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Orthokeratology
Introduction
Rather than using contact lenses to add a new refracting surface to the eye to correct ametropia, in orthokeratology (OK), specially designed rigid contact lenses are instead used to temporarily alter the shape and thereby refractive power of the anterior corneal surface.
History of Orthokeratology
was the first to describe how rigid lenses, then manufactured in polymethyl methacrylate, could be fitted steeper than corneal curvature to induce corneal steepening so as to correct hyperopia, or flatter than corneal curvature so as to flatten the cornea and correct myopia. Lenses were fitted with sufficient difference in curvature to the cornea to promote change in corneal curvature, but not so great as to destabilize lens fitting. Lenses were then worn during waking hours and progressively altered in curvature as corneal shape changed until the full refractive effect was achieved, at which stage daytime wear was continued with the final fit lens to retain the effect. In this format, OK remained a niche procedure practised by a small number of dedicated enthusiasts, limited by the need for daytime lens wear, the complexity in timing and the number of lens refittings that were required, alongside unpredictable refractive effects.
Modern Orthokeratology
The mid-1990s saw the convergence of three independent technologies that revolutionized OK. Computerization led to greater control over lens lathing and dramatically improved measurement of corneal topography, while improvements to polymer technology saw the introduction of gas-permeable contact lens materials.
Independently of each other, and reported that reverse-geometry lens designs, now made possible through computerized lathing, could be utilized to stabilize flat-fitting lenses. Reverse-geometry lenses have secondary curves that are steeper than the back optic zone radius ( Fig. 30.1 ). When utilized in a flat-fitting rigid contact lens, this enables peripheral realignment with the cornea in what would otherwise be a poor-fitting lens with excessive edge lift. When applied to OK this meant that the final fit lens could be worn at outset without compromising fit and centration. This new approach in OK lens fitting was given the name ‘accelerated orthokeratology’ because it provided rapid onset (within a few days) of effects that had previously taken weeks or months to achieve. Advances in lens material oxygen transmissibility added a further dynamic by enabling the refractive effect from accelerated OK to be obtained from overnight wear, with lenses removed during the day ( ). Overnight wear has since become the accepted modality for OK, with FDA approval for overnight wear of the Paragon CRT lens for OK (Paragon Vision Sciences, Mesa, AZ, USA) first issued in 2002.
International Orthokeratology Prescribing Trends
reported data on international OK prescribing up to 2018, and in Figs. 30.2 and 30.3 . These data have been updated to 2020. Contact lens fitting data were accessed by a survey from 71 countries over a 17-year survey period (2004–20). This generated a database of 338,446 contact lens fits, of which 3,857 were with OK lenses and 355,971 were with other lens types (non-OK). Fig. 30.2 shows data for OK fitting for the 39 countries where information about 1000 fits or more was available. For this subset of markets, the median number of lens fits evaluated was 6,817, ranging from 1085 in Belgium to 72,926 in Japan. This graph demonstrates a steady increase in OK fitting from about 0.2% of all fits in 2014 to around 3% in 2020.
Fig. 30.3 Shows the distribution of the average number of OK fits conducted in the 39 markets mentioned above. There is a significant difference in the extent of prescribing between nations ( P <.0001). There is clearly strong interest in European countries, as well as Hong Kong, presumably due to the high usage of OK for myopia control. On the other hand, OK prescribing was less than 1% in 22 of the 39 countries represented.
Ocular Changes Induced by Orthokeratology
The original concept of OK was based on the belief that nonalignment fitted rigid lenses would bend the cornea to flatten or steepen curvature to reduce levels of myopia or hyperopia, respectively. were the first to report that this is not the case, demonstrating that OK-induced reduction in myopia was accompanied by a flattening of the central cornea, thinning of the central cornea and thickening of the mid-peripheral cornea. By using Munnerlyns’ formula for calculating laser refractive surgery ablation depth, the authors were able to show a close correlation between the predicted refractive change from change to the central corneal thickness and actual refractive change.
