Differential Diagnosis for Weakness, Muscle Pain, and Elevated CK and LDH
Single Most Likely Diagnosis
- Polymyositis: This is an inflammatory muscle disease characterized by muscle weakness and pain, elevated creatine kinase (CK) levels, and the presence of specific autoantibodies. The high CK level of 672 and the presence of RNP antibodies support this diagnosis, as RNP antibodies are often found in patients with polymyositis.
Other Likely Diagnoses
- Dermatomyositis: Similar to polymyositis, dermatomyositis is an inflammatory muscle disease but is distinguished by the presence of skin manifestations. While the question does not mention skin symptoms, dermatomyositis could still be considered, especially if there are subtle skin findings not reported.
- Inclusion Body Myositis (IBM): This condition is the most common acquired myopathy in adults older than 50 years, characterized by progressive muscle weakness and wasting, particularly affecting the distal muscles. Although IBM typically has a slower progression and might not present with as high CK levels, it remains a consideration.
- Mixed Connective Tissue Disease (MCTD): Given the presence of high RNP antibodies, MCTD should be considered. MCTD can present with features of lupus, scleroderma, and polymyositis, including muscle weakness and elevated muscle enzymes.
Do Not Miss Diagnoses
- Lupus Myositis: Systemic lupus erythematosus (SLE) can cause myositis, presenting with muscle weakness and elevated CK levels. The presence of RNP antibodies, which can be seen in SLE, makes this a critical diagnosis not to miss due to its potential for multi-organ involvement and the need for specific treatment.
- Statins or Other Drug-Induced Myopathy: Certain medications, notably statins, can cause myopathy characterized by muscle pain, weakness, and elevated CK levels. Identifying and stopping the offending drug is crucial for treatment and prevention of further muscle damage.
Rare Diagnoses
- Nemaline Myopathy: A genetic neuromuscular disorder that can present with muscle weakness, although it typically starts in childhood. Adult-onset cases can occur, and while CK levels might not be as high as in inflammatory myopathies, it remains a rare consideration.
- Mitochondrial Myopathies: A group of disorders affecting the mitochondria, leading to muscle weakness among other symptoms. These conditions are rare and often have additional features such as neurological symptoms or multisystem involvement.
- Glycogen Storage Diseases: Certain types of these genetic disorders can present with muscle weakness and elevated CK levels, particularly after exercise. They are rare and often have distinctive features such as cramps or myoglobinuria after exercise.