The cornea is the transparent, dome-shaped front surface of the eye, functioning as the body’s outermost lens. This structure is responsible for the majority of the eye’s focusing power, contributing approximately two-thirds of the total light refraction. A flat cornea represents a structural deviation where the curvature is significantly less curved than the average human eye. This variation in shape directly impacts how light is bent upon entry, influencing overall visual function.
Understanding Corneal Curvature
The curvature of the cornea is a precise measurement that dictates its optical power, which is expressed in diopters (D). A typical, healthy cornea has a refractive power averaging around 43 Diopters, with a normal range generally falling between 41 and 47 Diopters. Corneas with measurements below 41 Diopters are often classified as flat.
Measuring this shape is performed using specialized instruments like a keratometer or a corneal topographer. Keratometry uses a small area to determine the main curvature, while topography creates a detailed, color-coded map of the entire surface. On these topographical maps, flatter areas are typically represented by cooler colors, such as blue or green.
Impact on Visual Acuity
The main optical consequence of a flat cornea is a reduction in the eye’s overall focusing strength. A less curved surface bends light less sharply, meaning the light rays are not converged quickly enough. This results in the image attempting to focus at a point theoretically located behind the retina.
This refractive imbalance is known as hyperopia, or farsightedness, where distant objects may appear clear but near objects are blurred. The eye must exert extra effort to accommodate, or increase the focusing power of the internal lens, to pull the focal point forward onto the retina. If the cornea is not uniformly flat, but rather flat in one meridian and steeper in another, it can also lead to an irregular shape known as astigmatism.
Etiology and Related Conditions
The condition can arise from either congenital factors (meaning a person is born with it) or from acquired causes later in life. The primary congenital form is known as Cornea Plana, a rare developmental defect characterized by a near-total absence of the normal corneal protrusion. This is categorized into two main inheritance patterns: autosomal dominant (CNA1) and autosomal recessive (CNA2).
The dominant form is typically milder, with a mean corneal power around 37.8 Diopters, and causes moderate hyperopia. The recessive form is significantly more severe, sometimes showing corneal power as low as 29.9 Diopters, and is frequently associated with other ocular anomalies like central corneal clouding. Acquired flatness is often the result of prior eye surgery, such as the now-obsolete procedure called Radial Keratotomy (RK). RK involved making deep radial incisions in the periphery of the cornea to flatten the central curvature and correct nearsightedness. Over time, these incisions can cause progressive corneal changes that result in a hyperopic shift and irregular astigmatism.
Correction and Management Strategies
The vision problems stemming from a flat cornea are commonly managed using conventional corrective lenses. Eyeglasses are prescribed with positive-power lenses to add the necessary focusing strength that the cornea lacks, successfully shifting the focal point onto the retina. Contact lenses are also a primary option, but their fitting requires careful consideration due to the abnormal curvature.
Standard soft contact lenses may not fit well on a flat cornea and can move excessively, causing discomfort and unstable vision. Specialized lenses are often necessary, such as those with a flatter base curve or, more commonly, rigid gas permeable (GP) lenses. The larger, specialized scleral lenses are particularly effective because they completely vault over the flattened cornea and rest on the white of the eye, creating a smooth, new refractive surface with a fluid layer underneath. While refractive surgery like LASIK can be used to correct hyperopia by steepening the cornea, this procedure can be limited in eyes that are already extremely flat.