Primary hydatid disease of the axilla: An unusual presentation

INTRODUCTION

Hydatid disease is a parasitic infection caused by tapeworm larvae of the Echinococcus granulosus and continues to be encountered in many livestock-raising regions across the world. Humans get infected incidentally through ingestion of the parasite eggs, following which the embryos penetrate the intestinal wall and enter the circulation. Due to their filtering function, the liver and lungs remain the most frequently affected organs. However, primary involvement of soft tissues is extremely uncommon and accounts for only a small proportion of reported cases.^[1,2]

Axillary hydatid disease is exceptionally rare and may come up with a significant diagnostic challenge. On clinical examination, it becomes difficult to differentiate it from other benign or malignant lesions due to its non-specific manifestation. Patients typically present with a gradually increasing swelling leading to consideration of entities such as lymphadenopathy, abscess, lymphangioma, seroma, epidermoid cyst, chronic hematoma, or soft tissue neoplasm. Consequently, the diagnosis remains frequently unsuspected at the initial clinical assessment.^[3,4]

Imaging plays a central role in establishing the diagnosis. Ultrasonography (USG) remains the initial imaging modality of choice. Characteristic findings such as daughter cysts, cysts with trilaminar walls, floating membranes, and the classic water-lily sign can strongly suggest hydatid disease before surgery. Computed tomography accurately delineates lesion morphology, calcification, and anatomical extent, whereas magnetic resonance imaging provides superior soft-tissue characterization, demonstrating internal cyst architecture and its relationship with adjacent neurovascular structures. Early recognition is very important because invasive procedures such as aspiration or biopsy may result in cyst rupture, dissemination of parasitic contents, recurrence, or even anaphylactic reaction.^[2,5,6]

This emphasizes the diagnostic value of radiological imaging in achieving an accurate pre-operative diagnosis, avoid in advent biopsy, and facilitating appropriate surgical management.

CASE REPORT

A 45-year-old female presented with a gradually increasing, painless swelling in the right axillary region for 4–5 months. There was no history of fever, weight loss, trauma, previous surgery, or known exposure to dogs or sheep. In Figure 1 clinical examination revealed a large, well-defined, non-tender, soft to firm swelling in the right axilla. The overlying skin appeared stretched, thinned, and mildly erythematous with focal superficial ulceration at the most prominent part, showing extrusion of the whitish membranous material [Figure 1a]. Surrounding skin was normal without any evident satellite lesion.

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Figure 1: (a) Clinical image of right axilla demonstrating well defined swelling with stretched, thinned and mildly erythematous overlying skin with focal superficial ulceration at the most prominent part with extrusion of the whitish membranous material (dotted black arrow). (b,c) Gross specimen of the axillary hydatid cyst following surgical resection showing multilobulated cystic mass with thick fibrous cyst wall and surrounding fibrofatty tissue (short yellow arrow). Adjacent extracted pearly white laminated membranes are seen separately (short red arrow), a characteristic gross feature of hydatid disease. (d) Intraoperative photograph showing well defined cystic lesion in right axilla following surgical exposure (dotted blue arrow).

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USG of the right axilla demonstrated a large, well-defined hypoechoic multicystic lesion with multiple daughter cysts and detached internal membranes, producing the characteristic “water-lily sign” [Figure 2a and b]. No significant internal vascularity was detected on the color Doppler study.

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Figure 2: (a,b) Ultrasonography of the right axilla showing a well-defined multicystic lesion containing multiple daughter cysts (dotted black arrow) and detached internal membranes, producing the characteristic “water-lily sign” (dotted yellow arrow). (c,d) Coronal MRI images demonstrating T2 hyperintense multicystic lesion with daughter cysts (short black arrow) and T2 hypointense detached internal membranes within (short yellow arrow).

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Subsequent computed tomography revealed a well-circumscribed, multiloculated, hypodense cystic lesion of size 7.5 × 7.0 × 9.5 cm in the intermuscular plane within the right axilla. Multiple daughter cysts were identified within the lesion. It did not show any osseous erosion, chest wall invasion, or encasement of adjacent neurovascular structures.

Magnetic resonance imaging further characterized the lesion which appeared predominantly hypointense on T1-weighted (T1W) images and markedly hyperintense on T2W images. Multiple daughter cysts and T2 hypointense crumpled internal membranes were clearly visualized, making it highly suggestive of hydatid disease [Figure 2c and d]. Based on the characteristic multimodality imaging features, a pre-operative diagnosis of hydatid cyst was considered.

Following this, the patient underwent complete surgical excision of the lesion with meticulous care to avoid intraoperative rupture and dissemination. Gross examination of the excised specimen revealed a multicystic lesion containing multiple daughter cysts and laminated membranes [Figure 3a].

