The cornea is the transparent anterior portion of the outer layer of the eye, corresponding to a watch crystal. It measures about 12.5 mm horizontally x 11.5 mm vertically and is thinnest in the center. The cornea consists of five layers: epithelium, Bowman’s layer, stroma, Descemet’s membrane and endothelium. Pathology can involve these layers individually, as in Fuchs’ dystrophy, a disorder of the endothelial layer, or in combination, as is the effect of a corneal ulcer. The principal functions of the cornea in vision are to permit transmission of light into the eye and to focus it onto the retina. Maintenance of corneal clarity is essential to both functions, and diseases associated with a loss of corneal clarity invariably reduce vision quality.
The cornea has numerous nerve fibers passing through it, so it is exquisitely sensitive. Nerve endings terminate primarily within the epithelium, making any disruption of the epithelial layer particularly painful. Corneal nerves play a role in maintaining a healthy ocular surface, and diseases that damage corneal nerves such as herpes simplex keratitis and diabetes may create long term difficulty. Corneal clarity is principally maintained by the function of the corneal endothelium. This cell layer pumps fluid out of the cornea and into the eye, which maintains corneal dehydration. Disorders affecting the endothelium, such as Fuchs’ dystrophy, result in corneal swelling and clouding.
The cornea’s front and rear surface curvatures are critical to its role as a lens focusing light into the eye. The cornea provides two-thirds of the focusing power of the eye, compared to one-third for the lens. A disturbance of normal corneal curvature creates blurred vision that may not be able to be effectively corrected with eyeglasses or contact lenses. Kerataconus is a corneal warping disorder for which glasses and soft contact lenses often cannot effectively correct vision. Kerataglobus, Terrien’s marginal degeneration, Salzmann’s nodular degeneration and pterygia also cause blurred vision by affecting corneal curvature.
The tear film and epithelial layer are the principal barriers of the cornea to external infectious agents. Disruption of these defenses makes the cornea more vulnerable to infection, which is why contact lens wearers are more prone to serious corneal infections. Lens-induced minor trauma to the cornea provides a route for bacteria to invade the tissue. A poor tear film in dry eye patients can induce epithelial breakdown that may be complicated by infection. Patients with conditions that result in poor healing of the corneal epithelial surface will frequently have their condition complicated by infection. Healing the epithelial surface is critically important in managing patients with corneal exposure, corneal erosions, chemical burns and neurotrophic disease.
Fuchs’ Endothelial Dystrophy
Fuchs’ endothelial dystrophy (FED) is a disorder of the corneal endothelium characterized by premature death of endothelial cells and development of characteristic nodules on Descemet’s layer, referred to as guttae. Loss of endothelial cells occurs normally with aging, but in FED the cell dropout is much more pronounced. This dystrophy is always a bilateral condition and is more common in women. If endothelial cell function is sufficiently compromised, corneal edema occurs. There is a reduction in vision associated with moderate stromal edema that is more severe when epithelial edema is noted as well. Patients with early epithelial edema often note fluctuation of their vision quality, with blurring more pronounced in the morning. Edema is more severe after overnight eyelid closure during sleep.
Patients with early epithelial edema may benefit from treatment with salt water eye drops and ointment. A bandage soft contact lens can relieve pain caused by epithelial edema and bullae formation. Corneas with FED may decompensate or develop severe edema after intraocular surgery such as cataract removal. For this reason, it is important for patients to recognize the risk of proceeding with intraocular surgery. If severe edema develops, the only treatment options become a full thickness corneal transplant or Descemet’s stripping endothelial keratoplasty.
Herpes Simplex Disease
Herpes simplex virus is a member of the family of viruses that also includes cytomegalovirus, varicella-zoster virus and Epstein-Barr virus. Herpes viruses have the unusual ability to cause latent (or dormant) infections. After primary infection at a peripheral site, herpes simplex virus resides in sensory nerve ganglia (most commonly the trigeminal), autonomic ganglia or the brain stem. In these reservoirs, the virus may survive for decades. Under certain conditions it then reactivates, travels down the nerve to the peripheral end organ and causes recurrent disease.
The eye is a favored end organ for herpes simplex disease. All ocular tissues are vulnerable to infection but keratitis, or corneal infection, is most common. The symptoms of keratitis include pain, redness, tearing and blurred vision. Episodes of infection may produce scarring and sensory nerve damage. Recurrent infection can lead to more serious stromal keratitis that may be complicated by corneal ulceration, deep scarring and vascularization.