This is porphyrin staining – a pigment in tears. Excessive tearing is often the problem underlying the problem. Excessive tearing may also just be relative since tears are drained through a duct that leads to the nose ( the reason your nose runs when you cry).
Excessive tearing can also be due to dryness or irritation to the eyes. If these ducts get plugged or blocked with matter or mucus, there is nowhere to drain the tears and the tears then drain from the inner angle of the eyes. Excessive fluid then causes the staining because of the pigment.
In many cases the problem may be cured by flushing out these ducts or using soothing eye drops to reduce the irritation. You will need to have your veterinarian guide you as to the best possible resolution. Often they can check whether these ducts are open by using a special stain in the eyes called fluorescein. This stain, if seen draining from through the nostrils is an indication that the duct is open.
Good Luck. Hope this helps. This problem is usually more a cosmetic problem but sometimes if there is excessive and persistent moisture at the inner angles of the eyes, it may cause some skin irritation or infection in the skin eyefold beneath the eyes.
Martin Slome DVM
Centre Street Animal Hospital
7700 Bathurst Street Units 40-42
Tel. (905) 771-9855
Fax (905) 771-7260