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HomemipatientOK: Getting the Mix Right

OK: Getting the Mix Right

A simple orthokeratology lens mix up, combined with Melbourne’s extended COVID-19 lockdown, had a significant but reversible (and irreversible) impact on the eye sight of young twins.

Identical twins Sarah and Tracy* first presented to the practice in September 2019, aged seven. They had recently immigrated from Shanghai and were referred for orthokeratology (OK). Their myopia was diagnosed at age six and a half. At diagnosis they were both -1.00 myopes, but rapidly progressed to be -3.00 in one year. Their last examination was six months prior to seeing me when spectacles of this prescription had been prescribed. As they had both exhibited fast progression, they were initiated on low dose Atropine treatment of 0.01% nocte.

while they could share clothes, shoes, and even spectacles, they could not share contact lenses

Figure 1. Sarah’s corneal topography.

Both parents are myopic; their mother is a -4.50 myope and their father is a -4.00 myope.

Sarah’s vision with presenting spectacles of prescription R & L -3.00DS was R 6/12, L 6/9, correcting to R & L 6/6 with R -3.75DS and L -3.50DS.

Tracy’s vision with presenting spectacles of prescription was R 6/9= and L 6/12=, correcting to R & L 6/6 with R -3.75DS and L -4.00DS.

Binocular vision testing revealed a near esophoria of one for Sarah and three for Tracy. Both had a lag of accommodation of +1.00DS.

Despite being identical in physical appearance and having very similar refractive errors, their corneal curvatures differed slightly:

Sarah had mild with-the-rule corneal toricity of R 0.8D, L 0.6D, with Flat K values of R 7.98mm and L 7.99mm.

Tracy also had mild with-the-rule corneal toricity of R 0.6D and L 0.9D, with Flat K values of R 7.80mm and L 7.85mm.

As their corneal shapes differed, the girls were prescribed lenses with differing parameters.

Figure 2. Tracy’s corneal topography.

They were prescribed Paragon CRT contact lenses, which are specified in sequence of Back Optic Zone Radius (BOZR), Return Zone Depth (RZD) and Landing Zone Angle (LZA).

Sarah was prescribed R 89 525 33, L 88 525 33, and Tracy was prescribed R 87 550 33, L 88 550 33.

These lenses performed well and the girls were able to achieve consistent unaided vision of R & L 6/6.

They were reviewed in November 2019, then delayed their next six month followup due to COVID-19. They returned towards the end of Melbourne’s COVID-19 lockdown in October 2020. Both reported clear vision, however Tracy complained of a sore left eye.

Vision unaided for Sarah was R 6/7.5=, L 6.7.5.

Subjective refraction was R -0.50DS (6/6), L -0.25 (6/6=).

Vision for Tracy was R 6/7.5, L 6/12=. This could not be improved with refraction.

Corneal topography with Medmont E300 topographer revealed good centration of the treatment zone in both eyes for Sarah, however there was a slight central island in the right eye (Figure 1).

Corneal topography for Tracy revealed good centration of the treatment zone in the right eye, and a superiorly displaced treatment in the left eye with significant central distortion (Figure 2).

Examination revealed no abnormalities in Sarah’s eyes, however Tracy exhibited mild corneal staining in the right eye and significant staining in the left eye (Figure 3). Examination of the contact lenses in situ revealed that the left contact lens on Tracy was excessively flat fitting (Figure 4).

Figure 3. Tracy’s left eye showing significant corneal staining.

Take a minute to consider what may have happened…

Sarah was wearing R 87 550 33, L 88 550 33.

Tracy was wearing R 88 525 33, L 89 525 33.

The girls had mixed up their lenses, so that Sarah was wearing Tracy’s and vice versa. But not only was Tracy wearing Sarah’s lenses – she had Sarah’s right in her left eye and left in her right eye.

And furthermore, their reduced vision was not only due to mixing up lenses – their myopia had progressed and both had exhibited axial elongation.

Axial biometry had increased for Sarah from R 25.28, L 25.08mm in September 2019 to R 25.84, L 25.68mm in October 2020.

Axial biometry had increased for Tracy from R 24.72, L 24.75mm in September 2019 to R 25.07, L 25.08mm in October 2020.

Both girls were refitted with lenses with a flatter BOZR for a greater refractive target and their lenses were ordered in differing colours. Fortunately the Paragon CRT lenses come in four different handling tints – green, blue, red, yellow as well as clear.


Mixing up lenses is one of the most common problems that causes a ‘sudden change’ in vision and/or comfort. Some patients manage to get them mixed up more often than not – usually the biggest perpetrators being teenage boys… So, when a patient reports that all was well until a definable moment, the first thing I do is check their lenses. It is a relief, and often patients are quite abashed, when I inform them that a simple lens mix up is the cause of their concerns.

For this reason, lenses with different handling tints are extremely effective for reducing errors. I originally prescribed lenses for the girls from my CRT fitting kit. Keeping OK stock is handy as there is no wait for the lenses to be manufactured. This enables immediate OK initiation, and fast replacement of broken and lost lenses. However, the downfall is that all the stock lenses are tinted green, which can result in patients mixing them up. Fortunately CRT lenses are all engraved with their parameters, for easy identification. I often dispense a hand held loupe magnifier to patients to assist with this.

Figure 4. Fitting pattern of Tracy’s left contact lens revealing excessively flat fit.

