Differential Diagnosis for Proximal Muscle Weakness

Single Most Likely Diagnosis

• Polymyositis: An inflammatory muscle disease characterized by proximal muscle weakness, which is often symmetric and progressive. It is one of the most common causes of proximal muscle weakness in adults.

Other Likely Diagnoses

• Dermatomyositis: Similar to polymyositis but also includes skin manifestations such as rash and heliotrope discoloration. It is another common inflammatory myopathy presenting with proximal muscle weakness.
• Thyroid Disorders (Hypothyroidism or Hyperthyroidism): Both conditions can cause muscle weakness, with hypothyroidism more commonly associated with proximal muscle weakness.
• Diabetic Amyotrophy: A complication of diabetes that affects the nerves going to the muscles, leading to proximal muscle weakness, typically in the thighs.
• Osteomalacia: Softening of the bones due to vitamin D deficiency, which can cause proximal muscle weakness among other symptoms.

Do Not Miss Diagnoses

• Lambert-Eaton Myasthenic Syndrome (LEMS): An autoimmune disorder that can cause proximal muscle weakness, among other symptoms. It is crucial to diagnose due to its association with small cell lung cancer.
• Myasthenia Gravis: An autoimmune disease that leads to fluctuating muscle weakness, which can include proximal muscles. It is critical to diagnose due to its potential for respiratory failure.
• Guillain-Barré Syndrome: An acute inflammatory demyelinating polyneuropathy that can start with proximal muscle weakness and ascend, potentially leading to life-threatening respiratory failure if not promptly recognized.

Rare Diagnoses

• Inclusion Body Myositis (IBM): The most common acquired myopathy in adults over 50, characterized by progressive proximal and distal muscle weakness, often with a slower progression compared to polymyositis.
• Mitochondrial Myopathies: A group of disorders caused by mitochondrial dysfunction, leading to muscle weakness among other symptoms. They are rare and often present with a mix of neurological and systemic features.
• Glycogen Storage Diseases: Rare genetic disorders that affect the body's ability to store glycogen, leading to muscle weakness, among other symptoms.
• Toxic Myopathies: Caused by certain drugs or toxins, these conditions can lead to proximal muscle weakness. Examples include statin-induced myopathy and alcohol-related myopathy.