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July 16, 2020
Managing the vision of an extremely myopic patient can present many challenges (Astin, 1999). The vision in spectacles is reduced due to the minification of the image, reduced effect of vertex distance of the lenses, and cosmetically, the thickness of the lenses, even with high index lens materials is unsightly.
Contact lenses are often the preferred option for the highly myopic patient, providing better vision due to the magnification compared with spectacles (Astin, 1999). However, due to the high power of the lenses required, increased thickness, oxygen transmissibility, and availability as a readily available disposable lens, fitting contact lenses to someone with extreme myopia can present many challenges.
Rigid gas permeable contact lenses have better oxygen transmissibility (Ichijima and Cavanagh, 2007), optical properties (Hong et al., 2001, Fonn et al., 1995), durability (Tranoudis and Efron, 1996), dimensional stability and deposit resistance than soft lenses, however, the thickness profile of a high powered minus lens can pose problems in achieving a successful fit.
This report presents the contact lens management of a patient with high myopia.
Mr DG, a 46-year-old printer, presented to the practice reporting headaches, sore eyes, and poor night vision with his current contact lenses. He reports wearing rigid contact lenses since he was a teenager for his high myopia. His current contact lenses were fitted in the UK and are over 5 years old. He is not taking any regular medication, reports no previous surgery, infections, or trauma to his eyes, and he has no family history of any eye conditions such as glaucoma, retinal detachment or macula degeneration. He would like to investigate refitting his contact lenses as he is finding his current lenses are causing regular headaches, sore eyes, and poor night vision.
Vision with his current contact lenses measured:
R -19.00/-2.25x180 6/20
His contact lenses were examined in situ with the addition of sodium fluorescein (NaFl). He displayed bilateral ptosis typical of long term contact lens wear and both lenses moved excessively with blinking and did not maintain good lens centration. The NaFl pattern showed heavy bearing in the periphery of the lens at 3 and 9 o’clock, with the right lens displaying excessive central clearance and the left central touch (See figure 1 below).
Both lenses tended to “see-saw” in the vertical meridian, and this lack of stability resulted in decentration of the lens edge past the limbus either inferiorly or superiorly. Both eyes displayed a tear film rich in mucous, however, it was absent of corneal or conjunctival staining or infiltrates. Upper eyelid eversion revealed mild papillae.
Figure 1 NaFl assessment of the right lens at presentation
Figure 2 Nafl assessment of the left lens at presentation
His contact lenses were removed, and his pupils were dilated with one drop of 1% tropicamide OU. While his pupils were dilating corneal topography was performed using the Medmont E300 topographer, and the lenses were inspected and measured using a radiuscope and vertometer.
Two corneal topography measurements were taken for each eye in a geometrically centred position to provide the greatest corneal area of measurement with the eyes held wide open without the use of topical lubricants. The key corneal shape descriptors such as Sim-K’s and eccentricity are displayed in figure 3 below.
Figure 3 Anterior corneal topography of the right and left eyes
The left cornea is significantly flatter than the right cornea centrally (Apical Corneal Curvature R 7.538 and L 7.755) and appears slightly distorted, the difference in sagittal depth between the two eyes also reflects the difference in central corneal curvature (8.00 mm Chord @180 R 1115 microns and L 1105 microns).
The flatter central cornea and slight distortion seen in the left topography is a result of the flat fitting contact lens. Both maps displayed WTR corneal toricity that extended from the centre to the limbus and was confirmed by the difference in eccentricity between the vertical and horizontal principal meridians, with the horizontal meridian displaying a significantly higher eccentricity than the vertical meridian.
Elevation maps further confirmed the corneal toricity with a difference in weighted average sagittal height between the two principal meridians at the 8.00 mm chord of R 52.6 microns and L 37.6 microns. The HVID’s were measured using the ruler in the Medmont Studio 5 software giving R 11.59 mm and L 11.53 mm.
Both contact lenses displayed surface scratches and were covered in lipid and mucous deposition. Measurement with the radiuscope and vertometer measured R 7.5 and -16.00D and L 7.65 and -16.50D. There was no indication or history of the type of lens material.
