A rare cause of soft-tissue mass in a hunter: muscular echinococcosis

Clinical History

The patient presented to the outpatient surgery clinic with long-standing swelling of the left gluteal region. He had no relevant medical history but mentioned that he was an amateur hunter.

Imaging Findings

The patient was referred for ultrasound of the left gluteal area. This examination revealed the presence of a cystic structure which contained multiple smaller cysts. Some of these cysts were in contact with the lesion’s wall. The structure’s long diameter was 27cm. (Fig. 1) This “cyst within the cyst” ultrasound image was characteristic and raised suspicion of the potential diagnosis. Colour Doppler examination showed increased peripheral vascular signals and hyperaemia of the septa between the cysts.
The patient subsequently underwent MRI examination in a different hospital which demonstrated a large, well-marginated, fusiform, multilocular cystic lesion which was situated within the borders of the left gluteus medius muscle. After IV contrast administration, the lesion was only slightly enhanced. Imaging of the surrounding tissues was normal. (Fig. 2)

Discussion

Echinococcosis constitutes a parasitic disease common in rural countries and caused by Echinoccocus granulosus. The parasite is acquired through faecal-oral transmission and affects mainly the liver and lungs and less commonly the brain, heart, kidney and the spleen. Skeletal muscles are the only organs affected in only 0.2%-3% of all patients. [1, 2] This can be explained by the muscular contractions and high concentration of lactic acid which prevent the parasite installation. [1] Moreover, the liver and the lung act as a filter in the spread of the parasite. The proximal muscles of the lower limbs are more commonly affected than the upper limbs, possibly due to their large volume and good blood supply. [3] The mean age of patients with muscular echinococcosis is reported to be 30 years. [4]
Muscles reported to be affected by muscular echinococcosis (ME) include quadriceps, gluteus, trapezius, psoas, erector spinae, biceps femoris, adductor brevis, infraspinatus, biceps brachii, triceps branchii and sternocleidomastoid. [3, 5]
ME usually presents as a soft-tissue tumour but can also cause pain or peripheral neuropathy due to compression of nerves by the cyst. There may be either motor or sensory deficit in a distribution according to the nerve affected. [1, 3]
Useful laboratory tests include eosinophil count, skin tests (like Casoni’s test) and serological tests like Western Blot assay for E. granulosus proteins. The serological tests may be falsely negative when there is no lung or liver involvement. [1, 6]
A hydatid cyst consists of the outer pericyst, a middle acellular layer and the inner germinative membrane. Ultrasound demonstrates its cystic nature and sometimes identifies the pericyst, floating membranes and daughter cysts whereas CT and MRI better describe its location and extent. Ultrasound has also been reported to be useful in evaluating the results of antihelminthic therapy. [7] CT demonstrates the calcification of the cyst wall. [3] MRI findings vary, according to the life cycle stage of the parasite, but typically include a cyst containing multiple cysts or a detached membrane and a low-intensity rim on T2-weighted images which corresponds to the pericyst and enhances after IV contrast administration. [1, 2] There may also be the characteristic “water-lily” sign. [8]
Surgical removal of the unruptured hydatid cyst is the treatment of choice. Possible complications include dissemination of the infection and allergic reactions which is also why needle biopsy should be avoided. Histology of the resected tissue is essential to establish the diagnosis. Albendazole is used when the cyst is ruptured during surgery. [1]

Differential Diagnosis List

Final Diagnosis

Histology and positive anti-echinococcus antibodies showed muscular echinococcosis.

URL:	https://www.eurorad.org/case/11775
DOI:	10.1594/EURORAD/CASE.11775
ISSN:	1563-4086