Hydatid cyst of thigh

A 30-year-old female patient presented with swelling on the anterior aspect of the right thigh for the past 3 months.

Initial ultrasonographic evaluation revealed a large multiloculated cystic lesion involving approximately the upper two-thirds of the anterior aspect of the right thigh.
Further MRI revealed a large multiloculated cystic lesion which appeared hyperintense on T2-weighted images and hypointense on T1-weighted images involving the anterior compartment of the right thigh in the intermuscular plane with internal small daughter cysts. It caused medial displacement of the rectus femoris muscle, lateral displacement of the vastus lateralis muscle, posterior displacement of the vastus medialis muscle. On post-contrast study, there was minimal peripheral rim enhancement.

Muscle hydatid disease is a very rare clinical and radiological diagnosis, because the hydatid has to cross the pulmonary and hepatic barriers with additional hindrance of high level of lactic acid in the muscle tissue that is unfavourable for its survival [1, 2]. Differential diagnosis of hydatid disease should be considered for every soft cystic mass in any anatomical location, especially in endemic areas. Hydatid disease of the skeletal tissue presents as large asymptomatic swelling which is fixed to the muscle, most commonly, in the thigh. The swelling being cystic in nature, can be uniloculated or multiloculated with multiple daughter cysts. Ultrasonography, MRI and CT reveal uniloculated or multiloculated cyst. The sensitivity of ultrasonography is 95% and if vesicular fibrils are present, the sensitivity of US increases to 100% [3]. Free floating membrane and multiple smaller daughter cysts maybe seen in it. MRI is reported to be the best for clear identification of involved structures and for surgical planning [4, 5]. MRI is capable of adequately demonstrating most features of hydatid disease, with the exception of calcification which is usually seen in a CT scan [6]. Eosinophilia due to parasitic infestation may or may not be present. Multiloculated lesion with internal daughter cysts is characteristically seen in hydatid disease [8]. Post-contrast peripheral rim enhancement can be seen in hydatid disease [8].

Other differentials with similar presentation include vascular malformation (cystic lesions showing avid contrast enhancement), haematoma (heterogeneous collection with variable internal signal depending on the age of the haematoma), abscess (heterogeneous collection with surrounding inflammatory changes) or myxoid tumours (which appear heterogeneous due to myxoid and solid components) [8]. Surgical excision (pericystectomy) is the treatment of choice in musculoskeletal hydatid disease [9]. Percutaneous aspiration and infusion of scolicidal agents may be considered in inoperable cases [9]. Adjuvant anti-helminthics are used in surgical treatment protocol to prevent dissemination and recurrence [9]. In our case, based on the imaging and clinical diagnosis of hydatid cyst, surgical excision was done, imaging findings of cyst including cyst being in inter-muscular plane were confirmed and cut section of cystic lesion showed multiple yellowish-white daughter cysts confirming hydatid disease. Adjuvant anti-helminthics were administered preoperatively.
Teaching point:
Considering the typical imaging findings of hydatid disease along with the clinical history, though rare, diagnosis of hydatid disease is quite straightforward.

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