SEPs and CNS magnetic stimulation in syringomyelia

The Role of Neurophysiology in Managing Patients with Chiari Malformations

Chiari malformation type 1 (CM1) includes various congenital anomalies that share ectopia of the cerebellar tonsils lower than the foramen magnum, in some cases associated with syringomyelia or hydrocephalus. CM1 can cause dysfunction of the brainstem, spinal cord, and cranial nerves. This functional alteration of the nervous system can be detected by various modalities of neurophysiological tests, such as brainstem auditory evoked potentials, somatosensory evoked potentials, motor evoked potentials, electromyography and nerve conduction studies of the cranial nerves and spinal roots, as well as brainstem reflexes. The main goal of this study is to review the findings of multimodal neurophysiological examinations in published studies of patients with CM1 and their indication in the diagnosis, treatment, and follow-up of these patients, as well as their utility in intraoperative monitoring.

Effects of syringomyelia on electrodiagnostic test results in Cavalier King Charles Spaniels

OBJECTIVE

To determine the effects of syringomyelia on electromyography (EMG) findings, somatosensory-evoked potentials (SEPs), and transcranial magnetic motor-evoked potentials (TMMEPs) in Cavalier King Charles Spaniels (CKCSs).

ANIMALS

27 client-owned CKCSs that underwent prebreeding magnetic resonance imaging screening or investigation of clinical signs consistent with syringomyelia.

PROCEDURES

In dogs with (n = 11) and without (16) magnetic resonance imaging-confirmed syringomyelia, the median nerve in each thoracic limb was stimulated and SEPs were recorded over the C1 vertebra; onset latency and latency and amplitude of the largest negative (N1) and positive (P1) peaks were measured. The TMMEPs were recorded bilaterally from the extensor carpi radialis and tibialis cranialis muscles; onset latencies in all 4 limbs were measured. Bilateral systematic needle EMG examination was performed on the cervical epaxial musculature, and the number of sites with spontaneous activity was recorded.

RESULTS

In dogs with syringomyelia, amplitudes of N1 and P1 and the amplitude difference between P1 and N1 were significantly smaller than those recorded for dogs without syringomyelia (approx 2-fold difference). No difference in SEP latencies, TMMEP latencies, or the proportion of dogs with > 2 sites of spontaneous activity detected during EMG examination was detected between groups.

CONCLUSIONS AND CLINICAL RELEVANCE

Results indicated that SEP amplitude at the C1 vertebra was a more sensitive measure of spinal cord function in CKCSs with syringomyelia, compared with results of EMG or TMMEP assessment. Measurement of SEP amplitude may have use as an objective assessment of the evolution and treatment of this disease.

The use of transcranial magnetic stimulation in the clinical evaluation of suspected myelopathy

Central motor conduction time (CMCT) and motor evoked potential (MEP) latencies measured by using transcranial magnetic stimulation (TMS) are parameters used to evaluate electrophysiologic function of the corticospinal motor tract. We present 5 cases to illustrate how the use of TMS had contributed to clinical management. CMCT and MEP latency measurements were found to be useful in determining the significance of lesions seen on neuroimaging and helped clinical decisions in the presence of multiple lesions or multiple clinical conditions that cause similar clinical manifestations. TMS study is particularly useful in localizing levels of conduction defect.

Cutaneous silent periods in intramedullary spinal cord lesions

OBJECTIVE

The neurophysiological assessment of intramedullary spinal cord lesions has been unsatisfactory. Previous studies in patients with syringomyelia suggest that testing of cutaneous silent periods (CSPs) may be useful to assess centromedullary lesions.

METHODS

The authors studied nine patients with intramedullary spinal cord lesions of different etiologies. Eight patients with cervical lesions presented with hypalgesia, hypothermesthesia, or pain in at least one upper extremity; five of them had also upper limb weakness or sensory impairment. One patient with a thoracic lesion had normal upper limb function. The authors recorded CSPs in abductor pollicis brevis muscle following digit II and digit V stimulation. Somatosensory evoked potentials (SEPs) were obtained following median and tibial nerve stimulation. Motor evoked potentials (MEPs) were obtained in biceps brachii, abductor digiti minimi and tibialis anterior muscles following transcranial magnetic or electrical stimulation.

