2.1 Keratoconus, Pellucid Marginal Degeneration & Keratoglobus
Key Takeaways
- Keratoconus usually produces focal corneal thinning and protrusion with irregular astigmatism; slit-lamp and map findings support, but do not individually establish, the diagnosis.
- Pellucid marginal degeneration classically has an inferior peripheral thinning band with protrusion above it; a crab-claw map pattern is suggestive but can occur in other ectatic shapes.
- Keratoglobus involves more diffuse limbus-to-limbus thinning and requires exceptional protection from trauma plus specialist oversight.
- Cone labels and staging systems describe shape or severity but do not automatically choose a lens modality.
- Sudden edema and vision loss in ectasia may represent acute corneal hydrops: stop lens wear and arrange urgent ophthalmic evaluation rather than recommending medication within technician scope.
2.1 Keratoconus, Pellucid Marginal Degeneration, and Keratoglobus
These ectatic disorders weaken and reshape the cornea, producing irregular astigmatism and reduced spectacle-corrected vision. The technician should recognize patterns, obtain repeatable measurements, support contact-lens fitting, and identify urgent findings. Diagnosis, progression treatment, and medication belong to the appropriately licensed eye-care clinician.
Keratoconus
Keratoconus commonly produces focal or regional thinning with anterior and posterior protrusion. It is often asymmetric between eyes and can progress at different rates. Findings that may support suspicion include:
- increasing irregular or oblique astigmatism;
- reduced spectacle acuity or scissoring on retinoscopy;
- localized steepening and abnormal elevation or pachymetric progression;
- epithelial thinning over a stromal cone on epithelial mapping;
- Fleischer ring, Vogt striae, or apical scarring in more developed disease; and
- frequent prescription change or increasing higher-order aberration.
None of these findings should be treated as a single pathognomonic test. Vogt striae may become less visible with gentle pressure, but pressure is not required to diagnose disease and should never be forceful. Map quality, tear-film artifacts, corneal warpage, and contact-lens discontinuation history matter.
Nipple, oval, and globus are traditional descriptions of cone size and distribution. They help anticipate decentration and bearing but use approximate boundaries and do not dictate one lens diameter.
Pellucid Marginal Degeneration
PMD classically has a crescent-shaped band of inferior peripheral thinning, often separated from the limbus by relatively uninvolved cornea. The area of greatest protrusion may sit above the thinned band. Refraction often shows high against-the-rule or oblique astigmatism.
A crab-claw, butterfly, or kissing-birds curvature pattern raises suspicion but is not exclusive to PMD; inferior keratoconus and other shapes can look similar. Confirm the location of thinning with slit-lamp examination, pachymetry, and tomography rather than diagnosing from an axial map alone.
The inferior distribution can make a small corneal GP unstable, yet corneal GP, intralimbal, hybrid, custom soft, piggyback, and scleral designs remain options depending on actual geometry and patient needs.
Keratoglobus
Keratoglobus is characterized by diffuse thinning and globular protrusion that may extend close to the limbus. The thin cornea can be vulnerable to trauma. Care should be coordinated with a corneal specialist. Protective eyewear is commonly advised, and any trauma, sudden pain, or vision loss needs urgent evaluation.
A large vaulting lens may provide optical rehabilitation for some eyes, but handling risk, corneal fragility, oxygen burden, limbal clearance, and removal technique require careful assessment. No lens makes the eye mechanically invulnerable.
Acute Corneal Hydrops
Acute hydrops results from a break in the posterior corneal complex that permits aqueous entry into the stroma. The patient may report sudden vision loss, pain, photophobia, and marked clouding. Findings can overlap with infection or other emergencies.
The technician should:
- stop contact-lens wear;
- avoid patching or recommending leftover medication;
- document symptoms, visual change, and visible findings;
- arrange urgent same-day ophthalmic or corneal-specialist evaluation; and
- give emergency instructions if the receiving clinician cannot be reached.
Medical management varies with severity and evolving evidence. Hypertonic agents, cycloplegia, pressure-lowering medication, anti-inflammatory therapy, gas procedures, or surgery are decisions for the treating ophthalmologist. Do not turn one regimen into an NCLE technician protocol.
Choosing a Lens Modality
Begin with visual goals, corneal and limbal shape, epithelium, tear film, handling, work environment, prior failures, and patient preference.
- A corneal GP can provide excellent optics and tear exchange when it centers and avoids harmful bearing.
- An intralimbal design may improve stability without full scleral landing.
- A hybrid or piggyback system may combine rigid optics with soft-lens comfort or protection.
- A custom soft lens may help when handling or tolerance makes a rigid option impractical.
- A scleral lens can vault irregular tissue and improve stability, but it adds reservoir and lens thickness and needs careful landing-zone assessment.
Verify every choice on eye. A descriptive cone label is a starting hypothesis, not a laboratory order.
Follow-Up
Document acuity, over-refraction, map quality, corneal and limbal relationship, staining, scars, neovascularization, lens centration, movement, clearance where applicable, wear time, and symptoms. Compare with baseline for progression or lens-related change. New pain, photophobia, discharge, epithelial defect, infiltrate, edema, or sudden vision reduction requires medical triage.
Exam Traps
- Crab-claw topography is suggestive, not diagnostic, of PMD.
- Vogt striae are supportive, not pathognomonic.
- Do not stage or fit from Kmax alone.
- Large ectasia does not automatically mean only one lens modality can work.
- Acute hydrops is an urgent referral problem, not a technician medication question.
Fine posterior stromal lines become less visible with gentle pressure in an eye with irregular astigmatism. What is the best interpretation?
A patient with known ectasia has sudden pain, photophobia, major vision loss, and dense corneal edema suspicious for acute hydrops. What should the contact-lens technician do?
An inferior thinning band with protrusion above it and a crab-claw map pattern most strongly raises suspicion for which condition, while still requiring clinical and tomographic correlation?