Maureen Hoirch

Questions regarding the sequential neural generator theory of the somatosensory evoked potential raised by digital filtering

Digital bandpass filtering (300-2500 Hz) designed for zero phase shift was applied to somato-sensory evoked potentials recorded with cephalic bipolar montages. Four consistent negative and corresponding positive peaks with latencies of about 16, 18, 19, and 20 msec were elicited with median nerve stimulation. Peroneal nerve stimulation also elicited 4 reproducible negative-positive peaks having latencies of about 24, 26, 28, and 30 msec. Interpeak latencies measured 1.3 +/- 0.2 msec and 1.8 +/- 0.25 msec for median and peroneal elicited SEPs respectively. Because cephalic bipolar recordings cancel most far-field potentials, multiple generators cannot account for all the additional components seen. It is hypothesized that some of the high frequency components recorded are due to activity in recurrent intrathalamic neuronal networks.

Evaluation of radiculopathies by segmental stimulation and somatosensory evoked potentials

Thirty-six patients with suspected or myelographically proven radiculopathies were investigated with motor and sensory conductions, F-waves, needle electromyography, and somatosensory evoked potentials (SEPs). SEPs were elicited by cutaneous nerve stimulation representative of input from individual cervical and lumbosacral dorsal roots. A myelographic defect was present in 83% of 30 patients who had myelograms. Overall 78% of patients had one or more abnormal electrophysiologic tests, the needle EMG giving the best diagnostic yield (75%). F-waves and SEPs were abnormal in 43% and 57% of cases respectively. Motor deficit correlated best with abnormal EMGs, whilst abnormal SEPs occurred most frequently when sensory deficit predominated. Prolonged latency of the SEP occurred rarely, reduced amplitude or abnormal morphology being the most useful characteristics. SEPs evoked by cutaneous nerve stimulation are a useful addition to conventionally available electrophysiological methods of evaluating radiculopathies, especially in the absence of motor deficit.

Central nervous system amplification: Its potential in the diagnosis of early multiple sclerosis

Ability to record a sizable somatosensory evoked potential (SEP) in the absence of a recordable sensory nerve action potential (SNAP) suggests a normally occurring central nervous system amplifying process. Increments in SEP and SNAP amplitude with increasing stimulus strength between threshold and 2.5 times threshold (maximum) were compared. At threshold (40% of maximum stimulus) and 50% maximum stimulus, amplification measured 2.3 ± 0.7 and 2.0 ± 0.6, respectively. In 21 MS patients, the SEP at threshold stimulation was absent in 15, but normal in 5 of these at maximum stimulation. It is postulated that normal central amplification is markedly attenuated in MS, and this may be a sensitive indicator of early disease.

The Electrodiagnostic Evaluation of Spinal Root Lesions

Available electrophysiologic techniques helpful in identifying root lesions are surveyed. Demonstration of fibrillation or positive sharp waves in paraspinal muscles occurs seven to ten days earlier than elsewhere and indicates a lesion at least as proximal as the root. Abnormalities of the F-wave (its absence, a prolonged latency, a significant side-to-side difference, increased dispersion, or decreased persistence) reflect proximal motor axonal dysfunction. The H-reflex is a measure of the integrity of the S1 sensory root. Somatosensory evoked potentials (SEPs) are particularly useful in indicating disease when sensory symptoms or signs predominate. There is a 70-80% correlation between clinical, myelographic, and electrophysiologic findings, but only one may be abnormal. These findings therefore complement each other in the overall evaluation. Electrophysiology has limited value in postlaminectomy assessment.

A new indirect method for measuring spinal conduction velocity in man

A non-invasive, indirect method for measuring spinal cord mixed afferent-efferent conduction is described. The method is based upon eliciting late reflex responses labelled R1 and R2 from voluntarily contracting thenar and tibialis anterior muscles by preferentially stimulating median and common peroneal sensory nerve fibres. The mean onset latencies of R1 measured 27.5 msec and 30.6 msec recorded from hand and leg muscles respectively. R2 mean onset latencies measured 46.0 msec and 65.1 msec respectively. R1 has characteristics similar to an H-reflex. R2 is a long-loop reflex of unknown pathway assumed to involve similar circuits and rostral turn around points when elicited by both arm and leg stimulation. Mean spinal cord conduction time between the seventh cervical and fifth lumbar spinous processes, is given by (formula; see text) It measured 7.95 msec and the calculated mean conduction velocity was 57.9 +/- 5.7 m/sec.

