Primary hydatid cyst in rectus abdominis muscle: a rare presentation on an unusual location

Abstract

Introduction:

Hydatid disease, caused by Echinococcus granulosus, commonly affects the liver (80%) and lungs (15%). Muscular involvement is rare, with primary hydatid cysts in skeletal muscles comprising only 2–3% of cases. This report presents the first documented case of a primary hydatid cyst in the rectus abdominis muscle in Nepal, and the 11th case globally.

Case presentation:

A 38-year-old female presented with a progressively enlarging epigastric swelling and localized pain over 1 year. On examination, a firm, non-tender 5 × 5 cm mass was palpated in the left rectus abdominis muscle. Imaging, including ultrasonography and contrast-enhanced CT, showed a well-defined cystic lesion without involvement of other organs. Serology for hydatid disease was negative. The cyst was excised surgically under general anesthesia, ensuring no spillage, and the surgical site was irrigated with hypertonic saline. Histopathology revealed acellular laminated keratin, protoscolices, and inflammatory infiltrates. Postoperatively, the patient received albendazole for 4 weeks and recovered uneventfully, with no recurrence on follow-up.

Discussion:

Primary muscular hydatid cysts are rare, as muscle contraction and lactic acid provide an unfavorable environment for the parasite. Imaging plays a vital role, as serology often yields false negatives. Surgical excision is the gold standard treatment, supported by anthelmintic therapy to prevent recurrence.

Conclusion:

This case highlights a rare presentation of hydatid disease in the rectus abdominis muscle, underscoring the importance of considering atypical locations in diagnosis. Early surgical intervention and postoperative medical therapy ensure positive outcomes.

Introduction

Hydatid disease is a parasitic infestation in humans, most commonly caused by Echinococcus granulosus, which primarily affects the liver (80%) and lungs (15%)^[1]. Unusual localization of cyst includes kidney, heart, spleen, pancreas, brain, bones and muscles where only few cases of isolated involvement of muscles have been reported in adults^[2]. We are presenting a rare case of primary intramuscular hydatid cyst of rectus abdominis muscle which is the 11th reported case of solitary abdominal wall hydatid cyst so far and the first from Nepal to the best of our knowledge. This case highlights an extremely rare presentation of hydatid disease in the rectus abdominis muscle, contributing to the limited global literature on such atypical locations. It underscores the need for clinicians to consider hydatid cysts in unusual sites for timely diagnosis and management. The work has been reported in line with the SCARE 2023 criteria.

Highlights.

• First reported case of primary hydatid cyst in the rectus abdominis muscle from Nepal, and the 11th globally.

• Rare presentation of hydatid disease in the skeletal muscle, with only 2–3% of cases involving muscles.

• Imaging (ultrasonography and contrast-enhanced CT) played a crucial role in diagnosis, with negative serology for hydatid disease.

• Surgical excision was performed without cyst rupture, followed by postoperative albendazole therapy for 4 weeks.

• Histopathological findings confirmed the diagnosis with laminated acellular keratin, protoscolices, and inflammatory infiltrates.

• Successful recovery with no recurrence on follow-up, emphasizing the importance of early intervention and appropriate treatment in rare cases of hydatid disease.

Case presentation

Patient information

A 38-year female presented to our outpatient department with complaints of swelling in the epigastric region for 1 year which progressed significantly over the last 3 months. She also complained of pain over the swelling for the last 3 months. There was no associated fever, vomiting, bowel or bladder abnormalities. She came from a rural background and had reared domestic animals at her house and had frequent contact with her pet dog and other street dogs.

Clinical findings

On physical examination there was a firm globular, non-mobile, non-tender mass over the epigastric region measuring ~5 cm × 5 cm, 2 cm left from the midline. Other findings were normal.

Diagnostic assessment

Laboratory investigations of blood and urine revealed no abnormalities. Ultrasonography (USG) showed a cystic lesion in the intermuscular plane of rectus abdominis muscle with intracystic echogenic debris within thickened and edematous floating intracystic debris and contrast enhanced computed tomography (CECT) of abdomen revealed a well-defined thin walled fluid density lesion of size ~40 mm × 48 mm in the intramuscular plane of left rectus abdominis muscle in the epigastric region. Post contrast study showed mild peripheral enhancement. The lesion was abutting the lesser curvature of the stomach inferiorly with maintained fat plane (Fig. 1a, b, and c). However no involvement of any other organs like liver or lungs was seen. Despite negative serological tests, imaging findings and histopathology confirmed the diagnosis of a primary hydatid cyst (Fig. 2).

Figure 1.

CT scan abdomen showing mildly peripherally enhancing well-defined thin walled fluid filled density lesion in left rectus muscle in epigastric region. (a) CT scan abdomen coronal view. (b) CT scan abdomen axial view. (c) CT scan abdomen sagittal view.

Figure 2.

Histopathology showing protoscolices and lamellated keratin.

Therapeutic intervention

Surgical removal of the mass was planned. Under general anesthesia the mass was excised carefully (Fig. 3a, b) to avoid any spilling of contents and the surgical field was irrigated with hypertonic saline. Histopathological examination of the sample revealed features consistent with hydatid cyst, i.e., acellular laminated keratin with few protoscolices and extensive areas of necrosis with dense mixed inflammatory cells comprising lymphocytes, neutrophils and few eosinophils. Hence, the diagnosis of primary intramuscular hydatid cyst of rectus abdominis muscle was confirmed. The patient was treated with Albendazole for 4 weeks post- operatively. The postoperative period was uneventful.

Figure 3.

(a) Gross specimen of cyst after excision. (b) Gross specimen of cyst after sectioning.

