Eye injuries are common in animals, including dogs. Determining if there is any erosion to the surface of the eye is an important part of evaluating the extent of injury. Fluorescein is an orange stain that is applied to the cornea to reveal corneal lesions. Once the stain is applied to the eye, the excess is rinsed off, and the remainder turns fluorescent green. The stain adheres to any areas where the surface layer of the cornea (the epithelium) is missing and where the underlying layer (corneal collagen or stroma) has been exposed. This test outlines the extent of any ulceration and permits more accurate assessment of the size, depth and type of ulcer.
A fluorescein stain is indicated anytime a dog has a red or painful eye, or if any corneal irregularities or trauma to the eye are noted. Fluorescein staining is also used to determine if the duct that allows passage of tears from the corner of the eye to the nose is open and working. (This duct is why our nose runs when we cry).
There are no real contraindications to performing this test in a dog with eye problems.
What Does a Fluorescein Stain Reveal in Dogs?
Fluorescein staining of the cornea identifies abrasions, scratches, ulcerations and lacerations present in the cornea on the surface of the eye. Early treatment is crucial in promoting repair and healing of corneal ulcers and injuries, and in preventing rupture of the eye. Fluorescein is also important in revealing if the duct from the eye to the nose is open and draining properly.
How Is a Fluorescein Stain Done in Dogs?
Initially, the surface of the eye is cleaned of any mucous or discharge. The eyelids are opened and a drop of fluorescein stain is dropped on the surface of the eye. The eyelids are then closed to allow the stain to flow over the entire surface of the eye. If the duct from the eye to nose is being evaluated, the opening of the nostril is examined for evidence of fluorescent green stain showing up at the tip of the nose.
If the cornea and surface of the eye are to be examined, the fluorescein is flushed out of the eye with eyewash. The eye is then examined with either a penlight or special cobalt blue filtered light to detect the presence of the green stain on the eye. In a normal undamaged cornea, no stain remains on the surface of the eye. If the surface layer of the cornea is damaged, stain remains and outlines the damage. This test usually takes less than five minutes.
Is a Fluorescein Stain Painful to Dogs?
Fluorescein staining is not painful. The test is frequently performed on an dog with a painful eye, however, so the animal may resist the test unless a drop of local anesthetic is applied first. The local anesthetic will numb the painful eye and make application of the fluorescein easier.
Is Sedation or Anesthesia Needed for a Fluorescein Stain?
Neither sedation nor anesthesia is needed to perform a fluorescein stain. Some dogs resent the veterinarian getting close to their face to place the stain, particularly if they are painful. In these dogs, sedation or ultra-short-acting anesthesia may be necessary.