hydatid disease is a parasitic disease caused by the larvae of a tapeworm belonging to the genus echinococcus .
it can affect any part of the human body , the liver and the lung being the most frequently involved organs .
an intra - thoracic but extra - pulmonary location is very rare , representing less than 0.1% of all cases .
mediastinal localization is exceptional with less than 150 cases reported in the literature so far .
we report an even rarer presentation of hydatid cyst in the posterior mediastinum causing erosion of the overlying ribs and adjoining vertebra with extension into the spinal canal , assuming a dumbbell shape , and simulating an aggressive malignancy .
there have been less than 10 cases of hydatid disease with mediastinal localization and intraspinal extension reported until date .
this case report emphasizes that a differential diagnosis of hydatid should be considered for a mediastinal cystic mass , however , aggressive it may appear , clinically or radiologically .
a 22-year - old female , farmer by occupation , presented with complaints of chest pain and progressive weakness of left lower limb that had persisted for 3 months .
respiratory system examination revealed dullness on percussion and tenderness in left infraclavicular , suprascapular , and interscapular regions .
radiographs of chest , performed in posterior - anterior ( pa ) and lateral projections , demonstrated well - defined homogenous soft tissue opacity in the posterior mediastinum .
there was erosion of the 4 rib posteriorly , scalloping of superior margin of 5 rib , and widened fourth intercostal space [ figure 1a and b ] .
contrast enhanced computed tomography ( ct ) of the thorax revealed a well - defined , multiloculated cystic mass in the left paravertebral space , extending into the spinal canal by causing erosion of the adjoining pedicle and lamina of fourth thoracic vertebra .
there was erosion of the overlying 4 rib with scalloping of the 5 rib and widened intervening intercostal space [ figures 2 and 3 ] .
chest x - ray ( a ) posterior anterior ( pa ) and ( b ) lateral views show well - defined homogeneous soft tissue opacity within posterior mediastinum , causing erosion of the 4 rib posteriorly , scalloping of the superior margin of the 5 rib and widening of the 4 intercostal space ( arrow ) .
contrast enhanced computed tomography ( ct ) thorax in ( a ) mediastinal and ( b ) lung windows show multiloculated cystic lesion in left paravertebral space , causing erosion of pedicle and lamina of the 4 dorsal vertebra , with intraspinal extension ( blue arrow ) .
( a ) three dimensional sagittal multiplanar reformation shows posterior mediastinal mass causing destruction of adjoining vertebra ( arrow ) with intraspinal extension .
( b ) volume rendered coronal image shows erosion of the 4 rib ( straight arrow ) and scalloping of the 5 rib ( curved arrow ) with widened 4 intercostal space .
based on the imaging findings , the possible differential diagnosis included neurogenic tumors , aggressive malignant masses like round cell tumors ( ewing 's sarcoma , primitive neuroectodermal tumors , and others ) , lymphoma , or metastasis .
since the mass was predominantly cystic with multiloculated appearance and considering the occupation of the patient , hydatid cyst was also considered a possible differential diagnosis .
the serological test with enzyme linked immunosorbant assay ( elisa ) was positive for echinococcosis , thus suggesting hydatid cyst as a possible etiology .
the patient was operated and the cystic mass along with the involved 4 rib were completely excised .
postoperative recovery was uneventful and the patient was discharged and put on adjuvant chemotherapy with mebendazole for 2 years .
the patient was monitored with routine follow - ups for 5 years with no evidence of recurrence .
hydatid disease continues to be a major health problem in underdeveloped areas where animal husbandry is common and no veterinary control exists . it is endemic in parts of south america , mediterranean regions , the middle east , africa , and australia .
humans are secondary hosts and are infected by the ingestion of food or water contaminated by eggs of these parasites . in humans ,
other sites in decreasing frequency include peritoneum , kidney , brain , mediastinum , heart , bone , soft tissues , spinal cord , spleen , pleura , adrenal glands , bladder , ovary , scrotum , and thyroid gland .
