Primary meningeal lymphoma in a horse.

This article discusses a case of primary meningeal lymphoma, a type of cancer affecting the protective outer layer of the brain and spinal cord, in an 18-year-old horse. The horse showed varying symptoms including locomotion difficulties, depression, and muscle atrophy and examinations led to an eventual diagnosis after death.

Initial Examination and Symptoms

• The 18-year-old horse, a Morgan gelding, began exhibiting a range of symptoms which instigated an initial examination.
• The horse had a progressive ataxia and weakness that had escalated over a 3-day period. Ataxia refers to a lack of muscle control or coordination which in this case resulted in gait abnormalities, particularly on the left side of the animal.
• Additionally, the pelvic limbs were more affected than the thoracic limbs further highlighting the asymmetrical nature of the condition.
• Additional symptoms included signs of depression, miosis of the left pupil (abnormal constriction of the pupil), ptosis of the left upper eyelid (drooping), and noticeable areas of muscle atrophy (wasting) particularly on the left side of the neck and over the dorsal aspect of the left scapula (shoulder blade).

Diagnostic Investigations

• Inflammatory changes were evident in the horse’s Cerebrospinal Fluid (CSF). CSF is the clear fluid in the brain and spinal cord which acts as a cushion for the brain, provides nutrition, and helps to remove waste. Such changes suggest potential problems with the nervous system.
• Upon the horse’s death, a necropsy— an autopsy for animals — was performed revealing additional findings.
• The autopsy shows diffuse and irregular thickening of the dura mater along the entire spinal cord. The dura mater is the outermost layer of the meninges, the three layers of protective tissue covering the brain and spinal cord.
• A histologic examination, a microscopic inspection of biological tissues, was also conducted. This examination revealed infiltration of the leptomeninges with neoplastic lymphocytes. Leptomeninges refer to the two inner layers of the meninges that are usually affected in meningitis. The infiltration of neoplastic lymphocytes indicates the presence of a lymphoma or cancer that begins in the cells of the body’s immune system.

Final Diagnosis

• Following all the examinations, an ultimate diagnosis of primary meningeal lymphoma was made. This is a rare condition which signifies cancer in the protective coatings of the brain and spinal cords, specifically originating from lymphoid tissue.
• This case is unusual due to it being found in a horse, the age of the animal and the asymmetrical presentation of the symptoms — which was more to the left.