Corneal Ulcer

Definition

A local necrosis of the corneal tissue, the clear front part of the eye through which light passes, that leads to scarring and reduced visual acuity.

Clinical Signs

May observe any of the following:

Squinting
Inflammation/Redness
Pain (for information on recognizing various signs of pain or discomfort refer to: Signs of Pain In Rats)
Excessive porphyrin drainage in one eye
Swelling of lid
Mucus
Persistent rubbing of the eye
Cloudiness or color change such as white or gray spot on cornea (termed a hypopyon is an accumulation of white cells, or pus in the anterior chamber)

Etiology

The cornea is the transparent membrane at the front of the eyeball, and which overlays the iris. Similar to the human cornea, the rat’s cornea also has 5 layers, from the outer most layers to the inner layer they are:

Epithelium
Basal membrane (Bowman’s layer which is adjacent in the human is absent in the rat)
Stroma
Descemet’s membrane
Endothelium

A corneal ulcer is an erosion or open sore that typically begins at the outer layer of the cornea.
The most common causes of corneal ulcers, in the rat, are due to bacterial infection, viruses or injuries (e.g., abrasions, scratches, foreign particles). Ulcers that are a result of viral infection tend to occur on an already intact cornea (e.g., SDA). Ulcers that occur from a bacterial infection often result when there is a small abrasion to the epithelium due to trauma.

Additional causes of ulceration to the cornea are:

Fungal infections
Chemical (e.g., smoke or cage sprays)
Disorders or conditions causing the cornea to dry out (e.g., decreased tear production, decrease or absent blink response such as when under anesthesia)
Inflammatory diseases
Allergy response
Eyelid abnormalities (e.g., entropion, a rolling in of the eyelid)
Malnutrition

Superficial corneal ulcers, that are noninfectious and that involve only partial loss of the outer most layer of the epithelium, can with early treatment heal with minimal to no scarring. Deep ulcerations that extend to the stromal layer and beyond into descement’s membrane (known as descemetoceles) result in not only severe scarring, but can result in perforation, rupture of the cornea and loss of the eye.

Corneal ulcers are a true emergency requiring early, prompt treatment.

Figures

Case History and Photos

Fig. 1: Corneal ulcer as a result of injury and infection in 3-month-old rat.

Diagnostic

Ophthalmoscopic exam

Use of fluorescein dye in affected eye and visualization with slit lamp.

Schirmer tear test to measure tear production.

Culture from the eye may determine if an infectious organism is present.

Treatment

Treatment should begin as soon as possible to prevent further injury to the cornea.

Application of a warm compress to the affected eye for 5-10 minute intervals, if the rat will tolerate, for comfort.

Application of a topical antibiotic or antifungal eye drop(s) or ointment as prescribed.

Pain control: acetaminophen (Tylenol) or ibuprofen (Motrin), or Metacam.

Where pain or spasm of the eye is present, the vet may instill Atropine eye drops to dilate pupil and relieve the discomfort and or spasm.

Systemic broad-spectrum antibiotics may be necessary for the prevention and or treatment of secondary infection.

*Note: medication such as topical corticosteroids are generally not recommended where corneal ulceration is present as they may prevent healing and lead to a greater bacterial or fungal infection.

Surgical enucleation of the eye may be necessary where there is severe perforation, uncontrolled infection, or pain.

In the event of surgery:

It is important to understand that preanaesthetic fasting of rats is not necessary since vomiting does not occur in this species. Free access to both food and water should be provided until just prior to anesthesia (Flecknell, 1991).

Also, rats do experience pain with surgical procedures. The type of pain medication used post-op should be determined based on extent of procedure and the anticipated severity of pain.

Recommended post-op analgesia that may be given:

For severe pain or first 24 hours post-op: Buprenex (buprenorphine), or Torbugesic (butorphanol).
For mild to moderate pain: Banamine (flunixin meglumine), Metacam (meloxicam), or carprofen. Do not use if a corticosteroid has already been prescribed.
*Note: for pain not controlled by the use of an NSAID (e.g., Banamine, meloxicam, or carprofen), alone, consider alternating or co-administering with a narcotic (e.g., buprenorphine or butorphanol) or narcotic-like (e.g., tramadol) medication.

In addition, a post op, broad-spectrum systemic antibiotic may be prescribed.

For information regarding medications refer to the Rat Medication Guide.

Nursing Care

Limit the spread of infection by washing your hands, and drying them before and after caring for the rat’s eyes.
Apply a warm compress to the eye for comfort if the rat will allow.
Administer any antibiotics, pain medication or antimicrobial ointments or drops as prescribed by the veterinarian.
Provide clean bedding daily while the eye is healing. Avoid the use of litter type bedding, thereby protecting the eye.
Keep nails clipped to prevent the rat from accidentally scratching eye with long pointed nails.
In the event there is dried or excess drainage from the eye, gently irrigate eye with normal saline or sterile water, or clean with a moistened cloth using warm water or normal saline.
Contact veterinarian if any of the following are observed: increased swelling, redness, pus formation, or signs of pain.

Provide post-op care, as listed below, in event of surgical removal of eye:

Provide a hospital cage during recovery, especially if there are concerns that their cage mates may groom sutures or wound site.
Provide clean bedding daily such as felt, soft t-shirt type material or ink-free paper towels. Avoid using material such as terry cloth type towels that can ravel. Also avoid litter-type bedding, post-op, until healed to prevent the chance of wound contamination or infection.
Provide additional warmth to maintain body temperature within normal limits. It is essential that the rat does not become overheated or dehydrated. The rat should also be able to move away from the heat source if it becomes uncomfortable. If the rat is unconscious or immobile extreme care must be taken to keep the heat low and stable.
- You can use an isothermic product that is heated in the microwave such as SnuggleSafe®. Make sure to follow the product directions carefully and wrap in a towel before placing in the cage. SnuggleSafe® will provide heat for 12 hours before needing to be reheated. Other similar types of products may vary in re-heat time. Check directions for individual product.
- If using a heating pad (good for long term use) use only the low heat setting, put a thick towel in between the pad and the cage bottom, and place beneath a corner of the cage.
- If none of these options are available you can use a plastic bottle filled with hot water, and wrapped in a towel, in the corner of the cage.
Medicate for post-op pain as needed.
Provide high calorie foods or food supplements such as Nutri-Cal Paste, canned Ensure, soy or soy formula during recuperation.
Encourage fluid intake while recuperating, such as water, Jell-O water, or electrolyte replacement drinks such as Pedialyte or Gatorade (which can be found in local grocery stores).
- Please note that Pedialyte is only good refrigerated for 24 hours after opened, but can be frozen as ice cubes and thawed as needed.
- *Note: Juicy types of fruit also provides an additional fluid source in the diet.

Outcome

Healing with minimal scarring
Pain relieved
Uneventful recovery in the event of surgery

Prevention

Early veterinary attention and appropriate treatment even for minor eye injuries or infections can help prevent the development of ulcerations, perforations, or loss of an eye.

Resources

Grieve, K., Dubois, A., Simonutti, M., Paques, M., Sahel, J., Gargasson, J. L., et al. (2005). In vivo anterior segment imaging in the rat eye with high speed white light full-field optical coherence tomography. Opt. Express, 13(16), 6286-6295. Retrieved February 3, 2011, from http://www.opticsinfobase.org/abstract.cfm?URI=oe-13-16-6286