What topometric indices summarize
Topometric indices compress aspects of anterior corneal curvature into numeric descriptors. Common labels include the index of surface variance (ISV), index of vertical asymmetry (IVA), keratoconus index (KI), central keratoconus index (CKI), index of height asymmetry (IHA), index of height decentration (IHD), and minimum sagittal curvature radius (Rmin).
Why the source matters
Topometric values should be read from the named topometric examination and assigned to the correct eye. A similarly named or positioned value on another Pentacam display is not a substitute. Device software versions and reference databases may affect displayed classifications.
Pattern before label
An index can indicate variance, vertical asymmetry, decentration, or localized curvature behavior, but it does not explain the complete corneal phenotype. Map morphology, bilateral symmetry, elevation, pachymetric progression, quality, and clinical history determine whether the numeric signal is coherent.
Use thresholds cautiously
Published or device-displayed reference bands are population- and platform-dependent. Borderline values should not be treated as universal hard stops, and a normal value should not erase a concerning pattern elsewhere in the examination.
What each 8-mm topometric index describes
| Index | Technical meaning | CER-AI role |
|---|---|---|
| ISV | Standard deviation of individual sagittal radii from mean curvature; a global surface-irregularity descriptor | Reported context; not an independent CER-AI disposition gate |
| IVA | Mean superior–inferior curvature difference relative to the horizontal meridian | Reported context; not substituted for signed I-S |
| KI | Ratio of mean radius values in the superior and inferior corneal halves | Reported context |
| CKI | Ratio of peripheral-ring to central-ring mean radius; emphasizes central steepening | Reported context |
| IHA | Mean superior–inferior difference in corneal elevation along the horizontal meridian | Reported context |
| IHD | Vertical decentration of elevation data derived by Fourier analysis on a 3-mm-radius ring | Reported context |
| RMin | Smallest axial/sagittal curvature radius across the measurement area | Center topometric index; distinct from posterior Cornea Back Rmin |
| TKC | Pentacam anterior-surface topographic keratoconus classification | Reported device classification; not an autonomous CER-AI diagnosis |
| KISA% | Composite of central K, I-S, corneal astigmatism, and SRAX | Reported device index; CER-AI does not reverse-engineer its inputs from KISA |
| Signed I-S | Printed inferior–superior dioptric asymmetry with sign preserved | Direct numeric input to CER-AI ERSS topography and NICE; never replaced by ISV/IVA/IHD/IHA/KISA |
Definitions follow the OCULUS Pentacam Interpretation Guide. Device colors and thresholds describe the Pentacam reference framework; CER-AI preserves these indices as source-linked context unless a separate canonical pathway explicitly names the field.
Selected sources
- OCULUS Pentacam manufacturer documentation
- Toprak et al., Turkish Journal of Ophthalmology 2023
- Maraghechi et al., Journal of Medicine and Life 2020
See the complete CER-AI medical reference registry for broader context.