Uncommon Presentation of Primary Hydatid Cyst in the Adductor Magnus Muscle: A Radiological Perspective

대내전근 내 원발성 포낭의 드문 발현: 방사선학적 관점

Introduction

Hydatid disease (HD), also known as cystic echinococcosis (CE), is a zoonotic infection caused by the larval form of Echinococcus granulosus. While the liver (65–75%) and lungs (25–30%) are the most commonly involved organs, primary involvement of skeletal muscle remains exceptionally rare, accounting for <1% of all reported cases. This article presents an in-depth analysis of a rare case of primary hydatid cyst (HC) in the adductor magnus muscle, highlighting imaging characteristics, differential diagnoses, pathology, and management strategies.

Clinical Case Overview

A 55-year-old woman presented with a painless swelling in her right thigh that had persisted for several weeks. The patient denied systemic symptoms such as fever, chills, itching, or a history of trauma. On physical examination, a nontender, ill-defined mass was palpated in the medial aspect of the thigh, with no signs of inflammation overlying the skin.

Radiologic Assessment

MRI Findings (Fig. 1–4)

Coronal and axial MR imaging sequences revealed a 55 x 40 x 76 mm complex cystic lesion localized within the adductor magnus muscle. The lesion exhibited the following features:

T1-weighted imaging: Daughter cysts appeared hypointense relative to surrounding muscle; the mother cyst was isointense.
T2-weighted imaging: Both mother and daughter cysts demonstrated hyperintensity.
Fat-suppressed T2: A low-signal-intensity peripheral rim (ectocyst) and high-signal-intensity pericyst were noted, typical of T2 rim sign.
Diffusion-weighted imaging (DWI) and ADC mapping: Daughter cysts were hyperintense on DWI, and the mother cyst showed diffusion restriction, suggestive of high cellularity or inflammatory activity.
Post-gadolinium fat-suppressed T1-weighted images: Peripheral enhancement and intermediate enhancement of the mother cyst matrix were observed.

Pathologic confirmation from resected specimens confirmed the diagnosis, revealing laminated membranes and internal germinal layers (Fig. 5).

Figure 1. Coronal T1 & T2-weighted MR image shows a multiloculated hyperintense cystic lesion with internal daughter cysts.

Figure 2. Axial MRI images reveal daughter cysts and an isointense mother cyst. Fat-suppressed T2-weighted image demonstrating low-signal rim (ectocyst) and high-signal pericyst.

Figure 3. DWI and ADC maps show restricted diffusion in the mother cyst matrix.

Figure 4. Post-contrast fat-suppressed T1-weighted coronal image with peripheral and matrix enhancement.

Figure 5. Histopathology (H&E x40) showing internal germinal layer and laminated wall structure.

Diagnosis

Final Diagnosis: Primary hydatid cyst (HC) of the adductor magnus muscle

Differential Diagnosis

Hematoma
Abscess
Benign or malignant soft tissue tumors
Complex ganglion cyst
Metastasis

While imaging patterns in HC may resemble abscesses or complex tumors, the presence of daughter cysts, peripheral rim enhancement, and specific MRI signal patterns (notably T2 rim sign) are crucial distinguishing features.

Discussion

Epidemiology and Pathogenesis

Hydatid disease is endemic in livestock-raising regions such as the Mediterranean, South America, the Middle East, and parts of Asia. The disease occurs through ingestion of E. granulosus eggs, most commonly via close contact with dogs. After intestinal penetration, larvae can disseminate hematogenously and implant in organs.

Muscle tissue is an unfavorable site for cyst development due to its contractile nature and high lactic acid levels. Hence, primary intramuscular HC is extremely rare and often misdiagnosed.

Radiological Hallmarks

MRI remains the modality of choice for musculoskeletal HC due to its superior soft-tissue contrast and multiplanar capabilities.

T2 Rim Sign: A hypointense outer rim (ectocyst) and hyperintense inner pericyst indicate fibrous capsule formation and are considered pathognomonic for HC.
Daughter Cysts: Hyperintense on T2, often clustered around the mother cyst.
Restricted Diffusion: Often observed in type 3 HC, possibly due to dense cellular or inflammatory content.
Post-contrast MRI: Shows peripheral and intramural enhancement.

Pathological Features

Histologically, HC exhibits:

Laminated ectocyst
Inner germinal layer with potential brood capsules
Absence of inflammation unless secondarily infected or ruptured

Treatment Approach

The optimal management includes:

Preoperative albendazole therapy (5 days minimum) to reduce cyst viability.
Complete surgical excision without rupture, to prevent spillage and secondary dissemination.
Postoperative albendazole treatment for at least 6 months, monitoring hepatic function.

Aspiration or marginal biopsy is contraindicated due to the risk of anaphylaxis and dissemination.

Quiz

1. What best characterizes the lesion seen on imaging?

(A) Purely cystic lesion with multiple locules
(B) An ill-defined lobulated solid mass
(C) Air-containing lobulated solid lesion
(D) Mixed semi-solid lesion with lipid content
(E) Multivesicular cyst with daughter vesicles

2. Where is the lesion located?

(A) Pectoralis major
(B) Obturator internus
(C) Iliacus
(D) Adductor magnus
(E) Gracilis

3. What is the final diagnosis based on imaging and pathology?

(A) Hematoma
(B) Abscess
(C) Metastasis
(D) Hydatid cyst
(E) Ganglion cyst

Answer & Explanation

1. Answer: (E) Explanation: MRI shows a well-defined multiloculated cyst with daughter cysts, classic for hydatid cyst (type 3).

2. Answer: (D) Explanation: The lesion was specifically localized within the adductor magnus muscle, confirmed by MRI anatomy and histopathology.

3. Answer: (D) Explanation: The Presence of daughter cysts, laminated membrane, and germinal layer is diagnostic of hydatid cyst.

Conclusion

Although extremely rare, primary hydatid cysts of the adductor magnus must be considered in the differential diagnosis of soft-tissue masses in endemic areas. MRI, especially with T2-weighted imaging and diffusion studies, is invaluable in distinguishing hydatid cysts from other soft-tissue lesions. Early recognition and complete surgical excision combined with antihelminthic therapy are crucial for optimal outcomes.

References

[1] F. Inan, et al., “Diffusion-weighted MRI in hepatic hydatid cysts,” Clin Radiol, vol. 63, no. 6, pp. 655–662, 2008.
[2] C. Oruç, et al., “Evaluation of hepatic hydatid cysts with diffusion-weighted MR imaging,” J Magn Reson Imaging, vol. 38, pp. 416–423, 2013.
[3] K. Polat, et al., “Imaging characteristics of muscular hydatid cysts: MRI and CT findings,” AJR Am J Roentgenol, vol. 185, no. 4, pp. 936–942, 2005.
[4] WHO Informal Working Group on Echinococcosis, “Guidelines for treatment of cystic echinococcosis in humans,” Bull World Health Organ, vol. 74, pp. 231–242, 1996.
[5] R. Pedrosa, et al., “Hydatid disease: radiologic and pathologic features and complications,” Radiographics, vol. 20, no. 3, pp. 795–817, 2000.
[6] M. M. Dziri, “Hydatid cyst of the liver: diagnosis and treatment,” World J Surg, vol. 25, pp. 1–7, 2001.
[7] T. D. Eckert and P. Deplazes, “Biological, epidemiological, and clinical aspects of echinococcosis,” Clin Microbiol Rev, vol. 17, no. 1, pp. 107–135, 2004.

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