Abstract
Objective: The aim of the study was to examine intracortical excitability in cerebellar patients.
Methods: Short-latency intracortical inhibition (SICI), long-latency intracortical inhibition
(LICI) and intracortical facilitation (ICF) to paired transcranial magnetic stimulation
(TMS) were investigated in 8 patients with ‘pure’ cerebellar syndromes and in 14 age-matched
normal controls. The conditioning stimulus for short-latency intracortical inhibition
and intracortical facilitation was set at 70% of the resting motor threshold (RMT)
and preceded the test stimulus (110–120% of the resting motor threshold) by interstimulus
intervals (ISIs) of 1–30 ms. For the long-latency intracortical inhibition determinations,
the conditioning stimulus was set at 120% of the resting motor threshold and preceded
the test stimulus (also 120% of the resting motor threshold) by interstimulus intervals
of 30–500 ms. Results: No statistically significant differences were found between patients and controls
as regards either short-latency intracortical inhibition or intracortical facilitation.
A significant prevalence of long-latency intracortical inhibition was present in cerebellar
patients at interstimulus intervals of 200–500 ms (conditioned MEP amplitude=29–41%
of test MEP) as compared to controls (71–96% of test MEP). The amplitude of conditioned
MEPs was persistently less than 45% of the test MEP in six patients, who were studied
at interstimulus intervals up to 1000 ms. Conclusions: Long-latency intracortical inhibition was prevalent and abnormally longer-lasting
in patients. Tonic hyperactivation of a subpopulation of GABAergic interneurons in
the motor cortex of patients may be the mechanism responsible for this abnormality.
Our findings seem to be specific to cerebellar diseases and are the opposite of those
found in movement disorders such as dystonia and Parkinson's disease. These data suggest
that the cerebellum and the basal ganglia may have opposite influences in tuning the
excitability of the motor cortex.
Keywords
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Article info
Publication history
Accepted:
October 16,
2003
Received in revised form:
September 2,
2003
Received:
May 14,
2003
Identification
Copyright
© 2003 Elsevier B.V. Published by Elsevier Inc. All rights reserved.
