Orthokeratology alias corneal refractive therapy is the use of specialized lenses for temporary correction of mild-to-moderate myopia. Modern orthokeratology involves the overnight use of reverse-geometry gas-permeable lenses as an alternative to refractive surgery and spectacle wear.

Orthokeratology is also often referred to as OK, ortho-k, corneal reshaping and colloquially rigid contacts. Modern orthokeratology or commonly overnight orthokeratology is now established worldwide as a viable alternative to refractive surgery.

Historical analysis of orthokeratology

The history of orthokeratology can be traced as far back as to the Chinese myth of the use of small weights or sandbags on eyelids during sleep to reduce myopia. This can be considered the basic principle behind orthokeratology. This was followed by another precursor to modern orthokeratology in the concept of flat-fitting glass scleral lenses. In 1888, the French ophthalmologist, Eugene Kalt, implemented these lenses to flatten the cone in keratoconus cases and hence reduce myopia.

Modern orthokeratology took its first step in 1962, when George Jessen, made an accidental discovery. While working on his application of the “orthofocus” method for fitting polymethylmethacrylate (PMMA) lenses, he chanced upon improvements in uncorrected vision once the lenses were removed. The 1960s and 1970s saw various researches on the conventional flat-fitting lenses.

In 1971, the International Orthokeratology Section of the National Eye Research Foundation defined orthokeratology as “the reduction, modification, or elimination of refractive anomalies by the programmed application of contact lenses.” However, this definition became redundant with time.

Problems in stabilizing flat-fitting lenses on the cornea led to various experiments to modify lens designs. Meanwhile, four studies on the effectiveness of orthokeratology continued using different lens designs and theories. There were four such studies with identical reports of modest success in reductions in myopia (averaging an estimated 1.00 D) but with erratic and individually variable patterns. Thereafter, orthokeratology took a backseat in refractive concepts until its resurgence in the mid 1990s.

Renewed interest occurred due to advancement in technology and a new concept of accelerated orthokeratology. This was triggered by production of reverse-geometry lens designs by Wlodyga and Stoyan; the invention of appliances for corneal topographic analysis; and the availability of high-Dk gas-permeable materials that made overnight lens use possible.

Hence, orthokeratology came about overnight, which is now the standard practice worldwide. In 2003, the Food and Drug Administration approved the Paragon CRT, the first of its kind, for use in overnight orthokeratology in the US.

The methodology behind overnight orthokeratology

In overnight orthokeratology, the specialized lenses are used during night sleep and taken off on awakening. This ensures clear vision without the help of spectacle or contact lenses all through the day.

In modern orthokeratology, the specialized lenses used are reverse-geometry gas-permeable lenses. The effect of the use of these lenses is noticeable almost instantly and the constancy of the therapy after 7 to 10 days of its use. The procedure is also totally reversible once the lens is discontinued.

The result of the process occurs through a fine reshaping of the anterior corneal layers. This results in thinning of the central corneal epithelium and thickening of the mid peripheral stroma. However, the cellular biology of these changes is still being investigated further.

Effectiveness of orthokeratology

Various clinical researches on overnight orthokeratology have been conducted by the application of various reverse-geometry lens designs. They have ascertained the process to be an effective corrective technique for myopia, to a maximum of around 4.00 D. Though the procedure has been tried on patients with higher myopia (mainly in Asia), there have been alarms about its safety and standard of vision in such cases.

The ability of orthokeratology to correct astigmatism, hyperopia and possibly presbyopia is still undergoing extensive research. The toric reverse-geometry lenses that are specialized to correct astigmatism are currently under development.

Pros and cons of overnight orthokeratology

The benefits of this process are its ease of use and non-invasive nature and its reversible potential. Moreover, once the lenses are removed after nightwear, it almost instantly removes any left over edema and a tear film fragment build up. This also ensures insignificant lens uneasiness generally associated with open-eye gas-permeable lens use. Hazards associated with daily wear for instance 3-and 9-o’clock staining or ocular dryness is also minimal.

Among its disadvantages is the lesser issue of the temporary nature of the therapy. If night uses of lenses are discontinued, the cornea reverts to its original shape soon. The biggest and very serious concern is the complication of overnight hypoxia. Though there is an antidote in the use of high-Dk materials, there has been a high incidence of severe corneal infections due to a lack of proper clinical standards. There has been a higher incidence of such infections in East Asia compared to Western nations. Worse still, many of the cases have been children below 15. Such cases have reported noticeable decrease in best-corrected visual clarity and have had to undergo keratoplasty to bring back eyesight.