To the Editor:
We congratulate Canales et al.1 on their innovative application of bedside ultrasound to identify frailty preoperatively. Their study adds to the accruing evidence base that patients with reduced skeletal muscle mass are at increased risk of poorer outcomes after surgery. The authors present results for raw unadjusted measures and for body surface area– and body mass index–adjusted measures, which is a helpful comparison. However, this highlights one frequent inconsistency emerging in studies of this type: how to adequately account for indexation of muscle size to differences in body shape, sex, and ethnicity and whether to undertake such indexation at all.
As an example, Mueller et al.2 used a coefficient adjustment of 1.484 for female participants. This figure was derived from a single study of healthy patients predominantly under the age of 60.3 The study by Salim et al.4 standardized rectus femoris and vastus intermedius to thigh length and lumbar skeletal muscle cross-sectional area to patient height. In another study investigating mortality in critically ill patients, rectus femoris muscle was adjusted for body surface area only.5 In other studies exploring the association between psoas muscle size and outcomes, muscle size has been stratified into tertiles or quartiles by sex or by both body surface area and sex.6
To further obfuscate the utility of such skeletal muscle measurements, ethnic disparities may occur.7 In all the above cited studies, where reported, the majority of patients have been Caucasian. The study by Canales et al.1 was a small pilot study, but it is interesting to note that all Hispanic patients (five participants) were classed as frail, when Hispanics, especially males, have significantly less skeletal muscle mass and a higher rate of decline with ageing when compared to other groups.7
Ultrasound measurement of major muscles is a conceptually and logistically attractive surrogate for sarcopenia and/or frailty and holds the promise of providing an objective outcome for prehabilitation programs. It is hard to envisage how such a measure will move beyond an interesting association found in the separate and disparate populations that have been studied. There is much work to be done to find, if at all possible, universally acceptable measurements that are generalizable. We hope future researchers report multiple methods of indexation and raw data for comparison, as Canales et al.1 have done, and that larger studies attempt to unravel some of these unknowns to enable a more evidence-based standard of reporting.
The authors declare no competing interests.