Orthokeratology is defined as the temporary reduction, modification, or elimination of myopia by the programed application of a rigid contact lens. This procedure was first reported in the early 1960s when the “orthofocus” technique was described. The technique of traditional orthokeratology contact lens fitting, where the base curve radius of lenses was designed to be flatter than the central corneal curve, had lost favor because earlier controlled clinical studies showed it was unpredictable and modest in its ability to correct myopia. However, with the advent of reverse-geometry contact lens designs and new rigid gas-permeable (RGP) materials with high oxygen transmissibility, clinical interest in orthokeratology has been renewed.
Mountford conducted a study of orthokeratology in which patients wore reverse-geometry contact lenses on an overnight basis. This strategy was reported to significantly enhance the predictability and efficacy of the orthokeratology in reducing myopia. Nichols and his co-workers also reported satisfactory results from overnight orthokeratology. The theory underlying overnight orthokeratology is that, as the RGP lenses are worn, the cornea is reshaped, and the level of myopia is reduced as the patient sleeps. The lenses are removed on waking, and good vision is maintained without correction through the day.
However, overnight wear is known to be a risk factor for infectious keratitis, a serious and vision-threatening complication, as in all contact lens use. Previous case reports have specifically documented the occurrence of this rare complication of overnight orthokeratology. We have reported 6 patients with pseudomonal keratitis related to overnight orthokeratology. Here, we extended the observation period to include 21 eyes in 20 subjects with infectious keratitis associated with overnight orthokeratology to investigate patient histories, microbial culture results, clinical courses, and visual outcomes.
DISCUSSION
Overnight orthokeratology is gaining increased popularity for the treatment of myopia in children because good vision can result without the aid of glasses or contact lenses in the daytime. Another reason for its usage becoming popular among children is that some parents often have the false belief that orthokeratology itself can halt or even reverse the progression of myopia, especially in our locality, where the myopia rate increases from 20% at 7 years to 61% at 12 years, 81% at 15 years, and 84% at 18 years. As is inherent in the practice of orthokeratology, subjects seen for correction generally are children with excellent best corrected visual acuity. Although the risk of infection with overnight orthokeratology is rarely reported, this complication can be devastating. Furthermore, it is likely that many cases of infection are not reported, or some mild cases may have been encountered and treated in local practices. Thus, the reported number of infections likely underestimates the true incidence of the problem.
Overnight wear is a principal risk factor for infectious keratitis among all types of contact lens users. Several studies have shown that overnight contact lens wear, including wear of high-oxygen-transmissibility RGP lenses, impairs the epithelial barrier because of the reduced oxygen transmission through contact lens. In addition, lack of eye movements that help disrupt the bacterial glycocalyx and spreads lysozyme over the corneal surface can render the eye more susceptible to bacterial infection.
Ren et al establish the importance of lens oxygen transmissibility in controlling P. aeruginosa binding to the corneal epithelium, not the lens. Their follow-up report shows that a hyper-oxygen-permeability (Dk) RGP polymer that produces no lens-wear-related increases in P. aeruginosa binding to the cornea in an alignment fit does produce increased P. aeruginosa binding in an orthokeratology reverse-geometry design. As described before, we were unable to get the details of orthokeratology protocols in our patients. The most popular overnight orthokeraology lenses in our locality are Paragon HDS (paflufocon B with a Dk of 58), Paragon HDS (paflufocon D with a Dk of 100), Boston Equalens II (oprifocon A with a Dk of 85), and Siflufocon A (fluorosilicone acrylate polymer with a Dk of 81). Further epidemiologic studies would confirm if the new hyper-Dk RGP lenses provide less risk for infectious keratitis.
The reverse-geometry orthokeratology lenses are similar to traditional orthokeratology contact lens, but the base curvature of these lenses is designed to be flatter than the central corneal curvature. The reverse geometry design means that the secondary curve radius is steeper than the base curve radius. At the secondary curve junction, the lens and cornea form a tear reservoir exhibiting a band of midperipheral fluorescein pooling. This design improves the centration and stability of the lens on the cornea. However, compressive forces exerted by these lenses in the redistribution of corneal epithelium may damage the corneal surface, especially when it is already compromised from overnight lens wear. This may lead to an increased risk for infections. The locations and sizes of the corneal ulcers complicating orthokeratology could imply an improper fitting or dislocation of the contact lens. Unfortunately, we were not able to get the premorbid lens-fitting conditions. Further study to evaluate the relationships among the locations of corneal ulcers, lens fitting, and topography of compressed cornea is warranted.
