On the relation between the EMG frequency spectrum and the duration of symptoms in lesions of the peripheral motor neuron.

Two samples of muscles, one with proved, the other with doubtful neurogenic lesions have been investigated with EMG frequency analysis. Confirming reports from previous authors it was possible to show that a large proportion of muscles with proved lesions have a displacement of the frequency spectrum towards low frequencies("bass" displacement). The displacements were principally found in cases with old lesions. In a group of cases in which the symptoms had lasted a few months only a displacement towards high frequencies ("descant" displacement) was revealed. In view of these findings it was surprising to note a relatively large number of spectra with bass displacement in a group of cases in which the symptoms had lasted less than one month. The probable explanation of this is to be found in the fact that the muscles with bass displacement in this group principally belonged to patients with systemic diseases such as diabetes, alcoholism, renal insufficiency, itc., who quite possibly might have had earlier neurogenic lesions with a subclinical course. In the group of muscles with doubtful neurogenic lesions there was also found a comparatively large number of bass-displaced spectra. It was possible to show that here also there was a large number of patients with systemic diseases.