What are Corpora Nigra?
Corpora nigra (also known as granula iridica) are pigmented, irregular, and roughed extensions of the posterior iris epithelium. They are present in a small group across the dorsal (upper) pupillary margin extending into the anterior chamber and small, less prominently across the ventral (lower) pupillary margin.
Corpora nigra are normal anatomical structures in horses that function to improve vision in bright light by reducing the glare from the sunlight.
What are Corpora Nigra Cysts?
Corpora nigra are lined by cells that can secrete a viscous substance causing enlargement and cyst formation. The normally, irregularly-shaped corpora nigra develop into large, smooth, and rounded cystic structures extending over the pupil. The cysts usually develop slowly, but can progress quickly in certain cases. Single or multiple cystic corpora nigra can occur in one or both eyes. Horses are not born with the lesions, but acquire (develop) them. They do not appear to be associated with intraocular inflammation or related to other ocular abnormalities.
Who is Affected by Corpora Nigra Cysts?
Any horse can develop cystic corpora nigra, but is more commonly seen in middle-aged to older horses. There is no known age, sex, or breed predisposition for cystic corpora nigra.
What are Common Signs of Corpora Nigra Cysts?
The majority of horses with cystic corpora nigra are not significantly affected by the small cysts and do not display any clinical signs. Some horses have larger cysts in certain locations that can decrease their vision and lead to abnormal clinical signs. Horses with large cysts may have impaired vision in bright light due to the large cysts extending over the constricted pupil and obstructing their visual field. Some horses appreciate the subtle movement of the suspended cystic structures leading to head shaking and erratic behavior changes like shying away on the affected side, refusing to jump or perform.
How are Corpora Nigra Cysts Diagnosed?
Cystic corpora nigra are typically diagnosed by the characteristic appearance on ophthalmic examination. It is
recommended to evaluate the cyst with a bright light before dilation in order to assess the extent of the visual field affected. Rarely, ultrasound is used to confirm that the structure is a fluid-filled cyst and not an inflammatory or cancerous mass.
How are Corpora Nigra Cysts Treated?
Most horses do not require treatment for cystic corpora nigra. However, treatment is recommended for horses with impaired vision due to cystic corpora nigra. Laser therapy (transcorneal) is the most effective and noninvasive treatment for cystic corpora nigra. The procedure does not require general anesthesia and is performed under standing sedation. The laser beam is focused on the cyst to cauterize and disrupt the cyst wall to prevent further fluid secretion. Horses are prescribed anti-inflammatory medication to control the mild postoperative intraocular inflammation that can occur following laser treatment. Laser photocoagulation to treat cystic corpora nigra is highly successful with minimal risks.
Normal corpora nigra appearance after laser treatment. This patient has no spooking behavior and sees normally.