Introduction
Orthokeratology is a widely accepted optical treatment for myopia control in children, especially in East Asian countries. However, this treatment has existed for many years, originally having been introduced by Jessen in 1962. At that time, the treatment was not well received, but later introduction of reverse geometry designs; sophisticated instruments, such as computerized corneal topography; and super high gas permeable rigid gas permeable (RGP) materials led to a rapid improvement in lens design and fitting. This led to resolution of the majority of problems associated with the older techniques, such as corneal edema, distortion, and poor vision. Modern orthokeratology allows lenses to be worn on an overnight basis only, with the full refractive change taking place and remaining stable within 2 to 4 weeks after commencing lens wear. It has been shown to be effective in slowing the progression of myopia. Earlier studies showed that one of the main causes of poor post-orthokeratology visual acuity was significant residual refractive error, the major contribution to which was uncorrected corneal astigmatism. Toric design orthokeratology lenses were therefore introduced to correct higher-astigmatic eyes. For high myopia, a pilot study on high-myopic children has demonstrated a higher level of myopia control using partial correction (PR) orthokeratology while lens designs for full correction are still in the pipeline.
Changes in Refraction and Visual Acuity
With accelerated overnight orthokeratology, refractive change can now be achieved after a short period of lens wear. Studies have shown significant refractive changes to occur after only 1 night of lens wear, with reports ranging from 42% to 69%.
Most studies aimed for a modest myopia reduction, as recommended by the manufacturers who designed these lenses for myopia up to 4.00 diopters (D) only. In effect, there is a mean decrease in myopia of 3.00 D, associated with improvement of unaided (post-orthokeratology) visual acuity to close to 20/20 Snellen or 0.00 logMAR. In subjects with no significant residual refractive error after stabilization of treatment, or with residual refractive error corrected, high- and low-contrast logMAR visual acuities have been shown to be comparable to those in spectacle-wearing controls. Changes induced by orthokeratology are temporary and questions have been raised as to how fast these changes return to baseline values after overnight lens wear. The retention rate is obviously important, as it reflects the quality and stability of the unaided visual acuity after lens removal. Many researchers have investigated and reported daytime regression to be in the range of 0.25 D to 0.50 D. Mountford suggested that the practitioner should therefore aim for an initial overcorrection of existing myopia by about 0.50 D to minimize the regression effect.
Topographic Corneal Curvature Changes
Computerized corneal topography plays an essential role in modern orthokeratology practice. It allows practitioners to assess the prefitting anterior corneal shape and monitor changes to the corneal topography after lens wear. Corneal mapping provides the only reliable means of knowing precisely where the lens is positioned during sleep. This information helps the practitioner determine the appropriate lens modifications necessary to achieve the desired outcome. Most researchers have found no change in the posterior corneal curvatures and their findings supported the hypothesis that orthokeratology effects refractive changes primarily through remodeling of the anterior corneal curvature and not by overall bending of the cornea.
Anterior Segment Changes
Alhabi and Swarbrick reported central corneal thinning of 9.3 ± 5.3 µm after one night of orthokeratology lens wear in 18 subjects wearing orthokeratology lenses. At the end of 90 days of lens wear, the central cornea thinned by 19.0 ± 2.6 µm. The corneal changes involved rapid thinning of the central corneal epithelium and thickening of the midperipheral stroma and probably account for the rapid refractive changes induced by orthokeratology lenses. Their results confirmed the findings of an earlier study by Swarbrick et al., which suggested that the refractive changes induced by orthokeratology lens wear were due to the redistribution of corneal tissue and not by overall bending of the cornea.
In more recent studies, a smaller amount of central corneal thinning has been reported. Cho and Cheung reported significant reduction of the central corneal thickness in their orthokeratology subjects. The average central corneal thinning was 9 µm after 6 months of lens wear, with no further changes in subsequent visits in their 2-year study.
Using four-zone orthokeratology lenses with a target of 4.00 D, Charm and Cho partially corrected 12 high-myopic subjects and found no significant central corneal thinning over the course of their study. At the end of 24 months of lens wear, changes in central corneal thickness ranged from –34.5 to +5.3 (median –4.6) µm (negative value indicating thinning) in their PR orthokeratology subjects and from –13.0 to +28.5 (median 3.5) µm in their control subjects.
