Decremental responses to repetitive nerve stimulation (RNS) in motor neuron disease.

OBJECTIVE

To clarify the features of decremental responses following repetitive nerve stimulation in patients with motor neuron diseases (MNDs), in comparison with myasthenia gravis (MG).

METHODS

The subjects consisted of 48 MND, 39 generalized MG and 19 ocular MG patients. Six muscles, both proximal and distal muscles, were tested.

RESULTS

Significant decrements (>5%) in at least one muscle were observed in 83% of the MND patients, and 74% and 47% of the generalized MG and ocular MG patients, respectively. Decrements were more frequently observed in the proximal muscles both in MND and MG patients (deltoid 76% and 62%, and trapezius 71% and 51% for MND and generalized MG, respectively), suggesting lower safety factors in neuromuscular transmission in those muscles. Decrements in the nasalis were rare in MND (8%) in comparison with generalized MG (54%).

CONCLUSIONS

Decremental responses were frequently observed in MND patients. There were small differences between MND and MG regarding the distribution and other features of decrements, such as the degree of the U-shape or the responses to different stimulus frequencies and to brief exercise.

SIGNIFICANCE: These results imply that the underlying mechanism regulating the decrements is common to MND and MG.