American Board of Internal Medicine (ABIM) Certification Practice Exam

What laboratory finding helps distinguish critical illness myopathy from critical illness neuropathy?

1. Decreased muscle strength

2. Normal EEG findings

3. Elevated serum CK level

4. Decreased sensory nerve conduction velocity

The correct answer is: Elevated serum CK level

The distinguishing factor between critical illness myopathy and critical illness neuropathy can often be identified through laboratory findings, specifically the serum creatine kinase (CK) level. In cases of critical illness myopathy, there is typically an elevation in serum CK levels due to muscle injury and breakdown. This is because myopathy involves damage to the muscular tissue, which allows CK, an enzyme found in high concentrations in muscle tissue, to leak into the bloodstream. In contrast, critical illness neuropathy generally does not cause significant muscle injury or a release of CK; instead, the primary issue is related to nerve function and integrity. Therefore, the elevated serum CK level serves as a useful biomarker to help differentiate between the two conditions. The other choices, while they may be relevant to the overall picture of critical illnesses, do not specifically provide the clear distinction needed between myopathy and neuropathy. For instance, decreased muscle strength is a common symptom in both conditions, making it non-specific. Normal EEG findings may indicate cerebral function but do not provide insight into muscle or nerve pathology. Decreased sensory nerve conduction velocity pertains more to neuropathy characteristics, and while it is relevant to identify neuropathies, it does not help distinguish between myopathy and neuropathy. Thus,