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The many ways to fix a cornea

Surface disease is very common in veterinary ophthalmology as our critters have a tendency to scratch, poke, gouge, lacerate and abrade the cornea on a regular basis. The cornea is the clear windshield in the front of our eye and is often affected directly or indirectly by these insults. Other primary disease processes, such as degenerative or inflammatory change, can also affect the clarity and integrity of the cornea. Throw in some infectious agents like bacteria and viruses and this can create a host of problems that we as veterinarians need to address either medically and/or surgically.

A clear cornea in a dog...with a small lid tumor on the lower lid

A clear cornea in a dog...with a small lid tumor on the lower lid

The uncomplicated cornea abrasion is simply treated with topical antibiotics to prevent infection along with an Elizabethan collar to prevent self trauma (yes, the dreaded collar is essential!). Healing of these superficial lesions is from side-to-side where the surface tissue (epithelium) slides over the abraded surface rapidly to heal the wound. This should occur within a week in most cases. Searching for a persistent underlying cause such as a foreign object or offending hair is required when initially presented. I see very few of these simple erosions here as most general practitioners can identify these lesions with a dye called fluorescein that stains the cornea and, if simple, go away prior to needing referral with the treatment above. It is the complicated or persistent ulcers that generally prompt a specialist’s evaluation and care.

Superficial corneal erosion in a cat

Superficial corneal erosion in a cat

Our focus for this posting will be for the ulcers that become infected or degenerate leading to a loss of tissue. The cornea is made up of layers of collagen, the bulk in the center called the stroma. If an injury penetrates the epithelium and bacteria are presented to this layer, some bacteria in the environment release enzymes called collagenases that can digest the collagen. This can happen rapidly which can progress to a full-thickness hole in the eye resulting in rupture of the globe. Contrarily, this can be slower process leading to a more sloping defect of variable depth. We can also see a similar process in old dogs with degenerative disease where tissue loss without overt evidence of bacteria can lead to a similar appearance. In general, if the depth of the divot is less than 50% of the corneal thickness then medical management is the norm. If the divot is greater than 50%, surgery may be considered due to the loss of structural support and thus an increased risk for rupture. This depth is evaluated by a tool called the slit lamp biomicroscope which is the workhorse for our examinations of the front part of the eye. Let’s look at how we identify and approach the non-surgical ulcers first.

Small but deep ulcer with an early vascular response

Small but deep ulcer with an early vascular response

Above is the photo of a young Pug-mix with a small but deep corneal ulcer. Different than the first photo of the cat, this ulcer looks deeper even with the naked eye. In addition, the eye is a little more angry with an increase in discomfort with redness to the whites of the eye along with small vessels at the edge of the cornea. This activity is consistent with infection and is certainly common in this breed and others with prominent globes like the Pug. With our slit lamp, we could assess that this ulcer was 50% of the way through the cornea. For perspective, the cornea is roughtly 1/2 millimeter thick, so high magnification, a steady hand and good training is needed to evaluate the depth.

Diagnostic tests often utilized in this scenario include corneal scraping to look for inflammatory cells and/or organisms (cytology) and culture of the lesion to assess for bacterial growth. These tests are helpful in that the cornea is typically devoid of inflammatory cells so seeing just a few is significant. If these are white blood cells typically seen in times of infection, the culture sample might be submitted to help determine if bacteria are present and what antibiotics may be useful in killing them. While awaiting for the results of the culture (cytology is done on site in many instances), antibiotics are typically started. Other medications that may be employed include drops that dilate the pupil to help with internal ocular discomfort, serum drops to try and stop the collagenase activity, oral antiinflammatories for intraocular inflammation and pain along with the aforementioned dreaded E. collar. Frequency of medication is variable but is usually higher in these cases (4-6x/day or more) with serious infection vs. the 2-3/day utilized in a non-infected ulcer for prophylaxis.

Initial healing of the stromal ulcer 3 weeks after initial visit

Initial healing of the stromal ulcer 3 weeks after initial visit

Healing of these ulcers is often facilitated, and frankly necessary in some cases, by vessels that advance into the region of tissue loss and encourage regeneration or rebuilding of the collagen. This may create more potential for scarring, however, is an acceptable trade off rather than rupture. Dogs in particular can have pigment migrate into these regions which, if large, can significantly affect clarity. However, when small like with this lesion, can be clinically insignificant. In the first photo, I think you may appreciate that the vessels have already migrated in and filled in our hole with some residual inflammatory material (white spot) in the deeper tissue. When healing was complete, a focal brown spot without any tissue loss was all that was left. In the final photo below, you can see this region along with some fine granular material unassociated with the ulcer. A good outcome without the need for surgery for him!

Dunham,Ingrid OD lessmineral 9-25-19

Now some dogs may epithelialize (the surface tissue slides over and covers the ulcerated area) without concurrent thickening of the underlying tissue. Thus, a divot may remain but the ulcer by definition has healed. The is called a corneal facet. An area of weakness my remain but may be safe if the depth remains less than 50%. It may slowly fill in given time or remain the same. We may have pets that come in with facets that are deeper and leave well enough alone especially if they have a relatively sedate lifestyle where repeat injury seems unlikely. The photo below shows an old dog with a shallow facet with a rim of vessels but reasonable clarity to not warrant any additional intervention.

Shallow facet after a healed stromal ulcer

Shallow facet after a healed stromal ulcer

These are just a couple of a examples of when and how medical management is chosen and the potential results of such an intervention. Some dogs and cats get more vessels, pigment and scarring and others get less. The challenge for us is to initially stop the digestion, encourage a reaction and then try to reverse the process to improve the clarity. Different species have different levels of reactivity and scarring that we try to manipulate as best we can. Next time, we will discuss the surgical choices in those patients where the risk of rupture is high without giving some structural support.

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