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Topography Guided PRK

 
Transepithelial PRK in keratoconus using the Schwind Amaris 750s excimer laser 

 

Spectacle correction still provides good vision for many patients in the early stages of keratoconus. When the corneal shape becomes more irregular however, spectacles become less useful, and Corrected Distance Visual Acuity (CDVA), or the number of letters you can read on a testing chart with spectacles on, decreases. At this stage, rigid contact lenses are required for good vision. Rigid contact lenses work where spectacles do not because they act to restore a regular focusing shape to the front of the eye.

Until recently, most surgeons avoided laser refractive surgery in patients with keratoconus for two reasons. First, laser refractive surgery, and LASIK in particular, can make keratoconus worse. Second, earlier lasers were only capable of correcting regular focusing errors. Treatment developments have now emerged which make these limitations much less relevant. Corneal Collagen Cross-linking (CXL) is effective in stabilising keratoconus and is now often combined with specialised excimer laser treatment designed to regularise the corneal focusing shape such with the aim of helping patients to see better without rigid contact lenses.

The Schwind Amaris 750s laser platform offers several advantages in the context of laser refractive surgery in keratoconus. These include corneal wavefront treatment programming (a custom treatment design based directly on an accurate map of your corneal surface shape), accurate eye tracking (helping to ensure that the laser corneal shape correction is accurately superimposed on the shape irregularity mapped out at corneal scanning), and transepithelial PRK (the ability to treat directly through the corneal skin layer). Transepithelial PRK is particularly important, since the skin layer is constantly remodeling so as to smooth corneal surface shape. It thins over the peaks and thickens over the troughs of an irregular cornea. Most other laser systems require skin layer removal prior to treatment – the surface treated is then no longer the surface mapped at scanning.

Transepithelial PRK is a ‘no touch’ procedure in which the area of corneal skin layer removal is minimised. This reduces the period of discomfort in comparison with other forms of surface laser refractive surgery (eg LASEK). The skin layer heals, restoring comfort, in most patients within 4-5 days of treatment. Patients having treatment in one eye only typically require one week off work.

More information about preparation for and recovery after surface laser refractive surgery is available under ‘Laser Refractive Surgery’ on this site.

The corneal shape often improves for up to 2 years after CXL. For this reason, it is logical to delay transepithelial PRK until corneal shape has stabilized. The counter argument there is that combining CXL with transepithelial PRK will mean that you only have one painful recovery period, since both treatments require corneal skin layer removal. The corneal shape may not change significantly after CXL, and PRK retreatment can be repeated later if further corneal shape changes do occur. At the time of writing, the question of whether CXL and PRK should be performed simultaneously or sequentially has not yet been answered.

Many patients with keratoconus also have high astigmatism and high myopia. The primary aim in laser refractive surgery for keratoconus is to restore a regular corneal shape and good spectacle corrected vision. Once this aim is achieved, residual refractive errors can be corrected using ICL implantation. This two stage approach is more accurate than attempting to correct high refractive errors and an irregular focus simultaneously. 

 

Appointments

If you wish to arrange a preliminary consultation, please telephone 020 7566 2156 or 07484 081815 (or from outside the UK +44 20 7566 2156 or +44 7484 081815) or email pp.bruce.allan@moorfields.nhs.uk