SPINAL CORD INJURY
Neonatal spinal cord injury (SCI) may occur as an obstetrical complication or as a result of a vascular insult to the spinal cord. Typical clinical presentation may include findings of diffuse hypotonia, possible respiratory distress, hyporeflexia, and urinary retention. Bilateral flaccid paralysis of the upper extremities may be a sign of neonatal SCI (62). An anterolateral SCI due to a vascular insult will produce EMG findings of severe denervation in diffuse myotomes. Typically, 2 to 3 weeks may lapse before fibrillations and positive sharp waves are elicited. Anterior horn cell and axonal degeneration will typically result in decreased CMAP amplitudes in multiple peripheral nerves. SNAP amplitudes are spared. Somatosensory evoked potentials may be spared if posterior columns are preserved.
Traumatic SCI often results in loss of anterior horn cells at a specific "zone of injury." For example, a child with C5 tetraplegia may have denervation present at the bilateral C6 and C7 myotomes. This zone of partial or complete denervation becomes particularly relevant in the evaluation of a patient for possible placement of an implanted functional electrical stimulation system for provision of voluntary grasp and release. The presence of denervation necessitates concomitant tendon transfers with electrical stimulation of the transferred muscle group.
Somatosensory evoked potentials (SSEPs) may help establish a sensory level in an infant or young child with SCI and is also useful in the evaluation of the comatose or obtunded child at risk for SCI without radiographic abnormality (SCIWORA). Somatosensory evoked potentials are discussed in the following.
Transcranial electrical MEPs to monitor the corticospinal motor tracts directly are now used routinely in addition to SSEPs for detection of emerging SCI during surgery to correct spine deformity or resect intramedullary tumors (63-67). Afferent neurophysiologic signals can provide only indirect evidence of injury to the motor tracts since they monitor posterior column function. Transcranial electrical MEPs are exquisitely sensitive to altered spinal cord blood flow due to either hypotension or a vascular insult. Moreover, changes in transcranial electrical MEPs are detected earlier than are changes in SSEPs, thereby facilitating more rapid identification of impending SCI.
BRACHIAL PLEXUS AND CERVICAL NERVE ROOT LESIONS
Traumatic obstetrical brachial plexopathy or "neonatal brachial plexus palsy" (NBPP) usually results from traction on the brachial plexus (predominantly upper trunk) and its associated spinal roots. This can lead to stretching or rupture of the trunks of the plexus and/or partial axonotmesis or avulsion of the spinal roots. The most common cause is a shoulder dystocia of the anteriorly presenting shoulder causing excessive lateral neck traction. Injury to the upper trunk of the brachial plexus, and/or C5 and C6 cervical roots is the more common injury known as Erb-Duchenne palsy. Damage to the lower trunk, and/or C8-T1 cervical roots, is referred to as Klumpke's palsy. Severe brachial plexus injuries may involve the entire plexus and C5-T1 nerve roots diffusely. Horner's syndrome due to injury of the C8 and Tl roots and the superior cervical sympathetic ganglion may be an associated clinical finding. An isolated Klumpke's palsy is rare in the setting of traumatic birth palsy and usually results from a fall onto a hyperabducted shoulder, penetrating trauma, or tumor.
EDSs help determine the location (root and/ or plexus), extent, and severity of the brachial plexus injury. Examination should be deferred until at least 3 to 4 weeks after the injury to allow for abnormal spontaneous rest activity (fibrillations and positive sharp waves) to develop in the setting of denervation and axon loss (see Figure 6.8). Complete injuries are characterized electromyographically by absent MUAPs and absent CMAP amplitudes in peripheral nerves supplied by the transected axons. In the setting of total motor paralysis, motor NCSs with measurement of the amplitude of the CMAPs in distal and proximal muscles provide useful prognostic information. For example, the preservation of the CMAP amplitude 10 days or more after the injury with complete clinical paralysis suggests that the damage is, in part, a neuropraxic injury with better prognosis. In this setting, F-waves are absent. If motor function is absent and no MUAPs are observed, examination of the amplitude of the SNAPs in the dermatomal distribution
FIGURE 6.8 Fibrillation potential (A) and positive sharp waves (B) indicative of acute denervation and axon loss.
of the branches of the affected brachial plexus trunks can help distinguish injuries to the plexus from severe cervical root injuries or avulsions. The sensory dorsal root ganglion lies in the intervertebral foramen distal to the damaged segment with a root injury, leaving the sensory axon projection from the dorsal root ganglion to the limb intact. Thus, the SNAP is obtainable in the setting of a root avulsion with absent clinical sensation.
