Malcolm Yeh

Topographic analyses of somatosensory evoked potentials following stimulation of tibial, sural and lateral femoral cutaneous nerves

Using topographic maps, we studied the scalp field distribution of somatosensory evoked potentials (SEPs) in response to the stimulation of the tibial (TN), sural (SN) and lateral femoral cutaneous (LFCN) nerves in 24 normal volunteers. Cortical peaks, i.e., N35, P40, N50 and P60 were generally dominant in the contralateral hemisphere for the LFCN-SEP, whereas all peaks except N35 had dominance in the ipsilateral hemisphere to TN- and SN-SEPs. The findings imply that ipsilateral or contralateral peak dominance for the lower extremity SEP is determined by where the cortical leg representation occurs. As a result, mesial hemisphere representation results in peak dominance projected to the hemisphere ipsilateral to stimulation. Representations at the superior lip of the interhemispheric fissure or lateral convexity lead to midline or contralateral peak dominance. These findings also suggest that the paradoxically lateralized P40 is not the result of a positive field dipole shadow generated by the primary negative wave in the mesial hemisphere, but is the primary positive wave, analogous to P26 of the median nerve SEP. Accordingly, contralaterally dominant N35 is likely equivalent to the first cortical potential of N20 in the median nerve SEP. The difference in vector directions of potential fields between N35 and P40 may account for the opposite hemispheric dominance for these peaks in TN- and SN-SEPs.

Dissociation between upper and lower neck N13 potentials following paired median nerve stimuli

Cervical N13 potential in response to the median nerve stimulation can be recorded either from upper (Cv2) or lower (Cv6) neck with almost equal amplitudes and latencies. It has long been debated whether they represent the same or different generator sources. Using a conditioning-test paired stimuli paradigm, we examined the differences of recovery function of Cv2- and Cv6-N13, anterior neck (AN)-P13, and scalp recorded P13/P14 in 6 healthy subjects. All cervical electrodes were referenced to the non-cephalic site. Scalp response was recorded with linked ear reference. The inter-stimulus intervals ranged from 4 to 20 ms with 2 ms increments. Throughout 4 to 18 ms ISI, Cv6-N13, AN-P13 and scalp P13/P14 were suppressed, whereas Cv2-N13 was facilitated. All but scalp P13/P14 returned close to the control at 20 ms ISI. The findings indicate that Cv2-N13, Cv6-N13 and scalp P13/P14 are independent each other and arise from different generator sources. The suppression of Cv6-N13 is consistent with a postsynaptic nature of this potential and may indeed be mediated through dorsal horn interneurons creating a current field orientation in the posterior-anterior direction. The facilitation of Cv2-N13 suggests that this is a presynaptic potential and may travel through the dorsal column with vertical orientation. The longer period of suppression of scalp P13/P14 suggests it to be of polysynaptic origin and to arise at least rostral to the cuneate nucleus.

Fundamental principles of somatosensory evoked potentials

Studies of SSEP provide unique opportunities for investigating physioanatomic substrates of sensory pathway and cognitive functions of the sensory system. Progress of clinical investigation and application of SSEP have been stalled in more recent years, although SSEP still remain a useful tool for diagnosis of various neurologic disorders and for the monitoring of spinal cord function during surgery. Reflecting complex sensory system in human, scalp-recorded SSEP consists of multiple waves, having different distribution, amplitude, and latencies among different electrodes. The physioanatomic significance of these multiple waves, especially the late components, is largely unknown. These should be explored further, especially in relation to cognitive function.

Locked-in syndrome after intrathecal cytosine arabinoside therapy for malignant immunoblastic lymphoma

A 22‐year‐old man with malignant immunoblastic lymphoma had “locked‐in” syndrome within 48 hours of receiving a single (100 mg) dose of intrathecal cytosine arabinoside (ara‐C) in conjunction with intravenous ara‐C, cisplatin, and doxorubicin. Eight hours after therapy, the patient had central hypoventilation and blurred vision that progressed to blindness within 3 hours. During the next 10 hours, he became completely quadriplegic but remained intermittently alert and was able to respond to commands by eye or head movements. Radiographic studies showed necrosis of the medulla and swelling of the entire spinal cord. The patient persisted in a locked‐in state until his death 3 weeks later, after removal of life support systems. Autopsy confirmed extensive necrosis of the lower medulla, optic chiasm, cranial nerves I and IV, and spinal cord. This case was unusual for its severity. The temporal relationship to ara‐C instillation favors a toxic idiosyncratic response to chemotherapy. The authors advocate caution when bolus intrathecal and intravenous ara‐C are administered to a patient within a short time of each other. Cancer 1992; 702504‐2507.

