Article

orthokeratology today

When the Ortho-k Magic Stops

orthokeratology today
When the Ortho-k Magic Stops
BY JOHN MOUNTFORD, DIP, APP, SC, FAAO, FCLSA

We've all had that previously successful ortho-k patient who presents with the complaint that his lenses "aren't working as well as before." The symptoms usually consist of discomfort, loss of the  ortho-k effect or simply blurred vision. Resolving the discomfort portion of this claim is usually the easiest, so I'll focus on resolving loss of the effect.

Decreased Effect = Discomfort

If you see the patient in the morning with the lenses in situ, you may immediately note fine (grade 1) corneal staining centrally that wasn't present at previous aftercare visits. This is almost universally associated with back surface deposits.

The patient will report a gradual decrease in the ortho-k effect of the lenses associated with an increase in discomfort. Gently drying the lens with a tissue reveals a grainy central deposit that may prove resistant to removal by lens polishing because the rotation speed of the polishing dome is close to zero centrally.

Losing the Effect

Loss of the ortho-k effect can occur either gradually or suddenly. Causes of gradual loss include:

  • The ortho-k lenses are in the wrong eyes
  • The underlying myopia has increased. Check the over-refraction with the lens in place. We initially prescribe the lens for emmetropia, so if we find minus with the over-refraction, then either the patient has become more myopic or the lens has steepened
  • It's rare, but the lens can steepen in the back central optic zone radius (base curve radius [BCR]) over time, so check it with the radiuscope
  • Warped lenses
  • Creeping decentration

Lenses that do not initially provide an ideal bull's eye response with a small degree of either superior or lateral decentration will always get worse over time and have an adverse impact on lens efficacy. Sometimes a BCR that has flattened with time causes creeping decentration.

  • Deviation from wearing schedule. If the patient drops the wearing schedule below that required for the maintenance of the effect, then his lenses will appear to have stopped working.

The following can cause rapid loss of the effect:

  • Lenses in the wrong eyes, especially if there is a marked difference in lens parameters
  • Accidental overnight decentration, which occurs when the patient accidentally causes lens decentration by (usually) sleeping in a position that allows his hand or pillow to exert pressure on his lid, causing the lens to move off center. The patient notices the blurred vision and assumes that the lens isn't working properly so he usually puts the lens in for a longer period of time the following night. This simply compounds the problem, as the lens will automatically center over the distortion zone induced the previous night
  • A successful patient either looses or breaks a lens and the replacement lens "doesn't work." The manufacturers automatically assume that every lens is perfect, which isn't true if the lens doesn't work. Unfortunately, the only parameter you can check is the BCR, and that's not the whole story. The critical curves in a reverse geometry lens (RGL) are the reverse curve and the alignment curve(s), which have a major effect on the lens sag and ultimately the lens effect.

Unfortunately, you can't measure these curves and what then follows is a frustrating lens exchange game until another "identical" lens arrives that works. On the positive side, you can cheaply and accurately check RGLs.

Coming Up...

My next article will deal with strategies for controlling vertical and lateral decentration as well as with checking reverse geometry contact lenses.

Dr. Mountford is an optometrist in private practice specializing in advanced contact lenses for keratoconus, post refractive surgery and pediatric aphakia. He is a visiting contact lens lecturer to QUT and UNSW, Australia.