Keratoconus and its cousins: meet the family
Conical cornea, or keratoconus, affects about 1 in 1,000 Australians and usually appears around puberty. It has much in common with a number of rarer corneal disorders.
This eye disease is much better understood than it was even a decade ago. It is now known that it occurs across all races and both genders. It occurs more frequently in African, Middle Eastern and Polynesian patients and seems to favour males slightly over females. It is defined as progressive, non-inflammatory thinning of the cornea, resulting in the characteristic conical shape that gives the disease its name.
Forme Fruste keratoconus
This is a sub-clinical form of the disease, which usually means you have no symptoms. It may in fact only be picked up in the course of a corneal assessment for laser eye surgery. Forme fruste keratoconus means the disease started in the eye but for some reason stopped progressing. It has a genetic component, so forme fruste can be present in up to 50% of family members of someone with frank keratoconus.
This is the rupturing of one of the cornea’s internal membranes and occurs rarely in advanced keratoconus. It occurs suddenly and often painfully, and leads to fluid leaking into the cornea, turning it milky white. Corneal hydrops can be triggered by mild trauma (such as eye rubbing) when the cornea is already very thin. It usually returns to clear again after a couple of months and is not necessarily a reason to proceed to a corneal transplant. Ironically contact lenses are easier to fit after an episode of corneal hydrops.
This disease is much rarer than keratoconus but shares many similarities. It is non-inflammatory and involves thinning of the whole cornea, which bulges outwards. It also progresses in the teens and twenties and usually stabilises by the thirties. Best visual correction is through using large diameter scleral lenses.
Pellucid marginal degeneration
If you’re over 45 and have just been diagnosed with a progressive corneal disease, chances are, it could be pellucid marginal degeneration. Another corneal thinning disease, this one involves thinning along a narrow mid peripheral band (usually inferiorly) rather than centrally (as in keratoconus). It is rare and usually diagnosed in middle age. It is progressive and may be corrected with glasses in early stages. Contact lenses may be required to provide functional vision as the condition progresses.
This is a very rare congenital abnormality where the inside surface of the cornea is abnormally thin, while the outside remains normal in curvature (see image). It can be across the whole inner surface or in just one part and in non-inflammatory and non-progressive, so you don’t have to worry about it getting worse over time. It tends to have less impact on vision because the integrity of the outer corneal surface is unaffected.
Corneal ectasia after laser eye surgery
The upper limits of LASIK are now well understood. During the evolution of the procedure, however, some patients developed inherently weak corneas after surgery, which mimicked keratoconus in presentation. This corneal ectasia was a rare but known complication of LASIK before it was fully understood. Thin or steep corneas are not suitable for LASIK, but may be suitable in some cases for another laser eye surgery procedure called PRK. Anyone with a subclinical, very mild form of keratoconus (called forme fruste keratoconus) should not have LASIK as it carries a significant risk of corneal ectasia for these patients. Corneal ectasia caused by LASIK has all but disappeared in quality practices. Unfortunately we are still seeing new cases of corneal ectasia in patients that have been overseas to get “cheap” laser eye surgery. For whatever reason the eyes have not been examined adequately before proceeding with the surgical correction.