Necrotizing myopathy (also called necrotizing autoimmune myopathy [NAM] or immune-mediated necrotizing myopathy [IMNM]) is a newly defined form of idiopathic inflammatory myopathy. In the past, all patients who presented with muscle weakness, elevated creatine kinase levels, and other symptoms of myopathy, but who didn’t have skin involvement, were classified as having polymyositis. Now, it is recognized that some of these patients have unique findings on their muscle biopsies that distinguish them from those with other forms of myositis.

Patients with necrotizing myopathy have muscle biopsies that show much less inflammation in muscle tissue than polymyositis patients, but they have increased evidence of muscle cell death (necrosis).

Researchers are also beginning to distinguish different categories of necrotizing myopathy that have different risk factors and different treatments. These distinctions are based on the presence of different autoantibodies in the patient’s blood and probably mean that these are different diseases. These different forms include:

  1. Patients with SRP autoantibodies
  2. Patients with HMGCR autoantibodies
  3. All other patients with necrotizing myopathy

Signs and symptoms

Like other forms of myositis, patients with necrotizing myopathy may experience:

  • Weakness in the muscles closest to the center of the body, such as the forearms, thighs, hips, shoulders, neck, and back
  • Difficulty climbing stairs and standing up from a chair
  • Difficulty lifting arms over the head
  • Falling and difficulty getting up from a fall
  • A general feeling of tiredness