Motor inhibition and excitation are independent effects of magnetic cortical stimulation

We administered magnetic cortical stimulation (MCS) during voluntary contraction of intrinsic hand muscles to 8 patients with motor neuron disease (MND), 5 patients with pure lower motor neuron syndromes (LMN), a patient with severe subacute sensory neuropathy (SSN), and 10 healthy volunteers. Patients with MND had clinical evidence of upper MND and elevated thresholds for (3 patients) or absence of (5 patients) motor evoked potentials (MEPs). MCS during sustained contraction inhibited electromyographic activity in 6 of 8 patients with MND, without preceding MEPs. MCS had no effect on the electromyogram (EMG) of the other 2 patients with MND. In normal subjects and patients with LMN, inhibition of EMG was never seen without a preceding MEP, regardless of stimulus intensity. In the patient with SSN, MCS elicited normal MEPs and inhibited the EMG in a pattern similar to normal subjects, whereas supramaximal electrical stimulation of median and ulnar nerves failed to inhibit the EMG despite normal M and F responses. Our findings indicate that the inhibitory effects of MCS on EMG are not dependent solely on changes in afferent feedback caused by the muscle twitch produced by the MEP, or on Renshaw cell inhibition. We suggest that some of the inhibitory and excitatory effects of MCS on the motor system are mediated by distinct cortical elements, which may have different susceptibilities to pathophysiological processes in MND.

Retinocalcarine function in Alzheimer's disease. A clinical and electrophysiological study

Impaired visual function in Alzheimer's disease (AD) could result from either precortical or cortical lesions, or both. In a parallel psychophysical study of visual function in AD, we found that contrast sensitivity function, color vision, stereoacuity, and backward masking were impaired relative to the performance of age-matched control subjects, whereas performance on a critical flicker fusion test was normal. The intent of the present study was to determine whether abnormalities of the retinocalcarine pathway contribute to visual dysfunction. We performed neuro-ophthalmological examinations on 38 patients with AD; from this group, 25 received additional psychophysical testing and 13 underwent electrophysiological testing. Clinical neuro-ophthalmological examinations, full-field electroretinograms, focal electroretinograms, and pattern visual evoked potentials were normal in all patients tested. There was no evidence of retinocalcarine abnormality specific to AD. We conclude that the visual impairment experienced by some patients with AD primarily results from involvement of the visual association cortices rather than from precortical damage, at least before the end stage of the disease.

Hemispheric threshold differences for motor evoked potentials produced by magnetic coil stimulation

A brief monophasic pulse through an electromagnetic coil preferentially activates motor pathways of each hemisphere, depending on the direction of coil current flow. Using the preferred direction for each hemisphere, the minimum stimulus intensity (threshold) that evoked compound muscle action potentials in the contralateral abductor digiti minimi (ADM) muscle was significantly less for the left hemisphere than the right. Threshold for biceps on each side was significantly higher than ADM, but there was no side-to-side difference. Assessing handedness using a standard handedness index, those who had less tendency to use the right hand for everyday tasks had greater differences between hemispheres for ADM thresholds. The lower threshold of the left-hemisphere projection to hand muscles is probably related to the asymmetry of corticomotoneuronal monosynaptic connections; a greater number project to the motor neuron pool of the right- than left-hand muscles.

