Needle electromyography: its effect on serum creatine phosphokinase activity.

The effect of needle electromyography on serum creatine phosphokinase (CPK) levels has remained unclear despite the diagnostic implications. Serum CPK was serially assayed for changes before and after electromyography (EMG) in two healthy persons and ten persons having low back pain, while they remained at total bed rest. Mean curves showed a peak at 6 hours post-EMG and a return to baseline 48 hours post-EMG. The mean peak value was 1.5 times the baseline. Only one sample exceeded the normal range and it was from a patient with frank positive waves and fibrillation potentials. Correcting for dilution of CPK released, by body weight and for total trauma, by number of needle insertions did not modify the results. The time course and magnitude of elevation of CPK parallel those reported for post exercise changes but are much less than those reported to follow myocardial infarction. In the differential diagnosis of myopathies, care must be taken to ensure that the combination of needle electromyography, exercise, diurnal variation and active denervation does not lead to a "false-positive" elevation of CPK above the normal range. Needle electromyography by itself should not induce a false-positive CPK level in a normal person and no significant changes in CPK levels are likely to occur within two hours after the study.