Overnight Orthokeratology-Associated Microbial Keratitis.

Orthokeratology is designed to reduce myopia and astigmatism temporarily by flattening the corneal curvature. Several investigations in the 1970s and 1980s using a flat-fitting polymethyl methacrylate (PMMA) lens for orthokeratology showed unpredictable and uncontrollable results. However, as the design and materials used for orthokeratology contact lenses have evolved, orthokeratology with overnight wearing of gas permeable material (overnight orthokeratology) has shown more promising results.

The new generation of designs, collectively termed reverse geometry contact lens, contained the base curve and reverse curve. The base curve is designed to be flatter than the central corneal curvature. The secondary curve radius is steeper than the base curve radius, which can be used to improve the centration and stability of the lens. Advances in oxygen transmissibility (Dk/t) of rigid gas permeable materials are further influencing the current practice of orthokeratology. All these current trends in orthokeratology contribute to more predictable, stable, and safer effect than the older version of designs.

Complications reported with orthokeratology include corneal epithelial edema, abrasion or staining of the cornea, keratoconic change, and induced astigmatism. Among these problems, corneal epithelial staining is the most common phenomenon observed in overnight orthokeratology. Corneal epithelial damage can cause the break down of the corneal epithelial barrier and increase the risk of corneal infection. Many cases of infectious keratitis associated with orthokeratology including Pseudomonas aeruginosa, Acanthamoeba, and Serratia marcescens have been reported.

As orthokeratology treatment of myopia has become increasingly popular in Taiwan during the past 10 years, many cases of corneal infection have been observed. This study was conducted to review the clinical and microbiologic features and the outcomes of overnight orthokeratology-associated microbial keratitis.

DISCUSSION

Orthokeratology lenses are designed to flatten the central cornea. Their base curve is usually flatter than the anterior corneal curvature. Alteration of the corneal shape and remolding of the corneal epithelium have been reported as the possible mechanisms in changing refraction. Changes in the cornea that include corneal epithelial edema, abrasion, and staining of the cornea can occur in orthokeratology lens wearers. In our study, 90% of microbial keratitis were located at the central cornea. The high incidence of microbial keratitis at this central location in orthokeratology lens wearers coincided with the design of orthokeratology lenses that most compressed the central cornea and damaged the central corneal epithelium, thus predisposing the central cornea to an increased risk of infection.

Besides lens design, the night-wearing strategy of orthokeratology lenses may be another important risk factor of infection. Wearing extended-wear lenses overnight, even if only for one or two nights at a time, increases the risk of ulcerative keratitis compared with use of the lenses solely during the day. Thus, the risk of infection in overnight orthokeratology should not be overlooked. Other possible complications associated with orthokeratology lenses include corneal scarring, permanent corneal warpage, keratoconic changes in the cornea, induced astigmatism, and corneal thinning.

The prevalence of myopia in school children is high in Taiwan. Although the role of orthokeratology in arresting or slowing the progression of myopia is still not clear, orthokeratology has gained popularity for school children in Taiwan because of its publicized claim of halting myopia progression. However, children may not be able to manipulate or clean the lenses properly. Poor lens hygiene and possible corneal trauma during lens insertion and removal may predispose children to increased risk of corneal infection.

Although the new generation of RGP lenses for overnight orthokeratology have higher oxygen transmissibility, and hence might have lower infection rates, several cases of microbial keratitis associated with overnight orthokeratology have been reported including Pseudomonas, Acanthamoeba, Fusarium, and Serratia infection. Pseudomonas aeruginosa and Acanthamoeba are the most important pathogens causing microbial keratitis associated with overnight orthokeratology, accounting for 47.1% (16/43) and 20.6% (7/34) of cases, respectively.

Presently, all the eyes displayed the complete disappearance of corneal infiltrates and re-epithelization after using antimicrobial medications. No urgent therapeutic surgery was needed to control the infections. Previous reports confirm the success of the strategy in the majority of patients. Acanthamebic infection associated with orthokeratology can require months to heal. In our series, the mean time for re-epithelization with acanthamebic infection (112 days) was much longer than cases with nonacanthamoebic infection (6.6 days). The poor epithelization of these cases may be caused by drug toxicity or concomitant steroid usage, because these patients had been treated empirically with a variety of commercially available antibiotics and antibiotic-steroid combination before being referred to our hospital.

In a previous study regarding the clinical characteristics of microbial keratitis, we demonstrated the better medical treatment result of bacterial infection compared with acanthamebic infection. Early diagnosis is essential for successful treatment of acanthamebic keratitis. Delays in diagnosis and treatment can result in an extensive involvement of ocular tissue and poor visual outcome, as exemplified by patient 8. Therefore, the possibility of acanthamebic keratitis in users of orthokeratology lenses could not be overlooked. Where the clinical presentations are characteristic, vigilance should be heightened for acanthamebic keratitis, and laboratory examination mandatory.

The visual outcomes in our study varied from 20/20 to hand motion only. Because all the patients had a sequela of corneal opacity, the density, size, and location of the corneal opacity are key factors influencing visual outcome. Other complications such as cataract and secondary glaucoma also affect the visual prognosis.

We attempted to identify the specific data of orthokeratology lenses, including the brand and type of lenses, fitting characteristics, diameter, power, and base curve of lenses, and the corneal topographic changes; however, the details were unavailable to us because the orthokeratology lenses were not fitted at our hospital. Although the design and material of orthokeratology lenses are continuously evolving, the long-term effect and safety of overnight orthokeratology warrants further evaluation and continuous monitoring.

CONCLUSIONS

Patients and ophthalmologists should not overlook the risk of microbial keratitis after overnight orthokeratology. Parents should be educated on the importance of proper lens hygiene and the need for immediate ophthalmic consultation when symptoms of keratitis occurred.

Acanthamoeba and Gram-negative bacilli, including Pseudomonas aeruginosa, are the most common pathogens causing microbial keratitis associated with overnight orthokeratology in this series. Patient education, early diagnosis, and proper antimicrobial treatment are helpful in preventing and eradicating the infection.