In the setting of Erb's palsy, assessment of a superficial radial sensory or median sensory response to the index finger is useful in making a distinction between a C6 root avulsion and a more distal lesion involving the trunk of the brachial plexus. The median SNAP to the middle finger provides information about the integrity of C7 axon projections distal to the dorsal root ganglion. The presence or absence of an ulnar SNAP can help distinguish a lower trunk injury from a C8 nerve root injury.
In perinatal traumatic brachial plexopathy, positive sharp waves and fibrillations, indicative of true denervation, can be found by 14 to 21 days after injury (68). The absence of fibrillations or positive sharp waves after this time frame suggests a neuropraxic lesion with intact axons. In this setting, the prognosis for recovery is favorable. Early in the course of recovery prior to reinnervation, the interference pattern usually is reduced or discrete and recruitment frequencies increased into the neuropathic range (often >20 Hz). A follow-up needle EMG evaluation 3 to 6 months after the injury is useful to determine subclinical evidence of reinnervation. Such reinnervation is typically characterized initially by "nascent" polypha-sic MUAPs (see Figure 6.9). With reinnervation, the numbers of positive sharp waves and fibrillations decrease over time, amplitude of MUAPs increases as collateral spouting occurs, and with evaluation of the interference pattern, there is an observed increasing number of voluntary MUAPs.
The author prefers to initially obtain sensory NCSs (occasionally with sedation) consisting of a median sensory NCS recorded from the index finger (C6 dermatome), a median sensory NCS recorded from the middle finger (C7 dermatome), and an ulnar sensory NCS recorded from the fifth digit (C8 dermatome). Median and ulnar motor NCSs are useful to evaluate the integrity of axons traveling through the lower trunk. Axillary
FIGURE 6.9 Polyphasic motor unit action potential (MUAP) with a neuropathic firing frequency at 25 Hz. These polyphasic MUAPs obtained 4 months after brachial plexus injury are indicative of reinnervation.
and musculocutaneous motor NCSs (with assessment of CMAP amplitudes) are useful if an upper trunk injury is suspected. These CMAP amplitudes may be compared to the intact side depending on patient tolerance of the study (69). A CMAP amplitude reduction of more than 90%, compared to the unaffected side, predicted severe weakness of the corresponding root level. During the EMG study of the deltoid, the examiner should assess the clinical sensation of the C5 dermatome. The use of dermatomal and mixed nerve SSEPs in brachial plexus injuries is discussed in the following.
In addition to a complete needle EMG screen of upper extremity muscles clinically affected, electromyographic examination of the infraspinatus or supraspinatus can help localize an upper trunk injury proximal to or distal to the takeoff to the suprascapular nerve. While the examination of the rhomboid can be difficult in the infant, a finding of fibrillations or positive sharp waves supports the presence of a C5 root injury. While in the adult, electromyographic evaluation of the cervical paraspinal muscles may help evaluate the extent and severity of cervical root injuries, generally these muscles are extremely difficult to study in the infant due to poor relaxation. In the young child, adequate relaxation of the cervical paraspinal muscles may be obtained with general anesthesia but this is usually not necessary and does not influence management. In addition, study of the serratus anterior and rhomboids (typically performed to assess involvement of C5 and C5-7 roots, respectively) may be technically difficult in the infant due to intact sensation, the presence of trapezius overlying the rhomboids, depth of the rhomboids and serratus anterior, and the risk that sudden movement may cause penetration of the needle into the pleural space. Usually a combination of needle EMG evaluation, sensory and motor conduction studies, and F-wave studies allows the electromyographer to determine the location and severity of the injury.