Cervical magnetic stimulation

We stimulated the cervical region with a 9-cm-diameter magnetic coil on centered on the spinous processes in 21 normal subjects. We obtained maximal amplitudes with clockwise coil current in right-sided upper extremity muscles and counterclockwise coil current in left-sided upper extremity muscles. Optimal stimulation sites for biceps, triceps, and abductor digiti minimi were C-3 or C-4, C-4 or C-5, and C-4, C-5, or C-6, respectively. The latencies of the muscle responses varied little in the same subject in spite of marked amplitude changes due to suboptimal position of the coil or submaximal stimulator output. In abductor digiti minimi, the amplitude of the muscle response on cervical magnetic stimulation was 9 to 100% of the supramaximal amplitude on wrist electrical stimulation. We established normal values for latency, amplitude, and interside differences for the above 3 upper extremity muscles. The findings were reproducible, and the latencies obtained with large coils from different manufacturers in the same subjects were comparable. We found no advantage in bipolar recording over tendon-belly montage. Comparison of magnetic and electrical needle root stimulation in the same subjects showed that the magnetic stimulus was more proximal in biceps and triceps, and that the site of excitation was approximately the same in abductor digiti minimi. Indirect assessment of the longitudinal site of excitation based on F-wave minimal latency indicated that excitation occurred within millimeters of the emergence of axon of the peripheral motor neuron.

Clinical correlations of photoparoxysmal responses

Reilly and Peters (1973) reported that photoparoxysmal responses (PPRs) that outlasted the stimulus correlated significantly with seizures as compared to self-limited PPRs. However, they defined a wide range of activity, including occipital spikes and slow wave bursts, as PPR. We examined EEGs of 3557 patients and 48 normal subjects for PPRs defined only as generalized spikes or spike-wave activity. None of the 48 normal subjects showed a PPR. PPRs were seen in 35 (1%) patients, 27 (77%) of these had a definite history of epilepsy, 3 (9%) had a questionable history, and 5 (14%) had had no seizures. PPRs were prolonged more than 100 msec beyond the stimulus in 11, and self-limited in 24. The incidence of seizures was not statistically different in these 2 groups. Furthermore, the classification of PPRs into these 2 groups could be influenced by the time at which the stimulus was stopped after the appearance of the PPR. PPRs are rare in normal subjects and non-epileptic patients and have a high correlation with seizures irrespective of their relationship to the strobe stimulus.

Computerized EEG frequency analysis: sensitivity and specificity in patients with focal lesions

We performed computerized EEG frequency analysis (C-EEGFA) in 69 controls and 20 patients with focal brain lesions and focally abnormal conventional EEGs. Individual channel EEG frequency analysis variables that were helpful in differentiating the 2 groups were absolute delta and theta band power, relative delta, theta, and alpha band powers, and median-power frequency. High-frequency beta band power (20 to 32 Hz) was not useful. Changes in EEG with age were seen only after age 50 and generally consisted of an increase in anterior alpha power, with no significant increase in slowing. Correlations of C-EEGFA variables with posterior alpha power were more significant than correlations with age. Calculating normative C-EEGFA data for 5 subsets of controls, each with a different amount of posterior alpha power, increased the sensitivity of the EEG frequency analysis test without altering the specificity. Even with this correction 2 of 20 patients with focal lesions and focally abnormal conventional EEGs had normal C-EEGFA studies. If these obvious focal lesions produced normal results, more subtle diseases might not be detected. A significant clinical utility of C-EEGFA remains to be proven.

Magnetic stimulation of the human motor cortex: facilitation and its relationship to a visual motor task

Transcranial magnetic stimulation of the motor cortex can evoke motor responses in small hand muscles. This response can be facilitated by a background muscle contraction of the target muscle, resulting in an enhanced compound muscle action potential (CMAP) with a shorter onset latency. A number of intracortical mechanisms may account for this facilitatory effect, including, in part, direct input from visual to motor cortex. We studied the facilitation produced by a visual-motor task and compared those results with the facilitation produced by the same task without the visual cues. No differences in facilitation of amplitude or latency were observed. This suggests that there is no direct influence exerted by the visual cortex upon those elements of the motor cortex activated by a tangential magnetic stimulus, i.e., corticocortical and corticospinal neurons and their processes. Also, the large majority of facilitation (90%) was produced by a very small background voluntary contraction (less than 5% of maximum), indicating that any mild-to-moderate contraction of the target muscles will produce a consistent response for clinical measurements.

