Pellucid Marginal Degeneration (PMD) is the second most common corneal thinning disease after keratoconus. Unlike keratoconus, PMD causes a crescent-shaped band of thinning on the lower half of the cornea. It typically occurs between the second and fifth decades of life and is not associated with any specific ethnicity. Treatments for PMD include eyeglasses, contact lenses (soft, gas permeable, scleral), and surgical intervention.

PMD causes similar visual effects as keratoconus, such as blur, glare, and distortion. To explain PMD using the car windshield analogy, imagine a crescent-shaped bulge emerging from the bottom half of the windshield instead of creating a cone shape in the center. The area of distortion is wider but can still result in severe blur, glare, and distortion similar to keratoconus.

Key terms:

Astigmatism = Directional shadows on objects

Myopia = Leads to nearsightedness = Distance is blurry = Near is better

Hyperopia = Leads to farsightedness = Near is blurry = Distance is less blurry/clear

Astigmatism

To understand the visual effects of astigmatism, let us first consider what the lack of astigmatism means. Without astigmatism, the shape of the cornea is spherical and evenly curved, like a basketball. It has the same prescription in all clock hours of an imaginary clock.

If you have astigmatism, your cornea is not evenly shaped (an analogy of a football is often used), resulting in a different prescription depending on the direction we’re referencing. In the image labeled “with astigmatism,” the horizontal prescription is -4.00, whereas the vertical prescription is -2.00. This is a form of astigmatism.

Myopia

When patients with severe myopia don’t wear their glasses or contacts, everything is blurry several inches or feet beyond their eyes. Patients with severe myopia have historically worn soft or rigid contact lenses, but they may still suffer from blurry vision due to limited correction, dryness, discomfort from outdated custom lens designs, or physical discomfort from the weight of their glasses.

To better understand myopia, it’s easier to understand how a perfect visual system works. In the picture above, the topmost projector represents a perfect eye. The projector is set to project a clear image at twenty feet and the screen is set at twenty feet, resulting in a clear picture due to the match between the projector’s focal distance and screen placement. The bottom projector represents a myopic eye. Myopic eyes are longer than their anatomically-perfect counterparts, which in this analogy represents a screen which is placed further than the focal distance of the projector. Blur results from the mismatch between the projector’s twenty foot focal distance and the twenty-five foot placement of the screen.

Hyperopia

Patients with severe hyperopia differ from myopic patients in that both far and near are likely to be blurry. They, too, wear glasses and contact lenses, but with more of a bias towards glasses as typical contact lens parameters are not always available to correct their higher prescriptions.

We can revisit the projector analogy for hyperopia as well. The topmost image still represents a perfect eye with a projector focused at twenty feet, where the screen is also placed. Hyperopic eyes are smaller than their perfect and myopic counterparts, which is represented by a screen placed at twenty feet. Hyperopic eyes are weaker, which represents the projector being focused at twenty-five feet instead of twenty feet. The clear image falls five feet beyond the screen, which results in blur.