Lymphangiosarcoma in the nictitating membrane of a horse.

This study chronicles a case of a 15-year-old horse affected by lymphangiosarcoma, a rare form of cancer found in the nictitating membrane, which eventually led to euthanasia due to poor prognosis after surgical intervention.

Case Description

• The subject of the study was a 15-year-old Haflinger gelding which presented with a troubling mass in the left nictitating membrane, also known as the ‘third eyelid’.
• Biopsies were taken and evaluated histopathologically at multiple points due to the mass recurring after surgical removal.
• Ultimately, the horse was euthanized due to a poor prognosis following the remanifestation of the tumor post-surgery.

Mass Examination

• Histological analysis revealed that the mass was comprised of dilated, thin-walled vascular clefts and channels which were lined with flat to cuboid endothelial cells with moderate cellular diversity.
• Only a solitary mitotic figure was observed per high power microscopic field. These channels were largely empty although a minority contained erythrocytes (red blood cells).

Stromal Analysis

• The surrounding collagen-rich stroma, the connective tissue framework of an organ, was not present in large quantities. What little was present manifested focal follicular lymphoid aggregations and marked lymphangiectasia (dilation of the lymph vessels), accompanied by few lymphocytes (type of white blood cell).

Immunohistochemical and Ultrastructural Features

• On staining for cell protein markers, the cancerous cells showed positive for vimentin (a type of intermediate filament protein often seen in connective tissues) and partially positive for factor VIII-related antigen, which is associated with blood clotting.
• Ultrastructural analysis with an electron microscope showed the endothelial lining of vascular channels was discontinuous and partially lacked a basal membrane, i.e., the cell membrane was intermittently missing.

Diagnosis

• Combining the histopathological, immunohistochemical, and ultrastructural findings, a diagnosis of lymphangiosarcoma was made. Lymphangiosarcoma is a rare form of cancer affecting the lining of the lymph vessels.