Cortical magnetic stimulation in amyotrophic lateral sclerosis.

Forty patients with ALS underwent cortical magnetic stimulation. Twelve had marked pseudobulbar signs; in these motor evoked potentials (MEPs) could not be elicited. Mean MEP latencies in the others, who had predominantly lower motor neuron signs, measured 23.3 +/- 2.1 msec (thenar), 18.7 +/- 5.3 msec (EDC), and 13.4 +/- 2.9 msec (biceps), respectively. These values were significantly longer (P greater than 0.001) compared with normal values (n = 35), which measured 20.2 +/- 1.6, 14.2 +/- 1.7, and 9.4 +/- 1.7 msec, respectively. MEP amplitude was often markedly reduced (less than 15% of the M wave) compared with a normal mean of 39.5 +/- 13.0%. Overall abnormal MEPs (delayed, absent, or reduced in amplitude) approached 100%. It is argued that measuring central motor delay, which was not significantly different in the patients compared with normals, is subject to error in ALS.

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.

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.

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 (R1) 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. Sensory-motor 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.

Sensory group Ia proximal conduction velocity.

The fastest median and ulnar velocities derived by recording motor and mixed nerve action potentials, F waves, H-reflexes, and somatosensory evoked potentials (SEPs) were compared. H-reflex recording was facilitated by employing selective group Ia excitation during voluntary muscular contraction. Mixed nerve, SEP, and H velocities, considered to predominantly reflect group Ia conduction, measured 63.2 +/- 3.2 m/sec, 63.4 +/- 4.5 m/sec, and 67.2 +/- 4.3 m/sec, respectively, between the wrist and elbow. Conventional motor conduction velocity was significantly slower (58.3 +/- 5.1 msec), but F velocity, which although nonuniform is also a measure of motor conduction, was 68.4 m/sec. Mean F latency was considered more reliable and representative than minimum F latency. F and H velocities accelerated proximally by 4.5 m/sec. They complement each other when evaluating motor and sensory group Ia conduction. The H-reflex and SEP use identical stimulus characteristics and when simultaneously recorded allow direct comparison of the fastest conducting peripheral and central sensory pathways.

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.

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.

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.

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.

Altered supernormality in multiple sclerosis peripheral nerve.

Supernormality is a regular part of the recovery cycle of nerve. Supernormality was expressed in terms of amplitude, and in 20 control subjects the mean (S2 conditioned/S2 unconditioned) ratio of the median sensory nerve action potential amplitude recorded at the elbow measured 285 +/- 174 (range 119 to 783). A supramaximal conditioning shock was delivered 6 msec in advance of the test pulse. Seventeen (42.5%) of 40 patients with multiple sclerosis (MS) failed to demonstrate supernormality, the (S2 conditioned/S2 unconditioned) ratio being less than 100. This abnormality probably reflects a delay in recovery cycle and is additional evidence implicating peripheral nervous system involvement in multiple sclerosis.

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. A 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.

Electrodiagnostic evaluation of radiculopathies and plexopathies using somatosensory evoked potentials.

Segmental sensory stimulation representing cervical and lumbar dermatomes was used to elicit SEPs in 27 patients with radicular symptoms and/or signs and 12 patients with plexopathies. The test was confirmatory and aided in the anatomical localization in 19 (70%) and 9 (75%) of the respective patient groups. It is concluded that this electrophysiological test usefully complements other electrodiagnostic means of evaluating radiculopathies and plexopathies.

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.