Refractive Surgery Enhancements
Refractive surgery constitutes of elective vision correction options to reduce the refractive error for an otherwise healthy subject. The currently relevant options can be divided into (a) substractive (excimer ablation, lenticule extraction), (b) additive (phakic IOL implantation) and (c) exchange based (clear lens replacement).
Refractive surgery has become consistent and reliable in the recent times. There is a high satisfaction rate among patients and the visual outcomes have been fairly standardized due to better fluence , improved laser beam configurations and delivery models, knowledge of operative suite environment control, iris recognition and eye tracking, and customized algorithms.
However, there is still a subset of patients who either do not achieve satistfactory uncorrected vision after refractive surgery due to under- or overcorrection (with all the techniques), regression (with excimer ablation) or increase in myopia (with implanted lenses).
Causes and risk factors of post surgery refractive error
There can be multiple causes of early residual error: improper refraction/data entry, learning curve, decentered ablation or blocked ablation, and fluence issues, to include a few. Over correction may be often due to incorrect pre-operative measurement, over or under done algorithm adjustment, or intra-operative dehydration of the cornea. Implantable collamer lens rotation can lead to unexpected astigmatism.To add to this, even advanced IOL formulas may be off the mark in extremes of axial length for lens exchange. Late changes can be due to increase in axial myopia, lenticular myopia, corneal reshaping or also due to pregnancy or endocrinal disorders. It is imperative to rule out post lasik ectasia in these cases as it could mimic post lasik regression especially in the early stages of ectasia when there are only subtle changes in topography. The above information is useful as it would help in deciding for refractive surgery enhancement.
Indications for refractive surgery enhancement
The indications for refractive surgery enhancement include
- Realistic patient and surgeon expectation: the primary aim of refractive surgery is to make a candidate free of refractive correction for most of his personal and professional work. If the patient is happy with a mild, occasional refractive correction at challenging but rare tasks, enhancement may not be needed in spite of residual refractive error. There is a strong role of preoperative and postoperative counseling here. However, if the refractive error/ post operative aberration profile is affecting the patient‘s daily activities, an enhancement should be strongly considered.
- A stable residual refractive error: First and foremost, the refractive error must be stable for at least 3 months. Some cases would be worth waiting six or more months, especially with borderline residual beds.
- Explainable and correctable visual deficit due to the refractive error: The residual refractive error should be able to explain the difference between the postoperative visual acuities: corrected and uncorrected. The aim should be to assess the spectacle corrected visual acuity and try to match it after the enhancement. In the simplest of cases, refraction may suffice. For example, a patient with a postoperative residual error of -1 D spherical which improves his current uncorrected vision from 20/40 to 20/20 fits the bill. However, a patient with a 20/120 UDVA, improving with correction of -1D to only 20/60 does not. There have to be other causes including retinal issues such as macular lesions to these situations other than just refractive error. In some cases with very distorted aberration profiles, the spectable correction may not be able to get the patient back to the preoperative BCVA. However in such cases also , the visual acuity will be fairly commensurate with the lower order aberration profile.
- Enough buffer for the enhancement: There should be enough scope of the additional surgical intervention. For example, there should be enough residual bed if a flap relift is planned. For cases with bioptics with the first surgery being a phakic IOL implantation or IOL exchange, the cornea should be healthy enough to allow an excimer ablation. Often phakic IOLs are done for high myopia. In such cases a top-up excimer ablation of 1-2 dioptres can be done if the cornea permits. However, in cases with thin/suspect/keratoconic corneas undergoing ICL, the corneal ablation would be a contraindication.
- No systemic or ocular contraindication for surgery: The enhancement should be considered as a full surgery like the first correction, and a detailed reassessment of the eye including ruling out retinal lesions from the first surgery is a good practice.
Decision making algorithm
There are multiple studies using methds like flap reflit (manually or after femtosecond side cut), surface ablation over a LASIK, wavefront or non wavefront guided ablation to include a few.          However, there is no published strong meta-analysis in literature establishing a clear cut prefered practice pattern when it comes to enhancement. The actual decision making algorithm can defer from surgeon to surgeon and with the availability or experience with a particular enhancement method.
However the guiding principles are the same:
- Stable refraction
- Correct elucidation of cause of error
- Maximum possible correction performed safely.
- Proper counselling
- No contraindication for the chosen method of enhancement
Example method for post phakic IOL implantation enhancement
Example post excimer ablation enhancement algorithm
In cases with judicious planning and selection of technique, the results are fairly satisfactory after the enhancement procedure.
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