Researchers found four fundamental parameters could be considered as diagnostic signs to characterize all stages of PMD, including subclinical disease:

  1.  An inferior flattening island, defined as the flattening area surrounded by steep areas inferior to the horizontal axis.
  2. An apple-shaped pattern formed by mean power area as a yellow strip.
  3. Superior flattest area, defined as the presence of the flattest area at the superior quadrant.
  4. Against-the-rule irregular astigmatism (flat meridians at 45-135 degrees).

Additionally, the investigators noticed that measuring the extent of the inferior flattening island could be helpful for staging PMD and could be considered a characteristic sign of early PMD.

Based on these parameters, investigators were able to differentiate 36 cases of PMD from keratoconus.

“Four main topographic signs including inferior flattening island, apple-shaped pattern, superior flattest area and against the rules astigmatisms can be considered as diagnostic signs for PMD,” the researchers wrote in their ARVO abstract. 

Safi S, Jafarinasab M, Feizi S, et al. A new topographic “quad signs” for diagnosis and grading of pellucid marginal degeneration. ARVO 2019. Abstract 2104 -B040.2.