mediastinal hydatid cysts are very rare , accounting for less than 0.1% of all cases and less than 1% of thoracic involvement .
the patients may remain asymptomatic , the lesion being discovered incidentally on a routine chest radiograph .
the patients may present with non - specific symptoms of chest pain , cough , and dyspnoea .
these symptoms may be produced as a result of involvement of neighboring structures and other etiologies like hemoptysis , superior vena cava syndrome , phrenic or recurrent laryngeal nerve compression , vertebral destruction with intraspinal extension and neurological symptoms , and bernard  horner syndrome .
the complications of mediastinal hydatid cyst can be significant and include rupture into the mediastinum , pleural cavity and right ventricle , cysto - aortic fistula with possible embolization , and infection and compression of vital structures .
posterior mediastinal hydatid cyst with intraspinal extension is an extremely rare entity , with less than 10 similar cases reported in the literature till date .
though the most common cause of dumbbell masses is neurogenic tumor , a large variety of unusual lesions may produce similar picture .
these include neoplastic lesions such as benign / malignant peripheral nerve sheath tumors , plasmacytoma , chondrosarcoma , chondroid chordoma , superior sulcus tumor , metastasis , and nonneoplastic lesions such as infectious process ( tuberculosis , hydatid cyst ) , aneurysmal bone cyst , synovial cyst , traumatic pseudomeningocele , arachnoid cyst , vertebral artery tortuosity , and hemangiomas .
paravertebral masses with vertebral destruction and intraspinal extension have been described as spinal hydatids by some authors , and belong to group 5 , paravertebral hydatid disease , according to the braithwaite and lee 's classification .
the differential diagnosis of posterior mediastinal cystic lesions includes bronchogenic cysts , cystic lymphangioma , enteric cyst , and neurogenic tumors .
presence of a posterior mediastinal mass with rib and vertebral destruction and intraspinal extension can raise the possibility of aggressive malignant masses like round cell tumor ( ewing 's sarcoma , primitive neuroectodermal tumor , etc . ) , lymphoma , metastasis , or neurogenic tumor .
serological tests can often be negative , especially when the cyst is intact and uncomplicated . in suspected cases , aspiration cytology under imaging guidance
ct defines the cystic nature of the mass and delineates its exact anatomical relations . though presence of multiloculated appearance and calcification may suggest the diagnosis , there is no pathognomonic sign of hydatid cyst on ct .
magnetic resonance imaging is indicated in defining the intraspinal extension , especially when patients present with neurological signs .
the gold standard for treatment of mediastinal hydatid cyst is radical removal of the germinative membrane and pericyst . if total excision is not possible due to invasion of vital structures , partial pericystectomy after removal of germinative membrane can be performed .
it has been proposed that better results are obtained by combining surgical procedure with antihelminthic therapy postoperatively .
to conclude , hydatid cyst should always be considered in the differential diagnosis of mediastinal cystic lesions , especially in endemic areas , even when the lesion appears aggressive clinically and on imaging .
 hydatid cyst is known to affect all possible anatomical locations of the human body . however , the mediastinal localization is extremely rare . 
 this benign , commonly asymptomatic and incidentally detected disease , at times may simulate an aggressive malignancy by its potential to cause osseous destruction and intraspinal extension . a young female , farmer by occupation , presented with complaints of left chest pain and monoparesis of the left lower limb . 
 radiograph followed by computed tomography ( ct ) of the chest demonstrated a cystic mass within the posterior mediastinum , eroding and scalloping overlying ribs and extending into the spinal canal by causing destruction of adjoining vertebra , and assuming a dumbbell shape . 
 the serology was positive for echinococcosis . 
 the patient underwent surgery and the postoperative histopathology confirmed the diagnosis of hydatid cyst . 
 the patient recovered with no complications or recurrence . 
 hydatid cyst should always be considered in the differential diagnosis of mediastinal cystic lesions , however aggressive the lessions may appear . 
INTRODUCTION
CASE REPORT
DISCUSSION
CONCLUSION