Teaching corner

PDA L-R corneal oedema is caused by the increased fluid content within the corneal stroma owing to a failure at the level of the corneal epithelium and/or endothelium. The endothelium pumps fluid out by the use of a NA/K pump and also maintains corneal deturgescence by the presence of a physical barrier formed by zonula occludens adhesions. A failure at the level of any of these structures will cause the formation of corneal oedema and the loss of corneal transparency. We need to remember that the endothelium does not have the ability to regenerate and, once the endothelial cells degenerate, a smaller number of cells has to perform the same action and they end up overcoming their ability to do so. This is how corneal oedema appears.

Reasons of the corneal oedema:

• Endothelial dystrophy: it affects breeds such as the Boston Terrier and Chihuahua. This pathology is similar to human Fuch’s dystrophy. It affects both eyes
• Age related degeneration.
• Trauma : for example, the one cause by an anterior lens luxation and by phacoemulsification
• Toxicity : for example, the one caused by the antiarrhythmic tocaidine
• Anterior uveitis : there is an increase in endothelial permeability and a decrease of the Na/K pump activity
• Infectious : canine infectious hepatitis. In natural infections and in infections caused by vaccines with attenuated type I Adenovirus
• Other causes : endotheliitis, glaucoma and corneal ulceration.

Corneal oedema management:

Corneal oedema management can be complicated. The only efficient way to correct it would be to perform a corneal or endothelial transplant. Both procedures are specialist procedures. We do not tend to perform these procedures in our patients yet although some studies on the use of endothelial transplants in dogs are being carried out by a couple of research groups. Our goal therefore, is to achieve ocular comfort.

The main consequence of corneal oedema is the vision loss in severe cases and the already discussed presence of epithelial bullae and corneal ulceration in some other cases, or both. Hyperosmotic drops can be used to improve the resolution/control of epithelial bullae. A recent study has demonstrated that their use can decrease the corneal thickness in normal dogs. Surgical treatments such as thermokeratoplasty or keratoleptynsis (“letter box” surgery) can improve the degree of corneal oedema and the epithelial bullae and, consequently, the comfort and vision of the affected eyes.

From Panacea-Vet we recommend managing the owners expectations appropriately in these cases. It is essential they understand that corneal oedema is a long term problem, in which management of the clinical signs, rather than cure, will be achieved. .

Bibliography

1. Samuel M, Thomasy SM, CalderonAS et al. Effects of 5% sodium chloride ophthalmic ointment on thickness and morphology of the normal canine cornea. Veterinary Ophthalmology; 2019;22(3):229-237.
2. Boo G, Whittaker CJG, Caruso KA et al. Early postoperative results of Descemet’s stripping endotheliatoplasty in six dogs with corneal endothelial dystrophy. Veterinary Ophthalmology; 2019;22(6):879-890.
3. ArmourMD, AskewTE, EghrariAO. Endothelial keratoplasty for corneal endothelial dystrophy in a dog. Veterinary Ophthalmology, 2019;22(4):545-551.
4. Giannikaki S, Escanilla N, Sturgess K, Lowe RC. A modified technique of keratoleptynsis (“letter-box”) for treatment of canine corneal edema associated with endothelial dysfunction. Veterinary Ophthalmology. 2020; 23:930–942.
5. Whitley RD, Hamor RE. Diseases and Surgery of the Canine Cornea and Sclera. In: Veterinary Ophthalmology, 6th edn. (ed. Gelatt KN) Wiley Blackwell. 2021. 1082-1155