Diagnosis · Chapters 05 → 07

Imaging & staging

Detecting the subclinical form means converging three readings: curvature (Placido), elevation and pachymetry (Scheimpflug), and the epithelium that "unmasks" (OCT). Then stage to decide — and above all detect progression.

Chapter 05

Topography, tomography & epithelial mapping

5.1 Placido topography (anterior curvature)

Signs: inferior steepening, asymmetric bowtie with skewed radial axis (SRAX/AB-SRAX), high central K. Rabinowitz indices: I-S value (infero-superior asymmetry), central K > 47-48 D; KISA% combining K, I-S, astigmatism, SRAX. Limitation: it explores only the anterior surface and is "masked" by epithelial remodelling.

5.2 Scheimpflug tomography (Pentacam) — the standard

Anterior AND posterior elevation (best-fit sphere / toric ellipsoid): the posterior surface is the earliest marker.
Complete pachymetry map: thinnest point, its location, pachymetric progression indices (PPI), CTSP/PTI.
BAD-D (Belin-Ambrósio Enhanced Ectasia Display): global multiparametric score, excellent for the subclinical form.
Always pair with biomechanics (Corvis TBI) for subclinical forms.

5.3 OCT epithelial mapping — the tool that "unmasks"

The epithelium actively remodels its geometry to compensate for an underlying stromal irregularity: it partially masks the anterior Placido topography. OCT epithelial mapping reveals the "true" stroma.

Epithelial “doughnut” map — focal thinning over the cone apex, surrounding thickening ring (original schematic).

Normal epithelium: ≈ 50-53 µm centrally, remarkably regular; thinner superiorly, slightly thicker inferiorly.
Indices: ↑ Min-Max range, ↑ standard deviation, minimum location shifted infero-temporally.
Patterns: regular (normal) · focal doughnut thinning (KC) · diffuse thinning / lens-related warpage (regresses on lens cessation) · inferior peripheral band (PMD).

Major applications: subclinical screening (the doughnut may precede overt tomographic signs), differentiating true KC vs warpage from lenses, guiding topo-guided PTK (the epithelial profile dictates ablation depth), and tracking progression.

Integrated strategy — subclinical form

Tomography (BAD-D, posterior elevation, pachymetry) + biomechanics (Corvis TBI) + epithelial mapping (doughnut). A single criterion may be missing; it is the concordance that makes the diagnosis.

Chapter 06

Classification & staging

6.1 Amsler-Krumeich (historical, 4 stages)

Based on K, refraction/induced myopia, pachymetry and presence of scars. Useful, but takes neither the posterior surface nor vision into account.

Amsler-Krumeich stages
Stage	K max	Pachymetry	Refraction	Scar
I	< 48 D	normal	myopia/astig < 5 D	no
II	< 53 D	> 400 µm	5-8 D	no
III	> 53 D	200-400 µm	> 8 D	no
IV	> 55 D	< 200 µm	not measurable	yes

6.2 ABCD (Belin) — recommended

Staging by 4 independent parameters, each graded 0-4, with a (-/+) suffix for scar/vision:

Anterior radius, 3 mm zone (ARC).

Posterior radius, 3 mm zone (PRC) → early sensitivity.

Thinnest pachymetry.

Best corrected acuity (Distance BCVA).

Advantage: it integrates the posterior surface (early) and function (D), and serves as a stage-by-stage progression reference.

Notion of progression — CXL decision

Documented progression = significant change in at least one criterion (K max, ABCD index, minimum pachymetry, astigmatism) across 2 comparable exams → indication for cross-linking.

Chapter 07

Pellucid marginal degeneration — specifics

Semiology: inferior band of thinning at 4 to 8 o'clock, 1-2 mm from the limbus, cornea bulging above the thin band ("beer-belly"), strong against-the-rule astigmatism.
Topography: "crab-claw" / "butterfly wings" / "kissing doves" — but beware, crab-claw is also seen in inferior KC → pachymetry decides (inferior peripheral = PMD; para-central = KC).

Management

Rehabilitation

Rigid / scleral lenses — often the best visual rehabilitation.

Stabilisation

Band CXL, protocols adapted to the peripheral topography.

Regularisation

ICRS: delicate asymmetric inferior placement (very peripheral segment, close to the thin zone).

Lamellar surgery

Crescentic lamellar keratoplasty, tuck-in DALK, or large eccentric graft; reserved for very advanced forms.

SILK (sclerocorneal)

Sclerocorneal intrastromal lamellar keratoplasty (Gabison et al.): a crescent-shaped stromal graft inserted through a scleral tunnel under OCT — re-thickens the thin band and corrects the against-the-rule astigmatism, even in advanced forms.

Technique — SILK (Gabison et al., Cornea 2016)

The SILK (Sclerocorneal Intrastromal Lamellar Keratoplasty) treats advanced PMD by correcting both the thinning and the induced astigmatism. Under intraoperative OCT, the thinnest zone is mapped; then, through a scleral tunnel (incision parallel to the limbus), an intrastromal pocket is created by lamellar dissection that spares the corneal centre; a crescent-shaped stromal graft (~300 µm) is inserted to re-thicken the inferior band. Closing the scleral tunnel (10/0 nylon) steepens the vertical meridian and enhances correction — up to ~11.6 D of astigmatism corrected, without complications. Advantages: endothelium preserved, no corneal sutures, graft in a closed pocket (lower infection/dehiscence risk), short steroid course; a femto-assisted variant is possible. Ref.: Guindolet, Petrovic, Doan, Cochereau, Gabison. Cornea 2016;35(6):900-3.

SILK technique schematic: crescent stromal graft placed in an intrastromal pocket through a scleral tunnel, under intraoperative OCT (original schematic).

Sizing pitfall — ICRS in PMD

The thin peripheral band forces a very eccentric segment: higher extrusion risk and a less predictable effect than a central cone. Prefer scleral lenses or an additive CAIRS if the cornea is too thin for safe PMMA.