Hypotonia of rickets: a sequential study by P-31 magnetic resonance spectroscopy

To address the role of high-energy phosphorus compounds in the hypotonia of vitamin D-dependent rickets, nuclear magnetic resonance spectra were obtained sequentially from resting gastrocnemius muscle of a 10-month-old infant with rachitic hypotonia during supplementation with vitamin D, calcium, and phosphorus. During the initial weeks of treatment, the hypotonia resolved before evidence of epiphyseal mineralization. Over the early treatment period, the muscle phosphocreatine/beta-adenosine triphosphate [PCr/beta-ATP] ratio increased from 2.7-2.8 [wk 1-2] to 3.9-4.5 [wk 7-9]. The PCr/beta-ATP ratio for 6-month-old normal infant gastrocnemius and adult forearm were 4.0 and 5.7, respectively. Muscle strength appeared to recover concomitantly with an increase in retained muscle phosphorus and high-energy phosphate compounds, and with relative increase in the muscle phosphocreatine to ATP ratio. The synchrony of clinical recovery may relate to the recovery kinetics of these metabolic changes.