Abstract
BACKGROUND: 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. METHODS: 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. RESULTS: 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. CONCLUSIONS: 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.