How can an ophthalmologist differentiate between dry eye and blocked tear duct?

Sometimes it is difficult to determine the cause for tearing. Both dry eyes and blocked tear ducts can result in tearing that overflows onto the cheek. However there are some differences between dry eyes and blocked tear ducts. In general, dry eye patients usually describe a sandy, gritty, foreign body sensation in their eyes that improves with the use of artificial tears. An ophthalmologist may notice that a patient has a low tear lake and fluorescein staining on the cornea. However, patients with a blocked tear duct often have a high tear lake and lack corneal staining. These patients often tear in just one eye. An ophthalmologist may perform a dilation and irrigation procedure in the office to test if the tear duct is blocked. In this procedure saline is injected into the tear duct. If the tear duct is not obstructed, the patient will taste the saline in the back of his/her throat.