Keratoconus is usually thought of a "bulging" out of the cornea into a cone shape due to thinning. This description implies the cornea actually stretches but in fact, work done by various researchers shows that in keratoconus, the surface area and volume are maintained – only in the condition keratoglobus does the cornea actually stretch. Instead, the cornea warps so that there is distortion centrally, but the periphery of the cornea is usually fairly unchanged. 

To understand what happens, we can look at topography which is a colour coded mapping of the corneal shape – the red colours are the steeper areas  whilst the greens and blues represent flatter areas.

    Normal cornea     Keratoconus     Pellucid Marginal Degeneration

 

Types of keratoconus

  

How keratoconus is managed
The topography of a normal cornea (above left) shows astigmatism where there is more curvature (and power) in one direction of the cornea than the other direction at right angles to it. This forms the classical "bow tie" pattern seen here.

The middle topography shows classical keratoconus where this bow tie pattern no longer exists and there is steepening of the cornea in the inferior region.
 

Due to the warpage effect described above, the flatter areas of a keratoconic cornea tend to be flatter than a "normal" cornea. This is even more obvious in the picture of PMD – Pellucid Marginal Degeneration – where the steepening tends to take place nearer the edge (limbus) of the cornea. This pattern shows the classical "crabs claw" picture on topography and the person with PMD can often see quite well due to the fact that there is a relatively normal central area in the pupil area.
            The effect of the warping caused by Keratoconus is to distort the cornea and thus distort images passed through the eye onto the retina at the back. Glasses simply pass on this distortion, so vision can be very blurred indeed and without help, many people with keratoconus can only see a hand moving in front of their face.

Up until now, the only way to help keratoconics is to wear rigid contact lenses (which smooth out the distortions) or have a corneal graft. Unfortunately, the condition is such that many people simply cannot tolerate rigid lenses and grafts are not currently available in TnT, so many are rendered effectively partially sighted or blind by this condition.

There are now new alternatives for keratoconics: Soft lenses are now available (from the UK and the US) and Corneal Cross Linking and other surgical techniques are now possible in TnT.