Although the amount of reported corneal thinning varied between studies, it is well documented that central corneal epithelium thinned in orthokeratology. The clinical implication of this thinning may be close and careful monitoring of the central cornea of orthokeratology patients. The nature of changes of thickness in other corneal regions in orthokeratology lens wear is still unclear; thus further investigation is warranted in this area.
There were initial concerns that orthokeratology leads to a backward displacement of the cornea, resulting in a decrease in anterior chamber depth (ACD) and giving a false impression of shorter axial length measured during lens wear. However, studies have shown that changes in vitreous chamber depth were consistent with changes observed in axial length in orthokeratology. Anterior segment length was not altered with orthokeratology lens wear and no significant changes in ACD was reported at the end of 2 years of lens wear in Cheung and Cho’s study. Other studies have also reported no significant changes in ACD, although González-Mesa et al. reported significant reduction in ACD with orthokeratology lens wear. Cheung and Cho concluded that axial length is valid for the determination of axial elongation in orthokeratology.
Orthokeratology for Myopia Control
In 2005, Cho et al. reported effective myopia reduction (46%) after 2 years of orthokeratology lens wear in children. Their results showed similar control of elongations of the axial length and vitreous chamber depth. Following this report, a number of other clinical studies were published, all demonstrating effectiveness of orthokeratology for myopia control, although the percentage control varied from 32% to 55%. Variations in the level of myopia control achieved may be attributed to differences in subject criteria and methodology used.
In 2012, Cho and Cheung published a randomized controlled trial providing further evidence on the effectiveness of orthokeratology for myopia progression in children. Their results showed that axial elongation in 37 children wearing orthokeratology lenses was 43% slower than that of 41 children wearing spectacles. Axial elongations at the end of 2 years were 0.36 ± 0.24 mm and 0.63 ± 0.26 mm in the orthokeratology and control groups, respectively, and were significantly correlated to the initial age of the subjects. Younger myopic children (7–8 years) demonstrated faster progression (increase of myopia > 1.00 D or axial elongation > 0.36 mm per year) compared to older children (9–10 years) in both groups of subjects; 65% and 20% of the younger children in the spectacle-wearing and orthokeratology groups, respectively, and 13% and 9% in the older children, respectively.
Charm and Cho reported 63% slower axial elongation in 12 high-myopic children using PR orthokeratology and spectacles for residual refraction in the daytime compared to 10 high-myopic children wearing single-vision spectacles. Chen et al. reported 52% slower axial elongation in 23 children wearing toric orthokeratology lenses and suggested that the higher level of myopia control demonstrated may be due to better orthokeratology lens centration from the use of toric design orthokeratology lenses.
Cho and Cheung analyzed the combined data from two studies (72 orthokeratology children and 64 spectacle-wearing controls) to investigate the protective role of orthokeratology in reducing fast progression in children. In children wearing spectacles, younger children (6–8 years old) had the greater and more rapid axial elongation (> 0.36 mm/y) compared to older children (9–12 years old). Moreover, orthokeratology treatment reduced the risk of fast progression in children of this younger age group by 88.8%. The 2-year number needed to treat (NNT) for the younger orthokeratology subgroup was 1.8, which suggests that treating just two younger children with orthokeratology would prevent one from experiencing fast progression.
Other Corneal Changes
Corneal Pigmented Arc
Brown-pigmented arcs are associated with the use of orthokeratology lenses and are suggested to be associated with a sudden change in corneal curvature that would allow pooling of the tears in that area.
Cho et al. reported that the intensity of the pigmented arc, once observed, increased with the period of lens wear. The pre-orthokeratology spherical equivalent refractive error (SERE), the amount of SERE reduction, and changes in corneal curvatures were significantly larger in subjects presenting the pigmented arc than in those without the pigmented arc. In their study of high-myopic children using PR orthokeratology, Charm and Cho reported a pigmented arc in 32% of the children at the 1-month visit. The incidence reached 92% and 100% after 6 months and 12 months of lens wear, respectively.
Rah and coworkers hypothesized that these pigmented arcs would disappear after cessation of lens wear. They suggested that, over time, normal corneal exfoliation would slough off the pigments. Cho et al. reported the disappearance of the pigmented arc in the corneas of two adult patients when examined 2 months after ceasing orthokeratology lens wear. Our understanding of the formation of the corneal pigmented arc is limited; further studies are certainly needed in this area.
