International Newsletter and Forum on Rigid Gas Permeable Contact Lenses, Corneal Shape, Health and Vision
July 2009
In This Issue
Column
Keratoconus
Orthokeratology
I(n)site-the-practice
(R)GP art
Agenda
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Column
Elevation!
 
U2 will be in town soon on their new '360�Concert Tour.' While I look forward very much to that gig, I still hear the lyrics from an older song 'Elevation' in my head; interestingly from the album called 'All That You Can't Leave Behind.' What is all this about? It's about corneal topography. It almost seems that the more I look at corneal curve maps, the less I understand them when I try to image the true shape of the cornea - particularly for the irregular cornea. Not a big surprise really: curves have nothing to do with shape, while elevation maps appear to be much more valuable when it comes to three-dimensional imaging. It is like hiking, for instance, in the (non-existing) Dutch mountains. With a curve map, all you know is that it will be flat where the map shows blue colors. But you have no clue at what height you are: you could be at 3,000 meters/10,000 ft or at (or in the Dutch case, below) sea level. With a height map, you know exactly where you are and how high the top is. Maybe it is time to actually 'leave behind' curve maps and start using height maps to elevate our understanding of true corneal shape.  
Eef van der Worp  
Keratoconus
Unilateral Keratoconus?
 
bridgeYou've all heard the myth of unilateral keratoconus - and the ongoing discussion surrounding it. The corneal Loch Ness Mystery: does it exist or not? A new study from Spain sheds some interesting light on this. Fifty normal eyes (refractive surgery candidates) were compared to 15 clinically normal eyes of subjects diagnosed with unilateral keratoconus. Corneal wavefront indices (root mean square and vertical coma) could discriminate between normal eyes and the fellow eye of the unilateral keratoconus patients. Corneal curvature and thickness indices demonstrated a poor capability to differentiate between the groups. Based on this it appears that keratoconus is typically bilateral but that the signs can sometimes be very subtle. It also shows that traditional methods of evaluating keratoconus - such as curvature and thickness - are not capable of establishing early stages of the disease. 
Orthokeratology
Hope for the Hyperope? 
 
For orthokeratology to be considered a serious modality, it should have correction options for a variety of refractive errors - including astigmatism (see below) and also hyperopia. Hyperopic orthokeratology is quite challenging though, as we know from hyperopic refractive surgery. We need to create a plus lens within the optical zone, which is not an easy task. Paul Gifford of the ROK (Research in Orthokeratology) group just finished his PhD on this topic under the supervision of Helen Swarbrick. First of all they found, as with myopic orthokeratology correction, that the success rate goes down as the prescription goes up. The group with +1.50D correction reached 98% of the target by day seven of orthokeratology treatment, while in the +3.50D group this was 56%. They also concluded that the corneal change in hyperopic orthokeratology may be driven by mid-peripheral compression rather apical suction. Hyperopic orthokeratology: a mouthful, but it is available and, within limits, it shows good success rates.
Optometry & Vision Science
I(n)-site-the-practice
Is Piggy Back?
 
Piggyback Contact Lens (PBCL) systems involve the use of two contact lenses: usually a soft lens directly on the cornea with a corneal (R)GP on top. Many cases in need of piggybacking are related to corneal problems such as scarring from keratoplasty, radial keratotomy, keratoconus or irregular topography from laser induced corneal ectasia in refractive surgery. Other cases such as tilted or proud grafts that cause centration problems or heavy bearing on such areas or old scars also benefit. Before 2000 PBCLs had severe limitations because of the low Dk of the soft lenses. Since the advent of high Dk silicone hydrogels - coupled with the high Dk (R)GPs we have had for the past decade or two - we now have decent levels of oxygen transmission available and thus piggybacking has a much wider potential and greater use these days. See this month's case report by Alan Saks (New Zealand) for full coverage of piggybacking on a bilateral penetrating keratoplasty. 
(R)GP-ART
The Scream? 
 
Is this the topographical version of 'the scream' by Edvard Munch? Or is this a happy camper? Both, most likely. It is a topography map taken after myopic orthokeratology on a cornea with moderate corneal astigmatism, showing decentration and an obvious suboptimal outcome. However, the good news is that toric orthokeratology lenses are becoming more widely available and demonstrating excellent results. 
Case reports can be found in the international contact lens journals such as Clinical and Experimental Optometry (summary) and in Contact Lens Spectrum
Photo: Patrick Caroline (Portland OR, USA)
 
International Agenda
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I-site is an educational newsletter that is distributed on a monthly basis and provides an update on rigid gas permeable related topics (scientific research, case reports and other publications worldwide). I-site is objective and non-politcal. Its editor Eef van der Worp is a lecturer and a consultant for a varity of industry partners, but is not related to any specific company. Please contact us at: [email protected].