Pattern ERG: effects of reference electrode site, stimulus mode and check size

We studied monocular pattern ERG (PERG) in 10 normal subjects and a patient with optic neuritis. No clinically significant PERG could be recorded from the occluded eye with any reference (ipsilateral ear or temple, or midfrontal), indicating that cross-contamination is not present with binocular testing. Ipsilateral temple reference minimized VEP (P100/N100) contribution to the PERG N95 which occurred with ipsilateral ear or midfrontal reference. The conclusions were confirmed by results from the patient, who had marked monocular delay of a normal amplitude P100. Twenty-four subjects were tested with monocular and binocular stimulation using an ipsilateral temple reference. There were differences in PERG latencies and amplitudes although the interside amplitude ratio showed smaller differences with binocular stimulation. Increasing check size (17, 35 and 70 min) decreased P50 and N95 latencies and increased P50 amplitude.

Use of evoked potentials for diagnosis of multiple sclerosis

The author discusses visual evoked potentials, brain stem auditory evoked potentials, somatosensory evoked potentials, motor evoked potentials, and MRI for the diagnosis of MS. Most patients with MS will eventually have an MRI scan. However, certain specific questions regarding function and anatomic regions are better studied with evoked potentials, especially those of optic nerve, brain stem, and spinal cord.

Short-term variability in EEG frequency analysis

Knowledge of short-term EEG variability in computerized analysis is important before interpreting spectral EEGs or assessing changes that may be due to inherent variability and not necessarily related to a task (e.g., listening to a story), therapy or changes in underlying disease. Eighty to 120 sec of 14-channel, edited, bipolar EEG were recorded in normal subjects and analyzed using an FFT. Absolute and relative power in 5 standard frequency bands, and median and peak power frequencies were obtained for each 4 sec epoch, and the mean and standard deviation calculated for each parameter. The average variation of the mean power, absolute and relative, in the frequency bands was less than 10% although some parameters varied by up to 50% in an individual subject. Median and peak power had the least variability, about 3%. Changes in total power correlated positively with relative alpha power, but negatively or not at all with the other relative power measures. This suggests that interpretation of relative measures of delta, theta and beta in individual spectra may be dependent on total power or absolute alpha power. In addition, mathematical transformations were necessary to normalize the epoch data, suggesting that the mean and standard deviation of data from a series of epochs may not have maximal value unless a transformation is used. These results also indicate that caution is needed in interpreting changes in EEG frequency analysis data that are of the same magnitude as spontaneous EEG variability.

Evoked potentials in motor system diseases

We studied pattern-shift visual (PSVEP), brainstem auditory (BAEP), and somatosensory (SEP) evoked potentials in 38 unselected patients with motor system diseases (MSD) (28 sporadic, 10 familial). PSVEPs were normal in all patients, and BAEPs were normal in all except one with clinical hearing loss who had absent waves I and III and prolonged wave V latencies. Median and tibial SEPs revealed definite CNS conduction abnormalities in only 1 of 30 and 1 of 18 patients, respectively. In addition, four patients had peripheral and four had peripheral or central delays on tibial nerve testing. There were no or only small group differences in central conduction SEP, BAEP, and PSVEP values in patients with normal studies compared with controls. This study suggests that central conduction SEP, BAEP, or PSVEP abnormalities can rarely be attributed to MSD and that their presence in patients suspected of having this disorder should prompt a search for an alternative diagnosis.

Electroencephalography in AIDS and AIDS-related complex

EEG records from 47 patients with the acquired immunodeficiency syndrome (AIDS) or AIDS-related complex (ARC) were reviewed retrospectively to correlate EEG findings with neurologic abnormalities. Abnormal EEGs were found in 22 of 33 (67%) patients with AIDS and 5 of 14 (36%) patients with ARC. Among 27 patients with abnormal EEGs, there were 9 patients with dementia, 10 with opportunistic infections of the CNS, and 6 with no apparent neurologic disease. AIDS dementia was associated with intermittent or continuous slowing, often most prominent anteriorly. Focal slowing or sharp activity was usually found in patients who had focal CNS processes, such as cerebral toxoplasmosis and CNS lymphoma. These findings suggest the EEG can be a useful diagnostic test for evaluating patients with AIDS and ARC, particularly when these patients present with seizures, psychiatric symptoms, or cognitive dysfunction. The significance of abnormal EEGs in patients who are neurologically asymptomatic is unknown.

