Ectasia imaging, cross-linking and rehabilitation

3. Diagnostic Corneal Imaging: Topography vs Tomography

Early diagnosis of keratoconus is essential because corneal collagen cross-linking aims to arrest progression before irreversible visual loss occurs.

Placido Disc Topography

Reflects concentric mire rings off the anterior tear film to map anterior corneal curvature:

• Axial (Sagittal) Curvature Maps: Displays classic asymmetric bowtie patterns with skewed radial axes (SRAX $> 22^\circ$).
• Inferior Steepening (I-S Value): Compares average keratometric power 3 mm inferior to the centre with 3 mm superior. An $I-S\text{ difference} > 1.4–1.9\text{ D}$ is diagnostic of early keratoconus.
• Central Keratometry: $K_{\text{max}} > 47.2–48.0\text{ D}$ is highly suspicious.

Scheimpflug Tomography (Pentacam / Galilei / Sirius)

Scheimpflug imaging utilizes a rotating camera to capture optical sections from the anterior corneal surface through the posterior lens capsule, generating true 3D elevation maps and pachymetric profiles:

1. Posterior elevation may reveal ectatic change missed by anterior-only topography; interpretation depends on the reference surface, scan quality and device.
2. Pachymetric progression measures how thickness changes from the thinnest region outward.
3. ART and BAD-D combine multiple measurements. Device warning colours and thresholds are screening aids, not standalone confirmation.
4. Compare repeatable maps, clinical thinning, refraction and family/atopic history. Dry eye and contact-lens warpage can produce misleading maps; obtain stable measurements before planning treatment.

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4. Pellucid Marginal Degeneration & Keratoglobus

Pellucid Marginal Degeneration (PMD)

PMD is a bilateral, non-inflammatory ectatic disorder characterised by a peripheral band of stromal thinning located 1 to 2 mm from the inferior limbus, typically spanning between the 4 o'clock and 8 o'clock positions.

• Biomicroscopic Hallmark: The cornea protrudes immediately superior to the band of thinning. Crucially, the thinnest area is completely clear, avascular, and lacks lipid deposition (differentiating it from Terrien marginal degeneration, which displays superficial vascularisation and lipid arcs).
• Topography: A suggestive "crab-claw", "butterfly", or "kissing-birds" pattern on sagittal curvature maps.
• Refraction: Marked against-the-rule astigmatism, with a relatively steep horizontal meridian.

Keratoglobus

A rare bilateral non-inflammatory disorder characterised by generalised, uniform thinning of the entire cornea from limbus to limbus, with maximum thinning in the periphery.

• The cornea assumes a smooth, globular, spherical protrusion. Because the entire corneal periphery is paper-thin ($< 200–300\,\mu\text{m}$), patients are at catastrophic risk of globe rupture following trivial blunt trauma.
• Strongly associated with blue sclera, hypermobility, and Ehlers-Danlos syndrome type VI.

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5. Corneal Collagen Cross-Linking (CXL)

Corneal collagen cross-linking is the only proven medical intervention capable of arresting the progression of corneal ectatic disorders.

Photochemical Mechanism

Riboflavin and ultraviolet-A exposure generate photochemical reactions that increase stromal cross-linking. Oxygen availability and protocol affect the response. Laboratory stiffness increases do not supply a universal percentage of clinical strengthening or a guarantee that progression will stop in every treated cornea.

Indications & Documented Progression Criteria

Progression is a consistent change beyond measurement noise in at least two of anterior steepening, posterior steepening and progressive thinning or altered thickness distribution. A repeatable change in refraction and acuity supports the assessment. Numerical trial criteria are not universal diagnostic laws. In children, rapid progression risk may justify early cross-linking without waiting a compulsory twelve months.

The Standard Dresden Protocol

1. Epithelial Debridement (Epi-Off): The central 8.0 to 9.0 mm of corneal epithelium is mechanically removed under topical anaesthesia. The intact hydrophobic epithelium possesses tight junctions (zonula occludens) that prevent riboflavin penetration and absorbs UV-A light.
2. Riboflavin Saturation: Iso-osmolar 0.1% riboflavin in 20% dextran T500 is instilled every 2 minutes for 30 minutes. Adequate penetration is verified on slit lamp by observing yellow riboflavin flare within the anterior chamber.
3. Safety Pachymetry Threshold: Minimum deepithelialised stromal thickness must be $\ge 400\,\mu\text{m}$. The cytotoxic threshold of UV-A radiation for corneal endothelial cells is $0.37\text{ mW/cm}^2$. A $400\,\mu\text{m}$ riboflavin-saturated stroma absorbs $> 95\%$ of incident UV-A, keeping endothelial irradiance below $0.18\text{ mW/cm}^2$.
4. Thin corneas require a specialist validated modified protocol, reassessment during treatment, or deferral. Transient swelling to 400 micrometres does not by itself establish that standard irradiation is safe.
5. UV-A Irradiation: $365\text{ nm}$ UV-A light delivered at an irradiance of $3\text{ mW/cm}^2$ for 30 minutes, delivering a total cumulative surface fluence of $5.4\text{ J/cm}^2$ ($3\text{ mW/cm}^2 \times 1800\text{ s} = 5400\text{ mJ/cm}^2 = 5.4\text{ J/cm}^2$).
6. Demarcation Line: Anterior segment optical coherence tomography (AS-OCT) at 1 month reveals a distinct hyper-reflective line at approximately $300\,\mu\text{m}$ depth, marking the transition between cross-linked and untreated posterior stroma.

Accelerated & Transepithelial (Epi-On) CXL

• Accelerated CXL: 9 mW/cm² for 600 seconds, 18 for 300 seconds and 30 for 180 seconds each deliver 5.4 J/cm². Equal fluence does not guarantee an equal biological effect: oxygen replenishment, riboflavin and pulsing influence cross-linking. Compare clinical evidence for the specific protocol.
• Transepithelial (Epi-On) CXL: Employs chemical penetration enhancers (benzalkonium chloride, EDTA, or iontophoresis) to deliver riboflavin across intact epithelium. Avoids postoperative pain and infection risk, but clinical stabilization rates have historically been slightly lower than epi-off.

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6. Optical & Surgical Visual Rehabilitation

• Rigid Gas Permeable (RGP) Lenses: The gold standard for visual rehabilitation in moderate keratoconus. The tear lens trapped between the rigid rear surface of the lens and the irregular anterior corneal surface neutralizes anterior irregular astigmatism.
• Scleral Contact Lenses: Large-diameter gas-permeable lenses that vault entirely over the ectatic cornea and limbus, resting exclusively on the relatively insensitive bulbar conjunctiva and sclera. Fluid-filled reservoir provides superb comfort, ocular surface hydration, and optical neutralization for advanced keratoconus.
• Intrastromal Corneal Ring Segments (ICRS / Keraring / Ferrara / Intacs): Arcuate polymethylmethacrylate (PMMA) segments implanted into channels dissected in the deep stroma (at 70% to 80% depth) using a femtosecond laser. By Barraquer's thickness law, adding volume in the corneal periphery flattens the central cornea, reduces refractive myopia and astigmatism, and improves contact lens tolerance in patients with clear central corneas.