Examination of the posterior pole revealed several large vitreous floaters in each eye, tilted optic nerve heads, peripapillary atrophy typical of myopic crescents, and hypopigmentation of the maculae. OCT’s of the posterior pole revealed significantly thinner maculae; however, no macula schisis or holes were visible. The crystalline lens was clear in both eyes. IOP’s were R 21 mmHg and L 19 mmHg, and CCT’s measured R 555 microns and L 564 microns
Following the initial assessment, it was communicated to the patient that both the best vision and fitting of the lens could be improved. Due to the high level of myopia, his contact lens options are limited to custom lathed soft or rigid lenses. Although soft contact lenses are available for the correction of his high myopia and astigmatism, a rigid lens provides better dimensional stability, oxygen transmissibility (Ichijima and Cavanagh, 2007), deposit resistance, durability (Tranoudis and Efron, 1996) and optical properties (Phillips and Speedwell, 2006, Hong et al., 2001, Fonn et al., 1995).
Due to the reasons mentioned above and his familiarity with rigid lenses, it was proposed new rigid lenses could be refitted with improved fitting characteristics and vision. In regards to the fitting of his lenses, his current lenses appear to be a rotationally symmetrical tri-curve design. Both the NaFl and corneal topography analysis would suggest that the fitting could be improved with a back surface toric design. A back surface toric design would result in a lens that fitted the corneal shape better, improving the corneal contact lens relationship, a decreased overall volume of tears under the lens, enhanced centration (Michaud et al., 2016), and more even edge lift 360 degrees around the lens.
The steep fitting right lens and flat fitting left lens was causing corneal distortion and altering the shape of the cornea. It has been shown contact lens wear may alter the corneal shape and induce corneal distortion (Wilson and Klyce, 1994, Wilson et al., 1990b), especially for steep and flat fitting lenses (Korb et al., 1982, McMonnies, 2004). The warpage topography is characterized by a relative flattening of the cornea underlying the resting position of the contact lens. Lenses that ride high, for example, produced flattening superiorly (Wilson et al., 1990a). After discontinuing contact lenses up to 6 months can be required for the corneas to return to a stable topography after contact lens wear is discontinued (Wilson and Klyce, 1994).
In regards to the vision, the ROL revealed not only residual myopia but also ATR residual astigmatism. Hence a rotationally stabilised, non-spherical power effect lens is required to provide optimal vision. Rigid lenses can be stabilised through the use of prism ballasting or back surface lens toricity fitted to the toricity of the cornea. It is challenging to achieve a thin, comfortable, and wide front optic zone for prism ballasted rigid lens with a high myopic back vertex power. Due to the existing corneal toricity, the best option for rotationally stabilising the lens is a toric back surface lens. A back surface rigid contact lens will achieve rotationally stability by reaching a pseudo equilibrium on the cornea (between blinks, and ignoring lid forces), by minimising its potential energy through minimising the tear volume between it and the cornea.
Various resources suggest that a back surface toric contact lens should be considered when the corneal toricity (ΔK) is anywhere from 1.50DC to 3.00DC (Phillips and Speedwell, 2006, Kajita et al., 1999). There doesn’t appear to be any consensus to the amount required, and the parameter ΔK is not a very accurate indicator as the central toricity may not be consistent with the peripheral corneal toricity where the contact lens aligns and achieves its rotational stability.
In modern contact lens fitting, corneal topographers are used in replacement to the keratometer. The corneal topographer not only accurately measures the central corneal curvature, but also the peripheral corneal curvature and sagittal height. Corneal height and elevation maps provide a better representation of the toric shape of a cornea, and the difference in the peripheral height of the cornea in the principal meridians gives a better indicator for fitting a back surface toric lens. In the experience of the author, there must be at least 30 microns difference in the sagittal height of the cornea in the principal meridians, at the 8.00 mm chord, to stabilise a typical corneal Gas Permeable lens. In this case, the corneal topographical data showed there are over 30 microns of toricity, which allowed the use of a back surface toric lens to achieve rotational stability of the non-spherical power effect lens.
The corneal topographic height data was exported from Medmont Studio 5 to EyeSpace contact lens fitting software for the simulation and calculation of the lens parameters. The EyeSpace Bespoke Toric, a 3 zone, zone-specific, peripheral aspheric alignment GP contact lens was chosen. By entering and adjusting the lens parameters, a lens fit was achieved through the observation of the NaFl simulation and tear layer thickness (TLT) profile.
To achieve an optimal fit, the central back optic zone (BOZ) must have an even tear layer thickness profile in all directions, a central TLT of 5 to 20 microns, the alignment curve (AC or peripheral curve) should prove wide even touch/bearing on the peripheral cornea, and the axial edge lift should be between 60 and 100 microns (Tomlinson and Bibby, 1977). Where the lens bears/touch on the horizontal peripheral cornea, the lens fitting profile should be different from the vertical fitting profile. In the vertical meridian, the lens should fit with a slightly flatter profile and more axial edge lift (See figure 4 and 5). This profile ensures the lens remains well centred horizontally, and moves vertically on blinking, maximising tear exchange and comfort.