Using optical pachometry, revealed that whereas central corneal thinning was mostly attributed to changes in epithelial thickness, the mid-peripheral thickening was mostly a stromal effect. It has been suggested that cell compression, rather than cell movement or loss, may be the primary mechanism of central epithelial changes ( ). Further research has shown that stromal, but not epithelial, thickness changes during overnight wear of OK lenses are related to oxygen transmissibility of the lenses ( ). There is conjecture on whether OK influences posterior corneal curvature. Mild posterior corneal curvature changes were found by , which would suggest that corneal bending does have some role in the mechanism of change. However, using a Pentacam analysis system, showed instead that overnight OK lens wear alters the anterior corneal shape rather than the posterior corneal curvature or the anterior-chamber depth.
The mechanism by which OK creates changes to corneal curvature, thickness and refraction is not fully understood. Intuition suggests that OK lenses impose a direct moulding effect on the corneal surface through eyelid pressure. has put forward an alternative hypothesis: that it is the tear reservoir generated by the steeper secondary curves that primarily leads to the pressure changes in the postlens tear film that is responsible for the corneal tissue redistribution.
Refractive Effect
Efficacy of OK to alter refraction has been evaluated in daily wear and overnight wear protocols. fitted reverse-geometry lenses (OK74; Contex) manufactured in Airperm material to 6 young myopic subjects. The lenses were worn on a daily-wear basis for 28 days and resulted in a myopia reduction of 1.71 ± 0.59 D. conducted a 100-day daily-wear trial on 14 subjects, again using the OK74 design in Airperm material. They reported a myopic reduction of 1.50 ± 0.45 D. conducted a 60-day trial on overnight wear of the same Contex OK lenses, with eight subjects completing the study. The reduction in subjective refraction 4 hours after awakening was 1.83 ± 1.23 D. Using Paragon CRT lenses, found a reduction in the myopia of 3.33 ± 0.96 D after 1 month of overnight lens wear.
These studies are reasonably consistent in their estimation of the magnitude of refractive change, and in the variability of the response. While correction of greater amounts of myopia is sometimes reported, and proposed by some OK lens designers, they are not what is to be expected in the repeatable application of current lenses. Although corneal irregularities such as a decentred treatment zone (TZ) are sometimes encountered, in general, the corneal changes do not lead to losses of high-contrast visual acuity, low-contrast visual acuity or contrast sensitivity for these experimentally repeatable magnitudes of treatment ( ).
Orthokeratology Induced Changes to Corneal Curvature
Changes in corneal curvature and shape have also been evaluated in many studies, although differing techniques have been employed. , using the EyeSys corneal topographic mapping system, found that the apical corneal power diminished by 1.19 ± 0.38 D. Over the central 5–6-mm zone of the cornea there was significant central corneal flattening, which diminished towards the mid periphery. used keratometry and the TMS-1 videokeratographer to report central corneal flattening of 0.14 ± 0.06 mm vertically and 0.12 ± 0.07 mm horizontally. , using a Humphrey Atlas topographer and an Orbscan slit-scan topographer, reported a significant flattening of the apical radius of 0.20 ± 0.90 mm, which again diminished towards the periphery.
Correlations Between Changes to Refraction and Corneal Curvature
The relationship between refractive changes and corneal curvature changes has been a matter of some conjecture. Numerous studies using early OK lens designs reported that the refractive change was greater than that expected from corneal power changes measured using keratometry. The reported ratio of these changes was often 2:1 or more.
This anomaly is most likely a consequence of the irregular nature of the induced corneal topographic changes. It is now apparent that the corneal shape changes are more marked in the central cornea and diminish towards the mid periphery ( ), and hence, keratometry results are somewhat unreliable. Similarly, the reported vision improvements were often greater than would be predicted from corneal curvature changes.