Fibrillary Lines
In the early days of orthokeratology using reverse geometry lenses, white fibrillary lines ( Fig. 23.1 ) in the corneas of some orthokeratology patients were reported. When there were many, they appeared in a concentric format in the center of the cornea. As with the pigmented arc, there seems not to be any clinical ramifications. These lines are similar in appearance to fibrillary lines observed in keratoconic corneas. The condition is presumed to be pressure related and is reversible after cessation of lens wear. The work of Lum et al. confirmed that these fibrillary lines are related to an altered corneal sub-basal nerve plexus associated with orthokeratology lens wear ( Fig. 23.2 ). Lum et al. also reported a reduction in corneal sensitivity in 16 adults who had worn orthokeratology lenses for 3 months. Corneal sensitivity recovered quickly, within 1 month, when lens wear was discontinued. However, the recovery of central nerve fiber density change was much slower, with full recovery observed only 3 months after cessation of lens wear.
Corneal Staining
With improvements in lens materials, lens designs, and fitting, corneal staining is not as frequently observed as previously reported. Most studies reported mild central corneal staining, especially at the commencement of lens wear, and the incidence declined with continued lens wear. However, Charm and Cho reported that the incidence of corneal staining was generally higher in PR orthokeratology subjects.
Walline et al. reported corneal staining in 58.5% of their 23 subjects in the morning visits and in 35.5% of the subjects in the afternoon visits. On average, the staining observed was less than grade 2 and the most common type of staining pattern was the punctate type. Central corneal staining made up 77.8% of the staining recorded in the morning visits and 47.5% of the staining recorded in the afternoon visits. Walline and coworkers asserted that, although the incidence of staining was high, the staining was not serious enough to threaten safe contact lens wear.
Hiraoka et al. reported that only three subjects had moderate superficial punctuate keratopathy and one mild corneal erosion in their orthokeratology subjects in their 5-year study and the staining resolved completely within a week after discontinuation of lens wear.
In orthokeratology, the prudent practitioner should monitor corneal staining diligently, especially central corneal staining, in view of evidence of thinning of the central corneal epithelium in orthokeratology and the potential threat to vision should the cornea be infected. The other important factor with respect to the incidence of staining is the level of expertise used in fitting the lens. Staining caused by an incorrect fit should always be corrected. Central corneal staining occurs only when the lens comes into direct contact with the corneal epithelium and is therefore indicative of a less than optimal lens/cornea fitting relationship.
Lens Binding
Lens binding after overnight RGP lens wear is not uncommon and has been reported in a number of orthokeratology studies. Cheung and Cho reported that this was the most common nonvision-related problem (73%) that subjects reported in their study. More recent studies have reported reduced incidence of lens binding, probably due to improved lens designs and lens fitting. According to Mountford (personal communication), from clinical experience, most bound lenses will automatically free up following active blinking (on awakening) which facilitates tear exchange. However, it is important that practitioners carefully instruct their patients about how to remove a bound lens in the morning.
In their study, Cheung and Cho reported that over 80% of the subjects who included lens binding as one of their problems did not attempt to loosen their lenses before removal. It is essential that patients do not remove bound lenses forcefully, especially if a suction holder is used to aid removal, as this can lead to significant corneal damage.
Microbial Keratitis in Orthokeratology
Several recent reports of corneal ulcers in patients receiving orthokeratology treatment have led to concerns about the use of this procedure. However, these are case reports, some of which have provided minimal patient or treatment information; as such, they may not reflect the true picture of the potential risk of orthokeratology. Nevertheless, there are concerns about the potential risk associated with this treatment, especially since overnight lens wear is required and the level of lens care compliance among child contact lens wearers is uncertain. Boost and Cho found no significant change to the ocular microbiota with orthokeratology lens wear over an extended period. However, the majority of their subjects had significant contamination of at least one item, the most frequently contaminated item being the lens case, followed by the lens suction holder. Lens case isolates were significantly associated with those from the lens, suggesting cross-contamination. The majority of subjects who reported good compliance had low or no contamination of their lenses and lens accessories. Their results also showed that the most frequent breaches in the lens care protocol were failure to clean, disinfect, and replace the lens case. Cho et al. reported frequent and heavy contamination of the lens suction holders and lens cases, in spite of monthly replacement. They were of the opinion that patients’ (or parents’) attitudes toward the care of accessories (e.g., lens cases) were not satisfactory. It is therefore important that practitioners emphasize proper care and replacement of lens accessories to patients and their parents and avoid prescription of unnecessary accessories, for example, a lens suction holder. To minimize complications in orthokeratology lens wear, it is important that practitioners are diligent in the care of their patients, especially as the treatment involves children and overnight lens wear. During sleep, the cornea becomes hypoxic owing to lid closure; this hypoxia is increased by the presence of the contact lens. The protective mechanisms of the eye, which include intact corneal surface and flushing of tears via blinking, are also absent during sleep. These factors associated with the use of contact lenses contribute to the cause of contact lens–induced complications.
