What it is
Corneal oedema is a pathological condition in which the cornea, the external and transparent part of the eye, has an excessive accumulation of fluid in its innermost layer, called the stroma. To maintain its characteristics of transparency, the cornea is maintained in a state of optimal hydration by the barrier activity of the corneal epithelium, the outer covering layer, and by the activity of the corneal endothelium, an active system of natural pumps that removes excess water.
However, these physiological processes can be altered in certain pathological conditions, in which the cornea can become excessively soaked in liquids, with a consequent increase in thickness and loss of transparency.
What it entails
Corneal oedema is certainly an unpleasant condition for the patient, as it can cause distorted and blurred vision (the impression of seeing through frosted glass), haloes around lights and rainbows of colours. In more advanced stages, it can also cause intense pain and/or permanent damage to the transparency of the cornea. Since its early symptoms are similar to those of a cataract, an eye examination is necessary for an appropriate diagnosis.
The main causes of the onset of corneal oedema are:
- Alterations of the corneal epithelium
When the cornea is deprived of the integrity of the superficial epithelial layer, the underlying stroma becomes soaked with lacrimal fluid, and after about 4-6 hours it can reach an increase in thickness twice the initial value. The epithelium acts as a barrier to the flow of water from the lacrimal fluid to the stroma and any event that alters its structure such as trauma, inflammation, infection, etc., causes swelling and opacification of the cornea. However, epithelial oedema is transitory: once the integrity of the epithelial layer has been restored, the oedema is resolved within a few days. Its severity also varies throughout the day: indeed, it causes greater disturbances in the morning after the patient awakens, while it progressively improves during the day. Epithelial oedema can be caused by all those situations that alter, even temporarily, the structure of the epithelium, such as continuous use of contact lenses, chronic use of eye drops containing substances such as benzalkonium chloride, ocular traumas and surgical interventions (e.g. refractive surgery or corneal cross-linking).
- Endothelial dysfunctions
The endothelium is a single layer of cells that regulates the optimal amount of water present in the cornea; as a matter of fact, it acts as a semi-permeable barrier and as an active pump that takes excess fluid out of the corneal stroma, guaranteeing the transparency of the cornea. Stromal hydration is also regulated by the barrier function of the endothelium, achieved through tight junctions between the endothelial cells themselves. These functions of the endothelium can be compromised, although temporarily, as in the case of cataract surgery in patients with particularly hard nuclei in which the surgeon has to employ a greater amount of ultrasound, or permanently, as in the case of Fuchs dystrophy, a congenital pathology characterised by an imbalance of the endothelium that undergoes a progressive deterioration with the formation of fluid "bubbles" in the stroma.
The endothelial oedema can also be caused by perforating traumas, by foreign bodies, by acute glaucoma, etc.
The corneal oedema treatment has two objectives: the restoration of visual acuity and the resolution of symptoms and pain, in more severe cases.
The therapeutic approach to corneal oedema primarily involves the elimination of the triggering cause. When the oedema is caused by high ocular pressure, such pressure must first be reduced with hypotensive drugs. If the cause of the oedema is severe inflammation, the oedema is reversible through the resolution of the inflammation itself (unless irreversible damage to the endothelium has occurred). Proper use of contact lenses will be the cure for those situations caused by improper use of the same. In a secondary oedema with endothelial decompensation, as in endothelial dystrophies (Fuchs), the solution is the surgical replacement of the cornea (keratoplasty) or its diseased part (endothelial keratoplasty).
At a topical level, the specific therapy for corneal oedema consists in the instillation of eye drops, called hyperosmotic, capable of “attracting” excess water from the cornea towards the surface. Hypertonic substances such as sodium chloride and mannitol, used topically, can be useful to accelerate the resolution of the oedema, for instance after particularly demanding phacoemulsification procedures or after keratoplasty. They can also be useful for improving vision in a chronic oedema.
Hyperosmotic agents, by removing excess fluid, produce a clearing of the cornea especially in the early stages of epithelial oedema. They do not have a pharmacological action but a normalising effect on the tissue osmotic pressure, in particular immediately after eye surgery. In decades of widespread use of preparations with hyperosmotic activity, used topically in the conjunctival sac, it has been proven that they show:
- no serious adverse reactions
- no toxicity even for prolonged treatments
- no irritating or allergenic activity
- an optimal local tolerability