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Figure 3: (a) Gross cut section specimen of excised axillary hydatid cyst showing the characteristic pearly white folded laminated membranes of hydatid cyst (short black arrows) within a thick walled fibrotic cavity surrounded by host-derived pericystic fibro-inflammatory tissue (dotted yellow arrows). (b) Histopathological (40×) image with H&E stain revealing characteristic features of a hydatid cyst, including a thick acellular laminated membrane and an inner germinal layer along with protoscolices confirming the diagnosis of echinococcosis.

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Histopathological examination demonstrated an acellular laminated membrane with focal areas of calcification. An outer fibrotic layer containing granulation tissue and focal mixed inflammatory infiltrates. Occasional protoscolices and hooklets were also seen Figure 3b. The histopathological findings in conjunction with the imaging features confirmed the diagnosis of hydatid cyst.

Chest X-ray and abdominal ultrasound were normal excluding lung and abdominal involvement.

DISCUSSION

Hydatid disease is an important parasitic disorder in endemic regions and is most commonly associated with hepatic and pulmonary involvement. Soft tissue localization is unusual because the biological environment of skeletal muscle and subcutaneous tissues is considered unfavorable for parasite survival and growth. Continuous skeletal muscle activity, elevated lactic acid levels, and reduced oxygen tension limit implantation of the larvae at these sites. Hence, isolated axillary hydatid cysts are exceedingly uncommon, and it is rarely reported in the literature.^[1-3,7]

The clinical presentation is usually nonspecific, with most patients presenting with gradually enlarging, painful or painless mass lesion. Without any systemic symptoms or a relevant exposure history, the diagnosis can easily be overlooked. The various differential diagnoses of a cystic axillary lesion include suppurative necrotic lymphadenopathy, necrotic metastatic lymph nodes, lymphangioma, seroma, chronic organized hematoma, epidermoid cyst, and cystic soft-tissue tumors. Therefore, imaging findings become valuable in narrowing down the diagnostic possibilities [Table 1].^[3,8]

USG remains the initial imaging modality of choice because of its easy accessibility and ability to clearly demonstrate the cyst architecture. The identification of daughter cysts, trilaminar cyst membrane, and detached endocyst membranes is highly suggestive of hydatid disease. In the current case, visualization of floating membranes producing the water-lily sign provided an important diagnostic clue. Computed tomography further delineated the lesion, its extent, and confirmed the absence of osseous erosion or chest wall invasion. Magnetic resonance imaging offered superior soft tissue characterization and accurately depicted the multiloculated cystic nature of the lesion, its internal architecture, and its relationship to adjacent neurovascular structures.^[2,5]

We encountered excellent correlation between imaging and pathological findings. The detached internal membranes observed on ultrasound and magnetic resonance imaging corresponded to the laminated parasitic membranes identified on gross examination. Similarly, the multicystic appearance seen radiologically reflected the presence of multiple daughter cysts within the parent cyst. Such radiologic–pathologic concordance strengthens diagnostic confidence and highlights the characteristic features of hydatid disease even at an unusual location [Figure 1b-d].^[2,5,6]

Definitive management is complete surgical excision while maintaining cyst integrity. Careful excision and avoidance of intraoperative rupture are essential because spillage of cyst contents may lead to local recurrence, secondary dissemination, or hypersensitivity reactions. Histopathological examination remains the gold standard for diagnosis, demonstrating the characteristic laminated membrane, germinal layer, protoscolices, and hooklets. When diagnosed preoperatively and treated appropriately, the outcome is generally favorable.^[1,6]

DIFFERENTIAL DIAGNOSIS

CONCLUSION

Isolated primary axillary hydatid cyst is an extremely rare manifestation of echinococcosis and may present as a nonspecific axillary swelling mimicking various other lesions. Recognition of characteristic imaging features, particularly daughter cysts and detached internal membrane, can facilitate establishing an accurate preoperative diagnosis and prevent potentially hazardous biopsy or aspiration. Multimodality imaging, along with histopathological confirmation, plays a pivotal role in the diagnosis and management of this uncommon entity.

TEACHING POINTS

1. Primary axillary hydatid disease is a very rare differential diagnosis for a painless axillary mass. It may present with a diagnostic challenge at an initial clinical evaluation. However, Radiological imaging can enable accurate diagnosis, guiding appropriate management.

2. Classic imaging features such as daughter cysts, detached membranes (water-lily sign), and T2 hypointense peripheral rim helps making definitive pre-operative diagnosis of hydatid cyst and avoid unnecessary biopsy and its complications.

MULTIPLE CHOICE QUESTIONS

Q1. Which is the most characteristic imaging feature of hydatid cyst?

1. Diffuse diffusion restriction

2. Daughter cysts within a larger cyst

3. Homogeneous enhancement

4. Fat-fluid level

Answer: B

Q2. Which are the most commonly involved organs in hydatid disease?

1. Kidney and spleen

2. Brain and bone

3. Liver and lungs

4. Heart and pancreas

Answer: C

Q3. Which procedure should usually be avoided before diagnosis?

1. Magnetic resource imaging

2. Computed tomography scan

3. Ultrasound

4. Unprotected biopsy or aspiration

Answer: D