Despite the growing recognition of LMSs in avian pathology, other soft tissue sarcomas, especially those occurring in less commonly examined anatomical regions, remain poorly documented. Retrobulbar rhabdomyosarcomas, for example, have been reported in birds but remain exceedingly rare among avian species³. The retrobulbar space—a confined anatomical region behind the eye—is particularly difficult to evaluate clinically in birds due to the compact structure of the avian skull and limited physical access during routine examination. Consequently, tumors in this region may be overlooked or misdiagnosed, especially in the absence of advanced imaging or postmortem histopathological investigation. The rarity of documented cases may reflect both true low incidence and significant diagnostic limitations. Subtle or nonspecific clinical signs, such as exophthalmos or periocular swelling, may be mistaken for more common conditions such as retrobulbar abscesses, trauma, or inflammatory granulomas.

This case report presents a rare case of retrobulbar sarcoma in a budgerigar (Melopsittacus undulatus), detailing the histopathological and immunohistochemical characteristics of the tumor.

Histopathology and Immunohistochemistry: During necropsy, tissue samples including the eyes were fixed in 10% formalin, processed using routine protocols, and stained with hematoxylin and eosin (H&E) for light microscopic examination.

Staining Protocol: Deparaffinization, rehydration, and antigen retrieval (specific retrieval method and buffer dependent on the antibody; typically heat-induced epitope retrieval using citrate or EDTA buffer) were performed according to standard protocols. Endogenous peroxidase activity was blocked using 3% hydrogen peroxide. Following appropriate blocking steps to prevent non-specific binding, sections were incubated with the respective primary antibodies (Table 1) at room temperature for 30 minutes. Antibody dilutions were optimized based on manufacturer recommendations and in-house validation using known positive and negative control tissues. Subsequent steps included incubation with appropriate secondary antibodies conjugated to a horseradish peroxidase (HRP)-labeled polymer detection system (e.g., Dako EnVision+System). The chromogenic reaction was developed using 3,3'-diaminobenzidine (DAB) as the substrate. Sections were counterstained with hematoxylin, then dehydrated, cleared, and mounted using a synthetic resin.

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Table 1: Antibody details

Analysis: Stained sections were evaluated by light microscopy.

Postmortem examination revealed a retrobulbar leiomyosarcoma causing periorbital swelling. The tumor was localized in the retrobulbar region, extending into adjacent eye muscles (Figure 1A). The mass exhibited distinct boundaries and a well-defined structure, deforming the globe (Figure 1B).

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Figure 1: Gross, histopathological, and immunohistochemical findings: A. Tumor enveloping the eye (arrow), causing exophthalmos. B. Tumor encircling and deforming the eyeball (arrows) in cross-section. C.Spindle-shaped cells with cigar-shaped nuclei in intersecting fascicles (arrow) (H&E). D.Multinucleated cells (arrows) and lymphocytic infiltrate around the tumor mass (H&E). E. Strong and diffuse immunoreactivity for SMA (arrow). F. Focal desmin immunoreactivity in neoplastic cells (arrow).

Macroscopic Findings: The tumor occupied the retrobulbar space, causing marked distortion and anterior displacement of the globe (exophthalmos).

Histopathological Findings: Microscopic examination revealed a hypercellular neoplastic mass composed of spindle cells arranged in interwoven fascicles (Figure 1C). Neoplastic cells exhibited size variability, elongated nuclei with blunt ends, prominent nucleoli, and eosinophilic cytoplasm ranging from fibrillary to granular. Infiltrative growth into periocular striated muscle was observed, with swirling growth patterns, mitotic figures, coagulative necrosis, hemorrhage, lymphocytic infiltrates and multinucleated giant cells (Figure 1D). Vascular congestion in the choroid was secondary to mass effect.

Immunohistochemical Findings: Neoplastic cells showed strong cytoplasmic positivity for SMA (Figure 1E) and, vimentin (Figure 1F) supporting mesenchymal and smooth muscle origin.

Immunohistochemical profiling corroborated the diagnosis: SMA+/vimentin+/desmin– aligns with poorly differentiated leiomyosarcomas in veterinary and human pathology^1,2. Desmin negativity, though unexpected, has been documented in undifferentiated avian cases^6,7. Postmortem findings revealed local aggressiveness, with tumor encasement of the globe and muscle infiltration. Similar invasive potential is reported across species, including dogs, birds, and humans^2,8-10.

While rhabdomyosarcoma has been reported in budgerigar retrobulbar space^3, this represents the first retrobulbar leiomyosarcoma in this species.

Leiomyosarcomas are more frequently documented in other avian locations/species, such as pheasant wings⁹ or cutaneous/visceral presentations in pigeons^6,7. Histologically and immunohistochemically, this tumor aligns with leiomyosarcomas in mammals and other avian cases^1,2,7-10.

Retrobulbar localization remains unique. Primary retrobulbar leiomyosarcomas are reported in dogs⁸ and humans^9,10, showing similar profiles and aggressive behavior. No previous avian cases with retrobulbar involvement exist, suggesting extreme rarity or underdiagnosis due to limited resources in avian medicine.

Differential diagnoses included granulomatous inflammation, lymphoma, and other more common avian soft tissue sarcomas. Leiomyosarcoma was initially ruled out due to its extreme rarity in this location/species. Retrobulbar neoplasms in birds are typically associated with rhabdomyosarcoma, lymphoma, or spindle cell tumors³.

This case highlights diagnostic features of poorly differentiated orbital leiomyosarcoma. Characteristic morphology combined with IHC profiling (SMA+/vimentin+/desmin–) is essential for diagnosis.

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