Paired transcranial magnetic stimulation in hemifacial spasm

The aim of this study was to investigate the changes in cortical excitability in patients with hemifacial spasm (HFS) in order to examine the physiological state of facial nerve nucleus.

Nineteen patients with HFS and 13 control subjects were examined at rest. The procedure was repeated during voluntary contraction in 6 of control subjects. By paired transcranial magnetic stimulation, conditioning and test motor evoked potentials (MEP) were recorded from bilateral orbicularis oris muscles at interstimulus intervals (ISI) of 20, 25, 30, 50, 75 and 100 ms.

In control subjects at rest, ISI's of 20, 25, 30 ms evealed facilitation of the test MEP and inhibition developed at ISI's of 75 and 100 ms. Test MEP facilitation in the contracting controls and on the symptomatic sides of HFS patients was significantly less than the resting controls. Asymptomatic sides of HFS patients behaved similar to the corresponding symptomatic sides and to the contracting controls, although the values were not statistically significant when compared to the resting controls.

The lessening of facilitation found in this study can be interpreted as an abnormal finding that reflects the changes in the excitability of facial motoneuronal system in patients with HFS. However it is difficult to exclude the possibility that this finding can occur due to the normal behavior of cortex during voluntary or involuntary contraction.