Proliferative myositis: a rare muscle inflammatory disorder

A 50-year-old man came to the emergency department with left lower arm pain for 4 days. The pain and swelling started slowly and increased until it prevented him from sleeping at night. The physical examination revealed painful swelling of lower arm. Plasma levels of the D-dimer were elevated and fever was not reported.

The X-ray (Fig 1.) showed swelling of the soft tissue with no calcification. The ultrasound (Fig.2), do it on the same day, revealed diffuse hyperechogenicity of muscle fibres and hypoechoic lines within the muscle fibers. The hypoechoic lines represent preservation of continuous muscle bundles. Venous thrombosis was ruled out. Three days after ultrasound, an MRI was performed. In MRI a soft tissue (Fig.3) mass within the brachioradialis muscle was seen. The T2-weighted images showed a hyperintense mass with hypointense structures intercalated due to preserved muscle fibers. T1-weighted image showed an ill defined mass which was isointense with the muscle. STIR sequence showed diffuse increased signal intensity of the muscle brachioradialis due to the inflammatory changes. The T1 weighted image after contrast showed homogeneous enhancement with non-enhancing geometrical web which represents endomysium and perimysium oedema.

A histologic biopsy was performed (Fig.4), where basophilic giant cells and proliferative fibroblasts was found, consistent with proliferative myositis (PM).

Proliferative myositis (PM) is a rare benign inflammatory myophaty. There are several theories about its etiology such as mechanical injury and ischemia, but none is conclusive (1-2). It occurs most commonly in adults of > 40 years, and occasionally in children(3) The most common localisation of PM are head and neck region or in the upper extremities (4).

Patients may present with a rapidly enlarging solitary, firm and painful soft-tissue mass which is able to cause local compression symptoms (5). Due to its symptoms, it may be confused with venous thrombosis or aggressive tumour.

PM shows no specific features upon laboratory, and radiographic features are frequently nonspecific. That is why the anatomopathological study of the lesion is essential for the diagnosis.

The anatomopathological analysis reveals a lobular and infiltrative mass. Its section surface is gray in color, with a solid texture. Pathologically, the proliferative mass is characterized by a checkerboard of myofibroblasts infiltrating muscle fibers on the transverse section, and ganglion-like cells (6).

The most useful diagnosis probe for PM diagnosis is MRI. The mass is hypointense to isointense on T1-weighted sequences. Contrast enhancement may be variable but is typically homogeneous.

The mass appears hyperintense on T2- weighted sequences with areas of low signal. Preserved muscle fibres can be seen within the mass, appearing as linear hypointense structures on T2-weighted sequences. This last finding is the key for the diagnosis, not described in other similar diagnoses or in soft tissue malignancies (7).The differential considerations of proliferative myositis are wide and contain a lot of entities such as trauma, myositis ossificans, neoplasm, inflammatory diseases (polimyositis), muscular infarction and lymphoma (8).

The initial treatment should be conservative due to PM tends to the spontaneous resolution, so watch and wait strategy is preferred. Since local recurrence is very rare and no cases of metastasis have been reported (9).

The diagnosis of PM may sometimes be difficult and, in some cases diagnosis is not confirmed until after surgical resection. Core-needle biopsy is helpful in the diagnosis, thus avoiding unnecessary surgical trauma.