A novel optical sign in keratoconus: the Halo Sign and its clinical characteristics

The Halo sign is a recently identified slit-lamp optical phenomenon observed in keratoconus. This study aims to systematically characterize this phenomenon and investigate its clinical features in eyes with keratoconus. This retrospective, observational case series included 11 consecutive patients (22 eyes) with a clinical diagnosis of KC from a single tertiary center. Slit-lamp biomicroscopy with the beam directed at the cone apex was performed to evaluate the presence of the Halo Sign. A quantitative Halo Morphology Index (HMI) was defined as the aspect ratio (major/minor axis; a/b) of an ellipse fitted to the segmented halo outline on slit-lamp photographs and measured in ImageJ. Clinical and imaging characteristics were summarized descriptively, including corrected distance visual acuity (CDVA, logMAR), Amsler-Krumeich (AK) stage, thinnest corneal thickness (Thk), keratometric indices (e.g., mean corneal refractive power and mean pupillary power), keratoconus vertex distance (KVD), and corneal higher-order aberrations (HOAs). AS-OCT structural staging was recorded when gradable. The Halo Sign was absent in 14 eyes and present in 8 eyes. Halo-positive eyes were predominantly advanced by AK staging (all 8 eyes were AK stage 4), whereas Halo-negative eyes spanned a wider range of AK stages (1–4). In descriptive comparisons, Halo-positive eyes tended to have worse CDVA (logMAR), thinner corneas, steeper keratometry, and a higher prevalence of corneal scarring than Halo-negative eyes. Cone decentration (KVD) was also greater in Halo-positive eyes. AS-OCT structural staging was available in 19/22 eyes, with Halo-positive eyes more frequently classified as Sandali OCT stage 2 than Halo-negative eyes. HMI values in Halo-positive eyes ranged from 1.07 to 3.93, reflecting increasing halo outline anisotropy/irregularity. The Halo Sign is a newly described slit-lamp optical phenomenon that was observed predominantly in advanced keratoconus in this exploratory case series. HMI provides an operational, image-based descriptor of halo outline anisotropy.