CASE 61

By: Dr. Ahmad M. Aljefri M.B.B.S

HISTORY: Weakness.

What are your findings?

What is the differential diagnosis?

What are the causes?

FINDINGS:

Axial T1-weighted and STIR images demonstrate bilateral patchy muscle and subcutaneous edema with no fatty infiltration or atrophy on the T1-weighted images.

DIFFERENTIAL DIAGNOSIS:

Inflammatory myositis.

DIAGNOSIS:

Dermatomyositis.

PEARLS AND DISCUSSION:

Patients presenting with proximal muscle weakness of the shoulder or pelvic girdle musculature raise the suspicion of myositis. The differential diagnosis of pathological process that might inflict the skeletal muscles include iatrogenic, infectious, inflammatory, neoplastic and neurological conditions. Treatment requires the proper clinical information with ancillary laboratory tests. Imaging can help in confirming the diagnosis and it is valuable in guiding the biopsy. Radiography plays a limited role in diagnostic workup and it is usually most suitable in detecting the sheet-like calcification seen in dermatomyositis. On magnetic resonance imaging, the normal signal muscle is high than water and lower than that of fat on T1-weighted images. On T2 weighted fat suppressed and inversion recovery imaging, they exhibit lower signal intensity than water and higher than fat. Pathological conditions do not only involve the muscles by altering their signal characteristics but also changing the morphology. Distributing of involvement carries an important diagnostic implication. Therefore, when imaging a patient with the clinical question of myositis it is important to perform a large field of view images of both limbs to establish the distribution. Additionally, fat infiltration and atrophy is crucial for biopsy planning in which a negative result is most likely if the biopsy is to be taken from an area with fatty infiltration. T1 weighted images are important in detecting fatty filtration and atrophy. In our institution, our myositis protocol includes an axial large field of view images of thighs in T1-weighted and STIR images.

Differential diagnosis of muscle edema includes inflammatory autoimmune conditions such as dermatomyositis, infectious myositis, early denervation, compartment syndrome, early myositis ossificans, rhabdomyolysis, radiation-induced, medications such as corticosteroids and cholesterol- lowering drugs, and sickle cell crisis. The list of the causative etiologies is extensive and therefore correlation with the clinical data is prudent.

Autoimmune myositis includes dermatomyositis and polymyositis. Both demonstrating ascending nature involving the thigh muscles and propagating proximally to involve the shoulder girdle, neck and pharyngeal muscles. Polymyositis involves the muscles only and usually involve patients of the fourth decade. Dermatomyositis on the other hand involves the muscle and skin and has a bimodal distribution, in fifth decade and childhood where the disease is most severe. In adults, dermatomyositis is usually associated with an underlying malignancy. The clinical severity of these entities parallels the severity of the muscle edema. The usual pattern of involvement is patchy bilateral symmetrical muscle involvement. In the thighs, the vastus lateralis and vastus medialis are commonly involved whereas the rectus femoris being usually spared.

FURTHER READING:

1. May, D. A., Disler, D. G., Jones, E. A., Balkissoon, A. A., & Manaster, B. J. (2000). Abnormal Signal Intensity in Skeletal Muscle at MR Imaging: Patterns, Pearls, and Pitfalls. RadioGraphics, 20(suppl_1), S295–S315. doi: 10.1148/radiographics.20.suppl_1.g00oc18s295

2. Schulze, M., Kötter, I., Ernemann, U., Fenchel, M., Tzaribatchev, N., Claussen, C. D., & Horger, M. (2009). MRI Findings in Inflammatory Muscle Diseases and Their Noninflammatory Mimics. American Journal of Roentgenology, 192(6), 1708–1716.

3. Mauricio Castillo. "MRI Findings in Inflammatory Muscle Diseases and Their Noninflammatory Mimics : American Journal of Roentgenology: Vol. 192, No. 6 (AJR).".