Corneal Neovascularization can be defined as the excessive growth of blood vessels that occurs at the level of the cornea. The blood vessels come from the limbal vascular plexus and the main cause of the problem is represented by the oxygen deprivation. In healthy individuals, the cornea is transparent and it lacks vascularization. Both ocular insults and ocular hypoxia are responsible for the appearance of corneal vascularization, as you will have the opportunity to read below. It is important to remember that the corneal neovascularization is always accompanied by an inflammatory process.
The condition is often encountered in those who wear contact lenses. Risk factors for the appearance of corneal neovascularization include high myopia, the dry eye syndrome and different conditions with ocular involvement (compromised immune system, Sjögren’s syndrome or acne rosacea). Hydrogel contact lenses present the highest risk for corneal neovascularization.
Picture of Corneal neovascularization
This condition is not life-threatening but it can lead to loss of vision in more severe cases. The vision can be threatened by the direct invasion of the blood vessels at the level of the cornea or because of secondary changes, such as the hemorrhage or scarring. There is no age, race or gender predilection for the corneal neovascularization to have been identified so far.
In trying to explain the mechanism behind the corneal neovascularization, several theories have been proposed. The chemical theory is based on the supposition that there is a vasostimulatory factor that leads to the corneal neovascularization. It has also been suggested that the excessive growth of blood vessels might be due to the destruction of the vasoinhibitory factor (chemical theory as well).
Another theory, the mechanical one, considers that in healthy individuals blood vessels cannot penetrate the cornea (compact). In the situation that the compactness of the cornea is affected, with edema being present, it is possible that the blood vessels penetrate the cornea. More recently, a combination of the chemical and mechanical theories have been proposed. It seems that the presence of the vasostimulatory factor, in combination with the affectation of the corneal structure and the associated edema, are responsible for the corneal neovascularization.
Both inflammation and hypoxia contribute to a disruption in the balance of the corneal immune system. The materials that are used for the making of contact lenses, such as the hydrogel, can lead to the mechanical irritation of the cornea. Apart from the actual hypoxia, the cornea suffers from the associated inflammation. Other changes that occur at the level of the cornea include the epithelial erosion and the hypertrophy.
Symptoms of Corneal Neovascularization
If the central visual axis is not affected, the corneal neovascularization does not cause any noticeable symptoms. Upon performing a routine eye exam, the following changes can however be identified:
- Limbal hyperemia – earliest sign of corneal neovascularization
- Superficial vessels identified at the level of the cornea (single or multiple)
- In the situation that the deep stromal vessels are responsible for the corneal neovascularization, these are identified at the level of the cornea (sometimes accompanied by nerve fibers)
- Active engorged vessels and lipid exudate
- Corneal edema and leakage
- Lipid deposition – yellow-white opacities (surround the stromal vessels).
What are the Causes of Corneal Neovascularization?
These are the most common causes that lead to the appearance of the corneal neovascularization:
- Wearing contact lenses
- Frequent cause
- Increased risk – hydrogel contact lens material (HEMA – 2-hydroxyethyl methacrylate)
- Encountered in contact lenses recommended for daily and extended wear
- Older materials used for the making of contact lenses have a low oxygen transmissibility – cornea starved for oxygen – increased blood vessels to satisfy the need for oxygen
- Viral infection
- Common – herpes simplex
- Corneal ulcers
- Infectious keratitis
- Immunological conditions
- Corneal trauma
- Alkali injury.
Wearing contact lenses is one of the most frequent causes that leads to the appearance of corneal vascularization. At the same time, it should be mentioned that the blood vessels that invade the cornea are superficial rather than deep. The invasion of deep stromal vessels is encountered in the situation that an ocular insult or hypoxia has resulted at the level of the cornea.
Aside from the actual wearing of the contact lenses, there are other risk factors that can increase the risk for corneal neovascularization. Among these factors, there are: poor compliance, inadequate hygiene of the contact lenses and poor follow-up care.
The following methods can be used for the diagnosis of the corneal neovascularization:
- Medical history of the patient
- History of contact lens wear
- Pre-existing conditions (especially with ocular involvement)
- Physical examination
- Analysis of the cornea with specific instruments (measurement of corneal sensibility and identification of vascularization at the level of the cornea)
- Laboratory studies
- Herpes cultures (suspicion of viral infection with herpes viruses)
- Tuberculosis skin test + chest radiography (suspicion of tuberculosis)
- Testing for syphilis.
The differential diagnosis can be made with the following conditions: contact lens complication, herpes simplex, herpes simplex keratitis, herpes zoster, interstitial keratitis, onchocerciasis, pediatric tuberculosis and conjunctival squamous cell carcinoma.
How to treat Corneal Neovascularization?
A study performed on lab rats has demonstrated that both triamcinolone and doxycycline can be used to reduce the neovascularization process at the level of the cornea (topical instillation). Other studies recommend using angiotensin II receptor blocker drug telmisartan as a prophylactic measure (prevention of corneal neovascularization). The topical application of bevacizumab (anti-VEGF) has also been recommended as a treatment for corneal neovascularization.
Other treatment measures include:
- Discontinuing the current contact lenses and replacing them with better ones (higher oxygen permeability)
- Refitting the current contact lenses
- Permanent discontinuation of the contact lenses in patients diagnosed with severe forms of corneal neovascularization (reducing the risk of losing one’s eyesight)
- Treatment of the underlying cause or of the associated conditions (acne rosacea, dry eye syndrome, blepharitis, viral or bacterial infections – particularly those with staphylococcus)
- Topical corticosteroids
- Recommended for active neovascularization
- Superficial keratectomy
- Recommended in combination with topical and subconjunctival bevacizumab
- Surgical intervention
- Corneal transplantation
- Recommended in patients who present central corneal scarring
- Also recommended in the situation that the medical management does not restore the normal visual acuity
- Cornea grafts present a high risk for rejection in severe cases of corneal neovascularization
- Corneal transplantation
- Photodynamic therapy
- Argon laser obliteration
- Performed at the level of the vessel lumen
- Hyperbaric oxygen treatment
- Limited success
- Purpose – supply the corneal tissue with the necessary amount of oxygen
- Amniotic membrane transplantation
- Regeneration of the ocular surface
- Prevention of corneal neovascularization
One of the best ways to prevent medical problems such as corneal neovascularization is to change the contact lenses. The best alternative is represented by the modern contact lenses that are made from rigid gas permeable and silicone hydrogel. These have a higher level of oxygen permeability, thus reducing the risk for the cornea being deprived of oxygen.
Left untreated, the corneal neovascularization can progress and have a negative impact on the eyesight. Those who wear contact lenses should visit the eye doctor on an annual basis, for a routine eye exam.
The invasion of the cornea by the deep stromal vessels presents the biggest risks, with the optical transparency being lost due to a series of changes. Among the most important changes, there are the hemorrhage and scarring that can occur at the level of the cornea. The deposits of lipids can also affect the optical transparency, complicating the already existent corneal neovascularization.