EXTRA-ADRENAL PARAGANGLIOMA (EAPG)

� Definition: adrenaline-producing tumors that grow in the cells of the sympathetic nervous system (which helps to monitor and control the internal organs) � Signs: Although EAPGs can occur anywhere along the sympathetic nervous system, in horses the tumors are more likely to become apparent when they develop in the orbit, the cavity in the skull that holds the eyeball and related tissues. They are reported to be the most common tumor of the equine orbit. The most common sign is exophthalmia, the protrusion of the eyeball outward from the socket. Bleeding or other discharge from the nose may be present. Vision is not commonly affected in earlier stages of the disease, unless the tumor affects the optic nerve. Once the protrusion is pronounced, the horse may develop chronic inflammation of the cornea and conjunctiva (the membranes that line the eyelids and eye socket). Older horses are more likely to be affected. � Causes: unknown � Diagnosis: observation of the signs, coupled with imaging to view the tissues behind the eyeball. Options include x-rays, ultrasound, CT (computed tomography) and MRI (magnetic resonance imaging) scans. � Treatment: Tumors that cause no pain or other serious difficulties for the horse can be left untreated. Surgery is the only option to remove a tumor, and removal of the eyeball is also usually necessary. Complete cure is unlikely with treatment, because tumors often extend deep into the orbit and toward the brain, but prognosis is fair since the residual tumor is slow growing.