The ADS may be a useful method to diagnose keratoconus and keratoconus-suspect cases and serve as a grading and follow-up method for tracking disease severity. Video available at www.youtube.com/watch?v=pO1PtM1oJKI. Photo: Nikhil Gotmare. Click image to enlarge.

Clinicians who examine a lot of keratoconus patients may be familiar with a visual phenomenon observed during direct ophthalmoscopy called the oil droplet sign, which manifests as an annular dark shadow (ADS) within the red reflex. In a recent study, researchers aimed to determine if ADS serves as a diagnostic technique for early keratoconus and may be used as a quantitative measure of severity.

A total of 58 participants were involved in this study: 37 healthy keratoconus subjects (37 eyes) and 21 keratoconus suspects (37 eyes), all of whom underwent corneal tomography and a full ocular examination. The ADS was present in all keratoconus eyes and keratoconus suspects but in none of the controls. The height of the ADS was significantly correlated with keratoconus severity.

“The ADS can be used as a quantitative measure of keratoconus severity since a significant correlation was found between the height of the ADS and the severity of the disease,” the authors explained. “Thus, as the disease progresses, the ADS becomes thinner.”

In addition, it can serve as a follow-up tool to track the severity of the condition. “These results illustrate that the ADS may enable long-term follow-up, even in areas where there is poor access to health care,” the authors explained. “We noted three cases of bilateral keratoconus, where visual acuity was worse in one eye, yet the ADS was thinner. This unexpected finding may be due to cone location that is inferior to the visual axis and the pupil margins seen in the tomography maps.”

The source of the ADS measured is not clear, nor is the reason why its height changes as the disease progresses. One possible factor could be that when light enters the eye through the pupil, it hits the retina and is reflected back through the pupil producing a red reflex seen with the ophthalmoscope. The authors noted that any disturbance of the light in the optical path will appear as a dark shadow.

“In patients with keratoconus, the disturbance could come from the anterior cornea or the stroma,” the authors explained. “The present study showed that anterior corneal parameters were the only ones that predicted the height of the ADS in the multiple regression model. Thus, this suggests that the source of the ADS may be due to changes in the anterior cornea. Alternatively, the distortion may be created by uneven distribution of collagen in the stroma and separation of collagen bundles in the Bowman layer.”

Further studies examining the use of ADS in a large population could add valuable information regarding the prevalence of keratoconus in areas with limited access to public health, the authors suggest.

Gideon Abou Said A, Pinero DP, Shneor E. Revisiting the oil droplet sign in keratoconus: utility for early keratoconus diagnosis and screening. Ophthalmic Physiol Opt. November 16, 2022. [Epub ahead of print].