Advanced Toric Orthokeratology Lens Design in Case of High Myopia and Astigmatism (>-6.00D): Melanie Frogozo, OD, FAAO, FSLS
Background
Since children are at greater risk for developing high myopia, it is important to prevent myopic progression as early as possible. Myopia control takes two forms: decrease of myopia progression or prevention of myopia onset. Soft dual focus contact lenses, topical antimuscarinics, and orthokeratology can potentially significantly decrease myopia progression. The following report discusses a case of high myopia and astigmatism treated successfully with an advanced toric orthokeratology (OK) design.
Patient Consultation and Education
In the past, the patient had a previous history of trying OK lenses to correct her myopia; however, none of them were successful since all the previous lenses caused harsh central punctate staining. The corneal insult was thought to be caused by insufficient sagittal depth of the lens which caused tilting of it, and consequential harm to the cornea. Despite several lenses trialed, the patient was still showing cornea insult. After careful consideration, a lens design that incorporates corneal eccentricity was chosen the patient. The patient understood that if this lens design was not successful she would most likely have to be refit into a soft toric multifocal or a daytime wear corneal GP lens with a front surface multifocal in order to decrease the progression of her myopia. Both the mother and the patient understood this, and were motivated to move forward with a refit in OK lens design.
Test Procedures, Fitting/Refitting, Design, and Ordering
Visit 1
The patient presented for a contact lens evaluation for her OK lenses. She stated that her vision was clear and comfortable through her glasses and did not have any problems. She was still healthy and not taking any medications. Her ocular history was remarkable for lattice degeneration that was treated with cryotherapy in both eyes.
During this visit, the patient presented wearing spectacles with a prescription of -8.50 -2.50 x 180 in both the right and left eyes. Through this prescription the patient was corrected to 20/25 in each eye. Axial length was not assessed in either eye since a measurement instrument was not available. Manifest refraction was –9.00 – 2.25 x 180 in both the right and the left eyes which gave the patient 20/20 vision in each eye. The patient’s horizontal visible iris diameter was 12.2 mm in both eyes.
This patient had a history of previous night-time orthokeratology wear; however, she has complications of central corneal punctate epithelial erosion when fitted with lenses that had a spherical base and peripheral curves, spherical base and toric peripheral curves, and toric base curve and toric peripheral curve OK lenses. All lenses initially fitted did not incorporate corneal eccentricity values of the patient. It was determined that the previous lenses she was fitted with did not provide sufficient sagittal depth. Thus, she was to be re-fitted into a OK lens with both toric base curve and peripheral curves which also incorporated eccentricity in order to optimize the alignment curve and the sagittal depth of the lens design. The DRL (Double Reservoir Lens) OK lens incorporates a secondary reverse curve that divides the alignment curve. The reasoning behind the second tear film reservoir is the combination of two tear reservoirs results in increased hydrodynamic forces. This is important as OK treatment is thought to be generated by hydrodynamic forces under the lens with areas of tear pooling that induce suction forces and thin tear zones generate push forces.
Placido disc corneal topography during this visit showed regular with-the-rule (WTR) astigmatism in both eyes. Her K values during this visit were 42.74 @ 180 / 45.33 @090 in the right eye and 42.86 @176 / 45.58 @ 086 in the left eye. Her eccentricity values were 0.68 @ 176 / 0.33 @ 090 in the right eye and 0.68 @ 176 / 0.34 @ 086 in the left eye. The patient’s horizontal visible iris diameter (HVID) was still the same from the previous visit and measured to be 12.2 mm. The lens designed by the software created a toric central base curve and toric peripheral curves. The parameters of the lens ordered this visit were: 10.09 mm / 11.17 mm Base Curve (BC), +1.62 D power, and 11.20 mm overall diameter (OAD) in the right eye and 10.02 mm / 10.11 mm BC, +1.50 D power, and 11.20 mm OAD.
