Myopathy: Differential Diagnosis

Myopathy refers to a group of muscle diseases that are not caused by nerve disorders. These conditions primarily affect the muscle tissue, leading to muscle weakness. The differential diagnosis for myopathy can be categorized as follows:

• Single Most Likely Diagnosis
  • Muscular Dystrophy: This is a group of inherited disorders characterized by progressive muscle weakness and degeneration. It is one of the most common causes of myopathy, especially in younger patients.
• Other Likely Diagnoses
  • Inflammatory Myopathies (e.g., Polymyositis, Dermatomyositis): These are conditions where the muscle tissue becomes inflamed, leading to muscle weakness. They are more common in adults and can be associated with other autoimmune diseases.
  • Metabolic Myopathies (e.g., McArdle Disease): These disorders result from defects in muscle energy metabolism, leading to muscle weakness and cramps, especially after exercise.
  • Toxic Myopathies: Certain drugs and toxins can cause myopathy, such as statins, alcohol, and steroids.
• Do Not Miss Diagnoses
  • Thyroid Disorders (Hypothyroidism or Hyperthyroidism): Both underactive and overactive thyroid glands can cause muscle weakness and should not be missed due to the availability of effective treatment.
  • Electrolyte Imbalances (e.g., Hypokalemia, Hyperkalemia): Abnormal levels of potassium can lead to muscle weakness and are potentially life-threatening if not promptly addressed.
  • Malignancy-Associated Myopathy: Some cancers can cause myopathy, either directly or through paraneoplastic syndromes.
• Rare Diagnoses
  • Mitochondrial Myopathies: These are a group of disorders caused by defects in the mitochondria, the energy-producing structures within cells. They can lead to a variety of symptoms, including muscle weakness.
  • Glycogen Storage Diseases: These are inherited disorders that result from defects in the processing of glycogen, a complex carbohydrate used for energy storage. Some types can cause myopathy.
  • Lysosomal Storage Diseases: Conditions like Pompe disease, where the lysosomes (cellular structures responsible for breaking down and recycling cellular waste) do not function properly, leading to the accumulation of toxic substances within cells, including muscle cells.

Each of these diagnoses has a distinct pathophysiology and requires a tailored workup, including clinical evaluation, laboratory tests (such as creatine kinase levels, electrolyte panels, and thyroid function tests), imaging studies, and sometimes muscle biopsy. The justification for including these conditions in the differential diagnosis is based on their potential to cause significant morbidity if not recognized and treated appropriately.