Differential effects of peripheral vibration on motor-evoked potentials in acute stages of stroke

The excitability of sensorimotor cortex and spinal motoneurones can be modulated by afferent signals arising from the periphery. Low- and high-frequency vibrations activate separate classes of afferent units in the periphery. Low-frequency vibrations (2-100 Hz) activate the type I fast adapting afferent units (FA-I), whereas high-frequency vibrations (60-1000 Hz) preferentially activate the type II units (FA-II). Muscle spindles are also sensitive to high-frequency mechanical vibrations. Motor-evoked potentials (MEP) generated in response to transcranial magnetic stimulation (TMS) can be modulated by afferent signals. However, it is not clear whether these interactions take place at cortical or spinal cord levels.

Cerebrovascular attacks resulting in stroke generally affect both sensory and motor systems. In eight stroke patients with partial motor deficit in the first two weeks of the incident we studies the effects of low- (30 Hz) and high- (130 Hz) frequency mechanical vibrations on the MEPs obtained in response to TMS. Recordings from the abductor digiti minimi muscle were carried out by TMS of both lesioned and intact hemispheres. Six patients were tested again four to eight weeks after the initial assessment. The results also were compared with data obtained from eight control subjects. MEPs were evoked by 50% above threshold intensities and for each testing condition initially five control MEPs were recorded. This was followed by consecutive MEPs obtained during vibration (N= 5) and between vibrations (N= 5), and the traces were averaged and analyzed.

In normal subjects both low- (30 Hz) and high- (130 Hz) frequency vibration resulted in shortening of MEP latencies. In patients, there was a similar effect on the affected side with 30 Hz, but not with 130 Hz. Stimulation of the intact hemisphere during high-frequency vibration in the second test revealed a latency shortening, which could be due to central reorganization. The amplitude of MEPs showed a stronger facilitation in the presence of low-frequency vibration in the early stage of stroke compared with normal subjects. However, in the second test the level of facilitation was reduced, indicating an effect at the cortical level.

The results suggest that a cerebrovascular accident influences the modulatory effects of afferent inputs at both spinal and cortical levels, and in time, as reorganization takes place, these altered influences settle towards normal levels.