Pellucid marginal degeneration (PMD) and keratoconus are two corneal ectatic disorders that can look remarkably similar, particularly when changes in the lower part of the cornea are involved. Both can cause irregular corneal curvature and may lead to significant changes in vision. Because their appearance can overlap on corneal imaging, distinguishing between them requires more than simply looking at a single topography pattern. A detailed examination of the cornea, along with corneal tomography and thickness mapping when appropriate, can help an eye care specialist determine which condition is present.
For patients, the distinction matters because the location and pattern of corneal thinning are different in PMD and keratoconus, and those differences can influence how the condition is monitored and managed. At Cali Eye Institute in Torrance, as well as Lancaster Eye Institute in Lancaster and Anacapa Vision in Ventura, evaluation of corneal shape and structure can help provide a clearer picture when these conditions appear similar.
The Main Difference Between PMD and Keratoconus
The most important distinction is where the cornea becomes thin and where the resulting bulging occurs.
In keratoconus, thinning is generally located in the central or paracentral cornea, and the cornea progressively develops an irregular, cone-like shape. The area of greatest protrusion is typically associated with or located near the thinnest portion of the cornea.
PMD has a different pattern. The thinning usually occurs as a narrow, crescent-shaped band in the inferior peripheral cornea, while a small area of relatively normal cornea remains between the thinning and the limbus. Interestingly, the area of greatest corneal protrusion is typically positioned above the thinnest region, creating what is sometimes described as a “beer-belly” appearance.
This difference in the relationship between thinning and protrusion is one of the most useful clues when specialists are trying to distinguish PMD from keratoconus.
PMD vs. Keratoconus at a GlanceA quick reference comparing two corneal ectatic disorders — Pellucid Marginal Degeneration and Keratoconus. |
||
| 📋 Feature | 🔬 Pellucid Marginal Degeneration | 👁️ Keratoconus |
|---|---|---|
| Primary area of thinning | Usually inferior peripheral cornea | Usually central or paracentral cornea |
| Pattern of thinning | Narrow, crescent-shaped peripheral band | More localized central/paracentral thinning |
| Location of protrusion | Typically above the thinnest area | Generally associated more closely with the thinnest area |
| Corneal shape | Inferior peripheral steepening with central relative flattening | Cone-like or asymmetric corneal protrusion |
| Astigmatism | Often significant irregular, against-the-rule astigmatism | Often irregular astigmatism with an asymmetric pattern |
| Topography | May show a crab-claw or butterfly-type pattern | Often shows asymmetric steepening or an asymmetric bow-tie pattern |
| Diagnosis | Slit-lamp examination plus topography/tomography and thickness assessment | Slit-lamp examination plus corneal topography/tomography and thickness assessment |
Why PMD Can Be Mistaken for Keratoconus
One of the biggest challenges in distinguishing these conditions is that their effects on corneal shape can overlap. PMD can produce substantial inferior steepening and irregular astigmatism, while keratoconus can also affect the inferior portion of the cornea. As a result, a topography map may not always provide an immediate or definitive answer.
The familiar “crab-claw” or “kissing birds” pattern is often associated with PMD, but research has shown that a similar pattern can also occur in inferior keratoconus. In one comparative study, inferior keratoconus was actually more common than PMD among eyes showing a crab-claw pattern. This is why looking only at the anterior curvature map may not be enough.
Modern corneal evaluation can instead incorporate several pieces of information, including the location of the thinnest corneal point, anterior and posterior elevation, pachymetry, and the distribution of epithelial and stromal thickness. A 2026 study specifically examining PMD and inferior keratoconus found meaningful differences in epithelial, stromal, and total corneal thickness across different areas of the cornea, demonstrating the value of detailed thickness mapping when the two conditions have similar topographic appearances.
How Corneal Topography Helps
Corneal topography is an important part of evaluating both PMD and keratoconus because it creates a detailed map of the cornea’s curvature. It can reveal areas that are unusually steep or flat and can help identify patterns that may not be apparent during a routine eye examination.
In PMD, topography may show inferior peripheral steepening with relative central flattening, producing patterns sometimes described as a crab claw or butterfly appearance. Keratoconus may instead demonstrate a more localized area of asymmetric steepening associated with the cone. However, these descriptions are not absolute. A crab-claw pattern, for example, does not automatically mean that a patient has PMD.