RESULTS

CSP abnormalities were found in all patients with cervical lesions, but not in the patient with a thoracic lesion. Cortical median nerve SEPs had normal latencies in all patients, while tibial nerve SEPs, upper limb MEPs, and lower limb MEPs were delayed in five patients each. In one patient, abnormal CSP were the only neurophysiological finding. CSP abnormalities were associated with hypalgesia and hypothermesthesia in 95% of the studies.

CONCLUSION

Upper extremity CSP testing is a sensitive neurophysiological technique for the assessment of cervical intramedullary lesions. In particular, abnormal CSPs are highly associated with spinothalamic dysfunction.

Attenuation of somatosensory evoked potentials during positioning in a patient undergoing suboccipital craniectomy for Chiari I malformation with syringomyelia

Intraoperative electrophysiologic monitoring can diminish the risk of neurologic injury by enabling the detection of injury at a time when it can be reversed or minimized. This report describes a 14-year-old girl with a Chiari's malformation type I and syringomyelia who underwent a suboccipital decompression and dural patch grafting with concurrent somatosensory evoked potentials. When the patient was turned into the prone position and the neck was flexed, the left-sided somatosensory evoked potential deteriorated. After the patient's neck was repositioned, the left median nerve potential improved but did not return to baseline. Postoperatively, the patient had decreased proprioception of her left arm, which completely resolved at 2-week follow-up. This single case report does not establish the need for routine somatosensory evoked potential monitoring. Nevertheless, deterioration of the potential in this case led directly to a change in the surgical positioning, which may have significantly reduced the chances of a permanent neurologic injury.

Periodic limb movements in syringomyelia and syringobulbia

Twenty-six patients with syringomyelia were studied with polysomnography to determine the frequency of periodic limb movements (PLM) and its relationship to the presence of a Chiari anomaly, the severity of corticospinal tract involvement, and localization of the syrinx. Sixteen patients showed PLM in stages I and II of non-REM sleep and three PLM also while awake. There were no statistically significant differences in overall disability, corticospinal signs, presence of an associated Chiari anomaly, and disease duration between patients with and without PLM, although there was a trend for patients with PLM to have more severe disease. There was preservation of the lumbosacral enlargement of the spinal cord by the syrinx in all patients with PLM. The latency delay between lower and upper limb muscles was suggestive of conduction along propriospinal pathways. Syringomyelia may lead to an abnormal state of spinal hyperexcitability favoring the appearance of PLM. Detailed magnetic resonance image studies of patients with different localizations of the syrinx cavities may help to determine which tracts are involved in the production of PLM.

The diagnostic value of motor evoked potentials

OBJECTIVE

To assess the diagnostic usefulness of motor evoked potentials (MEPs) and to identify the optimal method for calculating the central conduction time. The test results were evaluated in a prospective study of 1023 neurological patients.

METHODS

We evaluated the correlation between clinical and electrophysiological findings, the accuracy, the sensitivity, the percentage of subclinical abnormalities and the false negative rates of MEPs in different neurological disorders. In patients with lower motor neuron involvement, we compared the central conduction time calculated as the difference between the latency of the cortical and magnetic root stimulation responses with that calculated using the F-wave method.

RESULTS

The agreement index between electrophysiological and clinical findings was 87%. The overall accuracy of the test was 0.97. The higher sensitivity values were demonstrated in spinal cord disorders (0.85), hereditary spastic paraplegia (0.80) and motor neuron diseases (0.74). The higher percentages of subclinical abnormalities were found in motor neuron disorders (26%) muscular diseases (24%), multiple sclerosis (13.5%) and spinal cord diseases (12.5%). The higher false negative rates were found in sylvian stroke (0.36) and hereditary spastic paraplegia (0.16). Central conduction study using magnetic paravertebral stimulation but not using the F-wave method, resulted in 12% and 10% of false positive values in lower limb multiradiculopathies and in neuropathies, respectively.

