Enhancement of inhibitory mechanisms in the motor cortex of patients with cerebellar degeneration: a study with transcranial magnetic brain stimulation

Motor cortex excitability in patients with cerebellar degeneration

OBJECTIVES

To study motor cortex (M1) excitability and the effect of subthreshold transcranial magnetic stimulation (TMS) in patients with cerebellar degeneration and normals performing a reaction time (RT) task.

METHODS

Time to wrist flexion after a visual go-signal was measured. TMS was always delivered at 90% of resting motor evoked potential (MEP) threshold. In one experiment, test TMS was delivered at various intervals after the go-signal. In half the trials priming TMS was also given with the go-signal. A second experiment examined the effect on RT of M1 and occipital priming stimulation alone.

RESULTS

M1 excitability, measured as the likelihood of producing MEPs in the wrist flexor muscles, increased immediately after the go-signal in the patients and stayed high until movement. In controls, excitability rose gradually. This difference was largely eliminated by priming TMS. RT was longer in the patient group, but improved with priming TMS. Occipital priming produced less effect on RT than M1 stimulation in both controls (P=0.008) and patients (P=0.0004).

CONCLUSIONS

M1 excitability prior to movement in an RT task increases abnormally early in cerebellar patients. This may reflect compensation for deficient thalamocortical drive. Subthreshold TMS can partially normalize the prolonged RT and abnormal excitability rise in cerebellar patients.

Central motor conduction to lower limb after transcranial magnetic stimulation in spinocerebellar ataxia type 2 (SCA2)

OBJECTIVES

To evaluate central motor conduction to lower limbs in spinocerebellar ataxia type 2 (SCA2).

METHODS

Transcranial magnetic stimulation was performed to study the corticospinal tracts of 18 patients with SCA2.

RESULTS

Central motor conduction time (CMCT) to lower limbs and thresholds were abnormal in 8 patients (44%); CMCT and thresholds were significantly correlated with disease duration and disability.

CONCLUSIONS

Corticospinal tract involvement is more frequent than previously reported in SCA2. Prolonged CMCT and increased threshold should not be used to differentiate between various type of autosomal dominant cerebellar ataxia. Similar to that reported in Friedreich's ataxia, we suggest that examining central motor conduction to the lower limbs may assist in evaluating the progressive steps of neurodegeneration in SCA2.

Prolonged postexcitatory inhibition after transcranial magnetic stimulation of the motor cortex in patients with cerebellar ataxia

Postexcitatory inhibition after transcranial magnetic stimulation of the motor cortex (silent period, SP) is supposed to be predominantly mediated by the activation of inhibitory cortical interneurons. Cortical excitability seems to be reduced in patients with cerebellar ataxia. Motor threshold, central motor conduction time and the duration of the silent period after a single magnetic stimulus to the motor cortex on both sides were measured in five patients with cerebellar ataxia of different origin and 18 healthy controls. Duration of SP was highly significantly prolonged in patients compared with controls (P<0.001) while motor threshold and central motor conduction times were not different. Fifteen of 18 control subjects showed late EMG responses after magnetic stimulation but none of the patients did (P<0.001). These findings support the hypothesis that cerebellar lesions activate inhibitory cortical interneurons or cause a disruption of a normally tonic cerebellar excitation to the motor cortex. Silent period measurement appears to be a sensitive diagnostic tool in the neurophysiological examination of cerebellar diseases.

The diagnostic value of motor evoked potentials

OBJECTIVE

To assess the diagnostic usefulness of motor evoked potentials (MEPs) and to identify the optimal method for calculating the central conduction time. The test results were evaluated in a prospective study of 1023 neurological patients.

METHODS

We evaluated the correlation between clinical and electrophysiological findings, the accuracy, the sensitivity, the percentage of subclinical abnormalities and the false negative rates of MEPs in different neurological disorders. In patients with lower motor neuron involvement, we compared the central conduction time calculated as the difference between the latency of the cortical and magnetic root stimulation responses with that calculated using the F-wave method.

RESULTS

The agreement index between electrophysiological and clinical findings was 87%. The overall accuracy of the test was 0.97. The higher sensitivity values were demonstrated in spinal cord disorders (0.85), hereditary spastic paraplegia (0.80) and motor neuron diseases (0.74). The higher percentages of subclinical abnormalities were found in motor neuron disorders (26%) muscular diseases (24%), multiple sclerosis (13.5%) and spinal cord diseases (12.5%). The higher false negative rates were found in sylvian stroke (0.36) and hereditary spastic paraplegia (0.16). Central conduction study using magnetic paravertebral stimulation but not using the F-wave method, resulted in 12% and 10% of false positive values in lower limb multiradiculopathies and in neuropathies, respectively.

CONCLUSIONS

MEPs represent a highly accurate diagnostic test. MEP clinical value is maximum in motor neuron, muscle and spinal cord diseases. In patients with lower motor neuron involvement, the gold standard for central conduction determination is the F-wave method.

Reduced intracortical facilitation in patients with cerebellar degeneration

OBJECTIVES

Transcranial magnetic stimulation (TMS) was used to study intracortical inhibitory and excitatory phenomena in patients with cerebellar ataxia.

METHODS

Motor evoked potentials (MEP) following single and paired TMS were recorded from the first dorsal interosseus muscle (FDI) in 15 patients with autosomal-dominant or idiopathic cerebellar ataxia and 15 age matched normal controls.

RESULTS

MEP amplitudes after paired TMS with short interstimulus intervals (1-4 ms) showing intracortical inhibition in the control group were not significantly different in the patient group. In contrast, with longer interstimulus intervals (8-20 ms) mean MEP amplitudes were significantly reduced in the patient group, indicating a decrease of intracortical facilitation. The mean postexcitatory inhibition after TMS was also significantly prolonged in the patient group.

CONCLUSION

Our findings support the idea that the cerebellum physiologically exerts a facilitatory influence on the motor cortex which is decreased in patients with a cerebellar degeneration.

Cerebellar somatotopic representation and cerebro-cerebellar interconnections in ataxic patients

Different methods of functional neuroimaging were used for studying somatotopic encoding of function in the cerebellum and for investigating cerebro-cerebellar interconnections in patients with cerebellar degeneration. fMRI showed, that the center of activation for hand function was located in the intermediate hemispheric portion of Larsell lobules H IV-V. Foot movements activated areas medial and anterior to the corresponding hand areas within Larsell lobules II-III. Changed function in motor cortices could be demonstrated in patients with cerebellar degeneration as compared to normal controls by recording movement-related cortical potentials (BP). In patients the motor potential was almost lacking and transcranial magnetic stimulation demonstrated enhancement of inhibitory mechanisms (prolonged postexcitatory inhibition) in the motor cortex. PET-findings suggested, that both effects are correlated to increased activity of inhibitory interneurons. Cerebellar patients showed increased activation in relation to movements in the SMA and basal ganglia and reduced activation in the cerebellum and lateral premotor areas. It could be speculated, that compensatory mechanisms are the reason for a stronger activation of the medial premotor system, including SMA, in patients with cerebellar degeneration. On the basis of our results it appears, that the cerebellum facilitates the lateral premotor system areas much more than it does the medial areas.