Visit 2
The patient presented for her contact lens dispensing. The lenses were placed on her eyes and both lenses showed the same pattern. Both the right and the left lens were mildly decentered inferior-nasal and displayed a 5.00 mm area of apparent bearing centrally at the back optic zone with an adjacent area of pooling at the reverse curve about 1.00 mm in length in the pattern of a bulls-eye. Adjacent to the reverse curve pooling, was bearing-to-alignment at the landing/alignment curve which was about 1.00 mm wide. However, this alignment curve was interrupted by a secondary reservoir reverse curve that was only 0.2 mm wide. This secondary reverse curve exhibited 360 degrees of pooling. Mild pooling was present under the edge lift curve which was about 0.25 mm wide. Both lenses were judged to result in an adequate fit and the patient’s vision was 20/20 out of each lens. The patient was to wear these lenses at night-time only. Handling and care instructions were retaught to her during this visit. Again, she was prescribed Unique pH for cleaning, disinfection, and storage. The patient was to return in 3 to 4 days for a contact lens progress evaluation.
Visit 3
The patient presented for her contact lens progress evaluation. The patient stated that she had worn the lenses every night for about 8 to 10 hours. She related that she had no problems with handling or care of the lenses and followed the instructions that were given to her the last visit. She stated she had no discomfort while wearing the lenses. During this visit, the patient presented wearing no correction. Her unaided correction was 20/80 in both the right and left eyes.
Manifest refraction was –3.00DS OD and – 2.50DS OS which resulted in 20/20 in vision in each eye. Placido disc corneal topography showed an oblate corneal profile characteristic of treatment with myopic OK. The tangential power analysis showed flattening within the central 2.00 mm radius with an adjacent 1.00 mm of steepening at 4.00 mm from the corneal center. The tangential difference map of the pre-treatment and the post-treatment topography are shown in Figure 1 below. The refractive difference map showed that patient had about -5.34 D of correction OD and -5.28D of correction OS. The patient was to return in two weeks for a contact lens progress evaluation.
Visit 4
The patient presented for her contact lens progress evaluation. The patient stated that she had worn the lenses every night for about 8 to 10 hours. She related that she had no problems with handling or care of the lenses and followed the instructions that were given to her at the previous visit. She stated she had no discomfort while wearing the lenses. Her unaided correction was 20/50 and 20/60 in right and left eyes respectively.
Manifest refraction was –2.00DS OS and – 2.50DS OS which resulted in 20/20 visual acuity in each eye. Placido disc corneal topography showed an oblate corneal profile characteristic of treatment with myopic OK. The tangential power analysis showed flattening within the central 2.50 mm radius with an adjacent 1.00 mm of steepening at 4.00 mm from the corneal center. The steepening was approximately uniformed 360 degrees around in both eyes. The tangential difference map of the pre-treatment and the post-treatment topography are shown in Figure 2. The refractive difference map showed that patient had about- 6.00D OD and -6.00D OS.
Although the lenses showed a defined bulls-eye pattern they both appeared to decenter inferior and nasal. This decentration was due to haptic landing in both lenses were too flat and needed to be steepened in both eyes. The alignment curve was indicated to be steepened in both the flat and the steep meridians by 50 microns. This change additionally would steepen the central clearance by 8 microns, thus, the central base curve was also flattened. The following modified lens parameters were ordered for the patient: 10.20 mm /10.28 mm BC, +1.73 D power, and 11.20 mm OAD for the right eye and 10.30 mm / 10.39 mm, +1.36 D power, and 11.20 mm OAD in the left eye. The patient was to continue wearing her current lenses until her new lenses arrived.
Visit 5
The new lenses were placed on her eyes. Both lenses were judged to be an adequate fit and the patient exhibited 20/20 visual acuity with each lens. The patient was to return to clinic in three days for a progress evaluation.
Visit 6
The patient presented for her contact lens progress evaluation. The patient stated that she had worn the lenses every night for about 8 to 10 hours. Her unaided correction was 20/25 OD and 20/25 OS. Manifest refraction was –1.00DS OD and – 1.00DS OS which resulted in 20/25 visual acuity in each eye. Placido disc corneal topography showed an oblate corneal profile characteristic of treatment with myopic OK. The tangential power analysis showed flattening within the central 3.00 mm radius with an adjacent 1.00 mm of steepening at 4.00 mm from the corneal center. The tangential difference map of the pre-treatment and the post-treatment topography are shown in Figure 3. The refractive difference map showed that patient had about -7.35D of correction OD and -7.15D OS.
Per topography and on-eye assessment of the OK lenses the patient had adequate treatment. The patient was to return to two months for a contact lens progress evaluation.