CONCLUSIONS

MEPs represent a highly accurate diagnostic test. MEP clinical value is maximum in motor neuron, muscle and spinal cord diseases. In patients with lower motor neuron involvement, the gold standard for central conduction determination is the F-wave method.

[Correlations between clinical deficits, motor and sensory evoked potentials and radiologic aspects of MRI in malformative syringomyelia. 27 Cases]

Twenty-seven patients (15 males, 12 females, age range: 16-66 years) were admitted for malformative syringomyelia diagnosed on MRI with measures of syrinx extending and transverse diameter. Posterior tibial somatosensory evoked potentials (PT SEP), median (M SEP), trigeminal (V3 SEP), brain stem auditory evoked potentials (BEAP), cortical and cervical motor evoked potentials (MEP) were correlated with clinical and radiological findings. SEP abnormalities were not correlated with the duration of symptoms. PT SEP proved to be more sensitive than M SEP. MEP abnormalities were very frequent (87% of the cases), even without clinical motor deficits. Trigeminal SEP were more sensitive than BEAP which were not related to the presence of associated cranio-vertebral abnormalities. We found no significative relationship between clinical and radiological results. Moreover, there was a positive relationship between electrophysiological and radiological results: abnormal trigeminal SEP were detected in 85% of the patients with high cervical syringomyelia. In all cases, trigeminal SEP and MEP should be done in association with M and PT SEP as both of them detect subclinical evidence of spinal cord dysfunction in syringomyelia.

Median nerve somatosensory evoked potentials in cervical syringomyelia: correlation of preoperative versus postoperative findings with upper limb clinical somatosensory function

Median nerve somatosensory evoked potentials (SEPs) were recorded in 30 patients with cervical syringomyelia before and after surgery. The different SEP components were compared with clinical somatosensory findings. The N13 potential (generated in the dorsal horn at C5-C6) was pathological in 85% of the upper extremities, or 90% of the patients, and correlated with pain/temperature as well as vibration/joint position sense; it was of higher sensitivity in syringomyelia than any other clinical symptom or SEP component. P14 (brain stem) and N20 (postcentral cortex) were less often affected and correlated with only vibration/joint position sense. Short-term postoperative clinical or SEP changes were most often seen after syringoendoscopy and less often after syringostomy, resection of cerebellar tonsils, or tumor extirpation. Alterations of SEPs after surgery occurred in more patients (60%) than did changes in clinical condition (approximately 27%); there was, however, no general correlation between these findings. We conclude that median nerve SEP testing with a proper recording technique identifying the different subcortical components is a valuable supplement in the pre- and postoperative diagnostic evaluation of syringomyelia and is of higher sensitivity than clinical somatosensory examination alone.

Discrepancy between SEPs directly recorded from the dorsal column nuclei following upper and lower limb stimulation in patients with syringomyelia

Somatosensory evoked potentials (SEPs) in response to electrical stimulation of the median nerve (MN) and posterior tibial nerve (PTN) were studied in 2 patients with syringomyelia. Intraoperative recordings were made from the surface of the dorsal column nuclei as well as from the scalp. Following MN stimulation, there was a preservation of scalp-recorded P9, P11, P13, and N20, however, there was an absence of spinal N13-P13. The dorsal column SEPs to MN stimulation were normal, characterized by a major negativity (N1), preceded by a small positivity (P1) and followed by a large positivity (P2). On the other hand, there was little or no cortical response (P37) to PTN stimulation. The dorsal column SEPs to PTN stimulation showed a disappearance of the normal P1'-N1'-P2' configuration, being replaced by a series of small spiky waves. The syringomyelic cavity may have thus compressed the gracile dorsal column which courses more medially than the cuneate pathway, causing desynchronization of the dorsal column SEPs. These findings suggest that dorsal column pathway arising from the lower limb is more vulnerable than that from the upper limb when a cervical syrinx is present.