Normal temporal variability of the P100

Determination of clinically significant temporal changes in P100 latency requires knowledge of the degree of normal intraindividual variability. Checkerboard visual evoked potentials using 3 check sizes (17', 35' and 70') were performed serially on 20 healthy volunteers. Each subject was tested at least twice an average of 6 months apart. The P100 latency was measured at Oz with a forehead reference (Pz, O1 and O2 channels were also recorded). The overall average P100 latency change between studies for all check sizes and both eyes was 2.9 msec. However, the maximum absolute latency change was 11 msec. There was no significant difference between the average latency change for the 3 check sizes. The P100 interocular difference changed a mean of 2.5 msec (maximum 9 msec). Amplitude was more variable, with a mean change of about 1.5 microV or 25% (maximum was a 60% decrease in amplitude). A P100 latency change of up to at least 11 msec needs to be acknowledged as normal when assessing the clinical significance of changes in P100 latencies in patients. Also, P100 latency changes greater than 11 or 12 msec are very suggestive of an abnormality in the visual pathway.

Evoked potentials in Guillain-Barré syndrome

We studied evoked potentials (EPs) in 27 patients with typical acute Guillain-Barré syndrome and 3 with Fisher's syndrome. Three of 21 had BAEP abnormalities: 1 with bilateral I-III, 1 with unilateral I-III, and another with unilateral III-V interwave latency prolongations. Three with Fisher's syndrome had normal BAEPs (one had a poorly formed wave V unilaterally with one click polarity only). Ten of 21 median nerve EPs and 9 of 12 peroneal or tibial nerve somatosensory EPs were abnormal. Seven patients with normal somatosensory EPs had abnormal F waves from the same nerve; none had normal late responses and abnormal somatosensory EPs. These observations differ from previous reports on the frequency and interpretation of EP abnormalities in Guillain-Barré syndrome.

Early evolution and incidence of electroencephalographic abnormalities in Creutzfeldt-Jakob disease

The clinical and EEG findings in patients in the literature with Creutzfeldt-Jakob disease (CJD) were reviewed and compared with findings in 36 patients with CJD at the Massachusetts General Hospital (MGH). Twenty-one of the 36 MGH cases had histopathology, all with findings consistent with CJD. EEGs in 18 patients studied pathologically and in 10 without pathological investigation (28 of the 36) had periodic sharp wave complexes (PSWC) at some time during the clinical course. Of the other eight patients, two had only a single EEG early in the course of the illness, four experienced unusually long clinical courses, and two never showed PSWC despite numerous EEGs. PSWC made their appearance within 12 weeks of onset of clinical symptoms in 25 of 27 in whom EEGs were done during that period. In the early stages, EEGs in 14 of 28 showed focal PSWC or amplitude asymmetries of PSWC that corresponded well with focal myoclonus or other focal neurological abnormalities. In the literature, PSWC occurred within 12 weeks of the onset of the illness in 66 of 75 patients (88%) with CJD who had comparable clinical and neuropathological findings and adequate EEG data during the first 3 months of the illness. In the approximately 10% of patients who experienced unusually long courses, PSWC occurred in only about 55%. The presence of PSWC in association with the appropriate clinical, biochemical, cerebrospinal fluid, and neuroradiological findings is diagnostic of CJD. Brain biopsy is, therefore, unnecessary even when clinical therapeutic trials are undertaken and certainty of diagnosis is required. The absence of PSWC in the EEG after 12 weeks' duration of illness is a point strongly against the diagnosis of CJD unless it is a rare subtype of long duration. Only those patients without PSWC need to be biopsied.