Our study demonstrates the odds of inpatient mortality is three times higher for older trauma patients with PLVI sarcopenia compared to those without. We also found that PLVI sarcopenia is an independent risk factor for reduced survival two years following injury and is associated with reduced likelihood of being discharged home. Our findings are consistent with several previous studies examining psoas sarcopenia in trauma populations .
Correlation between M-CSA and PLVI was weakly positive but M-CSA was not a predictor of overall mortality or any other measured health outcomes in our population. There was no statistical association between sarcopenia defined by either muscle group and inpatient complications or length of stay. Our results contradict findings from other studies that have reported a positive association between M-CSA and mortality at different time points in the trauma population [9–11]. There may be several reasons for this, including differences in measurement of sarcopenia; we defined sarcopenia as the lowest quartiles in masseter and psoas populations regardless of sex. Sarcopenia was more prevalent amongst females in both groups including the PLVI group, in whom we adjusted for body stature. Other studies have defined sarcopenia with sex-based cut-offs below the median  or one standard deviation below mean . In our study, the average CSA in patients with masseter sarcopenia was 438.5 ± 49.1 mm2 in females and 420.7 ± 70.4 mm2 in males. Two other studies quoted average sarcopenic values as 224 mm2  and 343 mm2  in females, and 281 mm2  and 418 mm2  in males. This heterogeneity highlights the importance of establishing standardised cut-offs, ideally referenced by healthy, non-hospitalised populations to prevent variations and over-diagnosis.
Additionally, masseter area is associated with variations in dentition status , body surface area  and craniofacial structure . Similarly, the psoas area may be affected by other comorbidities such as osteoarthritis  and spinal disease . A limitation of our study is that due to its retrospective nature, we could not account for indicators of stature, nutrition, dentition, mobility and socioeconomic status. Adjusting for these variables, especially body habitus and stature has relevance in achieving reliable sarcopenia measurements. Height and weight can be easily recorded, but in the acute clinical setting, such as major trauma or emergency surgery, where the clinical application of sarcopenia measurement lies in augmenting emergent decision-making and prognostication, accurate measurements of these variables may not be readily available or feasible. Thus, sarcopenia measurement should ideally rely upon independent predictors of stature that can be measured on the same opportunistic imaging modality. We adjusted for this in our study using the L4 vertebral body CSA as part of the PLVI but we are not aware of an available target for adjustment for stature in masseter sarcopenia quantification.
Furthermore, our results may be impacted by exclusion bias; 34.2% of patients were excluded from statistical analysis due to inadequate visualisation of bilateral masseters, compared with only 6.6% in the PLVI group (Table I). We may have failed to capture patients with reduced muscle quality. M-CSA was measured along the longitudinal axis, which requires reconstructing the imaging plane to align with the proximal and distal attachments. If accurate M-CSA measurement relies on higher quality imaging or is technically more challenging, its viability as a metric for sarcopenia may be limited.
Our study is limited by virtue of its retrospective, single-centre design. We adjusted for injury severity but confounders such as comorbidity index, ethnicity and any operative interventions were not examined. While psoas is the most commonly used muscle group in radiological evaluation of central sarcopenia in the trauma population , there is only one other study that looks specifically at PLVI . Conversely, this showed an association of PLVI with morbidity but not hospital mortality. Differences in PLVI cut-offs and determinants of inpatient morbidity could explain this disparity.
The prospect of muscle segmentation on volumetric CT imaging using deep learning algorithms provides exciting opportunity for further work in this area and may overcome many of the challenges in sarcopenia measurement, improving precision and validity [19, 20]. The trauma population is unique in the challenges it imposes given the heterogeneity of injuries inflicted- in terms of severity, quantity and distribution of affected body areas. This makes prognostication and clinical decision-making more difficult. Given that many patients in a trauma or neurosurgical setting only undergo CT imaging of the head or neck, it is crucial that future studies focus upon cranial as well as abdominal imaging modalities to develop pragmatic clinical applications for opportunistic sarcopenia assessment. Some studies have indicated that morphometric analysis of temporal muscle thickness [21–22] or zygomatic thickness  may be suitable craniofacial surrogates of central sarcopenia. Composite analysis of all facial muscles may serve to enhance diagnostic accuracy. Combining sarcopenia as an objective metric with clinical frailty scoring may allow multi-dimensional frailty assessment that can augment prognostication and clinical decision-making. It may serve to identify patients that will benefit most from multi-disciplinary interventions and navigate decision making around procedural interventions, discharge planning and palliation.