Nicolas Mavroudakis

Effects of diphenylhydantoin on motor potentials evoked with magnetic stimulation.

We studied the effect of an acute loading dose of diphenylhydantoin (DPH) on motor responses elicited with cortical magnetic stimulation in normal subjects. DPH increased significantly the motor threshold activation of ADM, APB, FDI and biceps. The motor threshold increase was of greater magnitude for the proximal muscle. Spinal soleus alpha-motoneuron pool excitability assessed by H-reflex was increased significantly suggesting that the motor threshold increase is related to a supraspinal effect of the drug. Our study demonstrates that the motor threshold increase observed after DPH administration occurs not only in epileptic patients but also in normal subjects.

Brain function in the vegetative state.

Positron emission tomography (PET) techniques represent a useful tool to better understand the residual brain function in vegetative state patients. It has been shown that overall cerebral metabolic rates for glucose are massively reduced in this condition. However, the recovery of consciousness from vegetative state is not always associated with substantial changes in global metabolism. This finding led us to hypothesize that some vegetative patients are unconscious not just because of a global loss of neuronal function, but rather due to an altered activity in some critical brain regions and to the abolished functional connections between them. We used voxel-based Statistical Parametric Mapping (SPM) approaches to characterize the functional neuroanatomy of the vegetative state. The most dysfunctional brain regions were bilateral frontal and parieto-temporal associative cortices. Despite the metabolic impairment, external stimulation still induced a significant neuronal activation (i.e., change in blood flow) in vegetative patients as shown by both auditory click stimuli and noxious somatosensory stimuli. However, this activation was limited to primary cortices and dissociated from higher-order associative cortices, thought to be necessary for conscious perception. Finally, we demonstrated that vegetative patients have impaired functional connections between distant cortical areas and between the thalami and the cortex and, more importantly, that recovery of consciousness is paralleled by a restoration of this cortico-thalamo-cortical interaction.

Abnormal motor evoked responses to transcranial magnetic stimulation in focal dystonia

We evaluated motor responses evoked after magnetic cortical stimulation in dystonia, emphasizing the relationship between resting and facilitation state.We studied 15 normal controls (mean age, 37.9 years; range, 23 to 63) and 13 dystonic patients (mean age, 43.4 years; range, 20 to 56). Surface electrodes were placed over the right first dorsal interosseous muscle to measure motor evoked potentials and inhibitory silent periods obtained with magnetic stimulation. The amplitude ratio of motor evoked potentials measured during facilitation and at rest with low-intensity magnetic stimulation was significantly higher in dystonic patients (15.09) when compared with normal subjects (5.43; p equals 0.04). The ratio of duration of silent periods evoked with 120% motor threshold (MT) and MT plus 25% magnetic stimulus intensity was significantly higher in dystonic patients (78.4%) when compared with normal subjects (69.7%; p equals 0.04). We conclude that with low-intensity magnetic stimulation the relationship between amplitudes of motor potentials evoked at rest and during facilitation, as well as the responses of pathways that mediate silent periods, are disturbed in focal dystonia. NEUROLOGY 1995;45: 1671-1677

Effects of vigabatrin on motor potentials evoked with magnetic stimulation.

We studied the effect of an acute loading dose of vigabatrin on threshold of motor responses and duration of silent period elicited with cortical magnetic stimulation in normal subjects. In contrast to phenytoin, vigabatrin does not increase the motor threshold of first dorsal interosseus muscle. We also show that, although vigabatrin increases GABA concentrations in the central nervous system, duration of silent period studied at various stimulus intensities is not modified after vigabatrin administration.

Central cord syndrome due to a spontaneously regressive spinal subdural hematoma

We examined a patient suffering from a spontaneous spinal subdural hematoma which presented as a central cord syndrome associated with subarachnoid hemorrhage. MRI illustrated the spontaneous resolution of the hematom.

Spinal and brain-stem SEPs and H reflex during enflurane anesthesia.

Whereas cortical SEPs are altered by halogenated anesthetics, spinal and subcortical SEPs are thought to be hardly affected. In this study the spinal N13 potential (recorded with anterior neck reference) showed a significant delay with enflurane anesthesia. The P13 and P14 far-field potentials, however, remained unchanged. Our results indicate that oligosynaptic as well as polysynaptic pathways are influenced by halogenated anesthetics and that enflurane has different effects on spinal gray matter and cuneate synapses. Our data also demonstrate that earlobe reference recordings are not adequate to measure pharmacologic effects on subcortical SEPs.

Audiogenic startle reflex in acute hemiplegia

The generators of the audiogenic startle reflex (ASR) are located in the bulbopontine reticular formation. We studied the influence of acute vascular supratentorial lesions on ASR. Ten patients with hemiplegia due to hemispheric cerebral infarct were studied within 5 days of stroke onset. ASR and magnetic cortical stimulation were performed the same day. A muscle response to magnetic stimulation was not elicited over the plegic side in any patient. In four of seven patients, ASR was enhanced over the plegic side. We suggest that enhanced ASR is due to the loss of a predominantly inhibitory hemispheric drive on ASR generators.

Global aphasia without hemiparesis following prerolandic infarction

Acute global aphasia without hemiparesis (GAWH) is said to result from the association of 2 separate lesions in the anterior and posterior left hemisphere language areas. We present 2 unusual cases of GAWH whose CTs revealed only a single lesion of the posterior part of F2 and F3. There was good recovery of verbal communicative abilities. Functional disconnection of posterior language areas seems responsible for this syndrome in such cases.

Dissociation of P13–P14 far-field potentials: clinical and MRI correlation

P13 and P14 far-field potentials are recorded over the scalp with median nerve stimulation when non-cephalic reference is used to measure somatosensory evoked potentials. The dissociation of these 2 potentials is exceptional. Only 2 cases subsequent to pontine lesions have been described hitherto. We report the case of a 31-year-old woman with a low grade glioma located at the spino-medullary junction who presented a P13-P14 far-field dissociation. This case fully supports the independent nature of the P13 and P14 potential generators.

The effect of acute hemiplegia on intercostal muscle activity

We recorded the EMG of parasternal intercostal muscles in 25 patients with flaccid hemiplegia during quiet spontaneous breathing, voluntary hyperventilation, and CO2-induced hyperventilation. The respiratory drive was abnormal on the hemiplegic side and the function of the intercostal muscles was affected specifically during voluntary hyperventilation.