Ultrasound measurement of quadriceps muscle in the first year of life. Normal values and application to spinal muscular atrophy.

Using real-time ultrasound imaging normal values were obtained for quadriceps muscle thickness and subcutaneous tissue depth in the first year of life from 120 normal infants. These values were corrected for weight and height by introduction of a "correcting factor". In addition, a "modified muscle index" was calculated by multiplying the ratio of muscle to subcutaneous tissue thickness with the correcting factor and showed a steady increase from birth (mean 2.1) to the end of the first year (mean 4.8). The study of 12 patients with spinal muscular atrophy type I (SMA I) revealed that 9 out of 12 SMA I patients failed to reach the 3rd percentile for the muscle to subcutaneous tissue ratio and 10 out of 12 were well below the 3rd percentile for the modified muscle index due to their muscle atrophy. Only the two youngest SMA I patients aged 4 and 6 weeks reached values on the 3rd percentile. In contrast, 10 patients with myopathy and one patient with holoprosencephaly showed values within the normal percentiles even in the presence of severe clinical muscle weakness and signs of atrophy on muscle histology. In the differential diagnosis of early onset myopathy, atrophy of the quadriceps muscle during the first months of life in the absence of markedly increased echodensity seems to be a pathognomonic finding for SMA.