Presentation of hydatid disease in muscles is very rare and it is due to contractility of muscles which prevent cyst formation in an unstable area. Another reason could be high level of lactic acid in muscles that leads to not proper area to cyst growing [5, 6]. The theory of the larva escaping from two filter of pulmonary and hepatic circulation is penetrating of intestine wall by larva and entering to Inferior Vena Cava (IVC) and systemic circulation through venous plexus of intestine [1, 4, 6, 7]. Regarding slow growing behaviour of the cyst, it is possible to be asymptomatic even for 5 to 20 years. Symptoms of the cyst depend on the location, compressive effect on adjacent organs, probable rupture effect and immunological and infectious complications [1, 3, 8]. Therefore, intramuscular cysts could present with local swelling, pain and tenderness. Fifteen percent of spinal involvement was reported [6, 8]. Symptoms of spinal involvement are low back pain (85%), radicular pain (5% to 25%) and paraparesis (25% to 77%) [2]. In the presented case, a palpable mass between two scapulas without tenderness, erythematosus or signs of spinal involvement was detected. The most common differential diagnosis for these cases are pseudocysts, congenital cysts, cystic tumours, abscess haematomas [1] and myositis [6].

The main diagnosis methods are serology and imaging. ELISA (Enzyme Linked Immunosorbent Assay) for Echinococcus IgG with sensitivity of 95% and specificity of 94%, Immunoelectrophoresis (IEP) and Haemagglutination test are some of serological studies [3]. None of these studies were positive in our case and it could be explained by isolation of the parasite from the host immune system by cyst capsule (in 50% of cases). Additionally, inadequate T-cell activation and cytokine production is another explanation [1, 4]. About Imaging modalities, it is possible to benefit from plain X-Ray, Ultrasonography (USG), CT and MRI. There is Gharbi’s classification for hydatid cyst masses in USG. The main features of hydatid cysts on USG are daughter cysts, detached membrane, and double-line sign [6]. CT could discover smaller cysts which are in different organs simultaneously, differentiate the parasitic cysts from non-parasitic, detect invasion of cyst to osseous and other structures especially when the cysts calcified and utilize in treatment follow-up. Water lily sign is a characteristic feature of hydatid cyst in MRI as a detached laminated membrane which attenuation of linear area within the cyst was increased [6]. In this case, this feature was not detected and regarding high prevalence of hydatidosis in northwest of Iran and presentation of multiple cysts in various organs, Echinococcusis was the most probable diagnosis. Treatment of hydatidosis depends on various factors. Surgery is the most recommended and optimal method in cases like this [9]. In cysts with types 4 and 5 of Gharbi’s classification, posterior or centrally positioned, presentation of more than three cysts, large cysts, cysts with heavy calcification, biliary and pulmonary communication of cysts and peritoneal rupture are indications of open cystectomy. In Gharbi’s type 1 and 2 cysts, anterior or peripheral cysts, one or two cysts, small cysts and with or without minimal calcification, the laparoscopic approach is an optional method [5].The surgery could be done by En bloc method which is defined as excision of cyst with whole and continuous shell of healthy tissue or by simple deroofing and enucleation of the cyst; especially, in firmly embedded cysts such as intraperitoneal cysts attached to viscera [3]. Irritation of area with hypertonic saline 20%, silver nitrate 0.5%, formalin, aqueous iodine and etc. is beneficial to prevent secondary and recurrent cysts [6]. PAIR is an alternative therapy for hydatid cysts. It is defined as Puncture-Aspiration-Injection-Reaspiration and performed as USG-guided percutaneous aspiration, injection of 95% ethanol and reaspiration [3]. Small studies for PAIR advocated that this is a safe technique without complication or recurrence. However, in larger studies it has been not recommended. These differences could be explained by dependency of technique to how experted is the performer [1, 10]. In the presented case according to high prevalence of hydatidosis in the area, hydatid cyst considered as the most probable diagnosis preoperatively and after whole intact cystectomy, the area irrigated with hypertonic saline. Anthelminthic drugs are suitable in sporadic small cysts and to prevent the recurrence after surgery [4]. In this case 6 months of Albendazole prescribed.

There have been few literatures of paraspinal hydatidosis which two of them presented as primary and solitary cyst [1, 3]. Some of them raised in cervical [1,7] and some in lumbar [2,3,4,5,8,9,10] areas. But in our case the cyst presented in thoracic area. In conclusion, hydatid cyst should be considered as one of the differential diagnosis of all cystic lesions, especially in endemic areas which it provides the opportunity of being technically prepared for the type of intervention and the way of treatment.