Short-interval intracortical inhibition in Parkinson's disease using anterior-posterior directed currents.

Reduced short-interval intracortical inhibition (SICI) is reported in Parkinson's disease (PD) and is considered to reflect abnormal GABAergic inhibitory system of the primary motor cortex in PD. We have recently shown, however, that SICI using anterior-posterior directed currents in the brain was normal in focal dystonia even though that using posterior-anterior currents was abnormal, indicating that the GABAergic system of the primary motor cortex is largely normal in dystonia. Here, we studied SICI in PD to clarify whether the GABAergic system is completely impaired in PD. We used paired-pulse transcranial magnetic stimulation to study SICI at interstimulus intervals of 3 and 4 ms with anterior-posterior or posterior-anterior directed currents in eight PD patients and ten healthy volunteers. The amount of SICI with posterior-anterior directed currents was reduced in PD patients compared with healthy volunteers; in contrast, SICI studied with anterior-posterior directed currents was normal in PD patients. These observations may be due to the difference in I-wave composition generated by the two directed currents and/or the difference in responsible inhibitory interneurons for the inhibition between the two current directions. We suggest that some or a part of inhibitory interneurons are not involved in PD. This discrepancy between SICI using posterior-anterior and anterior-posterior directed currents experiments may provide additional information about the circuits of the motor cortex.

Triad stimulation frequency for cortical facilitation in cortical myoclonus.

BACKGROUND: Abnormally enhanced cortical rhythmic activities have been reported in patients with cortical myoclonus. We recently reported a new triad-conditioning transcranial magnetic stimulation (TMS) method to detect the intrinsic rhythms of the primary motor cortex (M1). Triad-conditioning TMS revealed a 40-Hz intrinsic rhythm of M1 in normal subjects. In this investigation, we study the motor cortical facilitation induced by rhythmic triple TMS pulses (triad-conditioning TMS) in patients with cortical myoclonus. METHODS: Subjects were 7 patients with cortical myoclonus (28-74 years old) and 13 healthy volunteers (30-71 years old). Three conditioning stimuli over M1 at the intensity of 110% active motor threshold preceded the test TMS at various interstimulus intervals corresponding to 10-200 Hz. The resulting amplitudes of conditioned motor evoked potentials recorded from the contralateral hand muscle were compared with those evoked by the test stimulus alone. RESULTS: The facilitation at 25 ms (40 Hz) observed in normal subjects was absent in patients with cortical myoclonus. Instead, triad-conditioning TMS induced facilitation at a 40 ms interval (25 Hz) in cortical myoclonus. DISCUSSIONS: This change in the timing of facilitation may be explained by a shift of the most preferential intrinsic rhythm of M1, or by some dysfunction in the interneuronal network in cortical myoclonus.

Paired stimulation study of the median nerve sensory action potential in diabetic patients.

OBJECTIVES: Conventional nerve conduction studies (NCS) are not sensitive to detect mild diabetic neuropathy. In order to detect subtle changes, we compared the conventional NCS with the relative refractory period (RRP) measurement of the median sensory nerve action potential by a paired stimulation method. METHODS: Subjects were 29 diabetic patients whose conventional NCS were all normal. They were divided into two groups: neurologically symptomatic and asymptomatic groups. Twenty-eight age-matched control subjects were also studied. RESULTS: The RRP of the symptomatic diabetic patients (5.9 +/- 0.5 ms) and that of the asymptomatic patients (5.6 +/- 0.5 ms) was significantly longer than that of the control subjects (4.9 +/- 0.6 ms). There was no significant difference in RRP between the symptomatic and asymptomatic patients. This may be due to the fact that NCS reflects mainly large myelinated fiber function and early symptoms represent mainly thin myelinated or unmyelinated fiber function. CONCLUSIONS: The RRP measurement could reveal some mild involvement of peripheral nerves undetectable by conventional NCS, even though they caused no clinical symptoms.