Note a BOZR of 7.65/7.4 (e=0) and AC 7.1/6.8 (e=0.9) produced the best fitting profile for the right eye.
EyeSpace software displays the area of corneal-lens touch/bearing in blue. Clinically, cornea-lens bearing or touch can be misrepresented by the dark area on NaFl assessment as it merely represents an area where the tear layer thickness is less than approximately 20 microns. The visibility of NaFl is also concentration-dependent (Carney, 1972) and time-dependent (Wolffsohn et al., 2015), which can further compound NaFL assessment misrepresentation.
Figure 4 EyeSpace NaFl simulation and TLT cross-section in the horizontal meridian of the right eye
Figure 5 EyeSpace NaFl simulation and TLT cross-section in the vertical meridian of the right eye
Figure 6 EyeSpace NaFl simulation and TLT cross-section in the horizontal meridian of the left eye
Figure 7 EyeSpace NaFl simulation and TLT cross-section in the vertical meridian of the left eye
Although fitting software, such as EyeSpace, runs mathematical optimisation algorithms to select the parameters for the user automatically, it is essential to understand how to assess and adjust these parameters if needed. EyeSpace allows the user to adjust the parameters and re-simulate the fitting pattern, similar to what a practitioner does when trial fitting. However, this can be achieved faster, with a broader and more sophisticated range of parameters and most importantly without the need for the patient to attend a lengthy fitting appointment that takes up chair time, and is an uncomfortable experience for the patient, especially with a neophyte rigid lens wearer (Szczotka, 1997).
The concept of Tear Layer Thickness (TLT) was first introduced by Townsley (Townsley, 1970). Acceptable central TLT can range from 10 to 30 microns (Tomlinson and Bibby, 1977). A BOZR that is too steep is characterised by a convex tear thickness (plus tear lens) profile, where the tear layer thickness (TLT) is greater centrally than at the BOZR and AC junction (See figure 8).
Note a BOZR of 7.4/7.2 fits too steep to the patient's right eye. Steep fitting BOZ will typically decentre inferiorly, lack good movement with blinking, have poor tear exchange and can cause transient steepening of the cornea (McMonnies, 2004).
Figure 8 An example of a steep fitting back optic zone
Conversely, a BOZR that is too flat is characterised by a concave tear thickness (minus tear lens) profile, where the central TLT is less than the TLT at the BOZ and AC junction (See figure 9).
Note the BOZR of 7.9/7.6 fits too flat to the patient’s left eye. A flat fitting BOZ may result in central corneal bearing, lens instability, corneal epithelial desiccation, and flattening of the central corneal curvature.
Figure 9 An example of a flat fitting back optic zone.
The peripheral/alignment curve should tangentially align to the peripheral cornea, providing wide and even bearing/touch in the horizontal meridian. An alignment curve that is too steep has a tangential angle greater than the tangential angle of the cornea at the corresponding midpoint of the alignment curve and that of the cornea, which is easily visualised in the TLT graph. If the tangential angle of the alignment curve is equal to the tangential angle of the cornea, the line of the TLT graph will be parallel to the x-axis.
A steep fitting alignment curve will result in the lens “tenting” with even central corneal clearance over the BOZR and lens bearing at the periphery of the alignment zone. The TLT graph will display the TLT decreasing towards the periphery of the alignment zone, and a decreased edge lift (see figure 10). Steep alignment zones typically result in a lens that will decentre inferiorly, lack movement with blinking, and have poor tear exchange.
Figure 10 An example of a tight steep fitting alignment zone
A flat fitting alignment curve will display excessive edge lift and a TLT profile where the TLT increases towards the periphery of the alignment zone (see figure 11). Flat alignment zones typically cause the lens to move excessively with blinking, result in nasal or temporal decentration, and cause increased edge awareness.
Figure 11 An example of a flat fitting alignment zone
The edge clearance (EC) is the distance parallel to the axis of symmetry of the lens, between the cornea and the extreme edge of the lens. In contrast, the Axial Edge Lift (AEL) is the distance parallel to the axis of the symmetry of the lens between the extreme edge and the projection of the Back Optic Radius (Guillon et al., 1983). The relationship between the AEL and EC is only a loose one affected by the way the lens rests on the cornea. The optimal edge clearance value is 80 microns, with an acceptable range between 60 and 100 microns (Tomlinson and Bibby, 1977).
