Sarcopenia (ICD-10-CM code M62.84) is a type of muscle loss that occurs with aging and/or immobility. It is characterized by the degenerative loss of skeletal muscle mass, quality, and strength. The rate of muscle loss is dependent on exercise level, co-morbidities, nutrition and other factors. The muscle loss is related to changes in muscle synthesis signalling pathways. It is distinct from cachexia, in which muscle is degraded through cytokine-mediated degradation, although the two conditions may coexist. Sarcopenia is considered a component of frailty syndrome. Sarcopenia can lead to reduced quality of life, falls, fracture, and disability. Sarcopenia is a factor in changing body composition. When associated with aging populations, certain muscle regions are expected to be affected first, specifically the anterior thigh and abdominal muscles. In population studies, body mass index (BMI) is seen to decrease in aging populations while bioelectrical impedance analysis (BIA) shows body fat proportion rising.
Signs and symptoms The hallmark sign of sarcopenia is loss of lean muscle mass, or muscle atrophy. The change in body composition may be difficult to detect due to obesity, changes in fat mass, or edema. Changes in weight or limb or waist circumference are not reliable indicators of muscle mass changes. Sarcopenia may also cause reduced strength, functional decline and increased risk of falling. Sarcopenia may also have no symptoms until it is severe and is often unrecognized. Hypertrophy may occur in the upper parts of the body to compensate for this loss of lean muscle mass. An early indicator for the onset of sarcopenia can be significant loss of muscle mass in the anterior thigh and abdominal muscles.
Causes There are many proposed causes of sarcopenia and it is likely the result of multiple interacting factors. Understanding of the causes of sarcopenia is incomplete, however. Changes in hormones, immobility, age-related muscle changes, nutrition, and neurodegenerative changes have all been recognized as potential causative factors. The degree of sarcopenia is determined by two factors: the initial amount of muscle mass and the rate at which muscle mass declines. Due to variations in these factors across the population, the rate of progression and the threshold at which muscle loss becomes apparent is variable. Immobility dramatically increases the rate of muscle loss, even in younger people. Other factors that can increase the rate of progression of sarcopenia include decreased nutrient intake, low physical activity, or chronic disease. Additionally, epidemiological research has indicated that early environmental influences may have long-term effects on muscle health. For example, low birth weight, a marker of a poor early environment, is associated with reduced muscle mass and strength in adult life.
Pathophysiology There are multiple theories proposed to explain the mechanisms of muscle changes of sarcopenia including changes in myosatellite cell recruitment, changes in anabolic signaling, protein oxidation, inflammation, and developmental factors. The pathologic changes of sarcopenia include a reduction in muscle tissue quality as reflected in the replacement of muscle fibers with fat, an increase in fibrosis, changes in muscle metabolism, oxidative stress, and degeneration of the neuromuscular junction. The failure to activate satellite cells upon injury or exercise is also thought to contribute to the pathophysiology of sarcopenia. Additionally, oxidized proteins can lead to a buildup of lipofuscin and cross-linked proteins causing an accumulation of non-contractile material in the skeletal muscle and contribute to sarcopenic muscle. In sarcopenic muscle the distribution of the types of muscle fibers changes with a decrease in type II muscle fibers, or "fast twitch", with little to no decrease in type I muscle fibers, or "slow-twitch" muscle fibers. Deinervated type II fibers are often converted to type I fibers by reinnervation by slow type I fiber motor nerves. Males are perhaps more susceptible for this aging-related switching of the myofiber type, as a recent research has shown a higher percentage of "slow twitch" muscle fibers in old compared to young males, but not in old compared to young females. Aging sarcopenic muscle shows an accumulation of mitochondrial DNA mutations, which has been demonstrated in various other cell types as well. Clones with mitochondrial mutations build up in certain regions of the muscle, which goes along with an about fivefold increase in the absolute mtDNA copy number, that is, these regions are denser. An apparent protective factor preventing cells' buildup of damaged mitochondria is sufficient levels of the protein BNIP3. Deficiency of BNIP3 leads to muscle inflammation and atrophy. Furthermore, not every muscle is as susceptible to the atrophic effects of aging. For example, in both humans and mice it has been shown that lower leg muscles are not as susceptible to aging as upper leg muscles. This could perhaps be explained by the differential distribution of myofiber type within each muscle group, but this is unknown.
Diagnosis Multiple diagnostic criteria have been proposed by various expert groups and continues to be an area of research and debate. Despite the lack of a widely accepted definition, sarcopenia was assigned an ICD-10 code (M62.84) in 2016, recognizing it as a disease state. Sarcopenia can be diagnosed when a patient has muscle mass that is at least two standard deviations below the relevant population mean and has a slow walking speed. The European Working Group on Sarcopenia in Older People (EWGSOP) developed a broad clinical definition for sarcopenia, designated as the presence of low muscle mass and either low muscular strength or low physical performance. Other international groups have proposed criteria that include metrics on walking speed, distance walked in 6 minutes, or grip strength. Hand grip strength alone has also been advocated as a clinical marker of sarcopenia that is simple and cost effective and has good predictive power, although it does not provide comprehensive information. There are screening tools for sarcopenia that assess patient reported difficulty in doing daily activities such as walking, climbing stairs or standing from a chair and have been shown to predict sarcopenia and poor functional outcomes.
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