Inclusion Body Myositis – Magnetic Resonance Imaging (MRI)

Magnetic resonance imaging (MRI) produces images of tissue slices, similar to CT scans, but uses magnets instead of x-rays. Images of the legs or forearms can detect which specific muscles are most affected by the disease. MRI is not routinely ordered for inclusion body myositis, but is often useful. Improved MRI techniques and recent research findings are increasing the role of MRI in inclusion body myositis.

 

What are the MRI Findings in Inclusion Body Myositis?

Types of Muscle Changes Visible on MRI

  • Muscle atrophy, visible as reduced size of the muscles
  • Fat accumulation within muscles, seen as signal hyperintensity on T-1 weighted sequences. As muscles are damaged, muscle fibers are replaced by fat.
  • Inflammation and muscle swelling, or edema, visible as hyperintensity on T-2 weighted sequences, such as when using Short Tau Inversion Recovery (STIR) or Spectral Attenuated Inversion Recovery (SPAIR).

 

MRI images of the Thigh in Inclusion Body Myositis vs. Normal

Anatomical Location of MRI Changes —Which Muscles are Affected?

MRI findings in the forearm and thigh correspond to physical examination findings, while lower leg MRI can reveal muscle damage that isn’t detectable on physical exam.

In the forearm, the flexor digitorum profundus, which flexes the fingers, is most commonly affected.

In the thigh, the quadriceps femoris is typically abnormal, especially the vastus lateralis and the vastus medialis, with relatively less involvement of the rectus femoris.

Lower leg MRI frequently shows abnormalities in the medial gastrocnemious. The gastrocnemious enables plantar flexion, the ankle movement that allows us to stand on our toes. Weakness of the medial gastocnemious may not be noticed during an office exam. This is because the lateral gastrocnemious and the soleus, additional muscles which also are used in ankle plantar flexion, are much less affected, and they cover-up, or mask, the weakness of the medial gastrocnemious.

Video Demonstrating the Gastrocnemius Muscle

 

Uses of MRI for Inclusion Body Myositis

 

MRI Can Help Confirm the Diagnosis of Inclusion Body Myositis

The types of muscle changes seen on MRI, combined with the specific muscles where these changes are seen, can distinguish IBM from similar diseases. Research has shown that diagnosing IBM based on MRI findings can be highly accurate. MRI can be especially helpful in early stages of the disease, or in cases that do not fit the typical, most common patterns of the disease.

MRI Can Help Select Which Muscle to Biopsy

When choosing which muscle to biopsy, the physician would like to select a muscle that is already affected by IBM, but not too severely. If a specific muscle is still healthy, the biopsy may not show clear evidence of IBM. On the other hand, if a muscle has already been badly damaged by IBM, scar tissue and loss of muscle fibers may also obscure the diagnosis. In some cases, MRI can help the physician select an appropriate muscle to biopsy.

 

MRI May be Helpful in Future Inclusion Body Myositis Clinical Trials

It is becoming increasingly clear that MRI is useful, not just to diagnose inclusion body myositis, but to monitor the disease and observe how it progresses. Using techniques of “Quantitative MRI,” doctors can precisely measure things like the amount of fat accumulation in a muscle—they can put a number on it. Studies show that these measurements are reliable, reproducible, and correlate to muscle strength. In the future, it is likely that MRI will be one of the clinical trial endpoints used to help determine if drugs are effective in treating IBM.
 

Academic Reference for More Information about MRI and Inclusion Body Myositis

The report of the 255th ENMC workshop: Muscle imaging in idiopathic inflammatory myopathies is an excellent resource for information about the use of MRI in IBM.
 
 

by Kevin Dooley, MD

Revised May 1, 2024

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