Comparison of amplitude and area decrement in repetitive nerve stimulation.

OBJECTIVE To access the percentage of the patients whose repetitive nerve stimulation (RNS) studies were negative for 10% amplitude decrement but positive for 10% area decrement and to compare these disagreed results with specialist physician's diagnosis. STUDY DESIGN Retrospective descriptive study. SETTING Electrodiagnosis laboratory, Department of Rehabilitation Medicine, King Chulalongkorn Memorial Hospital. MATERIAL AND METHOD All of the electromyography (EMG) reports of RNS studies were reviewed Both 10% amplitude and area decrement were used as criteria for diagnosis in each patient. The disagreed results would be compared to final diagnosis of the specialist physicians that were recorded in out-patient medical records. RESULTS Eighty-three reports were included in the present study. Nineteen records (22.9%) were negative for 10% amplitude decrement but positive for 10% area decrement. Three records (3.6%) were positive for 10% amplitude decrement but negative for 10% area decrement. Twenty-two patients had disagreed results. Sixteen disagreed out-patient medical records (72.7%) were available for review the final specialist doctors' diagnosis. About 69% of patients, whose test was negative for 10% amplitude decrement but positive for 10% area decrement, were diagnosed as myasthenia gravis (MG) or suspected MG. All of the patients, whose test was negative for 10% area decrement but positive for 10% amplitude decrement, were diagnosed as MG. The use of both 10% amplitude and area decrement instead of 10% amplitude decrement alone will provide additional diagnostic yields in about 13% of the cases. CONCLUSION Twenty-three percent of patients had disagreed RNS results that were negative for 10% amplitude decrement but positive for 10% area decrement. When these disagreed results were compared to the final diagnosis of specialist doctors, 69% of these patients were diagnosed or suspected and treated as MG. Using both 10% amplitude and area decrement may improve sensitivity of MG diagnosis in about 13% of the cases.