Needle EMG has a lower false positive rate than MRI in asymptomatic older adults being evaluated for lumbar spinal...

False positive imaging tests--disk herniation or spinal stenosis--occur in a significant number of asymptomatic persons, increasing with age. A similar or greater prevalence probably occurs in people who present to physicians with mechanical back pain, potentially causing therapeutic misadventure. Electrodiagnostic testing may be normal in persons with asymptomatic pathology, but has not been directly tested.

As part of a larger study of older persons with lumbar stenosis, 35 asymptomatic adults were evaluated by an extensive questionnaire, codified history and physical examination, masked electrodiagnostic testing, and masked lumbar magnetic resonance imaging, with repeated procedure at 18 months. Thirty-two subjects remained after removal of three with neuromuscular disease.

The radiologist characterized 18 (56%) asymptomatic subjects as having spinal stenosis. There was no relationship between electrodiagnostician diagnosis and radiologist diagnoses. Among the 13 whom the electrodiagnostician identified as abnormal, 2 had technical data within normal limits and the only abnormality in 5 was >2/10 polyphasic motor units (considered a 'soft' finding by many). One muscle in 1 subject had abnormal spontaneous activity, and 3 persons scored >4 on paraspinal mapping. Electrodiagnostic findings were normal in the 5 (16%) who had disk herniations. None of the 22 re-examined acquired symptoms over 18 months and follow-up electrodiagnosis was essentially normal (one muscle in 1 subject had 3/10 polyphasic motor units).

MRI changes, motor unit changes on EMG needle examination, and low paraspinal mapping scores are not uncommon in asymptomatic older adults with spinal stenosis or disk herniation and may lead to false positive tests. The stricter criterion of abnormal spontaneous activity on needle examination and paraspinal mapping scores greater than 6 offered in this paper lowers the risk of false positive EMG testing.

EMG is less likely to be abnormal (false positive) in asymptomatic adults than MRI.