Optimizing stimulating and recording parameters in somatosensory evoked potential studies

Methods to increase the signal:noise ratio of evoked potentials are reviewed, dividing them into three groups: (1) increasing the number of stimuli averaged; (2) decreasing the noise (muscle relaxation, filters, intermittent artifact reject methods); and (3) increasing the amplitude of the signal (stimulus intensity, nerves stimulated, bilateral versus unilateral stimulation, stimulus modality, and montage). Examples of these different methods applied to recording of somatosensory evoked potentials are given, and it is concluded that, for optimal recording of somatosensory evoked potentials, a combination of all these methods should be used.

Familial paroxysmal hypnogenic dystonia

We studied a family with dystonic spasms that occurred with non-REM sleep. This familial disorder may be related to the sporadic cases reported previously.

Preservation of voluntary saccades after intracarotid injection of barbiturate

We studied voluntary saccades in 44 patients undergoing Wada testing before surgery for intractable epilepsy. After intracarotid injection of barbiturate, and while they were hemiplegic, patients could still make voluntary saccades toward or away from the side of injection. Sustained ipsiversive deviation of gaze was not noted. Saccades made away from the side of injection were slower than ipsilateral saccades in only 3 of 10 tests. These data support the hypothesis of parallel, independent pathways from the frontal eye fields and from the superior colliculi to the brainstem reticular nuclei that generate saccades.

Pattern evoked potentials (PEPs) in Parkinson's disease

Controversy exists in the literature concerning whether Parkinson's disease (PD) results in prolongation of pattern evoked potential (PEP) responses. PEPs were obtained in 20 patients with PD. The latencies of the first major positive potential (P2) in response to independent left and right eye stimulation were evaluated. There was no statistically significant difference (group or individual) when patients with PD were compared with age-corrected controls. There was no difference in results in comparing patients with mild, moderate, and severe PD. These results contradict previous reports of markedly abnormal PEPs.

The second sensory area in humans: evoked potential and electrical stimulation studies

A patient with intractable seizures originating from a right frontal focus was evaluated for surgical treatment. This evaluation was carried out using a chronically implanted array of 96 stainless steel electrodes 1 cm apart and covering the perirolandic and frontal areas. Somatosensory evoked potentials and electrical stimulation of the subdural electrodes localized the primary sensory hand area. Evoked potentials of identical waveform but of lower amplitude and 2.4 ms longer latency were recorded in the inferior frontal gyrus immediately anterior to the face area of the motor strip. Electrical stimulation of that area elicited: (1) a "paralyzing" feeling in the left arm and face; (2) inhibition of rapid alternating movements of left fingers, left hand, and tongue; (3) inability to maintain a strong voluntary muscle contraction of the left hand or tongue; and (4) speech arrest. This appears to be the first report of a secondary sensory area in humans demonstrated by both electrical stimulation and evoked potential studies. Electrical stimulation showed that the secondary sensory area overlapped an area of complex motor control, suggesting that the secondary sensory area provides direct sensory feedback information for appropriate motor integration.

Serum prolactin levels after epileptic seizures

We prospectively studied serum prolactin (PRL) elevation after different types of documented seizures in 17 patients. Marked PRL elevations above normal and above three times baseline were seen at 15 or 30 minutes after 20 of 25 (80%) generalized tonic-clonic, 13 of 30 (43%) complex partial, and 1 of 10 (10%) simple partial seizures. Although marked postictal PRL elevation is a sensitive indicator of recent epileptic seizures, a normal 15- or 30-minute postictal PRL level does not exclude an epileptic seizure.

Cortical afterdischarge and functional response thresholds: results of extraoperative testing

We have evaluated the afterdischarge thresholds and functional response thresholds in 21 patients with chronically implanted arrays of subdural electrodes. Afterdischarge thresholds varied from 2 to greater than 15 mA over the tested cortex, by as much as 12 mA in individual patients, and by as much as 12 mA between adjacent electrodes. Thresholds for functional alteration varied from 2 to 15 mA in tested cortex, by as much as 9.5 mA in individually tested patients, and by as much as 6.5 mA between adjacent electrodes. We conclude that the optimal localization of functional cortical areas requires different stimulation intensities at different points. The use of too high an intensity would produce afterdischarges at some positions. The use of too low an intensity would falsely make others appear functionally "silent."

