Adverse Muscle Composition : Revisiting Sarcopenia in General Population and Liver Disease using Magnetic Resonance Imaging

Sammanfattning: Sarcopenia - from the Greek words 'sarx' (flesh) and 'penia' (loss) - was, when coined in 1989, a term denoting the decline in muscle mass and strength that occurs with aging. Such definition implies everyone suffers from sarcopenia to varying degrees, which naturally makes studying sarcopenia challenging. Early (practical) definitions of sarcopenia focused on identification of low muscle mass, while later definitions also include criteria of low muscle strength. Use of such definitions to study sarcopenia has shown that wasting is intensified in those suffering from metabolic diseases, and even more rapid in end-stage diseases. Although it is unknown whether sarcopenia accelerate disease or the other way around, detection of sarcopenia concurrent with other diseases clearly identifies a vulnerable subgroup of patients who may need more extensive care.In severe stages of liver disease, poor muscle health has been linked to higher morbidity and mortality, and may affect the outcome of liver transplantation. Sarcopenia is therefore recognized as an important factor that should affect both clinical decision-making and intervention in patients being evaluated for liver transplantation. However, sarcopenia is poorly understood (and commonly overlooked) in earlier stages of disease, where the potential of preventative care is greater. One challenge has been the prevalence of obesity in diseases that may precede more advanced disease, such as non-alcoholic fatty liver disease (NAFLD). Due to their larger body size, individuals with obesity need more muscle mass to maintain mobility function. Therefore, the threshold for what is considered ‘low muscle mass’ needs to be higher, or somehow adjusted for body size.This thesis started by applying the European definition of sarcopenia in 10,000 individuals aged 44-78 years volunteering for the UK Biobank imaging study. It was identified that current body size adjustments used to detect 'low muscle mass' were ineffective. The consequence of this was underdiagnosis of sarcopenia in individuals with overweight and obesity.Therefore, a more personalized muscle volume assessment, that was independent of body size, was developed with the aim to describe how much an individual is deviating from what is expected and address whether they have an 'adequate' amount of muscle volume - muscle volume z-score.Muscle volume was measured using magnetic resonance imaging and from the same images, muscle fat infiltration (indicating muscle quality) was also quantified. The first results indicated that muscle volume z-score and muscle fat infiltration were independently associated with mobility function and hospitalization, and that a combination of the two may identify the most vulnerable individuals. Therefore, thresholds were suggested to identify an adverse muscle composition (low muscle volume z-score combined with high muscle fat infiltration).Following studies investigated associations of adverse muscle composition with metabolic diseases, mobility function, and mortality in general population and NAFLD. Overall, the studies showed that adverse muscle composition was associated with increased morbidity and mortality independent of mobility function, and indicated that muscle composition assessment could provide clinically relevant information that may be useful in risk-stratification of heterogeneous disease populations like NAFLD.Today, the relevance of adverse muscle composition and potential clinical use cases are evaluated in the liver transplant setting through both European and American clinical studies.