Sarcopenia can be defined as a significant loss of muscle function as a result of decreased skeletal muscle bulk. The definition was originally restricted to elderly individuals only, however, many use the term for people of any age.

Pathology

Sarcopenia in its primary form is age-related (geriatric patients) but can be prematurely elicited by severe, chronic illnesses (e.g. cancer) too. Sarcopenia not only diminishes the quality of life itself but also increases the risk of other comorbidities such as trauma ². 

A direct evaluation of muscle mass can be performed using imaging. However, assessment of muscle mass alone is insufficient as an imaging correlate, as muscle function can significantly decrease due to fatty infiltration (myosteatosis) of the muscles, causing significant functional impairment often without volume loss ³. Conversely, reduced grip strength and low gait speed may be used as functional markers. 

Specific criteria include ¹:

• grip strength

  • <26 kg for men

  • <16 kg for women

• low lean body mass (appendicular lean mass modified for body mass index)

  • <0.789 for men

  • <0.512 for women

The most widely accepted definition, from the European Working Group on sarcopenia in older people (EWGSOP), suggests relying on both decreased muscle function and diminished muscle mass when diagnosing sarcopenia. For clinical management the following staging system has been proposed:

• presarcopenia

• sarcopenia

• severe sarcopenia ³

Radiographic features

Dual-energy x-ray absorptiometry (DEXA) is the gold standard imaging modality in the evaluation of sarcopenia and has been used for the greatest number of trials ¹. Ultrasound, CT and MRI have also been used in the assessment of appendicular skeletal muscle mass ¹.

DEXA

Appendicular skeletal muscle mass (ASM) represents the sum of the lean muscle mass of the arms and legs and is commonly used as a diagnostic index for assessing sarcopenia ³. Appendicular skeletal muscle mass may be adjusted for height, body weight, or BMI too. 

CT

Proposed assessments of psoas muscle using CT include the psoas index and Hounsfield unit average calculation ¹.

Total abdominal muscle area (TAMA) is a measure of the area composed by muscles measured in the axial plane at the level of the L3 vertebra. (NB: This measure can be performed by automated tools which compensate for issues of fatty infiltration of muscle. Recommended lower limits of TAMA for adults:

• men: >52.4 cm²/m² 

• women: >38.5 cm²/m²

Note that several other definitions for the normal range of TAMA exist, some also incorporating subcategories based on BMI, and the cutoff values are not yet standardised ³. The TAMA, when divided by the patient's height, gives the skeletal muscle index sarcopenia measure.

MRI

Due to its better intrinsic contrast between muscle and fatty tissue MRI is considered superior for the assessment of myosteatosis. MR spectroscopy (MRS) is also capable of differentiating intramyocellular and extracellular fat ³.