with hemiplegic cerebral palsy found a positive correlation between median nerve SSEPs and the affected side using the amplitude of the responses rather than the latency (151). Other studies have confirmed the prognostic value of SSEPs in infants at risk for neurodevelopmental impairment (152-156).
Traumatic Spinal Cord Injury
SSEP results combined with early American Spinal Injury Association (ASIA) motor scores have been shown to predict ultimate ambulatory capacity in patients with acute SCI (157,158). Other authors have shown that SSEP improvement over a 1-week interval during the first 3 weeks after SCI was associated with motor index score improvement over a 6-month period (159). Both ASIA scores and MEP recordings are similarly related to the outcome of ambulatory capacity and hand function in patients with SCI. Dermatomal somatosensory evoked potentials have also been shown to be more sensitive for the detection of sacral sparing and of more prognostic value than mixed nerve somatosensory evoked potentials (160). However, somatosensory evoked potentials and dermatomal SSEPs have been shown to add little or no useful prognostic information to the initial physical examination in either complete or incomplete SCI patient groups (161).
The author has a great deal of experience utilizing somatosensory evoked potentials in the pediatric ICU to evaluate for SCIWORA (162) in the situation where children are comatose, or too obtunded to cooperate with the examination, or the child's age precludes a detailed sensory examination. Figure 6.17A shows an example of a normal tibial SSEP, whereas Figure 6.17C demonstrates the impaired posterior column conduction between the lower cervical spinal cord and brainstem with a SCIWORA injury sustained by a 4-year-old child.