Leptomeningeal dissemination of a poorly differentiated carcinoma involving the trigeminal ganglion in a horse: a case report.

Overview

• This research article describes a case report of a horse diagnosed with a poorly differentiated carcinoma involving the trigeminal ganglion, accompanied by leptomeningeal dissemination, which is rare in horses.
• The study highlights the diagnostic challenges and expands understanding of the biological behavior of equine carcinomas related to cranial nerves and leptomeningeal tumor spread.

Introduction and Background

• Neoplastic conditions affecting cranial nerves in horses are not common, making diagnosis difficult.
• Leptomeningeal dissemination refers to cancer cells spreading within the leptomeninges, the membranes covering the brain and spinal cord. This is a recognized route of tumor progression in aggressive cancers but is seldom reported in horses.
• The trigeminal ganglion is a key neural structure involved in facial sensation, and tumors affecting it are particularly rare in equine medicine.

Case Description

• The subject was a 15-year-old mixed-breed horse with a history of periocular squamous cell carcinoma, which had been treated by removing the eye (enucleation) three years before developing neurological problems.
• Progressive neurological signs prompted consideration of differential diagnoses such as metastatic cancer spread or infection-related conditions like equine protozoal myeloencephalitis.
• Because the horse’s neurological condition worsened quickly, euthanasia was performed and a necropsy (animal autopsy) was conducted.

Findings at Necropsy

• A large, infiltrative mass was found involving the right trigeminal ganglion.
• Histological examination revealed the tumor to be a poorly differentiated carcinoma characterized by:
  • Marked pleomorphism (variation in cell size and shape)
  • High mitotic index (indicating rapid cell division)
  • Invasion of nearby neural tissues
• Neoplastic cells were found free-floating within the subarachnoid space (the area beneath the arachnoid membrane of the brain), indicating leptomeningeal dissemination.
• Importantly, these free cancer cells were located at multiple sites and did not invade the brain tissue (parenchyma) itself.
• Immunohistochemistry tests confirmed the tumor cells were of epithelial origin (carcinoma), differentiating them from other tumor types like sarcomas or glial tumors.

Significance and Conclusions

• This case report expands current knowledge about the biological behavior of carcinomas in horses, particularly their ability to spread via leptomeningeal dissemination, which is infrequently reported in equine patients.
• The involvement of the trigeminal ganglion in such a carcinoma is notable due to its rarity and the complexity of the horse’s neurological symptoms associated with this tumor location.
• The findings emphasize the importance of considering neoplastic leptomeningeal spread in horses presenting with progressive neurological signs and a history of carcinoma.
• The report also highlights the diagnostic challenges due to overlapping symptoms with other neurological diseases and the need for histopathological and immunohistochemical confirmation.