Diameter is an important consideration for optimal lens fitting and has a significant effect on the centre of gravity of the lens and ultimately, the centration and position of the lens on the eye. The further the centre of gravity moves behind the lens, the greater the area of support above it. As the centre of gravity moves towards the front surface of the lens, there is less support for the lens, and it will become unstable due to gravity, and the force of the eyelids (Phillips and Speedwell, 2006).
Other factors affecting the centre of gravity of a lens are the power, thickness, and curvature. Relative to their respective alternatives, high negative power, thin lenses, and steep curvature result in the centre of gravity moving further behind the lens. Due to the lens having a highly minus Back Vertex Power (BVP), and a thick lens periphery, a larger diameter will aid the lens centration. No longer constrained by decreased oxygen delivery to the eye, higher DK materials allow fitting of larger lenses in such a way that the superior lens edge is positioned under the upper lid, resulting in better comfort due to reduced interaction between the lens edge and lid margin. However, a corneal lens should never be larger than the corneal diameter, which is oval in shape with typically the horizontal visible iris diameter (HVID) larger than the vertical visible iris diameter (VVID).
To avoid the lens bumping into the limbus, and to allow for good lens movement while maximising the diameter to aid centration and comfort, a lens diameter that is 90% of the HVID is desirable. As the HVID was R 11.53 and L 11.59, a diameter of 10.40 mm was chosen.
There are two methods for calculating lens back vertex power (BVP). Using either the spectacle refraction and keratometry readings, or from a known lens' BOZR and BVP fitted on the eye and the resultant refraction over lens (ROL). Using the spectacle refraction and keratometry readings is an entirely appropriate way of calculating the BVP for low to moderate BVP. However, for high BVP, the vertex distance plays a significant role in the accuracy of the calculation. Due to the patient's high myopia, the best method is to use the known lens parameters BOZR, BVP and the ROL.
Lens: BOZR 7.5, BVP -16.00 ROL: -0.25/-0.75x100
Best fitting BOZR: 7.65/7.4 Theoretical Lens stabilisation: 10 degrees.
Flat meridian: BVP = Lens BVP - Change in tear layer power + ROL
= -16.00 - (337.5 / 7.65 – 337.5 / 7.5) + -1.00
= -16.00 - (-0.88) + -1.00
Steep Meridian: BVP = Lens BVP - Change in tear layer power + ROL
= -16.00 - (337.5 / 7.4 – 337.5 / 7.5) + -0.25
= -16.00 - (0.61) + -0.25
BVP= -16.37DS/-0.74DCx180 (axis relative to the flat meridian of the back surface)
BVP (Rounded to 0.25) = -16.25/-0.75x180
Lens BOZR 7.65, BVP -16.50 ROL: 1.50/-2.00x90
Best fitting BOZR: 7.7/7.5 Theoretical Lens Stabilisation: 180.
Flat meridian: BVP = Lens BVP - Change in tear layer power + ROL
= -16.50 - (337.50 / 7.70 – 337.50 / 7.65) + -0.50
= -16.50 - (-0.29) + -0.50
Steep Meridian: BVP = Lens BVP - Change in tear layer power + ROL
= -16.50 - (337.50 / 7.50 – 337.50 / 7.65) + 1.50
= -16.50 - (0.88) + 1.50
BVP= -15.88DS/-0.83DCx90 (axis relative to the flat meridian of the back surface)
BVP (Rounded to 0.25) = -16.00/-0.75x90
The lenses were inspected and delivered to the patient a week later. The lens fit was assessed with biomicroscopic examination using sodium fluorescein and a Yellow Wratten filter (See images below). Both lenses fitted with excellent centration. Rotational stability was good with the flat meridian of the lens locating along the flat meridian of the cornea, along with an intra-limbal fit and good edge lift. Vision with the lenses measured R 6/15, N10 and L 6/15, N10.
Figure 12 Right lens on the eye at delivery
Figure 13 Left lens on the eye at delivery
Mr DG reported good initial comfort at the delivery, and a review for two weeks was scheduled. Mr DG was given the lens care regime of AOSept hydrogen peroxide daily cleaner, Menicon Progent to use monthly, Lens Plus saline to rinse lenses, and Hylo Fresh non preserved lubricant eye drops to use as necessary.
Mr DG returned two weeks later reporting good comfort and improved night vision. ROL measured R +0.25/-1.00x180 6/12 and L plano/-1.25x13 6/10. New lenses were designed with the power correction and dispensed one week later. Further reviews were conducted three months later, and then annually after that.