Difficulties with correlating corneal curvature, refraction and vision were confirmed by ( ) using overnight wear of reverse-geometry lenses. Refraction was measured both subjectively and by autorefractor, with a larger reduction in myopia reported for subjective refraction than for autorefraction. The authors hypothesize that this finding is a consequence of the greater weight given to peripheral optical effects with autorefraction.
Time Course of Changes
It is well established that changes in corneal shape and refractive error can be induced by OK, with significant structural and optical change in as little as 15 minutes ( ). On average, 75% of the required refractive change is achieved with 1 night of lens wear with an end-point achieved within 7–10 nights of lens wear ( ). A model of the average induced refractive change from these reported studies, and its time course, is shown in Fig. 30.4 .
Regression of Effect
An appealing factor of OK is that changes to refractive effect and corneal curvature return to preleans-wearing values if wear is ceased; however, this regression of effect starts soon after lens removal leading to some unwanted loss of refractive effect observed during the day between wearing intervals ( ) ( Fig. 30.5 ).
, in a retrospective study of 48 patients, found that after 90 days of overnight wear the regression of apical corneal power over approximately an 8-hour period stabilized between 0.50 and 0.75 D per day, but with significant individual variation. concluded from their study that the refractive outcomes after 60 days of overnight wear were sustained over an 8-hour day. However, analysis of their findings shows that most changes were measured over only a 4-hour period, and some changes – as indicated by autorefraction data – did show regression up to 0.50 D ( ). On average, a regression of approximately 0.25–0.75 D is expected throughout the day after overnight lens wear, but this regression is greater in the early stages of OK lens use. Up to 90% of recovery towards the baseline refraction can be expected within 72 hours of lens discontinuation ( ).
Other Changes to Corneal Physiology
reported the appearance of fibrillary lines in the anterior stroma of a 29-year-old Asian woman after wearing overnight OK lenses for approximately 9 years. The fibrillary lines were fine, slightly curved and subepithelial, and were arranged in a band-like annulus in the corneal mid periphery. The lines were not associated with epithelial staining, although a marked Fischer–Schweitzer corneal mosaic was noted after blinking. Fibrillary lines are a relatively common finding in normal and keratoconic corneas and have been reported previously accompanying OK lens wear. went on to map the corneal subbasal nerve plexus (SBNP) for this patient using confocal microscopy to establish that the earlier reported fibrillary lines were the visible appearance of alterations to the SBNP induced by OK lens wear. The SBNP was also mapped for a 21-year-old Asian woman wearing OK lenses for approximately 1 year and a non-OK-lens-wearing 28-year-old Asian woman, to reveal that the SBNP took on a whorl pattern appearance in the OK-lens wearer, compared with a tortuous network of central nerve fibres and thicker curvilinear mid-peripheral fibres in the non-OK-lens wearer.
Following on from their earlier work, using confocal microscopy measured corneal nerve fibre density (NFD) over a 1-mm 2 area in OK-lens-wearing subjects and also looked at changes in corneal sensitivity. The study, involving 16 subjects, revealed a significant reduction in central NFD after 3 months of lens wear that improved but did not return to prelens-wearing values 90 days after lens wear was ceased. Corneal sensitivity was similarly reduced but, in this case, returned to prelens-wearing levels within 30 days of discontinuation of wear.
It is clear from this research that OK lens wear alters corneal nerve structure and that corneal sensitivity is reduced, after as little as one night ( ). However, there are currently no published studies to report on whether soft or rigid contact lens wear similarly alters corneal nerve structure. The limited current evidence suggests that in OK these changes begin to resolve once lens wear is ceased, leading to the recovery of corneal sensitivity over the short term, and corneal nerve morphology returns towards prelens-wearing values over the longer term.