Follow up and outcomes

Patient was followed up in outpatient department after 4 weeks of discharge and reported no complaints.

Discussion

Hydatid cyst is a zoonosis caused primarily by the cestode parasite Echinococcus granulosus^[1]. Most common organs involved are liver and lungs and involvement of the musculoskeletal system is rare with an incidence of about 1–5%. Primary involvement of muscles is even rarer with an incidence of about 2–3%^[3]. Frequent contractions of muscle and presence of lactic acid is one of the mechanisms of prevention of hydatid disease in the skeletal muscles^[3-12].

After transforming from an egg to an oncosphere in the human intestine, the larval stage of E. granulosus typically travels through the bloodstream and lymphatic system to primary target organs like liver and lungs. Occasionally, some larvae bypass the liver filter, entering the systemic circulation and spreading to other parts of the body, potentially causing disease in uncommon locations outside the peritoneum, such as muscle tissue^[13]. Extraperitoneal involvement is rare, though some researchers have documented a series of cases with this atypical manifestation. Aydinli, for example, reported 14 cases of primary retroperitoneal hydatid cysts^[14]. In such cases, hematogenous spread could explain the presence of a cyst in an unusual site, like the rectus muscle^[13,15]. Other possible route of abdominal muscle involvement include, penetration from the intestines into the peritoneal space and subsequent invasion of abdominal wall, lymphatic spread or direct parasite entry into the inferior vena cava and reflux implantation by valsalva maneuver during daily activity^[16].

Hydatid cysts often present with a range of clinical features that depend on their size and location. Many patients may remain asymptomatic for years, especially when the cysts are small. When symptoms do occur, they commonly include abdominal pain, particularly when the cysts are in the liver or other abdominal organs, as well as nausea and vomiting due to increased pressure. Jaundice can result if a cyst compresses the bile ducts, leading to obstruction. Intramuscular cysts present as progressively enlarging, slow-growing, and painless mass in the muscle. The cyst may cause localized swelling and tenderness, mimicking soft tissue tumors. Depending on its size and location, it may compress adjacent structures, causing discomfort or functional impairment. Other symptoms may include fever, especially if there is a secondary infection or if the cyst ruptures, along with unintentional weight loss due to chronic illness. In cases where cysts are located in the lungs, patients may experience respiratory symptoms such as cough, chest pain, or hemoptysis. Additionally, the rupture of a cyst can lead to allergic reactions, including anaphylaxis, due to the release of antigens. Larger cysts may also cause pressure effects on adjacent organs, leading to a variety of symptoms based on their location.

Patients with hydatid cysts may show mild and nonspecific increases in markers such as ESR, CRP, and eosinophil counts. While serological tests are crucial for distinguishing hydatid cysts from other conditions, they are not entirely reliable due to a high rate of false negatives, making them primarily confirmatory in nature. Imaging plays a critical role in both the diagnosis and staging of hydatid cysts^[17]. Ultrasound is often the first imaging modality used because of its ease of use, lack of ionizing radiation, repeatability, patient-friendliness, and lower cost compared to MRI and CT^[18]. It is effective in identifying hydatid cysts; however, additional imaging with MRI and CT is necessary to differentiate hydatid cysts from other soft tissue masses, cysts, infections, and tumors^[17,18]

Despite being late clinically acknowledged in case of occurrence in the usual position, our case presented because of evident swelling over epigastrium that raised concerns to the patient. To establish the diagnosis serology tests were done that did not play much role as it was negative. USG and CECT of abdomen revealed a well-defined thin-walled fluid density lesion in the intramuscular plane of left rectus abdominis muscle in the epigastric region without involvement of any other organs (Fig. 1).

Histopathological examination of a hydatid cyst typically reveals a fibrous capsule formed by the host’s immune response, along with a laminated acellular membrane that appears eosinophilic and glossy. Inside the cyst, hydatid fluid is present, accompanied by germinal membranes that may give rise to daughter cysts or protoscolices. Inflammatory infiltrates, such as lymphocytes and eosinophils, may also be observed, especially in cases of rupture or infection, aiding in the diagnosis of hydatid disease. In our case, acellular laminated keratin layer, few protoscolices and inflammatory cells were visualized (Fig. 2).

The treatment of cystic echinococcosis typically involves chemotherapy, cyst puncture, and PAIR (percutaneous aspiration, injection of chemicals, and reaspiration). However, surgical resection of the cyst is considered the definitive standard treatment^[19-21]. Anthelmintic medications such as albendazole, mebendazole, or praziquantel are commonly used in drug therapy. In our case surgical resection of cyst was performed. Medical treatment with albendazole was given both preoperatively and postoperatively.

Conclusion

This case highlights the importance of considering atypical sites like the rectus abdominis for hydatid cysts. Surgical resection and anthelmintic therapy proved effective, underscoring the need for clinical vigilance and a multidisciplinary approach.

Ethical approval

In our institution, case reports do not require ethical approval. Hence, approval has not been obtained for the same.

Consent

Written informed consent was obtained from the patient for publication of this case report and accompanying images. A copy of the written consent is available for review by the Editor-in- Chief of this journal on request

Sources of funding

The authors declare that they did not receive any funds for the report.

Author’s contribution

R.A.: identifying the case, writing the report, reviewing and editing; G.S.S.: writing the report, reviewing and editing; B.B., B.M., S.D.Y., and S.K.M.: reviewing and editing

Conflicts of interest disclosure

Not applicable.

Research registration unique identifying number (UIN)

Not applicable.

Guarantor

Rabin Acharya.

Provenance and peer review

Not applicable.

Data availability statement

Not applicable.