Patient Acceptance
The main motivation for children to accept orthokeratology was the convenience of spectacle freedom in the daytime. Aversion to lens-handling procedures may be a strong factor reducing the motivation in children. It is important that children should not be forced into orthokeratology; if they are unwilling, the chance of good compliance is likely to be reduced. It is equally important that parents are not only properly educated on lens care procedures but that they closely monitor their children on care procedures. Good and strong parental support is essential and cannot be overemphasized.
Safety was the major concern of the parents when considering options for myopia control, but the decision on myopia control treatments was nevertheless affected by a combination of confidence in safety, effectiveness, and any additional benefit(s) provided by the treatment.
Topographic Corneal Curvature Changes
Computerized corneal topography plays an essential role in modern orthokeratology practice. It allows practitioners to assess the prefitting anterior corneal shape and monitor changes to the corneal topography after lens wear. Corneal mapping provides the only reliable means of knowing precisely where the lens is positioned during sleep. This information helps the practitioner determine the appropriate lens modifications necessary to achieve the desired outcome. Most researchers have found no change in the posterior corneal curvatures and their findings supported the hypothesis that orthokeratology effects refractive changes primarily through remodeling of the anterior corneal curvature and not by overall bending of the cornea.
Anterior Segment Changes
Alhabi and Swarbrick reported central corneal thinning of 9.3 ± 5.3 µm after one night of orthokeratology lens wear in 18 subjects wearing orthokeratology lenses. At the end of 90 days of lens wear, the central cornea thinned by 19.0 ± 2.6 µm. The corneal changes involved rapid thinning of the central corneal epithelium and thickening of the midperipheral stroma and probably account for the rapid refractive changes induced by orthokeratology lenses. Their results confirmed the findings of an earlier study by Swarbrick et al., which suggested that the refractive changes induced by orthokeratology lens wear were due to the redistribution of corneal tissue and not by overall bending of the cornea.
In more recent studies, a smaller amount of central corneal thinning has been reported. Cho and Cheung reported significant reduction of the central corneal thickness in their orthokeratology subjects. The average central corneal thinning was 9 µm after 6 months of lens wear, with no further changes in subsequent visits in their 2-year study.
Using four-zone orthokeratology lenses with a target of 4.00 D, Charm and Cho partially corrected 12 high-myopic subjects and found no significant central corneal thinning over the course of their study. At the end of 24 months of lens wear, changes in central corneal thickness ranged from –34.5 to +5.3 (median –4.6) µm (negative value indicating thinning) in their PR orthokeratology subjects and from –13.0 to +28.5 (median 3.5) µm in their control subjects.
Although the amount of reported corneal thinning varied between studies, it is well documented that central corneal epithelium thinned in orthokeratology. The clinical implication of this thinning may be close and careful monitoring of the central cornea of orthokeratology patients. The nature of changes of thickness in other corneal regions in orthokeratology lens wear is still unclear; thus further investigation is warranted in this area.
There were initial concerns that orthokeratology leads to a backward displacement of the cornea, resulting in a decrease in anterior chamber depth (ACD) and giving a false impression of shorter axial length measured during lens wear. However, studies have shown that changes in vitreous chamber depth were consistent with changes observed in axial length in orthokeratology. Anterior segment length was not altered with orthokeratology lens wear and no significant changes in ACD was reported at the end of 2 years of lens wear in Cheung and Cho’s study. Other studies have also reported no significant changes in ACD, although González-Mesa et al. reported significant reduction in ACD with orthokeratology lens wear. Cheung and Cho concluded that axial length is valid for the determination of axial elongation in orthokeratology.
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