Orbital hydatid disease, an exceptionally rare parasitic infection, is attributed to the tapeworm Echinococcus granulosus [6].Like many zoonotic diseases impacting humans, orbital hydatid disease typically arises from inadvertent ingestion of eggs expelled by definitive hosts, primarily canines [7]. The lungs, brain, spine, and orbit are more frequently affected in children and young adults compared to older individuals [3]. The orbit is a particularly rare site for hydatid disease, even in regions where the condition is common, accounting for less than 1% of all cases [4]. The clinical presentation of hydatid disease primarily stems from the mass effect exerted by the cyst on neighboring structures(6). This effect is particularly notable in regions with restricted space, such as the orbit [1]. In our case, consistent with typical presentations of hydatid disease, symptoms of an orbital hydatid cyst manifested gradually. Specifically, unilateral proptosis without associated pain was observed. Other reported symptoms in previous studies encompass unilateral proptosis with or without pain, visual impairment, periorbital pain, chemosis, and headache [8]. Accurate preoperative diagnosis and detailed localization are crucial for the effective management of orbital hydatid disease. Therefore, differential diagnosis should include consideration of other cystic mass lesions such as abscesses, cysticercosis, orbital hemorrhagic cyst, intraorbital conjunctival cyst and venolymphatic malformations [9]. Although an orbital hydatid cyst was at the top of the list of differential diagnosis, we were uncertain due to absence of daughter cysts on imaging. However, given that iatrogenic rupture of cyst could lead to anaphylaxis, as reported in previous cases, we decided to do lateral orbitotomy. Under controlled conditions, we intentionally ruptured the cyst to prevent the leakage of antigenic material. The pathology report subsequently confirmed the diagnosis of a hydatid cyst. In our case, orbital hydatid cysts manifested as unilocular hypodense cysts on CT imaging. However, it’s noteworthy that atypical hyperdense hydatid cysts can mimic other soft tissue orbital tumors, as observed by Sperryn et al. [10]. On CT, typical features of orbital hydatid cysts include a unilocular, non-enhancing homogeneous cyst with low density, akin to the visualization of the vitreous body [11]. If there is suspicion of a hydatid cyst, MRI is preferable for the differential diagnosis of soft tissue lesions. Orbital hydatid cysts are almost invariably situated in the superolateral and superomedial angles of the orbit, lying in or close to the muscle cone [12]. However, in our case, the hydatid cyst was located in the medial rectus muscle and presented with restrictive exotropia. This is the second reported case of intramuscular hydatid disease in the orbit [2]. Surgery is the most effective treatment for an orbital hydatid cyst. While medical therapy can be considered, it is generally less effective in intraorbital echinococcosis compared to other locations of the disease. In our case, both surgery and medical therapy were used and proved to be effective.

For surgical treatment, Radical surgery of the cyst is one option. Another option involves intraoperative aspiration of the cysts, which has the advantages of aiding diagnosis, reducing the size of the cyst, and causing the inner germinative layer to collapse, allowing the outer fibrous wall to be safely tented and snipped open [13].In the event of intraoperative cyst rupture, irrigation with hypertonic saline and hydrogen peroxide is recommended to neutralize daughter cysts and mitigate the risk of further dissemination [14]. In our case, due to the large size of the cyst, intentional rupture followed by irrigation with hypertonic saline was deemed necessary.

After confirmation of the hydatid cyst, we initiated treatment with albendazole. Albendazole is potentially more suitable for treating orbital cysts, as it has a broad spectrum of antiparasitic activity [15]. In contrast, medical therapy with mebendazole is likely ineffective in orbital hydatid disease and has no known therapeutic effect on hydatid disease of the central nervous system. Mebendazole crosses the blood-brain barrier poorly and may also penetrate the orbital region insufficiently [16].

In conclusion, although orbital hydatid cysts are rare, they should be considered in differential diagnoses in endemic regions. Accurate diagnosis, combined with intraoperative cyst rupture, irrigation with hypertonic saline, and antiparasitic treatment, is essential for effective management and favorable long-term outcomes without recurrence.