What is muscle atrophy?
Muscle atrophy is a medical term which is used to describe the loss of muscle size or mass when concerning orthopedic injuries or conditions.
Atrophy may occur in any orthopedic area, but for the purposes of impairment rating in the the AMA Guides 5th Edition, the term is usually applied to describe muscle loss in the arms or legs.
It’s important to note that the AMA Guides 5th Edition does not formally define the term “atrophy” anywhere in the formal text, glossary or the errata papers.
What causes muscle atrophy?
Muscle atrophy may be caused by disuse of muscles, injury to the central or peripheral nervous system, or a primary disease of the muscle itself. In the context of injury, muscle atrophy is most often caused by pain limiting the exercise of muscles or nerve damage which decreases the signal allowing the muscle to contract and exercise normally.
How To Determine Muscle Atrophy in a Workers’ Compensation Exam
Muscle atrophy in the limbs are objectively measured (limb circumference) with a flexible tape measure. For impairment rating purposes it’s reported in centimeters (if you’re using the AMA Guides 5th Edition—which you should be, if you’re in California).
Muscle Atrophy in the Arms
When measuring the arms, measure at the biceps and forearms. You can look it up for yourself in Chapter 15, The Spine, which instructs the medical evaluator to measure the injured worker’s arms “at the same distance above or below the elbow;” (page 392).
Strangely, there is no rating provided for muscle atrophy in Chapter 16, The Upper Extremities.
Muscle Atrophy in the Legs
In the legs, measure at the thigh and calf. Chapter 17, The Lower Extremities, instructs the medical examiner to measure the leg “at equal distances from above the joint line or another palpable anatomical structure.”
You might notice that Table 17-16 specifically states that the thigh “is measured 10 cm above the patella with the knee fully extended and the muscles relaxed.” Also, it says the calf “is compared with the circumference at the same level on the affected side.”
Note: Nowhere in the AMA Guides does it mention the position for measuring atrophy e.g. sitting, standing, or prone.
- Muscle atrophy may be caused by disuse of muscles, injury to the central or peripheral nervous system, or a primary disease of the muscle itself.
- The AMA Guides 5th Edition does not formally define the term “atrophy” anywhere in the formal text, glossary or the errata papers.
- Muscle atrophy is a ratable finding in only two chapters of the AMA Guides 5th Edition, Chapter 15, The Spine, and Chapter 17, The Lower Extremities. (A ratable finding is a condition that affects the impairment rating.)
- Chapter 15, The Spine, instructs the evaluator to measure arms “at the same distance above or below the elbow;” (page 392). Chapter 17, The Lower Extremities, instructs the evaluator to measure the leg “at equal distances from above the joint line or another palpable anatomical structure.”
Impairment Reporting Tips
- Always define or look at the evaluators note for a description of where measurement for the assessment of muscle atrophy is performed e.g. Mid biceps or 10 cm above the knee joint line.
- Make sure muscle atrophy measurements are reported in centimeters. If they are entered in inches, convert the value to centimeters (cm) and round up to the nearest cm for 0.5 or greater values and down to the nearest cm for 0.4 or less values.
- Make sure that ratings assigned for muscle atrophy are reported and provided for the injured side.
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