An enucleation is the surgical removal of a horse's eye. There are many indications for which this surgery would be performed, including trauma, neoplasia (cancer), extensive infection, or any condition causing pain in a blind eye. In Misty's case, the procedure was recommended due to an acute worsening of uveitis and ulceration along with equine recurrent uveitis flare-ups that had been occurring over the last four years.
Sarcoids are the most common tumour that occurs in horses. They are locally invasive, and difficult to deal with because recurrence is common even with aggressive therapy. One study showed that 14% of sarcoids occur exclusively in the periocular region (near the eye), and these tumours can be particularly tricky to deal with as it is difficult to get good margins to remove all tumour cells during surgical excision.
In horses, the third eyelid is prone to developing squamous cell carcinoma. Squamous cell carcinoma is the second most common tumour in horses, and it is the most common tumour in the equine eye. It develops most commonly on areas lacking pigmentation, poorly haired regions, and skin near mucocutaneous junctions. It can be quite an aggressive tumour, spreading to nearby tissues and local lymph nodes. In the third eyelid, it often initially appears as a reddened area, then becoming raised and in some cases developing a wart-like appearance. In most other areas, recurrence is extremely common unless surgical excision is combined with another treatment such as chemotherapy or cryotherapy. Fortunately, the third eyelid can be removed in its entirety, and a success rate of 90% has been reported with removal alone.