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| Muscle assessment techniques | Advantages | Limitations | Relevance to COVID-19 | Reference |
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| Computed tomography (CT) | It reliably measures muscle quantity and fat infiltration. It can be applied to the whole body or specific muscles and serves as a base for several reliable muscle mass indices. CT routinely used for other purposes (patients with tumors) can simultaneously measure muscle mass. It predicts mortality in critical patients. | Relatively expensive and time consuming, with high radiation risk. No cutoff values for diagnosing sarcopenia. | It has successfully identified muscle loss in ICU-admitted COVID-19 patients screened for lung fibrosis, albeit patients still encounter radiation risk. | [42, 94, 102, 103] |
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| Magnetic resonance imaging (MRI) | It reliably measures muscle quantity and quality (e.g., fat infiltration). It has no radiation risk, and it can be applied to the whole body or specific muscles. | Expensive and not portable or widely available, with technical difficulties and space requirement. There are no cutoff values for diagnosing sarcopenia. Its use is limited to research facilities. | Despite its high accuracy, its use may be impractical because of its limitations. | [94, 99] |
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| Dual-energy X-ray absorptiometry (DXA) | It correlates with low muscle mass measured by MRI, and it is cheap, quick, and widely available, with low radiation risk. | Not portable and inaccurate in patients with obesity and edema, while variations between protocols limit comparison of results. | May be used as a cheaper and quick alternative of CT. However, it should not be used in obese or edematous patients. | [98, 99, 104] |
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| Ultrasound | Simple, cheap, safe, portable, and noninvasive, with a high reproducibility. | Its measurement lacks standardization, and it is not clear which anatomical sites can best predict total skeletal muscle mass. | Can be a safe and cheap method for frequent muscular assessment over the course of hospital/ICU stay. | [93, 94, 99, 105] |
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| Bioelectrical impedance analysis (BIA) | It yields results concordant with CT, and it is cheap, noninvasive, and widely available. | Results are confounded by body water distribution. It reports higher SMM in patients who are males or have edema than CT. Its equations and cutoff values are population and device specific. | Not preferred in ICU patients because of changes in the hydration status. | [95, 98, 106] |
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| Electromyography | It is a noninvasive way to assesses neuromuscular transmission denervation and deposition of endomysial connective tissue and fat. | Not widely available and requires special technical skills. | It has been used to measure maximal voluntary contraction for quadriceps and biceps in recovering patients. | [46, 99, 107] |
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| Total/partial body potassium (TBK/PBK) | It is a cheap and simple alternative of CT and MRI with less radiation exposure. It yields results consistent with DXA. | It is based on assumptions that may not hold in old and diseased conditions, e.g., fixed muscle content of nitrogen and hydration coefficient of lean body mass. | May not accurately reflect on muscle mass because critical COVID-19 patients exhibit nitrogen loss as well as multiple micronutrient deficiencies and electrolyte imbalance. | [28, 99, 108, 109] |
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| Anthropometric measures (body mass index and circumferences of the calf, thigh and mid-arm) | They are cheap and safe techniques that fit in low-resource facilities because they do not require special skills. | They do not accurately quantify muscle mass and may be confounded by edema and adiposity. | They detected malnutrition and wasting in ICU-admitted COVID-19 patients, especially among diabetics with cytokinemia and hypoproteinemia. | [21, 28, 87, 110] |
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