EyeWorld Asia-Pacific March 2016 Issue

12 March 2016 EWAP Feature Views from Asia-Pacific Views from Asia-Pacific Rohit SHETTY, FRCS, PhD Vice Chairman, Narayana Nethralaya 121/C Chord Road, Bangalore Tel. no. +91-80-9611102568 drrohitshetty@yahoo.com M anaging irregular cornea especially in keratoconic eyes has always been a challenge. We tried to simplify this in our published article “Current review and a simplified five point management algorithm for keratoconus”. 1 Here we devised a scoring system (Table 1) and based our management on the final score (Figure 1). Keratoconus patients who are intolerant to contact lenses and show progression of the disease will need to be considered for topography-guided ablation (T-PRK) with adjunctive CXL. Those with a preoperative thinnest pachymetry of 450 μm (after epithelial debridement) or a predicted postoperative thinnest pachymetry of at least 400 μm, can be considered for T-PRK, although some investigators have included patients with a minimum pachymetry of 300 μm as well. The ablative or “Q based” protocol we use is based on the cone location, the corneal asphericity (Q) value, refractive error correction required and the change it produces. Since it takes four different important parameters into consideration, we believe this to be the most comprehensive protocol. The maximum tangential curvature on corneal topography is used for classifying the cones. 2 The x and y coordinate of the location of the maximum tangential curvature is noted. The distance (L) from the geometric center of the corneal tangential curvature map is evaluated as the square root of the sum of squares of x and y coordinate. The refractive correction is based on the spherical equivalent and pachymetry. Corneas with a spherical equivalent <6 D and with a thinnest pachymetry of >475 μm can undergo partial refractive treatment, but in corneas with a thinnest pachymetry between 450 and 475 μm and a higher spherical equivalent, a refractive procedure in not advised to avoid excessive tissue ablation. If a refractive correction is not being attempted, the Q value alone can be reduced by 20–30%. Later, we neutralize the change in defocus to spherical aberration. The refractive error used to neutralize is then added to the final refraction to be corrected. This step is important as this compensates for a change in the asphericity and induced refractive change. We attempt to limit the refractive correction to maximum 40 μm. Simultaneous crosslinking leads to increase in the biomechanical strength of the cornea as measured by corneal hysteresis and corneal resistance factor. Though T-PRK has been used for refractive correction in patients with forme fruste keratoconus, in established cases its primary aim is regularization of the corneal surface. References 1. Shetty R, Kaweri L, Pahuja N, Nagaraja H, Wadia K, Jayadev C, Nuijts R, Arora V. Current review and a simplified “five-point management algorithm” for keratoconus. Indian J Ophthalmol . 2015 Jan;63(1):46-53. 2. Shetty R, Nuijts RM, Nicholson M, Sargod K, Jayadev C, Veluri H, Sinha Roy A. Cone location-dependent outcomes after combined topography-guided photorefractive keratectomy and collagen cross-linking. Am J Ophthalmol . 2015 Mar;159(3):419-25. Editors’ note: Dr. Shetty declared no relevant financial interests. Figure 1 Table 1 Getting started - from page 11