Antipsychotic effect of electroconvulsive therapy is related to normalization of subgenual cingulate theta activity in psychotic depression

OBJECTIVE Electroconvulsive therapy (ECT) is one of the most effective options available for treating depressive and psychotic symptoms in a variety of disorders. While the exact mechanism of ECT is unclear, it is known to increase metabolism and blood flow specifically in the anterior cingulate cortex (ACC). The ACC is a cortical generator of theta rhythms, which are abnormal in patients with depression and psychotic disorders. Since patients with psychotic depression are known to respond particularly robustly to ECT, we investigated whether the therapeutic effect of ECT in this population was related to normalization of abnormal theta activity in the ACC. METHOD We obtained 19-lead electroencephalography (EEG) data from 17 participants with psychotic depression before and 2-3 weeks after a full course of ECT. EEG data was analyzed with quantitative measures and low-resolution electromagnetic tomography (LORETA) compared to an age-adjusted normative database. RESULTS Quantitative EEG analyses revealed that theta band (4-7 Hz) activity was the only frequency band that changed with ECT. LORETA analyses revealed that the primary site of theta activity change was within the subgenual ACC (Brodmann area 25). There was a positive association between increased subgenual ACC theta activity and decreased psychotic symptoms. The degree of low theta activity in the subgenual ACC prior to ECT predicted the antipsychotic response of ECT. CONCLUSIONS The antipsychotic effect of ECT is related to normalization of subgenual ACC theta hypoactivity.

Effects of Stimulus Intensity on Latency and Conduction Time of Short-Latency Somatosensory Evoked Potentials

We studied the effect of stimulus intensity on latencies of short-latency somatosensory evoked potentials (SSEP) by measuring both onset and peak latencies individually. The latencies of N9, N13, N20 and N9-N13 peripheral conduction time (PCT) of median nerve (MN) SSEP, and N8, N23, P37 and N8-N23 PCT of tibial nerve (TN) and sural nerve (SN) SSEP significantly shortened with increasing stimulus intensity by onset latency measurement. However, those latencies by peak latency measurement were less significantly shortened or had only a trend of latency shortening without statistical significance. In contrast to PCT, N13-N20 central conduction time (CCT) of MN-SSEP and N23-P37 CCT of TN- or SN-SSEP showed no latency changes with the increased stimulus intensity by both onset and peak latencies measurement. As peak latencies had greater interindividual variability than onset latencies shown by larger standard deviation, shortening of onset latencies were more consistent than that of peak latencies. We think shortening of onset latencies indicates the recruitment of faster conduction fiber along with increased stimulus intensity. As the degree of latency shortening was less if stimulus intensity was above 2.5 times sensory threshold, the stimulus intensity greater than 2.5 times the sensory threshold should be used for clinical application.

Results of 110 Vitrectomies with a Portable Vitrectomy System

We retrospectively evaluated the VITAC (vitreous-tissue aspiration cutter), a portable vitrectomy system with an end-cutting vitrectomy probe with a self-sharpening oscillating blade and a monoblock design, in 110 vitrectomy procedures, 34 performed at the University of California Davis Medical Center and 76 performed at the Eye and ENT Hospital in Shanghai, China. The indications for vitrectomy included penetrating injuries (22 eyes), intraocular foreign bodies (28 eyes), vitreous hemorrhages (18 eyes), cataracts (17 eyes), endophthalmitis (seven eyes), pupillary-block glaucoma (five eyes), bullous keratopathy (five eyes), aphakic penetrating keratoplasty (three eyes), pupillary membranes (two eyes), massive preretinal proliferation (one eye), and cystoid macular edema (one eye). Vitrectomy resulted in visual improvement in 19 of 34 eyes in the California series (56%) and in 60 of the 76 eyes in the Shanghai series (79%). This difference was attributable to the higher percentages of cases involving the posterior segment and vitreous hemorrhage in the California series. When results from both institutions were combined, surgery with the VITAC produced visual improvement in 79 of 110 cases (72%), comparable to the results obtained with other vitrectomy systems.

Chapter 20 Applications of SSEP recordings in the evaluation of the peripheral nervous system

Publisher Summary This chapter describes the various methods of somatosensory evoked potential (SSEP) recordings using electrical as well as various natural stimuli, and discusses the usefulness and limitations of various modes of stimulation used to evaluate the central sensory pathways and function of peripheral nerves. Because SSEPs track the functional integrity of the peripheral and central nervous system, a lesion affecting any part of somatosensory pathway may alter the recorded SSEP. SSEP components, especially far-field potentials of short latency, prove useful in localizing lesions that affect the spinal cord, brainstem, subcortical, and cortical regions. SSEPs are conventionally elicited by electrical stimulation of either mixed or cutaneous nerves. Although more natural stimulations, such as pain, temperature, and pressure, can elicit SSEPs, the early responses are much smaller than those of electrically elicited SSEPs, because the onset and offset of natural stimuli are more gradual and the natural stimulus devices are more complicated. Thus, the use of SSEP studies to evaluate pain and temperature function has not been popularized as clinical diagnostic tests.

Chapter 19 Magnetic stimulation

Publisher Summary This chapter describes magnetic stimulation, which means the electrical stimulation from magnetic field-induced eddy-currents rather than from direct stimulation by the magnetic field. Current induced from magnetic stimulation can excite not only the motor cortex but also the motor roots in the region of the intervertebral foramina, as well as peripheral nerves and plexuses. With the use of a specially constructed coil, magnetic stimulation may activate the pyramidal decussations but not the spinal cord. This technique has limited application in the assessment of the peripheral nerve for two reasons: (1) the inability to pinpoint the exact activation site; and (2) difficulty to stimulate selectively a specific nerve under consideration. Advances in coil design may further improve the technical precision and clinical utility of transcranial magnetic stimulation in the study and diagnosis of the peripheral nerve diseases.