Case Report: Atypical Localization of a Pulmonary Hydatid Cyst

Introduction

Hydatid disease, or hydatid cyst (HC), is a metacestodiasis caused by the larval form of the small canine tapeworm Echinococcus granulosus. Hydatid cysts predominantly affect the liver and lungs, which filter, and prevent parasite dissemination into the systemic circulation. Mediastinal hydatid cysts are exceptionally rare, comprising 0%–4% of all cases.¹ These cysts often pose a diagnostic challenge as they can be confused with other mediastinal lesions.

Case report

A 34-year-old male smoker with a 16-pack-year history and no notable medical history presented to the emergency department with a week-long history of bilateral basithoracic pain radiating posteriorly, rated 7 out of 10, and exertional dyspnea (mMRC grade 1). On physical examination, the patient was afebrile and stable in terms of both hemodynamics and respiratory function. Laboratory tests revealed inflammatory syndrome with a CRP level of 240 mg/L. Thoracic CT angiogram revealed a right anterior mediastinal mass with multiple loculations associated with pericarditis. Abdominal imaging showed two hepatic hydatid cysts. Thoracic magnetic resonance imaging (MRI) confirmed a 54 × 48 × 60 mm multivesicular hydatid cyst in the right anterosuperior mediastinum, adjacent to the pericardium, with pericardial effusion and leaflet thickening (Figure 1). Lateral tracheal and subcarinal lymphadenopathies were also observed. The hydatid serology test results were positive. The patient underwent surgery through a right lateral thoracotomy, where a cystic formation in the mediastinum was found in proximity to the pericardium and supra-aortic vessels, extending toward the lung. Cystectomy and lymph node dissection at stations 2R and 4R were performed. The patient’s postoperative course was uncomplicated. Histopathological analysis revealed a pulmonary hydatid cyst with reactive lymph nodes, and no signs of malignancy.

Figure 1. Mediastinal hydatid cyst with pericardial effusion on MRI.

Discussion

Hydatid disease is a parasitic infection caused by the larval stage of Echinococcus granulosus. The liver and lungs are the most common locations and act as filters to prevent the systemic dissemination of the parasite. Mediastinal localization is very rare, occurring in 0.1% of cases in western countries and up to 4% in endemic regions.¹ However, the mechanisms underlying this localization remain unclear. One theory suggests that the parasite crosses the hepatic and pulmonary filters, enters the systemic circulation, and lodges in the mediastinum. Another possibility is that they migrate via the lymphatic and chylous pathways.^2–4 Mediastinal hydatid cysts can occur in any part of the mediastinum, with a preference for the posterior mediastinum.⁵

The clinical presentation is non-specific, with chest pain and signs of mediastinal compression (dyspnea, dysphonia, and dysphagia) being common symptoms. However, mediastinal hydatid cysts can be discovered incidentally or may present with complications, such as rupture of the heart or large vessels.^5–7 Radiological imaging is crucial for diagnosing and assessing the extent of disease. Chest radiography typically shows a rounded or oval opacity in the mediastinum with hydatid tone. Thoracic ultrasound is a useful tool for assessing cyst characteristics, including the liquid content and thin walls. Membranous or vesicular detachment is suggestive, but rarely seen.^2–4 Thoracic CT imaging demonstrates a well-circumscribed mass of liquid density that does not enhance with contrast.^2–4,8 In our patient, the mass appeared to be an anterior mediastinal mass with multiple loculations. MRI provides detailed topographical information, helping to define the lesion’s relationship with neighboring organs, and may also reveal daughter cysts inside the main cyst.^1–4

The treatment of choice is surgical excision of the cyst, which is associated with favorable outcomes in most reports.^1,3 In our patient, the postoperative course was uncomplicated, and histopathological findings confirmed the diagnosis of a pulmonary hydatid cyst with reactive lymph nodes without malignancy.

Conclusion

Although rare, mediastinal hydatid cysts should be considered in the differential diagnosis of mediastinal masses, particularly in endemic regions. Imaging is essential for the diagnosis and evaluation of disease extent. Surgical excision remains the treatment of choice, with good outcomes in most cases.

Ethical approval

Ethical approval were not required.

Consent for publication

Written informed consent was obtained from the patient for the publication of this case report and any accompanying images. The patient was informed that all personal information would be removed to ensure confidentiality and anonymity. The patient acknowledged that the case details and images were freely available online and accessible to the public.