The natural history of conservatively managed brachial plexus birth palsy has been reported (69). Seventy-two percent of those referred for rehabilitation evaluation showed stable functional status at follow-up. There has been a resurgence of interest in surgical exploration of obstetrical brachial plexus palsy with external and internal neurolysis, neurotization, and in selected cases, nerve grafting (70-76). EMG evaluation at approximately 4 to 9 months postinjury may support the possible utility of a surgical exploration for neurolysis, neurotization, and/or nerve grafting if there is limited electrophysiologic evidence of reinnervation. Some authors suggest a repeat study within 3 months of the injury (77). Preoperative EDSs and intraoperative NCSs and somatosensory evoked potentials are helpful in the surgical decision making. Preoperative and/or intraoperative somatosensory evoked potentials may provide evidence of upper cervical root avulsion versus partial trunk and nerve root integrity as discussed in the following.
More recently, EMG and NCSs have been used in a more systematic fashion for surgical decision making. In two recent reports (78,79), infants with NBPP with severe lesions could be identified at 1 month of age by testing elbow extension and elbow flexion, and recording MUPs in the biceps muscle. Forty-eight infants were prospectively studied. The presence or absence of flexion paralysis at around 1 week (median 9 d; range 5-17 d), 1 month (median 31 d; range 24-53 d), and 3 months of age (median 87 d; range 77-106 d) was noted for clinical (shoulder external rotation, elbow flexion, extension, and supination) and EMG parameters (denervation activity, MUPs, and polyphasic MUPs in the deltoid, biceps, and triceps muscles). At 1 month, the absence of biceps MUPs had a sensitivity of 95% for later flexion paralysis, and the absence of deltoid MUPs had a sensitivity of 100% for flexion paralysis; the false-positive rates for the same findings were 21% and 33% respectively. EMG at 3 months was highly misleading as MUPs were seen in 19 of 20 clinically paralytic biceps muscles. These authors have implemented a decision rule that children without active elbow extension at 1 month should be referred to a specialized center, while children with active elbow extension as well as active flexion should not. When there is active elbow extension, but no active elbow flexion, the authors concluded an EMG was needed; the absence of MUPs in the biceps muscle was an indication for referral.
In another retrospective study of preoperative EDSs and computed tomography myelogram (CTM; 80) conducted in 21 children, the sensitivity of EDSs and CTM for detecting a postganglionic rupture was 92.8% (95% CIs [0.841-0.969]) and 58.3% (95% CIs [0.420-0.729), respectively. The sensitivity for EDSs and CTM for preganglionic nerve root avulsion was 27.8% (95% CIs [0.125-0.509]) and 72.2% (95% CIs [0.491-0.875]), respectively. In cases in which both CTM and EDSs gave concordant results, the sensitivity for both modalities combined was 50.0% (95% CIs [0.237-0.763]) for avulsion and 80.8% (95% CIs [0.621-0.915]) for rupture. Overall, EDSs were most useful in identifying ruptures, particularly in the upper plexus, whereas CTM was most sensitive in identifying avulsions in the lower plexus. It was concluded that both EDSs and CTM scans must always be interpreted in the context of a comprehensive evaluation of the patient. They provide supplemental information (in addition to the physical examination) for early detection of nerve root rupture and avulsion injuries, aiding surgical decision making and preoperative planning for neonatal NBPP
Chin and colleagues (81) recently reported on the prognostic significance of intraoperative EMG in NBPP. They investigated the predictive value of intraoperative neurophysiologic investigations in a total of 32 infants of 206 referred to their center who underwent exploration of the plexus, including neurolysis. The findings from intraoperative electromyography, sensory evoked potentials across the lesion, and gross muscular response to stimulation were evaluated. Outcomes were assessed with the modified mallet score at 3 to 4 years. The positive predictive value and sensitivity of the intraoperative EMG for C5 grafting and neurolysis were 100% and 85.7%, respectively, in infants without concurrent shoulder pathology. The positive and negative predictive values, sensitivity, and specificity of the three investigations combined were 77%, 100%, 100%, and 57%, respectively. In all, 20 infants underwent neurolysis alone for C6 and three had reconstruction. All of the former and one of the latter achieved biceps function of Raimondi grade 5. The positive and negative predictive values, sensitivity, and specificity of electromyography for C6 were 65%, 71%, 87%, and 42%, respectively. The method was concluded to be effective in evaluating the prognosis of C5 lesions. Neurolysis was preferable for C6 lesions.