Hans Lueders

Cortical somatosensory evoked potentials in response to hand stimulation

Somatosensory evoked potentials were recorded from chronically implanted subdural electrodes in six patients with intractable seizures. The following conclusions were reached: 1) The initial cortical negativity-positivity (N1 with a latency of about 20 msec and P2 with a latency of about 24 msec) recorded in the postcentral area was an expression of the classical primary surface positivity, but N1 was generated by the posterior pole of an early horizontal dipole in area 3b, and P2 was generated by the positive pole of a slightly delayed vertical dipole in area 1 and 2.2) P2 permitted the most accurate localization of the primary somatosensory area. 3) No potentials were elicited in the primary somatosensory area by stimulation of the ipsilateral hand. 4) No cortical potentials were seen at stimulation intensities below the sensory threshold. The cortical distribution of evoked potentials evoked by weak and strong intensities had significantly different distribution. 5) The recovery function of cortical evoked potentials showed a U-curve with an early period of facilitation (10 to 30 msec) followed by a prolonged period of subnormality which peaked at about 50 msec. The recovery curve at different cortical loci differed.

Evidence for epilepsy is rare in patients with psychogenic seizures

We documented psychogenic seizures in 50 of 79 patients referred for differentiation between psychogenic and epileptic seizures. Only 5 of these 50 had, in addition, EEG evidence for epilepsy. In patients with documented psychogenic seizures, a second diagnosis of epilepsy should not be made without definite EEG support.

Origin of far-field subcortical potentials evoked by stimulation of the posterior tibial nerve

Evoked potentials to stimulation of posterior tibial nerves were recorded from cervical-scalp derivation (cervical electrode: surface electrode on spinal process of the fifth cervical vertebrae; scalp: vertex). Four subcortical potentials labeled as N24, P27, N30 and P32 were identified. Evidence indicating that these components are originated in the following structures is presented: N24, posterior columns at the level of C5; P27, high cervical posterior columns or brain stem; N30, medial lemniscus or thalamus; P32, thalamo-cortical radiations.

Early somatosensory potentials evoked by median nerve stimulation: intraoperative monitoring.

we identified the sites of origin of the somatosensory evoked potentials to median nerve stimulation by recording directly from the cervical cord in the course of intraoperative monitoring. The N9 potential occurred before any potentials recorded from the cord or dorsal roots. Potentials with latencies corresponding to N11 were recorded at the median nerve root entry zone of the lower cervical cord. High-amplitude potentials were recorded at the level of the foramen magnum, with latencies approximating or following P13, suggesting that this potential is generated at the cervicomedullary junction.

Brain stem auditory evoked potentials in posterior circulation surgery

The objectives of this study were to evaluate the use of brain stem auditory evoked potentials (BAEPs) in 10 adult patients with vascular disorders of the posterior circulation that were treated surgically and to compare the BAEPs with the neurological findings. The vascular lesions included basilar artery stenosis in 3 patients, vertebral artery stenosis in 1 patient, brain stem/cerebellar arteriovenous malformation in 2 patients, and basilar artery aneurysm in 4 patients. Measurement of BAEPs were carried out during operation in all cases. Eight patients had BAEPs measured before operation, and 9 patients had BAEPs measured after operation. Repeat postoperative studies were performed in patients with changes in neurological status. In general, BAEP abnormalities correlated with the neurological findings before and after operation. Six patients had normal intraoperative studies. None of them had clinical findings of pontomesencephalic dysfunction after operation. Three patients with significant BAEP abnormalities during operation had neurological findings of pontomedullary ischemia after operation. Transient BAEP changes in 1 patient were thought to be the result of brain stem retraction. The BAEPs were lost in 3 patients who died. The use of BAEP measurement did not lengthen the operative procedures. However, the technique used in this study required 4 to 8 minutes for the accumulation and interpretation of each average, thereby delaying feedback to the surgeon. The results of this study suggest a potential role for BAEP monitoring in identifying brain stem injury during posterior circulation surgery.

Sensory seizure mimicking a psychogenic seizure

A patient had episodes of bilateral paresthesias with retained consciousness. The attacks were clinically considered to be psychogenic seizures. Electroencephalography indicated that the attacks were epileptic, perhaps originating from the second sensory area. Electroencephalographic recording of a seizure is essential in differentiating epileptic from psychogenic episodes.

Simple Technique for Monitoring Intraoperative Spinal Cord Function

Interference with normal spinal cord functioning is an important, although uncommon, complication of spinal surgery. Spinal evoked potentials have been advocated as a means of monitoring spinal cord transmission during operative procedures. We have developed a simple technique using electrodes inserted into the interspinal ligaments for monitoring purposes. This has produced stable and reproducible recordings with obvious clinical benefit in the first 50 cases in which it has been used.

Surgical monitoring of spinal cord function: Cauda equina stimulation technique

Spinal cord and subcortical brain stem evoked potentials had an amplitude at least 2 times higher when the cauda equina rather than bilateral peripheral nerves was stimulated. Cauda equina stimulation is indicated when potentials to peripheral nerve stimulation are absent or are too low in amplitude to permit reliable surgical monitoring. The technique is essentially without risks, but should be performed with a small lumbar puncture needle (21 to 22 gauge), and is contraindicated in patients with general infections, increased cerebrospinal fluid pressure, or a hemorrhagic tendency (thrombocytopenia or anticoagulant therapy).

Paradoxical lateralization of parasagittal sharp waves in a patient with epilepsia partialis continua

Summary: We present a patient with epilepsia partialis continua involving the right leg who demonstrated sharp waves paradoxically distributed over the vertex and right hemisphere. Posterior tibial nerve evoked potentials also showed a similar paradoxical lateralization. We postulate that generators situated on the mesial surface of the left hemisphere projected their activity obliquely, leading to paradoxical lateralization of the recorded electrical activity.