Pediatric refractive surgery for myopia management – Q&A with Prof. Deepinder Dhaliwal and Prof. Aleksandar Stojanovic

Pediatric refractive surgery is the surgical correction of refractive error in children under age 18 using various techniques including excimer laser-based procedures such as LASIK and PRK; femtosecond laser-based procedures such as SMILE; and insertion of phakic IOLs. There are three modifications that need to be made when performing refractive surgery in pediatric patients. These are:

1. Refractive target: in pediatric patients, we need to modify the refractive target. In 1999 to 2000, we performed a pediatric LASIK study and treated only the amblyopic eye of five children aged 5-8 with myopic anisometropia. Our refractive target was balance with the fellow eye, not emmetropia.
2. Anesthesia:  another important consideration is type of anesthesia. Working in conjunction with anesthesiologists from the Children’s Hospital of Pittsburgh, our subjects were placed under general anesthesia using a laryngeal mask airway (LMA). We learned the hard way that mask ventilation was problematic because of leakage of anesthetic gases (specifically nitrous oxide) that caused malfunction of the excimer laser.¹ Therefore, we switched to LMA, and cases could then be performed successfully. 
3. Surgeon Fixation: in adults, refractive procedures are usually performed under local anesthesia and patients are usually asked to fixate on a target light. Children are given general anesthesia for refractive surgery because it can be challenging for them to stay still, calm, and cooperative during such procedures. However, this means that they cannot be asked to fixate and so surgeon fixation is necessary instead.

We were able to perform 16-year follow-up on two of the subjects and were pleased to see that the patients had stable corrected distance visual acuity, balanced refraction between the two eyes, and good visual quality of life. There was no evidence of progression of anisometropia or amblyopia. Importantly, there was no corneal ectasia.²

Children who are candidates for pediatric refractive surgery include those with refractive amblyopia who have failed or are non-compliant with conventional amblyopia therapy, possibly due to spectacle-induced aniseikonia, or comorbid neurobehavioral issues preventing use of spectacles or contact lenses.³ Another potential group may be myopic children who are rapidly progressing (not responding or non-compliant with myopia control therapy).⁴ Important considerations (similar to adults) to assess candidacy for any refractive surgical procedure include careful evaluation of the eyes: assessment of adequate corneal thickness or anterior chamber depth, ocular surface evaluation, optic nerve and retinal evaluation, and measurement of intraocular pressure. Since careful follow up and postoperative medications are necessary to success of refractive surgery, assurance that children will have adequate support at home is mandatory.

The alteration in the anterior corneal shape resulting from myopic laser vision correction (LVC) aims to reduce the central corneal refractive power, facilitating emmetropia and compensating for diminished distant vision. This process involves laser ablation, guided by the Munnerlyn formula, which for treatment of myopia dictates the removal of more tissue from the corneal center, gradually decreasing towards the periphery of the desired ablation zone. The ablation depth linearly increases with the degree of myopia corrected and exponentially with the ablation diameter. Hence, careful consideration is crucial to optimize the ablation zone size, preventing excessive tissue removal that could weaken the cornea. This approach may lead to an "under-corrected" periphery, resulting in a cornea characterized by oblate asphericity and increased positive spherical aberration, especially in higher levels of myopia correction. These corneal optics, akin to those induced by Ortho-K treatment, create myopic peripheral retinal defocus, potentially beneficial for myopia control.

However, myopic LVC using current ablation profiles with large optical zones may necessitate more than 5 diopters of treatment to achieve the desired myopic peripheral retinal defocus (see Figure 1). Furthermore, existing LVC treatments are typically approved for patients aged 18 and above, the time when myopia control is not typically pursued.

Addressing the first concern, assuming myopic laser ablation is recognized as a viable myopia control method, laser manufacturers could design special ablation profiles that induce myopic peripheral retinal defocus, similar to the original myopic ablation profiles from the 1990s, that may be used for small prescriptions. This effect would mirror Ortho-K, multizonal contact lenses, or spectacle lenses, all proven effective in myopia control.

Regarding the issue of LVC’s limitation to patients aged 18 and above, it's important to note that myopia progression beyond that age remains a significant concern due to its potential to result in high myopia, with axial length greater than 26mm, and associated complications later in life. Studies have reported myopia progression of more than one diopter over five years in 21.3% of cases after the age of 21.⁵ 

We may conclude that LVC for myopia may currently be considered for myopia control in young adults with myopia exceeding 5 diopters, and the current practice of waiting for myopia stabilization should evolve into a clear indication for myopic LVC in cases of ongoing progression from age 18 throughout the early twenties.

Currently, there are only two relevant studies published: one describes the effect of myopic LVC on myopia progression but only includes three cases;⁴ the other describes the peripheral retinal defocus profile after LASIK for myopia.⁶ Only the first study addresses the impact of myopic Laser Vision Correction (LVC) on myopia progression.⁴

At the University of Tromsø, Norway, we are currently working on a PhD project investigating the hypothesis that myopic LVC induces peripheral retinal defocus, similar to the multizonal optics produced by myopia-controlling contact lenses, spectacles, and Ortho-K. This mechanism is believed to provide a stimulus to decelerate axial length growth and myopia progression in young adults. The primary objective is to prospectively evaluate the impact of LVC on peripheral retinal defocus. The secondary objective involves a retrospective assessment of the long-term effects of LVC surgery on myopia progression in young adults treated for myopia in the ages of 18 to 21.

1. The initial study involves a comparison of spatial refraction before and after Laser Vision Correction (LVC) in both the cornea and the central retina. The corneal analysis is conducted using topographic/tomographic ray-tracing analysis, while the assessment of the central retina is performed using multispectral refractive topography. The latter utilizes the MRT by Thodar to carry out the analysis.^7-11  
2. The second study is a historical longitudinal, matched-control investigation that examines the extended progression of myopia in the eyes of young adults who underwent myopic Laser Vision Correction (LVC) at least five years prior. This will be contrasted with myopia progression in eyes from a matched control group corrected with ordinary spectacles or contact lenses during the same period.

In addition, we have initiated a pilot study involving ten patients aged between 18 and 20, exhibiting progressive myopia that exceeds five dioptres and continuous axial length growth of more than 0.1mm per year. These patients are undergoing myopic LVC treatment, and we plan to monitor their progress for two years.

LVC has been extensively scrutinized for almost thirty years now and has been shown to be highly effective and safe, with tens of millions of satisfied patients. Its effect on myopic eyes is comparable to that of optical aids – the only difference is that it is irreversible. In this regard, there should be no issues with treating patients over 18 years of age with progressive myopia using the current laser ablation patterns. Treating young children with myopia higher than five diopters, where myopic peripheral defocus appears to occur with the use of current ablation patterns, may also be highly justifiable because these patients would have to rely on optical aids, i.e., a changed natural optical system, in any case, due to myopia. Besides, our clinical experience with children's acceptance of aspheric optics, which cause myopic peripheral retinal defocus (both multizonal contact lenses and spectacles), has consistently been excellent, presumably due to the high plasticity of their neural/perceptual system. If we consider the long-term implications of having an aspherical (hyper-oblate) cornea, it may also prove beneficial in presbyopic age due to the extended depth of field.

Current methods of myopia control including orthokeratology and multifocal soft contact lenses have associated risks such as infectious keratitis^12-14 and can risk permanent vision loss. The risks, benefits, and alternatives of any refractive correction should be discussed in detail with patients and families. Larger, prospective trials of refractive surgery in the pediatric age group will be important to help guide our patients.