Discussion
With the exception of two studies which examined muscles of mastication and the study which used DXA imaging modality, there was a majority across studies to define sarcopenia in the acute setting by measuring muscle mass with an axial section of CT imaging at the level of the lumbar spine, with the psoas measured either in isolation or in conjunction with other muscle groups. The anatomic level of the imaging analysis centered around the L3-L4 vertebrae, but there was no clear preference for the exact level. Establishment of a standard anatomic reference site for evaluation of patients for sarcopenia would facilitate future research by removing variation in measurement technique, thereby permitting direct comparison of findings between studies. Based on our findings, we recommend that further investigation into radiographically assessed sarcopenia measure uses CT modalities with caudal end of the L3 vertebrae serving as the anatomic reference point.
SMI was the predominant mode of assessing sarcopenia among the reviewed studies, but there were several variations on this theme to account for morphological features of the patient. Studies like Chang et al21 calculated SMI using lean muscle mass, obtained by setting density thresholds to exclude fatty infiltrate in the imaging software used to calculate CSA. A more holistic approach was taken by Leeper et al, normalizing psoas CSA using a calculated value for body surface area (BSA).6 Akahoshi et al similarly used calculated BSA to generate an estimate for psoas CSA, and defined sarcopenia as a difference between estimate and actual psoas CSA.22
It is clear that muscle CSA alone is not sufficient to define sarcopenia, but exactly how to account for body morphology remains an area for future research. However this is done should be practical for the patient who is acutely ill. The majority of the studies reviewed in this article used height, and as such we recommend that future studies account for patient morphology using patient height. However, Ebbeling et al23 normalized psoas CSA against the CSA of the vertebral body, which has the benefit of being measured accurately and contemporaneously to psoas CSA, rather than a previously recorded height.
The definition of a sarcopenia threshold for nine of the studies was made in relation to the cohort studied, whereas five studies set the sarcopenic threshold using values obtained from other publications. Both intrinsically and extrinsically defined sarcopenic thresholds deserve close scrutiny, while acknowledging that a true reference standard is not possible until normative population data are available.
An intrinsically defined sarcopenia threshold suffers from a reduced ability to generalize findings, as the portion of the study population arbitrarily defined as sarcopenic may not be representative of the wider sarcopenic population. The best example of this is Shibahashi et al,24 where the sarcopenia cut-off value was established using a best fit analysis in which sarcopenic status reflected worse patient outcomes.
By contrast, an extrinsically defined sarcopenia threshold faces scrutiny as to the similarity between the study population and the populations for which those thresholds have been established. Both Deren et al25 and Kaplan et al18 used sarcopenia cut-off values from a 2008 study by Prado et al, which examined sarcopenic obesity in patients with solid tumors of the respiratory and gastrointestinal tract.8 26 27
The majority of the studies reviewed found a correlation between sarcopenia and outcome, despite a wide variability in methods used to measure sarcopenia as well as populations sampled. In total, 16 articles reported the presence of correlation with three reporting no correlation, lending credence to the usefulness of radiographic assessment in predicting the stratification of patient healthcare outcomes. However, clinical adoption of radiographically assessed sarcopenia is predicated on comparison between the patient’s psoas CSA and a normal distribution of psoas CSA which is representative of the population served. Therefore, we recommend further retrospective study using large pools of CT imaging to develop population-specific values for the mean and normal distribution of psoas CSA based on patient demographic factors such as age and sex.
The three studies, by Couch, Ebbeling, and Mccusker, that found no correlation between sarcopenia and measured outcomes had a few shared characteristics. All three studies were retrospective in nature and were performed in Western countries. All three had reasonable sample sizes with 225, 180, and 325, respectively. All three also had a very similar mean age and percentage of male participants, with the mean age of the studies being 76.9, 74 and 76 and the male percentage being 55%, 57%, and 64%. They each used abdominal CT scans to measure the psoas in assessing sarcopenia, although they differed in anatomic references used and muscle characteristics measured.