Safety of Orthokeratology
Corneal complications can occur in OK, as in any form of contact lens wear. For example, corneal staining can be present, particularly after overnight lens wear. The significance of epithelial thinning, as reported after the wear of reverse-geometry lenses, in the occurrence of serious complications remains to be established . Overnight wear of any contact lens has previously been shown to be a major risk factor for keratitis (Holden & Lazon de la Jara, ), so the finding of complications in OK is perhaps not unexpected. Furthermore, OK lenses have been shown to retain more bacteria than alignment-fit rigid lenses after bacteria-loaded overnight lens wear ( ), which may increase the risk for an infection in OK patients if suitable conditions are present.
investigated trends in microbial keratitis (MK) associated with OK by performing a meta-analysis of cases that were reported in ophthalmic journals or presented at conferences; 123 cases were reported from 2001 to 2007. Most cases were found to be in female, East Asians aged between 8 and 15 years. In 41 (33%) cases, Acanthamoeba was implicated as the causative organism, and Pseudomonas aeruginosa in 46 (36%) cases. Their analysis revealed a peak of cases in East Asia, particularly in China and Taiwan during a short period when regulation of the modality was limited. Acanthamoeba keratitis in OK has been shown to be related to the use of tap water for rinsing , so the simple act of avoiding tap water could well have prevented a large number of the cases that Watt and Swarbrick assessed.
In Australia, sent a questionnaire to members of the Orthokeratology Society of Australia (now the Orthokeratology Society of Oceania, OSO) and received a response from 33 optometrists who had been fitting OK lenses for a median period of 7.5 years. OSO membership requires completion of an accredited certification course for OK. The authors combined their findings with two previously reported cases to reveal nine cases of MK (four P. aeruginosa ; two Acanthamoeba spp.; three unknown organisms). Poor patient compliance was implicated in most cases of presumed MK in OK. Although speculative, the low magnitude of reported cases, particularly those involving Acanthamoeba , suggests that better regulation of OK lens fitting is beneficial in reducing the rate of infection.
investigated the risk of MK with overnight OK by asking randomly selected practitioners to report cases of patients who attended unscheduled visits for a painful red eye. Of the 1317 (640 adults; 677 children) patients who were ultimately sampled, 8 events of corneal infiltrates associated with a painful red eye were reported (2 adults; 6 children). Two of these, both occurring in children, were classified as MK but did not result in loss of visual acuity. The overall estimated incidence of MK established by their study was 7.7 per 10,000 years of wear overall, and 13.9 per 10,000 years of wear in children, leading the authors to conclude that the risk of MK with overnight OK is similar to that with other overnight-wearing modalities. By comparison, investigated cases of MK presenting in Australia over a 12-month period to report an incidence of 19.5 cases of MK per 10,000 years of overnight wear of soft contact lenses, and 25.4 cases of MK per 10,000 years in overnight wear of silicone hydrogel soft contact lenses. Recently, evaluated paediatric patient records from 13 eye clinics in Moscow to estimate that 23,049 OK fits had been performed across the city throughout 2018. In the same year there were 5 reported cases of MK, equating to 4.9 and 5.3 cases per 10,000 patient years assuming an OK wear discontinuation rate of between 7% and 10%. This incidence of MK in children wearing OK is lower than the earlier USA cohort study ( ) and is similar to reported rates associated with daily soft contact lens wear ( ).
Although these studies suggest that overnight OK offers no greater risk than does traditional overnight lens wear and may be similar to daily soft contact lens wear, accurate information on the prevalence of serious complications in OK is still limited, and the potential for their occurrence must be recognized and protected against ( ). To this end, provided a comprehensive guideline for practitioners to improve their OK practice and minimize unnecessary or preventable complications as they believe that the key to safe OK lens wear is to update knowledge in the field continually and to practise to the highest professional standards.
Orthokeratology Lens Fitting
There are numerous OK lens designs, each with different approaches to lens fitting, which makes this an extensive subject to cover in a small space. Rather than describe all of these approaches, this section instead offers an overview of the principles of myopic OK lenses that are common to all designs.
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