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Biomedical subjects

K H Chiappa

Publications and source records attributed to K H Chiappa.

At least 37 records · Page 2Linked to original sources
PubMed1994

Effects of routine hyperventilation on PCO2 and PO2 in normal subjects: implications for EEG interpretations.

There are few data in the EEG literature describing the time course of hyperventilation-(HV) induced changes in blood gases, despite this being a routine activating procedure. We studied changes in blood gases and EEG in nine normal adult subjects before, during, and after HV. The mean PCO2 fell 18 mm Hg from the baseline during HV and recovered in 7 min. The mean PO2 rose 7 mmHg during HV and fell to 25 mm Hg below baseline 5 min after HV. The PCO2 recovery period is longer than is usually assumed in clinical EEG. The PO2 fall to a nadir at 5 min after the end of HV suggests that close attention should be paid to this period, as is confirmed by the re-buildup seen in moyamoya disease. Despite uniform changes in blood gases, the EEG median power frequency change showed marked variability; on average, it dropped by 1 Hz during HV and returned to baseline within 2 min of resumption of normal respiration. The EEG root-mean-square power showed a 200% increase during HV and also had returned to normal within 2 min.

Adolescent↗
PubMed1994

Electrophysiologic monitoring in the intensive care unit.

Electroencephalography (EEG) and evoked potential studies are established monitoring tools in the neurological intensive care unit (ICU). These neurophysiologic techniques provide information on physiological state and response to therapy, and may aid diagnosis and prognosis. Serial studies or continuous monitoring may enable changes to be detected prior to irreversible deterioration in the patient's condition. Current computer technology allows simultaneous display and correlation of electrophysiologic parameters, cardiovascular state and intracranial pressure (ICP). Continuous EEG monitoring in the ICU has been shown to have a decisive or contributing impact on medical decision making in more than three-quarters of patients. In addition, continuous EEG monitoring has revealed previously unsuspected non-convulsive seizures in one-third of patients. SEPs and BAEPs can provide useful prognostic information in coma-however, these tests are etiologically nonspecific and must be carefully integrated into the clinical situation. Motor evoked potentials offer a potentially useful tool for evaluating motor system abnormalities in the ICU.

Brain Stem↗
PubMed1993

Cortical and spinal motor excitability during the transcranial magnetic stimulation silent period in humans.

We investigated the electromyographic silent period in abductor pollicis brevis (APB) and flexor carpi radialis muscles following transcranial magnetic stimulation of human motor cortex. In APB, we measured cortical stimulation silent period (CSSP) duration as a function of stimulus intensity, motor-evoked potential (MEP) amplitude and muscle twitch force. We used peri-stimulus-time histograms to study the effect of cortical stimulation on single-motor unit firing patterns. We compared F-waves, H-reflexes and magnetic MEPs elicited during the CSSP to control responses elicited at rest and during voluntary contraction. CSSP duration depended on the intensity of cortical stimulation. However, we found no relationship between CSSP duration and MEP amplitude or muscle twitch force, thus the CSSP is not dependent solely on Renshaw cell inhibition or on changes in Ia and Ib afferent activity following the cortically induced muscle twitch. At low intensities of stimulation, the interval to resumption of motor unit firing following the peak in the peri-stimulus-time histogram corresponding to MEP latency sometimes exceeded that which could be accounted for by the motor unit's firing rate prior to the stimulus, suggesting that synchronization of motor unit firing by cortical stimulation cannot account for the CSSP. We found brief inhibition of F-waves during the CSSP in some subjects, reflecting activation of inhibitory corticospinal projections or segmental effects. In contrast, we observed longer inhibition of H-reflexes during the CSSP in all subjects, perhaps resulting from presynaptic inhibition of Ia afferents. Magnetic MEPs also were inhibited during the CSSP, suggesting inhibition of cortical elements by transcranial magnetic stimulation.

Adult↗
PubMed1993

Variability of motor potentials evoked by transcranial magnetic stimulation.

