Irregular Astigmatism

[caption id="attachment_5662" align="alignnone" width="750"] Section of superior cornea: A (red line) - Corneal epithelial surface; B (green line) - Stromal surface; C (blue line) - Stromal surface after PTK (replicating epithelial surface morphology); Yellow area - Epithelium; Striped area - Stroma subjected to remodelling by the epithelium; Area between the red and blue line - Tissue ablated by the PTK; Blue area - Stroma[/caption] Custom corneal ablation guided by stromal topography may correct irregular astigmatism better than ablation guided by anterior corneal topography, Aleksandar Stojanovic MD told the 2016 ASCRS•ASOA Symposium & Congress in New Orleans, USA. The result may be better corrected visual acuity outcomes for patients with keratoconus, corneal refractive surgery complications or corneal scarring from trauma or keratitis. Corneal optic asymmetries, such as coma and trefoil, are readily diagnosed by anterior corneal topography, noted Dr Stojanovic, of the University Hospital of North Norway, and SynsLaser in Tromsø, Norway. However, these higher order aberrations (HOAs) measured at the corneal surface usually are less pronounced than the stromal defects underlying them. “If the anterior corneal surface is irregular, the stromal surface is even more irregular, since the epithelium is always attempting to smooth the stromal surface up to its compensatory capability,” Dr Stojanovic explained. Therefore, using ablation profiles based on surface topography to guide a custom ablation after removing the epithelium will not remove stromal tissue correctly to regularise its shape. The magnitude and axis of astigmatism corrections also are incorrect due to compensatory epithelial remodelling. One solution is to apply phototherapeutic keratectomy (PTK) to the greatest epithelial depth, as measured by ultrasound or optical coherence tomography (OCT), to replicate the anterior surface morphology on to the stromal surface (see figure). Hence, this PTK will include the epithelium as well as the remodelled stroma (the yellow and the striped area on the figure). Applying custom ablation generated upon the anterior topography map will be viable only if such PTK is performed first, Dr Stojanovic said. Theoretically, the PTK ablates the surface topography into the remaining stroma where the custom ablation corrects it. But in practice, surface topography is not always accurately replicated on the stroma because the epithelium and stroma ablate at different rates, which introduces smaller or larger errors depending on the laser used. More elegantly, PTK can be integrated with topography-guided custom ablation for a one-step surgery, though differential ablation rates may still reduce the accuracy of this approach as well, especially if the laser used is not primarily made for such a purpose. One solution to the problem is offered by a new Scheimpflug topography-tomography system that generates a stromal topographic map by subtracting its epithelial map from its corneal surface map. Programming this data into the laser ablation planning software will generate stromal topography-guided custom ablation, which can be accurately performed after epithelial removal, Dr Stojanovic said. Accurate and mutually registered data for the corneal topography and the epithelial thickness map can be obtained only by using a single instrument for both measurements performed at the same time. In that case any laser can be programmed for separate ablations of the epithelium and the stroma, circumventing the issue of the different ablation rates. Because irregular astigmatism typically occurs in already compromised corneas, the trade-off between correcting corneal optics and preserving corneal integrity must be carefully balanced, Dr Stojanovic said. He treats HOAs first because they cannot be corrected with spectacles or soft contact lenses. Lower order aberrations are addressed only if enough corneal tissue is available. He targets a reduced optical zone if necessary. Aleksandar Stojanovic: aleks@online.no