Cornea, External Disease and Anterior Segment
The most anterior, clear portion of the eye is called the cornea. It is the first structure light traverses on its way into the eye. Any condition that affects the cornea can affect the cornea’s ability to help focus light on the retina, and thus affecting one’s ability to obtain a clear image. Since the cornea creates two thirds of the focusing power of the eye, even a small change in the cornea may lead to changes in one’s vision. Conditions of the cornea may be treated with medications or surgery.
Common Conditions that Affect the Cornea
Dry Eye: Due to the desert environment we live in, many people suffer from a feeling of eye dryness, or “dry eye.” Symptoms range from an irritated eye in certain conditions to blurriness, redness and foreign body sensation. These symptoms may only mildly interfere with one’s life or in some cases cause a severe disability. This irritation of the surface of the eye can have many different causes, and is important for us to determine which ones apply to you. A variety of treatment options are available, the choice of which depends on the severity of the disease and what is seen on examination.
Inflammation and Infection: Inflammation may affect the conjunctiva, a highly specialized skin covering the front of the eye. It may be caused by allergies, reactions to chemicals or infections. Infections may be caused by viruses, fungi, bacteria or protozoa. If the infection affects the cornea, it may cause a corneal ulcer. Corneal infections are associated with redness, pain, light sensitivity, and reduced vision in the affected eye. Any infection in the cornea can lead to scar formation. Scarring can reduce the quality of vision, so prompt diagnosis and treatment of infections is critical. Risk factors for corneal infections include contact lens wear, injury or eye trauma.
Fuchs’ Dystrophy: Fuchs’ dystrophy is a non-inflammatory, inherited condition that may be progressive and may affect both eyes. The internal cell lining of the cornea, called the endothelium, becomes damaged. If the endothelium does not function properly, fluid accumulates in the cornea, which causes clouding and a slow decrease in vision. Initially patients will notice blurred vision in the morning and a glare or halos around lights. As the dystrophy progresses, one’s vision can be blurred all day long. Early Fuchs’ dystrophy can be treated with hypertonic saline drops to help remove excess fluid from the cornea. In more advanced cases, surgery to replace the damaged cells may be necessary.
Keratoconus: Keratoconus is a common corneal condition in which the cornea begins to thin. As it thins, it loses strength, and aspects of the cornea start to bulge. The cornea changes into an abnormal shape, and one’s vision becomes affected. Initially, the vision can be corrected with glasses or contact lenses. If these are no longer effective, surgical intervention may be necessary. The options range from implantable corneal ring segments (called Intacs) to a corneal transplant. Approximately 20% of patients with keratoconus seeing a corneal specialist will eventually need a partial or full thickness corneal transplant in order to restore good vision.
Issues That Can Affect The Cornea
Cornea Surgery Options
Full Thickness Corneal Transplant (Penetrating Keratoplasty):
A full thickness corneal transplant can be used to treat a wide variety of corneal conditions. In this procedure, a central, full thickness button of cornea is removed and replaced by donor cornea of similar size. The transplanted tissue is typically sutured into position. The full thickness corneal transplant is an excellent tool to restore vision, but recovery of best vision can take 6 or as long as 12 to 18 months. The corneal transplant can be assisted by the femtosecond laser (femtosecond laser-assisted keratoplasty). The laser helps create a custom fit of the transplant tissue. The transplant tissue and the cornea are shaped to create an interlocking pattern. This pattern, reminiscent of furniture constructed with dove tail joints, creates a stronger wound and thus a safer surgery. It potentiates earlier suture removal and less astigmatism. This procedure is not for every patient, and only a detailed exam with special imaging studies of the cornea can determine if a patient is a good candidate for a laser-assisted corneal transplant.
Partial Thickness Corneal Transplant:
Instead of replacing the full thickness of the cornea, some patients benefit from a partial replacement. Two types of partial thickness corneal transplants exist. In deep anterior lamellar keratoplasty, or DALK, only a very thin layer of tissue is left and the rest is replaced with donor tissue. Visual rehabilitation is similar to a full thickness corneal transplant. The advantage of a DALK is the lower rate of rejection as compared to the full thickness transplant. In descemet’s stripping automated endothelial keratoplasty, or DSAEK, a smaller amount of tissue is transplanted. This more recently developed technique offers faster visual recovery for patients with corneal swelling caused by endothelial conditions such as Fuchs’ dystrophy. Only a very thin layer of cornea is removed and then replaced with a thin layer of donor cornea. Vision can be restored as quickly as 2 to 3 months.