At his annual appointments, he typically requires replacement of his lenses with minimal changes to the lens parameters due to poor wetting of his contact lenses despite his best efforts to clean and maintain the lenses.
Following contact lens fitting and adaptation, patients with high degree myopia can obtain great satisfaction with regards to improved optical correction, vision quality comfort and improved cosmesis. These factors can enhance their self-esteem and allow a wider variety of pursuits. At the initial consultation, it is essential to explain the advantages and disadvantages of lenses for these patients, to stimulate their motivation when adapting to lens wear and care.
Particularly for GP lens fitting, extra time and patience are required. Collecting as much information at the initial visit is vital to facilitate the smooth progress of discussion, lens fitting and ordering.
Advantages of contact lenses for high degree myopia are; the patient has good unaided near vision, so can easily see the lens on their finger when learning contact lens handling. They can easily examine the lens condition before lens insertion and can detect the presence of protein film, damaged edges, and cracks. They have high motivation to wear contact lenses and so are more likely than people with low degree ametropia to persevere through the initial adaptation phase.
In addition, people with extreme myopia have several reasons for preferring contact lenses over spectacles. These reasons include; the high prescription limits their choice of spectacle frames, they have limited choice of spectacle lens types, high refractive index lens materials are expensive, and thick spectacles are heavy and press on the nose, ears and sometimes also onto the face causing discomfort. Cosmetically, thick spectacle lenses look unsightly, and their eyes appear hidden and minified by the spectacles.
Optically, high powered myopic prescriptions will minify the image so the patient may not attain their maximum potential visual acuity. High powered spectacle lenses give more aberrations, distortions, and lens surface reflections, particularly at their edges, hence the patient will feel 'blinkered' and need to move their head more to observe objects at the side. With a high power prescription, if the spectacles move out of correct position by even a minor amount, changes in back vertex distance and angle can result in reduced vision and greater image distortions. If spectacle optical centres are not positioned precisely, there is a higher risk of relative prismatic effect between the two eyes, possibly leading to binocular strain, diplopia and headaches.
Disadvantages of contact lenses in extreme myopia include the need for aspheric or lenticular designs, hence the lenses are more costly, and supply is slower. The lenses cannot be fitted from stock, either initially or as a replacement. More complex lens fitting is required. Therefore, an experienced lens practitioner and longer chair-time are needed.
A high power lens is usually thicker than a low power lens, so the oxygen transmissibility (DK/t) is lower. As a consequence, the practitioner must be more careful regarding the oxygen supply to the cornea when deciding on which lens specification and material to order. The lens edge and lenticular transition are also thicker and, therefore, are less comfortable for the lid to glide over and potentially delay the initial lens adaptation and discourages full blinking, which can cause 3 and 9 o'clock epithelial staining and symptoms of dryness.
As a result of the extra lens thickness, the upper lid may constantly push the lens into a static low riding position resulting in 3 and 9 o'clock staining, lower limbal staining, indentation, and increases the likelihood of poor pupil coverage by the optical portion. Alternatively, a tight lid may grab the thick lens edge, pulling the lens into a high riding position, also causing pupil coverage problems. If only one lens rides high due to lid attachment, there may be some relative prismatic difference between the two eyes, leading to binocular strain and sometimes diplopia. High power lenses usually also have a lenticular front optic zone diameter and, therefore, a reduced back optic zone diameter, hence the patient may complain of image flare when the pupil is large.
This patient was unable to wear lenses for more than 12 months without the need to replace them due to poor wettability. As contact lenses age, their physical and clinical performance deteriorates, resulting in reduced comfort, vision, and wettability (Jones et al., 1996, Guillon et al., 1995). This has resulted in the wide acceptance of planned replacement for soft contact lenses. The concept of GP contact lenses having a longer life expectancy is based upon anecdotal or clinically intuitive assumptions, with a life of 5 to 10 years being reported (Yokota et al., 1992). Contrary to this, it has been reported that with high Dk GP’s (Dk greater than 90) patients should expect to replace them after approximately six months (Jones et al., 1996), providing an argument for planned replacement, rather than replacing based on clinical need.
In conclusion, while the fitting of a patient with extreme myopia can pose more complexity to achieve a correctly fitting lens, and requires a sound knowledge of contact lens optics and geometry. The benefits to the patient are numerous, and contact lens fitting with GP lenses should be offered as a vision correction option.
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