Magnetic stimulation in Alzheimer's disease

Mamede de Carvalho (1), Alexandre de Mendonça (2), Pedro C. Miranda (3), Carlos Garcia (2), Maria Lourdes Sales Luís (1)

(1) Department of Neurology, EMG Laboratory, Hospital de Santa Maria, Av. Prof. Egas Moniz, P-1600 Lisbon, Portugal
(2) Centro de Estudos Egas Moniz, Hospital de Santa Maria, Lisbon, Portugal
(3) Biomedical Engineering and Biophysics Institute of Faculty of Sciences, University of Lisbon, Portugal

Received: 9 August 1996 Received in revised form: 14 February 1997 Accecpted: 20 February 1997

Abstract Alzheimer's disease (AD) is a common cause of dementia in which some clinical motor abnormalities have been described. We used transcranial magnetic stimulation in order to test the hypothesis that the change in the motor cortex might cause modifications in motor excitability. Fourteen mildly to moderately affected AD patients were compared with 11 controls matched for age, height and sex. The motor evoked potential threshold value for the relaxed abductor digiti minimi was lower in the AD patients than in the control group for both left and right hemispheres (P < 0.05). No statistically significant difference was found comparing the left and the right hemispheres thresholds in each population. The mean interside threshold differences were small and not significantly different between patients and controls. The spinal motor neuron excitability, as evaluated by F/M and H/M waves amplitude ratios, showed no difference between the groups, reinforcing the motor cortex increased excitability hypothesis to explain this difference. Degeneration of inhibitory gabaergic terminals might be the basis for the increased cortical excitability in the motor cortex of the Alzheimer patients; postsynaptic changes in the GABA_A receptors might also affect inhibitory gabaergic transmission. The increased excitability found by transcranial magnetic stimulation in the motor cortex is important for understanding the emergence of seizures and myoclonus in this disease.

Key words Alzheimer's disease · Threshold · Transcranial magnetic stimulation

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