Slit-lamp examination remains one of the most accessible and widely used tools in ophthalmology, yet its diagnostic potential in keratoconus (KC) continues to evolve. A recent study has sought to investigate a novel optical phenomenon – the “Halo Sign” – which may offer additional clinical insight, particularly in advanced cases of the disease.
First described by He and Qu in a 2025 BMC Ophthalmology case report, the Halo Sign appears as an annular light projection on the iris when a slit-lamp beam is directed at the cone apex. Unlike classic signs such as Fleischer’s ring or Vogt’s striae, which reflect biochemical or biomechanical changes, the Halo Sign is thought to arise primarily from optical effects within the ectatic cornea.
In a small observational case series of 22 eyes from 11 KC patients at a single tertiary center, the Halo Sign was identified in eight eyes, being notably confined to advanced keratoconus (Amsler–Krumeich stage IV). By contrast, eyes without the sign spanned the full spectrum of disease severity. This suggests that the Halo Sign could potentially serve as a marker of advanced structural deformation, rather than an early diagnostic indicator.
The morphology of the halo varied considerably, with halos ranging from regular, well-defined rings to highly fragmented and irregular patterns. Using a newly proposed Halo Morphology Index (HMI), the investigators quantified these differences based on the aspect ratio of an ellipse fitted to the halo outline. The lower HMI values corresponded to more circular, well-defined halos, while higher values reflected increasing distortion and fragmentation.
Clinically, halo-positive eyes demonstrated worse visual and structural parameters. These eyes had poorer corrected distance visual acuity, thinner corneas, steeper keratometry, and significantly higher higher-order aberrations compared with halo-negative eyes. Corneal scarring was also markedly more prevalent. These associations all reinforce the impression that the Halo Sign reflects advanced disease and optical irregularity.
In terms of the clinical value of such a sign, qualitative recognition of the Halo at the slit lamp examination could prompt further evaluation or referral, particularly useful in resource-limited settings where corneal tomography is not readily available. However, the authors emphasize that it should act as a complementary observation rather than as a standalone diagnostic or staging tool.
Ultimately, the phenomenon highlights how optical manifestations of corneal pathology can translate into visible clinical signs. While preliminary, these findings suggest that careful slit-lamp observation – augmented by emerging descriptors such as HMI – may still yield new insights into keratoconus.
First described by He and Qu in a 2025 BMC Ophthalmology case report, the Halo Sign appears as an annular light projection on the iris when a slit-lamp beam is directed at the cone apex. Unlike classic signs such as Fleischer’s ring or Vogt’s striae, which reflect biochemical or biomechanical changes, the Halo Sign is thought to arise primarily from optical effects within the ectatic cornea.
In a small observational case series of 22 eyes from 11 KC patients at a single tertiary center, the Halo Sign was identified in eight eyes, being notably confined to advanced keratoconus (Amsler–Krumeich stage IV). By contrast, eyes without the sign spanned the full spectrum of disease severity. This suggests that the Halo Sign could potentially serve as a marker of advanced structural deformation, rather than an early diagnostic indicator.
The morphology of the halo varied considerably, with halos ranging from regular, well-defined rings to highly fragmented and irregular patterns. Using a newly proposed Halo Morphology Index (HMI), the investigators quantified these differences based on the aspect ratio of an ellipse fitted to the halo outline. The lower HMI values corresponded to more circular, well-defined halos, while higher values reflected increasing distortion and fragmentation.
Clinically, halo-positive eyes demonstrated worse visual and structural parameters. These eyes had poorer corrected distance visual acuity, thinner corneas, steeper keratometry, and significantly higher higher-order aberrations compared with halo-negative eyes. Corneal scarring was also markedly more prevalent. These associations all reinforce the impression that the Halo Sign reflects advanced disease and optical irregularity.
In terms of the clinical value of such a sign, qualitative recognition of the Halo at the slit lamp examination could prompt further evaluation or referral, particularly useful in resource-limited settings where corneal tomography is not readily available. However, the authors emphasize that it should act as a complementary observation rather than as a standalone diagnostic or staging tool.
Ultimately, the phenomenon highlights how optical manifestations of corneal pathology can translate into visible clinical signs. While preliminary, these findings suggest that careful slit-lamp observation – augmented by emerging descriptors such as HMI – may still yield new insights into keratoconus.
