Post-Laser Vision Correction (LVC) ectasia is part of a wider group of pathologies named as “ectatic ocular disorders”, in which the cornea progressively undergoes a process of thinning and steepening. Corneal ectasia strongly impacts vision and, if not timely treated, leads to corneal transplantation.1 Although rare, this complication is of great concern to refractive surgeons as it can lead to poor visual outcomes. Recent research estimated the percentage of corneal ectasia patients progressing to corneal transplantation at between 8 and 30%.2 Since its first report in 19983, much effort has been made by the scientific community to understand and prevent its development. Even though any laser refractive surgery procedure, including PRK4 and SMILE5, may lead to ectasia, research over the last 20 years has focused on cases after LASIK, the refractive procedure which seems to have the highest rate of this complication.6,7 This research includes a retrospective study, published in 2008, which highlighted that 96% of the ectasia cases analyzed occurred after LASIK, while only 4% took place after PRK.4,8
In theory, three possible contexts are recognized in which iatrogenic keratectasia can arise9:
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When a cornea that is biomechanically fragile and inclined to develop ectasia is further weakened by surgery. This is usually the case of patients who have not yet shown topographically noticeable signs of a corneal disease (e.g. keratoconus) but corneal biomechanics is already altered.
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When the surgery severely weakens a normal cornea below a safe limit.
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Intense eye rubbing
Regarding the first context, considerable progress was made in identifying the preoperative risk factors for post-LVC ectasia, which improved patient selection for various refractive procedures, and led to effective prevention of the condition.10 These risk factors included, most notably, corneal topography, tomography and epithelial maps.6,8,10,11 Additionally, the assessment of corneal biomechanics is now considered an essential screening tool to identify patients with increased susceptibility to post-LVC iatrogenic ectasia 6,12−14. Recent studies further confirmed the importance of corneal biomechanics in the diagnosis of keratoconus15,16, even in its early stages17, as for many it represents the “primum movens” in the development of the disease.
However, while substantial improvements in pre-LVC keratoconus screening and risk factor identification were made over the last 20 years, the same progress was not achieved in the diagnosis and early detection of ectasia post-op. In fact, the main diagnostic method for post-LVC ectasia remains clinical recognition of its signs and symptoms, based on corneal topographic and tomographic alterations. Timely detection of post-LVC ectasia is of foremost importance given the possibility to promptly treat these patients with cross-linking in order to stabilize the cornea.18
A recent step forward was the introduction of the CBI-LVC provided by the Corvis ST (Oculus, Germany) – a new biomechanical index that has shown high sensitivity and specificity in separating stable post-LVC eyes from post-LVC eyes with ectasia.19
Phototherapeutic keratectomy (PTK) is a surgical procedure that uses the excimer laser to treat corneal pathologies rather than changing the patient's refraction.20,21 Potentially, iatrogenic ectasia can occur after PTK22, particularly because PTK patients, on average, present a lower postoperative pachymetry relative to refractive patients, and their corneas are frequently weakened by previous ocular pathologies or surgeries. The aim of this paper is to assess the specificity of the CBI-LVC in eyes that remained stable after PTK when compared to those that underwent PRK.