The cornea is the transparent, dome-shaped outer layer, playing a major role in focusing light onto the retina. For clear vision, this tissue must maintain a smooth and precise structure, allowing light to pass through unimpeded. Corneal haze is a condition where the corneal tissue loses this transparency, appearing as a clouding or scarring that develops following an injury or disease process. This clouding scatters incoming light, which significantly interferes with the eye’s ability to create a sharp, clear image.
The Biological Mechanism of Corneal Opacity
Corneal haze is a biological response to injury, primarily involving the two layers beneath the protective outer epithelium: Bowman’s layer and the corneal stroma. The stroma, which makes up nearly 90% of the cornea’s thickness, is composed of highly organized collagen fibers and specialized cells called keratocytes. This precise arrangement of fibers is what allows the stroma to remain transparent.
When the cornea is injured, such as by trauma or surgery, the keratocytes near the wound undergo a transformation into activated cells known as myofibroblasts. This cellular change is triggered by signaling molecules like transforming growth factor-beta (TGF-β), released during healing. Myofibroblasts are contractile cells that produce and deposit a disorganized type of extracellular matrix, including abnormal collagen fibrils.
This newly deposited, haphazard matrix lacks the uniform structure of the native stroma, causing light to scatter instead of passing straight through. The degree of opacity is directly proportional to the number of myofibroblasts and the density of the abnormal collagen they produce. In mild injury cases, these myofibroblasts eventually disappear through programmed cell death, allowing the tissue to clear. However, with severe injury, the haze can become permanent.
Primary Triggers and Underlying Risk Factors
Corneal haze is a common final outcome of various types of damage that disrupt the corneal structure. A frequent trigger is laser refractive surgery, specifically Photorefractive Keratectomy (PRK), which involves removing the surface layer and ablating the underlying stroma. The risk of developing haze after PRK is higher with corrections for high degrees of myopia or astigmatism, although prophylactic measures like Mitomycin C have greatly reduced the incidence.
Infections of the cornea, known as infectious keratitis, are a major cause of haze, as the body’s intense inflammatory response creates scarring. This includes bacterial infections, as well as viral infections caused by the Herpes Simplex virus. Physical trauma, such as foreign bodies or chemical burns, can also lead to deep stromal scarring that results in permanent opacity.
Certain genetic conditions known as corneal dystrophies represent an internal risk factor for haze formation. These disorders cause abnormal material deposits within the corneal layers, which progressively cloud the vision. Examples include Lattice, Granular, and Macular corneal dystrophies involve the accumulation of foreign material that obstructs the cornea’s natural clarity.
Recognizing Visual Symptoms and Impact Severity
Corneal haze is characterized by several distinct visual complaints that reflect the scattering of light within the eye. The most common symptom is reduced or blurred vision, often feeling like looking through a permanent veil or a smudged lens. This loss of clarity is frequently accompanied by increased sensitivity to light, known as photophobia.
Patients often report visual disturbances in low-light conditions, such as driving at night, due to increased glare and halos around light sources. This is because the hazy tissue disrupts the light waves, preventing them from focusing cleanly on the retina. In some cases, particularly if the surface of the cornea is also irregular, a persistent foreign body sensation or mild discomfort may occur.
Visual impairment severity correlates with two factors: the density and the location of the opacity. Haze that is faint and located in the peripheral cornea may cause minimal or no noticeable symptoms. However, a dense patch of haze located centrally, directly over the visual axis, can severely compromise visual acuity, contrast sensitivity, and functional vision.
Management and Therapeutic Approaches
The treatment approach for corneal haze depends entirely on its cause, severity, and depth within the corneal tissue. For mild, non-progressive cases, particularly those seen temporarily after refractive surgery, observation is often the first step, as the haze may spontaneously resolve over a period of many months. When intervention is necessary, medical and surgical options are considered.
Topical medical management involves the use of corticosteroid eye drops, which suppress the inflammatory response and inhibit the transformation of keratocytes into myofibroblasts. These drops are used long-term to reduce the scarring process or prevent its recurrence after surgery. However, prolonged corticosteroid use requires close patient monitoring due to potential side effects, including an elevated intraocular pressure, which can lead to glaucoma, and the possible development of cataracts.
For visually significant haze that is superficial, a surgical procedure called Phototherapeutic Keratectomy (PTK) may be performed using an excimer laser. PTK precisely ablates a thin layer of the corneal surface to remove the scarred tissue and smooth the corneal contour, often resulting in significant improvement in vision. This treatment is frequently combined with Mitomycin C, an anti-metabolite drug applied briefly to the surface to prevent keratocyte activation and reduce the risk of haze recurrence.
If the corneal scarring is too deep, widespread, or unresponsive to laser treatment, a corneal transplant (keratoplasty) becomes the necessary intervention. A deep anterior lamellar keratoplasty (DALK) is the preferred technique when the innermost layer, the endothelium, is healthy. This partial-thickness procedure replaces only the scarred anterior layers with donor tissue, avoiding the higher risk of graft rejection associated with a full-thickness replacement. If the scarring extends through the entire cornea, a penetrating keratoplasty (PK) is performed, replacing the entire central cornea with a full-thickness donor graft.