Differential Diagnosis for Proximal Muscle Aching and Elevated CK

• Single most likely diagnosis
  • Polymyositis: This is an inflammatory muscle disease characterized by proximal muscle weakness and elevated creatine kinase (CK) levels. The symptoms and lab findings match closely with this condition, making it a strong candidate for the single most likely diagnosis.
• Other Likely diagnoses
  • Dermatomyositis: Similar to polymyositis, dermatomyositis involves inflammation of the muscles but is distinguished by the presence of skin rash. Elevated CK and proximal muscle aching are common, but the absence of a rash does not rule out an early stage or a variant of the disease.
  • Statin-induced myopathy: The use of statin medications can lead to muscle pain and elevated CK levels. Given the widespread use of statins, this is a plausible diagnosis, especially if the patient has recently started or increased the dose of a statin.
  • Hypothyroid myopathy: Hypothyroidism can cause muscle aching and elevated CK levels. Although less common than other causes, it's a consideration, especially if there are other symptoms suggestive of hypothyroidism.
• Do Not Miss diagnoses
  • Rhabdomyolysis: Although often presents with more severe symptoms, rhabdomyolysis can start with muscle aching and significantly elevated CK levels. It's crucial to identify this condition early due to the risk of acute kidney injury.
  • Inflammatory myopathies due to other autoimmune diseases (e.g., lupus, rheumatoid arthritis): These conditions can present with muscle involvement similar to polymyositis and dermatomyositis. Missing these diagnoses could lead to delayed treatment of a systemic autoimmune disease.
• Rare diagnoses
  • McArdle disease (Glycogen storage disease type V): A genetic disorder that affects the muscle's ability to break down glycogen, leading to muscle cramps and elevated CK after exercise. It's rare but should be considered in patients with a long history of exercise-induced muscle symptoms.
  • Mitochondrial myopathies: A group of disorders that affect the mitochondria, leading to muscle weakness and other systemic symptoms. These are rare and often involve other systems, but could present with proximal muscle aching and elevated CK.
  • Toxic myopathies (e.g., due to alcohol, cocaine, or certain toxins): Certain substances can cause muscle damage, leading to elevated CK and muscle aching. These are less common but important to consider, especially with a relevant exposure history.