Irregular Corneas

For the purposes of KeraSoft^® fitting, irregular corneas fall into two main categories, ectasia and non-ectasia, as illustrated in the diagram below. Kerectasia is a bulging of the cornea and can be due to Keratoconus, PMD or in some cases can occur after laser corrective surgery. Non-ectasia conditions include some post laser surgery cases, trauma and post corneal graft.

When fitting an irregular cornea, it is important to establish which of these conditions the patient has as each has different characteristics. Topography images are a very useful tool in the investigation

Keratoconus vs. Pellucid Marginal Degeneration

First we will look at the difference between Keratoconus and PMD as these are often difficult to differentiate.
Keratoconus is characterised by a steepening of the cornea that tends to be more centrally positioned. In PMD, the steep area is more peripheral and may even extend to the limbus and is characterised by a central flat area with a “kissing birds” or “crabs claw” appearance as can be seen from the topographies below.

 

Keratoconus

Pellucid Marginal Degeneration (PMD)

Pellucid Marginal Degeneration (PMD)

Keratoconus usually presents with medium to high myopia, whereas PMD refractions typically exhibit a low sphere with high minus cyls in the vertical axis.  The PMD example on the far right in Fig 2 could be mistaken for a typical inferior keratoconus with central K of around 6.15, however by studying the patterns it can be seen to present more of a PMD appearance with a flat superior corneal curvature of 8.20mm.  PMD cases can give relatively steep central Ks using a normal keratometer which can be misleading and may sometimes lead to a misdiagnosis.

Post Laser Corneal Surgery & Post Graft

Patients who have undergone laser refractive corrective surgery are usually very happy with their sight. In some cases, where the cornea becomes irregular and the vision is reduced, the practitioner needs to diagnose if there is ectasia or not. If there is ectasia, there will be a steepened cone within a much flatter central area and the peripheral region will often be characteristically steep.

Post graft patients will typically have a central flatter cornea with steepening in the periphery.

Options Available

For the above conditions, the following options are available:
•    RGP Contact Lenses
•    Piggy Back Lenses
•    Hybrid Lenses
•    Scleral Contact Lenses
•    Semi scleral lenses
•    Cross Linking (CXL)
•    Corneal Transplant
•    KeraSoft^® contact lenses