Here is a similar case where the diagnosis was confirmed at post-mortem. Whar could be causing the conjunctival swelling and exophthalmos in this ten year old thoroughbred?
Infection or tumour could be causing this, but the most likely diagnosis is that of lymphoma – neoplastic lymphocytes fill the orbit giving rise to a rapid presentation just like this. The reference here is Rebhun and Del Piero (1998) Ocular lesions in horses with lymphosarcoma: 21 cases (1977-1997) J Am Vet Med Assoc 212:852-4.