Tomography can provide additional information by assessing both the front and back surfaces of the cornea as well as its thickness distribution. This broader assessment can be particularly valuable when the front-surface appearance does not clearly distinguish PMD from inferior keratoconus.
The Importance of Corneal Thickness
Corneal thickness mapping is particularly useful because PMD and keratoconus tend to have different distributions of thinning.
With PMD, thinning is characteristically concentrated toward the inferior peripheral cornea, while the central cornea may remain comparatively thicker. There is also typically a small zone of relatively unaffected cornea between the thinned region and the limbus.
With keratoconus, the thinnest point is generally more central or paracentral and is associated with the area of corneal deformation. Identifying the location of this thinnest point and understanding how thickness changes across the cornea can therefore help specialists differentiate the two conditions.
This is also one reason why a patient should not assume that an unusual astigmatism prescription automatically indicates one condition or the other. Irregular astigmatism can occur with both PMD and keratoconus. If you are trying to understand how corneal irregularity differs from more common forms of astigmatism, our discussion of how keratoconus differs from ordinary astigmatism provides additional context.
Does PMD Progress Like Keratoconus?
PMD and keratoconus are both considered progressive corneal ectatic disorders, but their typical patterns of development can differ. PMD generally appears later in life than keratoconus and has often been described as progressing more slowly. However, progression varies from person to person, so age or the apparent severity of the condition should not be used by itself to determine whether a patient has PMD or keratoconus.
Regular monitoring can be important when an ectatic corneal disorder is suspected. Changes in corneal curvature, thickness, elevation, and vision over time can provide valuable information about whether the condition is remaining stable or showing evidence of progression.
Can PMD and Keratoconus Occur Together?
The relationship between PMD and keratoconus is more complicated than simply treating them as two completely unrelated conditions. The two disorders share several characteristics, and some research has reported cases in which features associated with both conditions are present.
This overlap is another reason why diagnosis should be based on the overall clinical picture rather than a single sign on a topography map. A specialist may need to consider the patient’s examination findings, corneal shape, thickness distribution, age, refraction, and changes over time before reaching a diagnosis.
Does the Difference Affect Treatment?
The treatment approach depends less on the name of the condition alone and more on factors such as visual quality, corneal stability, the degree of irregularity, and whether there is evidence of progression.
For patients with either PMD or keratoconus who have relatively good functional vision, observation and appropriate vision correction may be considered. Spectacles can be useful in earlier or less irregular cases, while specialized contact lenses may provide better vision when corneal irregularity makes conventional glasses insufficient.
If there is documented progression, corneal cross-linking may be considered in appropriately selected patients to strengthen the corneal structure and slow or halt further ectatic progression. More advanced cases may require other specialized approaches, depending on the amount of corneal irregularity and the patient’s visual needs. Management should therefore be individualized following a detailed corneal evaluation.
It is also important to recognize that PMD and keratoconus are not simply refractive errors that can be approached like routine nearsightedness or astigmatism. The underlying corneal structure needs to be evaluated before considering procedures intended to reshape the cornea.
Why an Accurate Diagnosis Matters
PMD can be relatively easy to overlook when the corneal changes are subtle, while advanced PMD may be mistaken for keratoconus. Conversely, inferior keratoconus can resemble PMD on corneal topography. A diagnosis based solely on one characteristic pattern can therefore be misleading.
A comprehensive corneal evaluation allows the specialist to look beyond the appearance of the cornea and determine where thinning occurs, where the cornea is most elevated, how the front and back surfaces are shaped, and whether there are signs of progression. These details can make a significant difference when PMD and keratoconus are being considered.
Getting a Corneal Evaluation
If you have been told that your cornea has an unusual shape, irregular astigmatism, or a topography pattern suggestive of keratoconus or PMD, a specialized corneal evaluation can help clarify the diagnosis. The distinction is not always obvious from symptoms or a routine refraction, and detailed corneal imaging may be necessary.
Cali Eye Institute serves patients in Torrance, while Lancaster Eye Institute provides care in Lancaster and Anacapa Vision serves patients in Ventura. Across these locations, patients can seek evaluation when there is concern about corneal irregularity, ectatic disease, or uncertainty between conditions such as PMD and keratoconus.
The goal is not simply to put a label on an abnormal corneal shape. An accurate diagnosis helps determine how the cornea should be monitored, whether it is stable or changing, and which vision-correction or corneal treatment options may be appropriate for the individual patient.