Recovery functions of spinal cord and subcortical somatosensory evoked potentials to posterior tibial nerve stimulation: intrasurgical recordings

The recovery function of evoked potentials to posterior tibial nerve stimulation was studied. Intrasurgical recordings were made from interspinous ligaments at lumbar levels and from high thoracic-low cervical level. In addition, surface recordings were obtained from neck-scalp derivations. The recovery function of the potentials recorded from lumbar and from high thoracic-low cervical spinal cord were very similar, showing an early period of supernormality (5-20 ms) followed by a period of subnormality which reached its lowest point at 40-60 ms. Assuming that the potentials recorded at the lumbar level reflect activity in the cauda equina, we conclude that the results support the hypothesis that the potentials recorded from the thoraco-cervical level reflect activity in the dorsal columns. The recovery curve of the amplitude between the far field potentials P27 (which most probably reflects activity of the afferent volley at the level of foramen magnum) and N30 (which, by latency criteria, would reflect lemniscal or thalamic activity) showed a similar shape but with a shorter duration of the periods of super- and subnormality. It is likely that this modification was due to the synapse at the gracilis nucleus. The first cortical component (P32) recorded in the neck-scalp derivation was totally abolished within the recovery period studied (50 ms interval).

Transient neuropsychological abnormalities (including Gerstmann's syndrome) during cortical stimulation

A patient with intractable partial seizures was intensively studied before surgical removal of the epileptogenic focus. A subdural electrode array was surgically placed over the left temporoparietal cortex to better localize the epileptogenic focus and localize cortical function. In addition to speech and sensory findings, acalculia, agraphia, right-left confusion, and finger agnosia were transiently produced by electrical stimulation in the perisylvian area. These findings and their relationship to the controversy surrounding Gerstmann's syndrome are discussed.

Subcortical and cortical somatosensory potentials evoked by posterior tibial nerve stimulation: normative values

Cortical somatosensory evoked potentials to posterior tibial nerve stimulation were obtained in 29 normal controls varying in age and body height. In obtaining these potentials we varied recording derivations and frequency settings. Our recordings demonstrated the following points: N20 (dorsal cord potential) and the early cortical components (P2, N2) were the only potentials that were consistently recorded. All other subcortical components (N18, N24, P27, N30) were of relatively low amplitude and not infrequently absent even in normals. All absolute latencies other than N2 were correlated with body height. However, interpeak latency differences were independent of body height. Below the age of 20, subcortical but not cortical peak latencies correlated with age, but this appeared to be due to changes in body height in this age group. Absolute amplitudes and amplitude ratios (left/right and uni/bilateral) showed marked interindividual variability and have very limited value in defining abnormality. The use of restricted filter windows facilitated the selective recording of postsynaptic potentials (30-250 Hz) and action potentials (150-1500 Hz).

Effect of stimulus intensity on subcortical and cortical somatosensory evoked potentials by posterior tibial nerve stimulation

The effects of stimulus intensity on subcortical and cortical somatosensory evoked potentials (SEPs) to posterior tibial nerve (PTN) stimulation were studied in 16 normal controls. Stimulus intensity was evaluated as a function of sensory threshold (S). Motor threshold (M) varied between 1 S and 2 S. The amplitude of N18 (afferent volley immediately before it enters the spinal canal) increased approximately linearly up to at least 4.5 S. N20 (dorsal cord potential) also demonstrated a linear increase up to at least 4 S but the rate of increase was significantly smaller. All central components (subcortical brain-stem components P27 and N30, and cortical components N1 and P2) showed an even smaller rate of increase which was non-linear and reached a plateau at 3 S. The relatively higher rate of increase of N18 as compared with N20 was most probably due to the recording of sensory impulses plus antidromic impulses in motor fibers. The smaller rate of increase and early saturation of all the central components compared with N20 suggests that of all the afferent fibers generating N20 only the low threshold fibers participate in the generation of more central components. Stimulus intensities of 3 S are recommended for clinical studies of the central SEPs to PTN stimulation.

High-dose monotherapy in treatment of intractable seizures

We evaluated the therapeutic efficacy and toxicity of high-dose monotherapy, using carbamazepine or phenytoin, in patients with previously uncontrolled seizures. Treatment with a single drug was equal to or better than polypharmacy, but only a few patients became free of seizures. Toxicity was mild and associated with higher total plasma levels after polypharmacy. Free fractions ranged from 0.14 to 0.30 for carbamazepine and from 0.60 to 0.13 for phenytoin. Toxicity was associated with free phenytoin levels above 3 micrograms/ml; there was no clear relationship between free carbamazepine level and toxicity.

Far-field auditory evoked potentials in near-miss sudden infant death syndrome

Auditory evoked potentials (AEPs) were determined in 16 patients with near-miss sudden infant death syndrome. The AEPs were normal in all patients except one with prolonged 1 to V interpeak latency (bilaterally) and another with an asymmetry of interear 1 to V interpeak latency difference. These results indicate that AEPs do not permit detection of infants at high risk for crib death.