We studied the effect of stimulus intensity, coil size, mental alertness and prestimulus muscle contraction on the variability of motor evoked potentials (MEPs) produced by magnetic cortical stimulation (MCS). In 5 healthy subjects we delivered MCS either with a circular coil centered at the vertex or a figure-8 coil centered over the motor cortex hand area, recording from first dorsal interosseous. With the subject at rest or exerting 5% maximum voluntary contraction, 30 consecutive stimuli were given at 4 stimulus intensities (SIs) in 10% increments above resting motor threshold. Concurrent mental arithmetic constituted mental alertness. Spectral analysis was performed on data from 300 consecutive stimuli. The variability of MEP response size was inversely related to stimulus intensity, prestimulus voluntary muscle contraction, the recruitment of motoneurons and the size of the field generated by the magnetic coil. The MEP variability was larger than and not correlated with the variability of the H-reflex. Fast Fourier transformation and cross-correlation analysis did not identify a consistent dominant frequency, suggesting that the variability in MEP size is essentially random. We suggest that the variability in MEP response is caused by constant, rapid, spontaneous fluctuations in corticospinal and segmental motoneuron excitability levels. Any maneuver that raises this level or increases the probability of motoneuron firing will decrease MEP variability.

Adult↗
PubMed1993

Facilitation of magnetic motor evoked potentials during the cortical stimulation silent period.

We investigated the relationship between stimulus intensity and magnetic motor evoked potentials (MEPs) elicited 100 msec after a conditioning stimulus that was 25% of stimulator output above resting motor threshold (RMT) during tonic contraction of abductor pollicis brevis. In five subjects, MEPs elicited with stimuli less than 25% above RMT were inhibited during the EMG cortical stimulation silent period (CSSP) produced by the conditioning stimulus, relative to MEPs elicited with the test stimulus given at rest. However, increasing the intensity of the test stimulus increased the amplitude of MEPs elicited during the CSSP relative to MEPs elicited at rest, such that MEPs elicited with stimuli 30 to 45% above RMT were facilitated during the CSSP. Increasing the intensity of the test stimulus also increased the amplitude of MEPs elicited with paired stimulation at rest, and caused facilitation in one subject. Since facilitation of MEPs was never accompanied by shortening of MEP latency, our observations point to supraspinal facilitory mechanisms. We suggest that facilitation of MEPs during the CSSP reflects temporal and spatial summation of conditioning and test stimuli.

Adult↗
PubMed1993

Ifosfamide causes a diazepam-sensitive encephalopathy.

An encephalopathy characterized by confusion, stupor, and mutism frequently occurs during administration of the chemotherapeutic drug ifosfamide (IFX). We investigated two patients who developed encephalopathy during IFX infusion. Both exhibited a rapid and near-complete restoration of baseline mental status functioning concurrent with EEG improvement after administration of intravenous diazepam. We recommend an EEG and trial of benzodiazepines in patients with IFX encephalopathy.

Adult↗
PubMed1992

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.

Aged↗
PubMed1992

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.

Adult↗
PubMed1991

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.

Action Potentials↗
PubMed1991

Magnetic stimulation of the human motor cortex: ipsilateral and contralateral facilitation effects.

Voluntary contraction of a muscle greatly increases the amplitude and decreases the latency of the motor potentials evoked by electromagnetic coil brain stimulation (facilitation). Facilitation has also been observed with contraction of a nearby ipsilateral and a contralateral homologous muscle. We studied these facilitatory relationships in 5 normal subjects in small hand, forearm and upper arm muscles using surface-recorded compound motor action potentials, single motor unit recordings, and post-stimulus time histograms. There was no evidence for spread of facilitation between any pair of muscles if the muscle from which motor evoked potentials (MEPs) were recorded was completely at rest during brain stimulation, although this sometimes required training to accomplish or could not be achieved. Thus, although spread of facilitation has been observed by others under these conditions, we did not find this effect. There may be significant interindividual variations in the degree of facilitatory spread.

Action Potentials↗
PubMed1990

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.

Adolescent↗
PubMed1990

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.

Adult↗
PubMed1989

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.

Adult↗
PubMed1989

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.

Electromyography↗
PubMed1988

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.

Adolescent↗