Occult Fifth Nerve Dysfunction In Multiple Sclerosis

Somatosensory evoked potentials (SEPs) were recorded following trigeminal nerve stimulation in 25 normal subjects. Mucosal stimulation of the lip resulted in a reduced stimulus artefact. The three initial peaks, N13, P19, and N30, measured respectively 12.8 +/- 0.9 S-3, 19.3 +/- 1.4 S-3, and 28.6 +/- 1.7 S-3. Blink reflex studies were also performed in most of these subjects. In 41.4% of 29 patients with established or suspected multiple sclerosis, the trigeminal SEP was abnormal. Additional use of the blink reflex raised the overall incidence of trigeminal nerve dysfunction to 51.7%. None of the patients had clinical evidence of fifth nerve involvement either historically or on examination. Four of seven patients with progressive spinal MS and two patients whose only deficit was that of optic neuritis, had abnormal trigeminal SEPs. It is concluded that occult involvement of the pontine fifth nerve structures occurs frequently in MS despite the rarity of corresponding clinical findings. The trigeminal SEP is a useful additional neurophysiological method.

Enhancement and Dynamic Time Warping of Somatosensory Evoked Potential Components Applied to Patients with Multiple Sclerosis

A zero-phase, high-pass digital filter at 300 Hz was used to emphasize the higher frequency, smaller peaks of the somatosensory evoked potential (SEP). Using the digital filter, a consistent pattern of peaks was observed in normals. A dynamic time-warping technique was created to align the peaks of digitally high-pass filtered SEPs. Based on this technique, a clinical test for abnormality of the SEP was developed and applied to 25 multiple sclerosis patients. The results showed that three times as many multiple sclerosis SEPs were identified by the dynamic time warping as by the traditional latency measurement.

Far‐field potentials from peripheral nerve Generated at sites of muscle mass change

Using a forearm-sternal referential recording montage, three peripheral nerve far-field potentials, P4, P7, and P9, were elicited by median nerve stimulation. The latencies correlated with the distances between the stimulating cathode at the wrist and the proximal forearm, mid-upper arm, and the shoulder, respectively. At these sites, the electrophysical characteristics of the surrounding volume conductor suddenly changes. Altered girth of muscle mass seemed to be a factor.

Paraspinal stimulation to elicit somatosensory evoked potentials: an approach to physiological localization of spinal lesions

Somatosensory evoked potentials (SEPs) were elicited by stimulation of the paraspinal region. Simultaneous bilateral stimulation, 2 cm lateral to the midline, sufficient to induce a visible muscle twitch, was applied opposite vertebral levels L3, T12, T6 and T1 and intervening segments in some subjects. The potentials were recorded over the scalp (Cz-Fz). The stimulus excludes most of the peripheral nervous system; the volley being initiated in the cutaneous branches of the primary dorsal root rami with some contribution from paraspinal muscle Ia afferents. In normal subjects, paraspinal evoked SEPs are easily elicitable and measurable. Mean spinal cord conduction velocity between T12 and T1 measured 64.1 m/sec (N = 25). The upper thoracic cord propagated faster than the lower thoracic cord which conducted faster than the lumbar segment. The technique was used to confirm the approximate level of radiologically visible spinal lesions that were surgically treated and to identify diffuse, focal or multisegmental spinal conduction slowing in patients devoid of radiologically visible lesions. The method has potential for intraoperative spinal cord monitoring.

Noninvasive measurement of central sensory and motor conduction

Potentials evoked by median and peroneal nerve stimulation were digitally filtered between 300 and 2,500 Hz to measure early latency components and assess sensory cord conduction velocity. Short (Rl) and long (R2) latency reflex responses were recorded from contracting thenar and tibialis anterior muscles. R1 is considered a spinal reflex akin to the H-reflex. Clinical evidence suggests that R2 involves a reflex arc with turnaround at the motor cortex. Sensorymotor cord velocity was derived from the latencies of R1 and R2. The method can be used to compare peripheral and central sensory conduction or conduction in central sensory and motor pathways.