Follow-up Care and Final Outcome
After the initial finalization of OK lenses, she was followed for a three-month followed by a six-month progression evaluation. After her year anniversary of wearing her lenses she was graduated to six month scheduled follow-up visits. The patient continued to wear her OK lenses overnight which provided acceptable uncorrected daytime vision.
Discussion
Investigations of OK contact lenses have found that they slow the growth of the eye on average 43%[1-3]. OK utilizes specially designed rigid contact lenses to reshape the corneal contour in order to temporarily modify or eliminate refractive error. Currently, the most common application for OK is the reduction of myopia; however, other designs target reduction of hyperopia, astigmatism, and even presbyopia.
Modern OK lenses are reverse geometry in design and worn overnight. The primary measurements needed to empirically design and order a modern OK lens include manifest refraction, horizontal visible iris diameter, and corneal topography. Some lenses are designed from flat K measurements, while custom lens design software may require eccentricity and apical radius of curvature measurements. These lenses are large in overall diameter and often 0.8 to 1.2 mm smaller than the horizontal visible iris diameter. The back surface of this modern OK lenses has between four to six curves. This section will describe each curve in detail from the center towards the periphery.
Patients who have a larger HVID will require a larger lens as their cornea has a greater sagittal depth in comparison to those with a smaller HVID measurement. The patient from this case report had an HVID of about 12.2 mm and, therefore needed a lens with an OAD of between 11.00 mm to 11.40 mm. The preceding lenses fitted on this patient before the finalized lens brand ranged from 10.50 to 11.00 mm in OAD. The patient ended up with central corneal insult with all of these smaller diameter lenses trialed. The last lens was able to achieve sufficient sagittal depth in order to safely treat the patient’s high myopia and astigmatism.
Toric Orthokeratology Lenses
Spherical OK lenses on astigmatic corneas usually result in poor centration, leading to induced irregular astigmatism, glare, and poor visual outcome. To determine whether a toric design is indicated corneal topography may be used in order to evaluate the sagittal height difference between the flat and steep corneal meridians. A 30 micron difference or greater at the landing chord length of the alignment curve may benefit from adding lens toricity. The chord length at which the lens lands is defined the alignment curve and will therefore vary depending on the particular lens design used; nevertheless, it is typically 8.0mm to 9.0mm. Kojima et al.(4) reported that even patients who have relatively low amounts of corneal astigmatism may have height differentials that exceed 30 microns, thus requiring a toric OK lens design.
This patient needed a full toric OK lens which incorporated her eccentricity values in order to create a lens that would give her acceptable vision correction and not harm her corneal health. Corneal eccentricity is considered to be the important variable to use, as it contributes significantly to ocular sagittal depth, even more so than does central corneal radius [5]. Ideally, corneal eccentricity values and apical corneal radius would be used to design OK lenses. Nevertheless, errors in eccentricity measurements from corneal topographers can arise from the corneal shape reconstruction algorithm used, measurement accuracy, and the fixation and tear film quality [6]. Characterizing the cornea by eccentricity may also contribute to error in interpreting corneal flattening, as this model may not describe corneal shape completely[7, 8].
References
- Charm J, Cho P. High myopia-partial reduction ortho-k: a 2-year randomized study. Optom Vis Sci 2013;90(6): 530-539.
- Chen C, Cheung SW, Cho P. Myopia control using toric orthokeratology (TO-SEE study). Invest Ophthalmol Vis Sci 2013;54(10): 6510-6517.
- Cho P, Cheung SW. Retardation of myopia in Orthokeratology (ROMIO) study: a 2-year randomized clinical trial. Invest Ophthalmol Vis Sci 2012;53(11): 7077-7085.
- Kojima R, Caroline P, Morrison S, Kinoshita B, Andre M, Lampa M. Should all orthokeratology lenses be toric?Presented at the Global Specialty Contact Lens Symposium, Las Vegas, NV, January 2016..
- Caroline P, Andre M. Effects of Corneal Eccentricity.Contact Lens Spectrum 2010;25(11).
- Mountford A, Caroline P, Noack D. Corneal topography and orhokeratology: Pre-fitting evalution. Contact Lens Spectrum 2002;38(4).
- Swarbrick HA. Orthokeratology review and update. Clin Exp Optom 2006;89(3): 124-143.
- Korszen E, Caroline P. The Anatomy of a Modern Orthokeratology Lens.Contact Lens Spectrum 2017;32(3): 30-32,34,35,40.
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