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Corneal Edema: What is it?

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An overview of corneal edema - its causes, symptoms and treatment.

The cornea is a protective, transparent dome-shaped layer of clear tissue that covers the front of the eye. In corneal edema, it becomes excessively hydrated due to the accumulation of fluid. 

An adult cornea is about 0.5 millimeters thick. It consists of five major layers: epithelium, Bowman's membrane, stroma, Descemet's membrane, and endothelium. 

The five membranes of the cornea are important for bending the rays of light entering the eye. The cornea and lens are responsible for focusing light in the eye but the cornea is twice more effective as the lens for this function.

To properly function, the cornea needs to be clear. The clarity of the cornea is determined by the regular arrangement of its protein fibers and its extent of dryness. 

As there are several nerve endings in the cornea, it is overly sensitive. Furthermore, it is devoid of blood vessels, hence it appears translucent. 

In this article, we will discuss Corneal Edema, its various symptoms, and how it can be treated. 

What is Corneal Edema?

The cornea works like a powerful refracting medium, contributing to the focusing power of the eyes. It is kept clear by pumping water from the endothelium layer.

When there is water retention in the cornea area, it causes corneal swelling, which leads to corneal edema. 

To keep the cornea dry and clear, the endothelium cells regularly drive out fluids from the cornea.

This endothelium is made of a single layer of cells. These cells do not regenerate when injured. When a lot of them are damaged, the cornea will be flooded by fluids which causes edema and the subsequent loss of vision.

Therefore, corneal edema therapy aims to keep the cornea dry (with concentrated saline solutions and other drugs with local diuretic actions) and to fight infections (with antibiotics) brought on by the fluid buildup.

Corneal endothelial cells rarely divide. Therefore, when some of them are lost to injury, the surviving cells in the single-layer change shape and grow larger to fill the spaces left by the destroyed cells.

However, if the number of destroyed cells exceeds the threshold necessary to maintain the normal functioning of the cornea, fluids seep through and hydrate the corneal stroma.

This excessive hydration interferes with the normal spacing of the proteins (Type I collagen fibrils) of the corneal.

Optical performance is diminished because:

  • The surviving epithelial cells change size and shape to close the gap created by the destroyed cells. These changes affect the light-bending properties of the endothelial cells
  • The excess fluid reaching the stroma scatters incoming light and reduces the ability of the cornea to focus the light properly

Causes of Corneal Edema: 

This medical condition is caused by problems associated with dehydration, endothelial disorder, ocular surgery, viral infections, traumatic injury, increased ocular pressure, and toxins. 

Endothelial Fuch's dystrophy is the most common cause of this problem. It is a genetic disorder, which is characterized by the gradual loss of endothelial cells. Women are usually at a higher risk of endothelial dystrophy than men. 

Infection by the herpes virus also results in inflammatory responses in the cornea, leading to corneal edema. This condition may also occur right away or after few years of undergoing any kind of ocular surgery. 

This medical condition may also arise due to damage of the Descemet's membrane, injury of the endothelial layer by ultrasound radiation, and/or infusion of toxic medications in the cornea. Additionally, powerful topical and systemic medications may also cause corneal edema.

Corneal edema can develop as a post-operative condition after eye surgery. Cataract surgeries are the most likely eye surgeries to present with corneal edema complications.

Corneal edema can also be caused by the poor design of lenses used in corneal transplants, surgical tools used in the transplants, an immune reaction to corneal transplants, and failing transplanted cornea.

When corneal edema develops after surgery to remove cataracts, it can be described as pseudophakic corneal edema, pseudophakic bullous keratopathy, or aphakic bullous keratopathy.

Corneal edema is more commonly found in people who are 50 years and older.

Symptoms of Corneal Edema: 

Distorted or blurred vision is the most notable early symptom of corneal edema. It may often become worse when the person wakes up after a night’s sleep and then improves throughout the day. 

Other notable symptoms of corneal edema are eye discomfort, halos around light, sensitivity to foreign particles, and photophobia (increased sensitivity to light). 

As corneal edema progresses, it may become more painful and include blisters that form on the surface of the eye. The disorder may cause corneal nerves to rupture, resulting in severe pain. 

Corneal Edema Treatment: 

Since the early symptoms are similar to cataracts, it is necessary to go through an ocular evaluation for an accurate diagnosis of this problem. 

The eye care expert may use ultrasound, specular microscopy, and optical pachymetry (a measurement of corneal thickness) to confirm the condition. 

The treatment is based on the exact cause of the edema. If corneal inflammation is caused by ill-fitting or continuous wearing of contact lenses, the use of correct contact lenses is recommended. 

If the swelling is due to a post-surgery complication, it can be improved by using over the counter diuretics prescribed by the doctor. The eye care expert may also recommend antibiotics and anti-inflammatory medications to treat corneal edema.

There is no treatment to promote the healing of the endothelial cells. However, the extent of the corneal edema can be controlled.

Patients who are suffering from mild corneal edema can get relief from using hypertonic solutions and ointments such as 2% and 5% solutions and ointments of sodium chloride.

These hypertonic solutions work by creating an osmotic gradient via a tear film outside the cornea that pulls fluid from the cornea. The fluid can then evaporate from the eyes.

However, evaporation is limited at night because the eyes are closed during sleep. Therefore, the effects of corneal edema are worse in the morning. To reduce this discomfort, 5% sodium chloride ointment can be applied at night and/or a hypertonic solution early in the morning to remove the fluid buildup during the night.

Patients who are experiencing significant pain can be treated by a technique called Anterior Stroma Puncture. This involves making multiple, shallow punctures at or below the Bowman layer of the cornea.

To help the epithelium affix to the cornea, a bandage lens will be placed on the cornea for some time. Usually, the kinds of lenses used are thin, permeable, and with high-water content so that they can allow the passage of oxygen.

However, a bandage lens can worsen corneal edema, especially at night.

It can also increase the risk of infections. Therefore, antibiotics are prescribed for corneal edema patients treated with bandage lenses. A broad-spectrum antibiotic such as Polymyxin B eye ophthalmic preparation is recommended.

Corticosteroids such as Prednisolone can also be used to reduce inflammations associated with corneal edema.

To reduce intraocular pressure caused by corneal edema, patients are given selective alpha 2-adrenergic agonists such as Brimonidine or beta-adrenergic blockers such as Timolol ophthalmic preparations. These drugs also reduce the production of aqueous humor.

When other treatments fail, corneal transplantation is recommended.

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