What are the two broad causes of brown or black corneal opacities?

Brown or black corneal opacities can primarily be attributed to two significant factors: pigment deposition and the presence of feline corneal sequestrum.

Pigment deposition refers to the accumulation of melanin or other pigments in the cornea, which can result from chronic irritation or inflammation. This may occur due to conditions such as chronic keratitis or exposure to allergens, leading to the deposition of pigment.

Feline corneal sequestrum specifically pertains to a condition commonly seen in cats, characterized by a localized area of necrotic tissue in the cornea that appears as a dark brown or black opacity. It often arises from chronic irritation, such as that caused by herpesvirus infections or environmental factors.

This answer encompasses both the general phenomenon of pigment accumulation in the cornea and a specific condition prevalent in felines, making it a comprehensive choice among the options. Each of these causes relates directly to observable changes in the cornea, which is critical for accurate diagnosis and treatment in ophthalmology.