While mixing up right and left lenses is common, mixing up lenses between kids is less intuitive. However in this case, from experience, I knew the outcome as soon as I saw Tracy’s eyes. I was also quietly relieved that the cause of the corneal staining was not from ill-fitting lenses. I had a brother-sister duo years ago, who accidentally managed to mix the nine-yearold brother’s -4.00 and the eight-year-old sister’s -6.00 OK lenses. Of course the brother was under-corrected and the sister was over-corrected. I’ve also had a girl turn up having broken her lens. She thought she was wearing her spare lens, however was actually wearing her father’s old spare lens. Fortunately, kids have an amazing ability to tolerate change. Nevertheless, where possible, I try to ensure members of the same household who wear contact lenses have different contact lens cases.

There is often the misconception that OK involves mechanically ‘pushing’ the cornea flatter, however OK actually uses ‘squeeze film forces’ that work to push and pull the corneal epithelium into a different shape. For myopic OK, the back optic zone radius (BOZR) is responsible for the ‘treatment zone’ on the cornea, and is selected by the formula BOZR = Flat K + Target Rx + Jessen Factor.1

The Jessen Factor is what I describe as the ‘fudge factor’ – basically an over-shoot factor to ensure the desired treatment is achieved for the entirety of the day as OK is completely reversible and there will be a tendency for the cornea to drift back towards its original shape.

As the BOZR is flatter than the cornea, if we were to employ a ‘standard’ rigid gas permeable contact lens design, the lens will be excessively flat on the cornea and may result in decentration, corneal insult and can lead to eventual scarring. Thus, modern day OK lenses are reverse geometry designs to ensure consistency of treatment as well as safe lens wear. A reverse geometry design lens is one that has a secondary curve steeper than the central curve, which will elevate or lower the lens onto the eye, with the goal of apical clearance. If the reverse curve is too flat, the lens has insufficient sagittal depth and will land on the cornea and can result in decentration and or apical insult, as was seen with Tracy. If the reverse curve is too steep, the lens has excessive sagittal clearance, which may result in insufficient or uneven treatment.1

So although the girls had very similar refractive errors, their corneal curvature was different enough to require different contact lens shapes. I explained this to them at length at the initial consultation as, while they could share clothes, shoes, and even spectacles, they could not share contact lenses.


To slow the spread of COVID-19, it was important to employ strict measures to prevent its spread, however lockdown did not come without other costs – with massive economic and health implications, of which myopia is one.

Technology has been a saviour during 2020, with the world having to pivot quickly to adapt to this unforeseen pandemic to allow for remote work and learning. However, there has been much concern raised about increased near work, screen time, time indoors, and its implications for myopia, as these activities are strongly correlated with myopic progression.2-4 Concerns for myopia have been growing worldwide in recent years, with myopia now being labelled as an epidemic. Half the world is predicted to be myopic by 2050, and an alarming 10% of the world’s population is predicted to suffer from high myopia,5 which comes with increased risks for significant vision impairment from pathological conditions.

Sarah and Tracy were at particularly high risk for myopia progression, given their early age of diagnosis, strong family history with both parents being myopic, and their binocular vision profile. Both were esophores at near and demonstrated accommodative lag at near, which has also been associated with higher risk for myopia.6 

The girls were among many I have seen coming out of lockdown exhibiting myopic progression, not just among my previously stable kids and teenagers, but also my adult patients. Melbourne had the harshest lockdown in Australia, lasting seven months. We were also subject to periods of curfew and limitations of one hour maximum outdoors each day. It will be interesting to see the effect of COVID-19 on myopia in Melbourne, and in particular, whether our rates of myopia progression were higher during 2020 compared to the rest of the country.

*Patient names changed for anonymity. 

Jessica Chi is the director of Eyetech Optometrists, an independent speciality contact lens practice in Melbourne. She is the current Victorian, and a past national president of the Cornea and Contact Lens Society, and an invited speaker at meetings throughout Australia and beyond. She is a clinical supervisor at the University of Melbourne, a member of Optometry Victoria Optometric Sector Advisory Group and a Fellow of the Australian College of Optometry and the British Contact Lens Association. 


  1. Mountford J, Ruston D, Dave T. Orthokeratology: Principles and Practice. London, Butterworth-Heinemann, 2004.
  2. Huang H, Chang DS, Wu P. The association between near work activities and myopia in children a systematic review and meta-analysis. PloS one. 2015;10(10):e0140419. 

3. Rose KA, Morgan IG, Ip J, Kifley A, Huynh S, Smith W, et al. Outdoor activity reduces the prevalence of myopia in children. Ophthalmology. 2008;115(8):1279–85. 

  1. Wai WONG C, TSAI A, Jonas JB, Ohno-Matsui K, CHEN J, ANG M, Wei TING DS, Digital Screen Time During COVID-19 Pandemic: Risk for a Further Myopia Boom?, American Journal of Ophthalmology (2020) 
  2. Fricke TR, Jong M, Naidoo KS, et al. Global prevalence of visual impairment associated with myopic macular degeneration and temporal trends from 2000 through 2050: systematic review, meta-analysis and modelling. Br J Ophthalmol. 2018;102(7):855-862. 
  3. Gwiazda, J, Thorn, F, Bauer, J, Held, R. (1993) Myopic children show insufficient accommodative response to blur Invest Ophthalmol Vis Sci 34,690-694


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