Inclusion Body Myositis – Diagnosis
Magnetic Resonance Imaging (MRI)
Magnetic resonance imaging (MRI) produces images of tissue slices, similar to CT scans, but uses magnetism 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 sometimes useful.
MRI can help:
- Diagnose inclusion body myositis
- Select which muscle to biopsy
MRI findings include:
- Muscle atrophy, visible as reduced size of the muscles
- Fat accumulation within muscles, seen as signal hyperintensity on T-1 weighted sequences
- Muscle swelling, or edema, visible as hyperintensity on T-2 weighted sequences, such as when using short tau inversion recovery (STIR).
MRI of the Thigh
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 is not usually 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.