|Year : 1999 | Volume
| Issue : 2 | Page : 108--11
Somatosensory evoked potentials by paraspinal stimulation in acute transverse myelitis.
Department of Neurology, Nizam's Institute of Medical Sciences, Panjagutta, Hyderabad, 500082, India., India
J M Murthy
Department of Neurology, Nizam«SQ»s Institute of Medical Sciences, Panjagutta, Hyderabad, 500082, India.
Somatosensory evoked potentials by paraspinal stimulation were studied in 6 patients with acute transverse myelitis. In one patient in whom posterior tibial somatosensory evoked potentials were not recordable, a poorly formed response was seen with paraspinal stimulation. Slowing of conduction across the involved segment was seen in the remaining 5 patients and fairly correlated with the clinical localization.
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Murthy J M. Somatosensory evoked potentials by paraspinal stimulation in acute transverse myelitis. Neurol India 1999;47:108-11
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Murthy J M. Somatosensory evoked potentials by paraspinal stimulation in acute transverse myelitis. Neurol India [serial online] 1999 [cited 2020 Nov 30 ];47:108-11
Available from: https://www.neurologyindia.com/text.asp?1999/47/2/108/1642
Acute transverse myelitis (ATM) is characterized by bilateral motor, sensory, and autonomic dysfunction, resulting from the involvement of both halves of the spinal cord, in the absence of preexisting neurologic or systemic disease. ATM, a fragment of disseminated vasculomyelinopathy, is pathogenetically identical with acute disseminated encephalomyelitis. Magnetic resonance imaging (MRI) is the premier modality of investigation for demonstrating structural lesions. The functional alterations can be studied by somatosensory and motor evoked potentials.,6], Goodridge et al have shown the usefulness of SEPs evoked by paraspinal stimulation in the localisation of spinal lesions. This study presents the usefulness of this technique in localisation of spinal lesions in ATM.
Six patients with dorsal ATM were studied. Diagnostic criteria for ATM included acutely developed paraparesis or quadriparesis affecting motor and sensory system as well as sphincters, in the absence of involvement of other parts of nervous system clinically and electrophysiologically. Electrophysiological evaluation included posterior tibial and paraspinal somatosensory evoked, visual evoked, and brainstem auditory evoked potentials. Studies were done during the active phase of the disease.
Paraspinal SEPs were elicited by the method described by Goodridge et al. SEPs were elicited by stimulation of the paraspinal region. Simultaneous bilateral stimulation (20-30 mA), 2 cm lateral to the midline, sufficient to induce a visible muscle twitch was applied opposite the vertebral levels T12, T6 and Tl. The cathodes were directed proximally. Stimulus rate was between 1 and 3 Hz. A ground was placed midline at the mid cervical level. In addition, SEPs were also elicited by stimulation of posterior tibial nerve at the ankle percutaneously, with current sufficient to induce clearly visible flexion of the toes. The potentials were recorded over the scalp (Cz-Fz) in both the procedures [Figure 1]. Both the procedures were done within 7 to 14 days. During this period all the patients except one were having florid signs.
All the six patients were men and the age ranged between 16 to 48 years. In four patients ATM developed following viral infection. In all the 6 patients the posterior tibial SEPs were abnormal. Interside difference was significant in 3 patients and in 2 patients latencies were prolonged on either side. Posterior tibial SEPs were not recordable in one patient [Table I]. Paraspinal SEPs showed slowing of conduction velocities across the involved spinal segment and fairly correlated with the clinical localization [Figure 2]. In 2 patients, as no clear morphology could be obtained, conduction velocities were measured between Tl and T12 segments. In the patient in whom posterior tibial SEPs were not recordable, paraspinal stimulation recorded poorly formed and low amplitude responses [Figure 3].
Functional alterations resulting from white matter lesions of the neuroaxis can be evaluated by various evoked potential studies. Evoked potential studies will differentiate ATM from multiple sclerosis (MS). In ATM, the abnormalities in somatosensory evoked potentials included absent or prolonged lower limb somatosensory evoked potentials.,, In one patient of ATM, Wulff10 found that the cortical potential had returned with normal latency, whereas in patients with MS, evoked potentials usually remained clearly delayed after clinical recovery.
Paraspinal SEPs provide localising information in both radiologically visible and non-visible spinal lesions. There are hardly any studies which evaluated the value of paraspinal SEPs in patients with ATM. Of the 5 cases of spinal lesions studied by Goodridge et al there was only one case of myelitis. Posterior tibial SEPs were not recordable in this patient whereas a small response of normal latency was seen with stimulation at T7. The response at T2 stimulation was normal. In the present study there was fairly good correlation between the segmental conduction slowing and clinical localisation. In two patients cortical potentials could not be recorded on mid thoracic paraspinal stimulation. However, there was conduction slowing across a wide segment of spinal cord. In ATM, magnetic resonance imaging studies have shown lesions involving several segments. Barakos et al reported that in four of their five patients, the signal abnormality extended over at least six spinal segments. In another study the mean extent of signal alterations on MRI was 19 segments.
In conclusion paraspinal SEP study is a useful technique to determine the extent of lesion and segments involved in patients with ATM.