Pediatric Sarcopenia: Hidden in Plain Sight?

See “Establishing Reference Values for Lean Muscle Mass in the Pediatric Patient” by Metzger et al on page 316. We live in an era where advances in healthcare have resulted in significant reductions in mortality from pediatric illnesses. Therefore, interventions must be aimed at improving functional outcomes of illness rather than mere survival. The skeletal muscle, an important component of human body composition, plays a critical role in growth, childhood developmental milestones, day-to-day functions, and metabolism. Sarcopenia, loss-of-muscle function and mass, an underrecognized phenomenon in childhood illnesses, is often hidden in the setting of obesity and is associated with poor outcomes (1–3). The critically ill child is particularly at risk of deterioration of muscle mass and function because of escalated muscle breakdown that cannot be matched by muscle protein synthesis (4). Immobilization, sepsis, organ failure, systemic inflammation, and suboptimal protein intake further contribute to the muscle wasting, leading to acquired weakness, and impaired physical functions for years after discharge. Thus, preservation of muscle mass and function is an important goal of nutritional and metabolic support during pediatric illness. Reliable tools for measurement of muscle mass and function in children are desirable but remain elusive. Ultrasound, computerized tomography (CT) scan and magnetic resonance imaging (MRI) are imaging modalities that have been applied in adults and children to measure and track regional muscle thickness or area as a surrogate for whole body muscle mass. These techniques vary in terms of their ease of use, cost, image resolution, radiation exposure, validation, and available normative data or reference values in children (5). Accurate and serial assessment of the muscle mass will allow early detection of muscle depletion during pediatric illness. Comparison of individual patient measurements to reference values derived from normative data will provide thresholds to diagnose sarcopenia. In the present issue of the journal, Metzger et al (6) describe sex-specific and age-based reference charts for total and height-normalized psoas muscle area (PMA) in healthy children. The authors used a trauma registry to identify previously healthy children who presented with level 1 or level 2 trauma and in whom an early abdominal computed tomography (CT) scan with appropriate landmarks was available. Using a tracing tool software, the psoas muscle was outlined on axial CT images at the level of L3-4. Muscle area of the left and right psoas muscle were added to obtain total psoas muscle cross-sectional area (cm2), which was normalized to height (divided by height in meter2) to obtain the psoas muscle index (PMI). Quantile regression modeling was used to create sex-based curves for total psoas muscle mass and PMI predictive of the 25th, 50th, and 75th percentiles for ages 0 to 17 years. Similar normative data and reference curves for CT-derived PMA in children have been, however, generated in 2 prior studies (7,8). Patient population and the lumbar landmark used for the cross-sectional CT imaging varied across these studies. Harbaugh et al (7) published age-based growth curves for psoas muscle area using measurements derived from CT images at the L4 vertebrae in 2591 children who presented with acute appendicitis or trauma. Lurz et al constructed age-specific and sex-specific growth curves for PMA using CT images at L3-4 and L4- 5 levels in 779 children from a single-center trauma database (8). The populations for the 3 studies differed based on their geographic location, nutritional status, and their ethnic mix. Data from the present study by Metzer et al enrich the available normative reference curves for CT-derived PMA in children, and provide PMA standardized to height (PMI). Measurements of skeletal muscle from cross-sectional CT imaging at the lower lumbar region are correlated with total body skeletal muscle over a range of muscle mass in healthy adults (9). Hence, CT imaging for body composition analysis has been increasingly used in clinical and research applications in adults. Despite being a surrogate for total body muscle mass, their widespread application for skeletal muscle assessment in children will remain limited. Unlike MRI that has high-image resolution and no risk of radiation; the associated ionizing radiation exposure makes CT a poor test for routine muscle mass assessments in children. The most reliable lumbar landmark for CT cross-sectional image used for PMA measurements is unclear and has been variable across prior studies. Compared with L3-4 landmark used in the present study, measurement of the more rounded cross-sectional psoas muscle shape at L4-5 has been reported to have a higher interrater reliability (8). Despite these limitations, when cross-sectional lumbar CT images are acquired as part of standard care, they provide convenient access to reliable body composition information (10). Loss-of-muscle mass and function are important predictors of poor outcome, and therefore, relevant targets for nutritional, pharmaceutical, and rehabilitative therapies. Hence, there is heightened interest in skeletal muscle mass assessments in children. With recent advances in imaging and other techniques, the toolkit available for muscle mass assessments continues to expand but an ideal method for pediatric application remains elusive (5). The ideal method for muscle mass assessment in children must have normative data, high-image resolution, easy availability, relatively low cost, validation against a gold standard, and preferably correlation with muscle function. It could be used as a legitimate biologic marker of muscle health, help early detection of hitherto elusive diagnosis of sarcopenia, and grade its severity. Serial imaging of muscle mass could help monitor the impact of ongoing illness on muscle mass and examine the efficacy of therapies that are aimed at preserving muscle function and improve patient outcomes.