Keratoconus and corneal
ectasia

Keratoconus is a bilateral condition, but in most cases, asymmetrical. It typically occurs in early adolescence and progresses in the 20s or 30s. The less affected eye may show a high amount of astigmatism or mild steepening. There is often a history of frequent changes in glasses prescription that do not adequately correct vision. Patients manifest with high or progressive astigmatism, which may be an early sign of the condition. More advanced cases manifest with blurry and decreased vision.

Patients with keratoglobus represent a bilateral decrease in visual acuity, which cannot be improved with spectacles or contact lenses. Those who develop hydrops (stromal edema due to leakage of aqueous humor through a tear in Descemet’s membrane) or corneal rupture are represented with eye pain.

Patients with PMD normally present asymptomatically, but cases can also present with progressive visual deterioration despite spectacle correction due to irregular astigmatism.

Patients who underwent laser refractive surgery at a younger age might have an increased risk for ectasia.

There are early and late clinical signs which indicate keratoconus.(8)

Early signs

• Scissoring of the light reflex on retinoscopy: occurs due to high irregular corneal astigmatism.
• Corneal thinning on a slit lamp, particularly in the inferior cornea.

• Rizzuti’s sign: conical reflection on the nasal cornea when a penlight is shone from the temporal side.
• Fleischer ring: an iron deposit often present within the epithelium around the base of the cone. It is brown and best visualised with a cobalt blue filter.
• Vogt’s striae: fine, roughly vertically parallel striations in the stroma. These generally disappear with firm pressure applied over the eyeball and re-appear when pressure is stopped.

Late signs

• Munson’s sign: a protrusion of the lower eyelid in downgaze.
• Breaks in Bowman’s membrane.
• Acute hydrops: a condition where a break in the Descemet’s membrane allows aqueous to enter into the stoma, causing severe corneal thickening, decreased vision, light sensitivity, tearing, and pain.
• Stromal scarring after the resolution of acute hydrops, which paradoxically may improve vision in some cases by changing corneal curvature and reducing irregular astigmatism.

Patients with keratoglobus represent the globular corneal appearance and diffuse corneal thinning, mostly in the peripheral retina. They may also develop hydrops and corneal scarring.

Patients with PMD manifest inferior corneal thinning. Their visual acuity cannot be improved with spherocylindrical lenses. However, it can be improved with a pinhole. The steepest corneal protrusion in PMD occurs above (central to) the area of stromal thinning, appearing like a ”beer belly” in the cross-section. This results in high and irregular ”against-the-rule” astigmatism of up to 20 diopters and a flattening of the vertical meridian. Astigmatism can instead be ”with the rule” if the inferior curvature is steeper than the horizontal curvature. The thinning area in PMD is always epithelialised, clear, avascular, and without lipid deposition (distinguishing it from Terrien marginal degeneration).

Slit lamp examination is the first step to diagnose corneal ectasia (mostly moderate and advanced stages), following the specific signs and checking corneal thickness using light beams.

Pachymetry is the measurement of corneal thickness, which can be done in several ways (i.e., using anterior OCT). It is very useful as additional proof of corneal thinning after slit lamp examination.

Computerised corneal topography is a computer-assisted diagnostic imaging technique in which a special instrument projects a series of light rings on the cornea, creating a colour-coded map of the corneal surface and cross-section profile.(16) Those medical devices produce a 3D image that characterises anterior and posterior corneal curvature and thickness distribution. They provide data on central corneal thickness, thinnest corneal thickness, changes in corneal thickness over the entire cornea, anterior curvature radius, and posterior curvature radius. It is crucial in preoperative refractive surgery diagnosis. It recognises early forms of corneal ectasia and monitors post-refractive surgery patients

Refractive error must be considered in conjunction with corneal thickness to determine if the patient will have an adequate residual stromal bed thickness. Approximately 15 µm of stromal tissue is ablated for every diopter of myopia correction, and a low residual stromal bed thickness (250–300 µm) increases the risk for postoperative ectasia.(17)

Some mild cases of keratoconus can be treated with soft toric lenses. However, most cases require rigid gas-permeable contact lenses or scleral lenses to neutralise irregular astigmatism.

The primary treatment for progressive keratoconus is corneal cross-linking. Corneal collagen cross-linking is a minimally invasive treatment using riboflavin and UV light to induce corneal stroma stiffening by forming additional cross-link bonds within the extracellular matrix of the stromal collagen. There are different protocols used. Nonetheless, the conventional epithelium-off procedure showed a high success rate. Patients are monitored every 3-6 months.

Patients who develop active hydrops require cycloplegics to relieve pain and hypertonic eye drops/ointment (sodium chloride 5%).

In most advanced cases, when corneal cross-linking does not provide satisfactory results, surgical options may be considered (intrastromal corneal ring segments (ISCR), deep anterior lamellar keratoplasty (DALK), or penetrating keratoplasty (PK).