Inclusion Body Myositis – Diagnosis

Introduction

The typical patient with inclusion body myositis does not receive an accurate diagnosis for at least several years after symptoms begin. Why?

  • Early symptoms are subtle and worsen gradually. Mild muscle weakness is not very noticeable, and patients can lead a normal life and perform normal day-to-day activities in spite of some loss of strength. A person can lose half his leg or grip strength and not even know it.
  • Patients often do not go to their doctor to discuss their early symptoms, believing they are just a sign of aging. A man who plays basketball with his grandchildren and finds that he can no longer shoot the ball far enough to make free throws is unlikely to see his doctor to complain.
  • Most of the symptoms are not specific to inclusion body myositis. If a woman has an unexplained fall or two, it is unlikely that her doctor would think of IBM. Falling is common, and has many causes.
  • IBM is rare, and most doctors are not familiar with it. IBM was not even clearly identified as a disease until several decades ago, and the typical primary care physician has never had a patient with an inclusion body myositis diagnosis.
  • There is no simple, reliable test for IBM. The most helpful test, a muscle biopsy, requires surgery, and even a biopsy does not always give a definite answer.

Although the diagnosis of inclusion body myositis is often difficult and complicated, an experienced doctor can frequently make a tentative diagnosis by simply listening to the patient’s story and performing a physical examination. Knowledge of key findings can often point the way to the correct diagnosis. Several additional tests are often used to help confirm the diagnosis.

Symptoms include a long, gradual history of slowly worsening muscle weakness, often demonstrated by frequent falls, difficulty standing up from a chair, and decreasing hand grip strength. Trouble swallowing, known as dysphagia, is sometimes the first symptom.

Physical Examination reveals weakness which is often greatest in certain typical areas, such as the fingers and the thighs.

Blood tests include serum creatine kinase and an anti-cN1A (NT5C1A) antibody test. The creatine kinase level can raise suspicions about muscle disease, and the antibody test can help confirm the specific diagnosis of inclusion body myositis.

Electromyography (EMG) and nerve conduction studies measure electrical activity in the arms and legs and can help determine if the disease is primarily affecting the muscles or the nerves, or both, in a pattern typically seen in IBM.

Magnetic Resonance Imaging (MRI) can show edema (swelling), atrophy (muscle wasting), and replacement of muscle tissue by fat.

Muscle biopsy is the best available test for diagnosing inclusion body myositis.

Misdiagnosis is common, but can usually be avoided if attention is paid to key findings.

The European Neuromuscular Centre (ENMC) has recently developed new guidelines for the diagnosis of inclusion body myositis. These criteria are important for determining who is eligible to participate in IBM research studies and clinical trials.

The doctors who are most expert at diagnosing inclusion body myositis are generally neurologists who specialize in neuromuscular diseases like Parkinson’s disease, multiple sclerosis, and amyotrophic lateral sclerosis (ALS). In addition, rheumatologists, who specialize in musculoskeletal diseases and systemic autoimmune diseases, may have experience with IBM.

Special Note

Each patient visit to a physician produces a list of the diagnoses responsible for the visit. These diagnoses are documented using diagnosis codes known as International Classification of Diseases codes, or ICD codes. Inclusion body myositis didn’t have an ICD code until several years ago. Use of the proper code helps IBM researchers, and allows the true prevalence of the disease to be more accurately known. Cure IBM encourages all IBM patients to politely ask if the proper ICD code for IBM, G72.41, is being used.

by Kevin Dooley, MD

Revised April 29, 2024

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