Electrocardiographic abnormalities in childhood spinal muscular atrophy.

Tremors of the isoelectric line in routine electrocardiograms have been described in patients with spinal muscular atrophy and have been interpreted as fasciculations of denervated muscles. In order to evaluate this phenomenon, 13 patients with spinal muscular atrophy have been studied (average age: 37.3 months). A first electrocardiogram was recorded routinely; a second tracing was then recorded with double sensitivity and double speed. In addition, all patients were evaluated clinically and had M-mode and cross-sectional echocardiography. Regular and constant spikes on the isoelectric electrocardiographic line were recorded in 12 patients (93.4%); their frequency ranged from 39 to 48 cycles/sec (average: 42.08 +/- 2.64). Contrary to previous reports, we found an "abnormal" electrocardiogram in all our patients with severe spinal muscular atrophy. The only patient with a normal electrocardiogram had a mild and clinically stable form of spinal muscular atrophy. We did not find any significant structural cardiac abnormality by clinical and echocardiographic evaluation. We conclude that continuous tremor on the isoelectric line of electrocardiogram represents a characteristic of spinal muscular atrophy in these patients. It is a result of muscle fasciculations and does not imply any abnormality of the heart.