Lens hygiene is important in contact lens wear and so would be expected to play a role in the infection related to orthokeratology. Most of our patients claimed they cleaned the lens strictly. Indeed, the fact that 12 patients had worn RGP lenses for more than a year attests to the diligence of their cleaning regimen and handling of the lenses. However, sampling of 4 available contact lens solutions produced positive culture results. Similar results have been published. Instilling in young children, who are enrolled in an orthokeratology program, and their parents the sense of the importance of proper lens hygiene and having them habitually use the standard cleaning procedure is thus very important.
Concerning the bacterial infections observed in the present study, the English literature review contains several case reports of infectious keratitis occurring after overnight orthokeratology. Of these 16 reported cases, 9 were caused by P. aeruginosa, one was Serratia marcescens, and 5 were Acanthamoeba; one had a negative culture result. A 16-case series of infection after orthokeratology has been reported in the Chinese literature. Taken together with our observations, P. aeruginosa is the most frequently isolated pathogen. This is not surprising, because P. aeruginosa is also the most common pathogen of contact lens-related corneal ulcers. Adherence of P. aeruginosa to RGP lenses and exfoliated epithelial cells has been studied.
Whereas Acanthamoeba keratitis was observed in only a single instance of overnight orthokeratology in the present study and in a previous study, Sun, et al from China reported 4 cases of Acanthamoeba keratitis complicated with overnight orthokeratology, and a report in the Chinese literature described a higher ratio (8 of 16 cases) of Acanthamoeba keratitis infection after orthokeratology. The known risk factors include swimming with contact lens still in place, washing lenses with tap water, or irregular disinfection for the patients with Acanthamoeba keratitis were not elaborated in these studies. The high ratio reported in the Chinese study might reflect geographic variation. However, Acanthamoeba should be taken into consideration in a patient with an atypical keratitis after overnight orthokeratology treatment. The early diagnosis and the prompt initiation of treatment with PHMB could make the visual outcome of Acanthamoeba keratitis favorable, as in our patient 13.
Our culture-positive rate (13/21, 61.9%) is consistent with that observed in other studies of contact lens-related corneal ulcer. Among the 8 culture-negative corneal ulcers, 7 of the involved eyes had received prior topical antibiotic therapy elsewhere. The negative culture results may have been a consequence of the tendency of local ophthalmologists to prescribe fluoroquinolones or other antibiotics for treatment of a corneal ulcer before referral of the patient to our hospital, a tertiary center.
When a contact lens wearer presents with a small corneal ulcer, it is crucially important to distinguish a noninfectious corneal infiltrate from infectious microbial keratitis. The presence of a moderate amount of pain, an overlying epithelial defect, surrounding corneal edema, or anterior chamber reaction in the patients with small corneal ulcers in our series suggested the diagnosis of infectious keratitis. This diagnosis was bolstered by the response of the patients to antibiotic treatment.
We reported a big series, may be the biggest one so far, of cases of infectious keratitis associated with overnight orthokertology. As overnight orthokeratology increases in popularity, the incidence of related infections will also likely increase. The long-term safety of overnight orthokertology should be evaluated thoroughly. Eye care practitioners who perform overnight orthokeratology have an obligation to warn their patients of this potential vision-threatening complication. Part of this obligation entails educating patients to the importance of strict adherence to proper contact lens hygiene. Patients must be informed to seek prompt medical attention when they experience signs or symptoms of infectious keratitis. Once an infection develops, the ophthalmologist needs to be aggressive in culturing and identifying the responsible organism. Prompt and aggressive management of this potentially vision-threatening complication can result in preserving useful vision.
Conclusions:
Infectious keratitis is a potential complication of overnight orthokeratology that may cause significant visual impairment. Parents of children who consider overnight orthokeratology should evaluate the benefit of temporary myopia reduction and the risk of infection.
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