From the Guidelines
The most likely abnormal diagnostic study in this patient is serum creatinine kinase concentration (option C). This patient presents with a clinical picture consistent with polymyositis, an inflammatory myopathy characterized by progressive, symmetric proximal muscle weakness. The history of difficulty carrying groceries, carrying children, climbing stairs, and rising from a seated position all point to proximal muscle weakness. The patient also experienced transient joint pain, which can occur in inflammatory myopathies. On examination, there is tenderness of proximal muscles (deltoids and iliopsoas) with reduced strength (4/5) in proximal muscles of both upper and lower extremities, while neck flexion and sensation remain normal. In polymyositis, creatinine kinase (CK) is typically elevated due to muscle inflammation and damage, as noted in the 2021 study by 1. CK is an enzyme released from damaged muscle tissue and serves as a sensitive marker for muscle injury.
Some key points to consider in this diagnosis include:
- The patient's symptoms of progressive weakness, particularly in the proximal muscles, are consistent with polymyositis.
- The presence of mild joint pain, which resolved without treatment, does not rule out polymyositis, as joint pain can be a feature of inflammatory myopathies.
- The physical examination findings of tenderness and reduced strength in the proximal muscles support the diagnosis of polymyositis.
- The 2018 study by 1 notes that myositis is a rare but potentially severe complication of immune checkpoint inhibitor therapy, and can present with progressive weakness and elevated CK levels.
The other options are less likely:
- Acetylcholine receptor antibodies are elevated in myasthenia gravis (which typically causes fluctuating rather than progressive weakness).
- AST may be mildly elevated in muscle disorders but is less specific than CK.
- IgM elevation is not characteristic of inflammatory myopathies.
- Troponin is specific for cardiac muscle damage, which is not indicated in this patient's presentation.
Overall, the clinical presentation and examination findings in this patient are most consistent with polymyositis, and the serum creatinine kinase concentration is the most likely to be abnormal, as supported by the 2021 study by 1.
From the Research
Diagnostic Studies for Muscle Weakness
The patient's symptoms of progressive weakness of the arms and legs, mild joint pain, and muscle tenderness are indicative of an inflammatory myopathy. The following diagnostic studies are relevant to this condition:
- Serum creatine kinase (CK) concentration is a key laboratory finding in inflammatory myopathies, including polymyositis (PM) and dermatomyositis (DM) 2, 3, 4, 5, 6
- Elevated CK levels are a characteristic feature of these conditions, and monitoring CK levels can help assess disease activity and response to treatment 5
Relevant Diagnostic Options
Based on the patient's symptoms and the evidence from the studies, the most likely abnormal diagnostic study is:
- Serum creatinine kinase concentration (Option C) 2, 3, 4, 5, 6 The other options are less relevant to the patient's symptoms and the diagnosis of inflammatory myopathy:
- Serum acetylcholine receptor antibody assay (Option A) is more relevant to myasthenia gravis, a different condition
- Serum AST concentration (Option B) is a liver enzyme and not directly related to muscle weakness
- Serum IgM immunoglobulin protein concentration (Option D) is not a specific marker for inflammatory myopathy
- Serum troponin concentration (Option E) is more relevant to cardiac damage and not directly related to muscle weakness