Couch and Ebbeling both measured the psoas using an axial slice at the L4 inferior aspect of the vertebral body whereas Mccusker measured at the most superior aspect of L3. Couch assessed lean psoas muscle area, Ebbeling measured psoas:lumbar vertebral index, and Mccusker measured total psoas area. In terms of adjustment for body morphology, Couch had none, Ebbeling used L4 vertebral body area and Mccusker used height. Cut-offs for sarcopenia diagnosis varied, with Couch not specifying their criteria, Ebbeling using patients with lower index values compared with cohort median, and Mccusker using the lowest quartile of total psoas CSA. It is not clear if these differences in methodology could account for the shared findings of no correlation between sarcopenia and outcome in these three studies.
There were some limitations to this review. First, most of the studies were performed in Western and English-speaking countries and examined trauma populations, all factors which limit the generalizability of conclusions drawn from such a narrow population. As the purpose of this review was to critically assess radiological measurement of muscle mass as an indicator sarcopenia in the setting of traumatic injury, other methods for diagnosing sarcopenia and non-acute settings were excluded. Furthermore, the results presented would carry more weight were this study conducted as a meta-analysis. However, a meta-analysis was not considered appropriate at this time due to the heterogeneity of method, classifications, and outcomes measured in the published literature.
We also must consider that sarcopenia largely affects the elderly, a population with more comorbidities and, given the opportunistic nature of data collection in the acute care setting, it is not possible to determine if patients included in each study were otherwise healthy prior to the event which resulted in their presentation for medical care. Any unknown or unlisted comorbidities at the time of patient presentation would confound assessment of sarcopenia status as well as outcome, increasing the apparent incidence of sarcopenia in the population studied, while also potentially artificially inflating any relationship between sarcopenia and morbidity or mortality. Finally, there is a bias against publication of research which does not demonstrate a significant relationship between an exposure and an outcome of interest, thus the association between sarcopenia and clinical outcomes may be weaker than the current literature suggests.
As previously noted, radiographic assessment of features like psoas CSA is a surrogate for total muscle mass, which is only one of several elements used in current clinical definitions of sarcopenia, such as the EWGSOP.28 The degree to which radiographic assessments of sarcopenia alone align with clinical definitions has not been clearly established. This is also a direction which we recommend future research focuses on, to clarify the association between clinical and radiographic assessments of sarcopenia.
Of note, the demographic characteristics and outcomes of patients with sarcopenia broadly parallel outcomes seen with the syndrome of frailty, with most frail patients exhibiting some evidence of sarcopenia.29 Although a uniform definition of frailty has yet to emerge, it has come to be recognized as a syndrome broadly characterized by diminished strength, endurance, and reduced physiologic function that thereby increases a person’s vulnerability for developing increased dependency and/or death.30 Although the Trauma-Specific Frailty Index has been used to predict discharge disposition in geriatric patients, it is limited by the requirement of a reliable historian who is able to answer lifestyle questions.30 Thus, in both frailty and sarcopenia, there remains a need for an assessment method for elderly patients who are acutely ill, whose condition may prevent full and accurate assessment using many of the current assessment tools. Radiographic measurements, despite their limitations, offer a first approximation of sarcopenia and the associated syndrome of frailty, and have demonstrated potential to inform admissions and care planning.
Establishing a standardized process for the radiographic assessment of sarcopenia is crucial to any deeper understanding of sarcopenia and its utility as a predictive marker in the clinical space. Further studies to elucidate the ideal measurement process are therefore needed, as it is still unclear which anatomic reference points, muscle characteristics, and body morphology adjustments are most salient. Additionally, there is still a wide variance in the criteria for the acute diagnosis of sarcopenia. Therefore, it would be useful to obtain a normative age and sex-adjusted data curve from a healthy adult population to establish both a uniform diagnostic threshold and methodology for the assessment of sarcopenia.
The majority of the articles analyzed in this review suggested a correlation between sarcopenia and outcomes such as morbidity, mortality, and complications. Taking this relationship into account, assessment of the effects of incorporating radiographic assessment of sarcopenia into patient management plan is needed. Furthermore, a consistent definition of sarcopenia strengthens its applicability to clinical care and ideally contributes to the optimization of hospital resource allocation. Establishing a consistent definition of radiographic sarcopenia also paves the way for future clinical trials to stratify acutely ill populations by degree of sarcopenia, which could increase the predictive capability of this measure, and by extension the strength of recommendations for therapy for these patients.
