Crab claw pattern on corneal topography: pellucid marginal degeneration or inferior keratoconus?

Purpose To evaluate the topographic, tomographic, and densitometric properties of patients with pellucid marginal degeneration (PMD) and inferior keratoconus. Patients and methods Retrospective, comparative case series. Forty-seven eyes of 32 patients with crab claw patterns were identified from 2751 patients with corneal ectasia. They were divided into two groups, inferior keratoconus and PMD, based on clinical findings. The topographic, tomographic, and densitometric measurements were analyzed. Results PMD was detected in 11 eyes of eight patients (mean age 50.2 +/- 11.1 years), and inferior keratoconus was detected in 36 eyes of 24 patients (mean age 34.7 +/- 10.1 years). The control group consisted of 40 patients (33.1 +/- 4.6 years). The thinnest corneal point and maximum anterior and posterior elevation points were located lower in the PMD than in the inferior keratoconus (P < 0.01). In the PMD, all deviation indices were higher than the controls (P < 0.01), whereas the deviation indices, except Dt (P=0.960), were lower than the inferior keratoconus (P < 0.01). The densitometry values of PMD were significantly higher than those of the controls in all zones and layers (P < 0.01) and significantly higher than the densitometry values of inferior keratoconus in the 6-10 and 10-12mm zones (P < 0.05). Conclusion There is a higher probability of a patient with crab claw pattern on the topography of having inferior keratoconus than having PMD. Therefore, analyzing only the anterior corneal surface is not sufficient in differential diagnosis. Tomographic and densitometric evaluations may facilitate the differential diagnosis.