Myosteatosis

Myosteatosis is excessive fat accumulation within skeletal muscle. The term is used for lipid located in and around muscle fibers and for imaging patterns that indicate greater fatty infiltration of muscle tissue. It is a feature of muscle composition, not simply a measure of how much muscle or total body fat a person has .^1,2

Myosteatosis can be assessed with imaging methods such as computed tomography or magnetic resonance imaging. On CT, lower muscle attenuation is often used as an indicator of greater lipid content, while MRI can quantify fat fractions more directly with specific sequences. Definitions and cutoffs vary across studies, body regions, populations, and imaging methods, so there is not one universal numerical threshold for myosteatosis .^1,2

The concept should be distinguished from sarcopenia, which centers on low muscle strength and, in many definitions, low muscle quantity or quality. A person can have substantial muscle mass but still have high fat infiltration. Myosteatosis is also not identical to intermuscular adipose tissue, which is visible fat between muscle groups; the broader literature may include several lipid compartments under the idea of muscle fat infiltration. Precise interpretation therefore depends on how a study measured and defined the fat .^1,2