By Dr. Renee Reeder
RT is a 36 year old Hispanic male who was interested in improving visual acuity. His current prescription (SRx) was 2 years old. He had a history of high astigmatism. His entering acuities were 20/20 OD, and 20/200 OS. His current prescription was -0.50-2.50×180 OD and +2.00-4.00×180. A dry refraction revealed an SRx of -1.50-2.50×180 with 20/20 vision OD, and -1.75-6.50×175 with vision 20/30 OS. A cycloplegic retinosopy was +0.75-1.75×010 OD and +6.00-7.50×170 OS. Keratometry was 41.00/44.25 @091 OD and 39.50/46.00 @ 081 OS. Orbscan topography revealed SIM K’s of 40.00/43.10 and pachymetry of 415 microns OD and SIM K’s of 35.90/45.10 with a pachymetry of 435 microns OS. Worth4dot showed diplopia at near and left eye suppression at distance. With dry SRx, RT showed OS suppression at distance and near. However, the patient was capable of forms when he was not suppressing.
Ocular health showed mild injection temporally in each eye, lids, lashes, cornea and lens were all clear OU. Due to the high amount of cylinder, thin corneas, and suppression, it was determined that he was not a candidate for LASIK. Alternatively, GP lenses were recommended and the patient was scheduled for a contact lens (CL) fitting.
RT returned for a CL fit 10 days later. Initial diagnostic lenses from our fitting set were:
|BV Power:||pl / -2.00||+3.75/-3.50|
The right lens centered slightly temporally, had a very slight apical bearing but overall showed good movement and lid attachment. It moved 0.75 mm with each blink and covered the pupil fully. Over refraction (OR) varied from pl to -1.00 with 20/20 vision. The initial left lens decentered inferiorly with slight apical pooling, mild mid-peripheral bearing and the pupil was fully covered. A second lens was applied to the left eye of the following parameters:
|BV Power:||pl /-3.00|
OR was unstable but showed approximately 20/20 acuity. The lens decentered inferiorly, but was aligned centrally. It also had significant inferior edge lift with bubbles. The following initial lenses were ordered:
Center thickness was ordered to be as thin as possible with a #1 blend, 7.7 front optic zone with a -3.00 carrier was requested OU.
RT returned two weeks later for a CL dispense. Spectacle acuity was 20/20 OD and 20/100 OS. Anterior segment was healthy. Lenses were applied to each eye. Both lenses showed full pupil coverage, slight superior decentration with 1 mm of movement with blink. There was mild mid-peripheral bearing and slightly excessive edge lift. Visual acuity was 20/25 OD, OS. There was no significant over refraction. The patient was trained on insertion and removal. He was instructed to wear the lenses for two hours for the first 2 days and then increase wearing time by 2 hours every day with a maximum wearing time of 10 hours. It was noted that future peripheral curves may be need to be steeper.
RT returned 2 weeks later in November 2000. He was complaining of discomfort and excessive edge movement. Patient reported an average wearing time of 1 hour with the maximum being 5 hours. Entering acuity was fluctuating at 20/30 OD, OS. Retinoscopy showed a variable reflex result of -0.50-0.75×115 and +1.75-0.75×180. It was apparent that the patient’s accommodation was unstable. OR was -0.50-1.50×100 20/20 vision OD, +0.25-1.50×165 20/30 vision OS. Anterior segment was healthy while the central fluorescein pattern was aligned there was excessive edge lift, OS>OD. For the right eye, a lens with tighter peripheral curves and more minus power was ordered. For the left eye, a lens was ordered with an increased cylinder span in order to lessen the movement.
blend #2 & -2.00 carrier
RT presented in December 2000 for a CL dispense and had lost the previous left lens. Entering VA with glasses was 20/20 OD and 20/200 OS. Anterior segment was stable and the lenses were applied. Acuities with lenses were 20/20 OD and a variable 20/25 OS. The right lens centered slightly superior and temporal, pattern was even and there was good lid attachment. There was full pupil coverage, good tear exchange and an occasional inferior bubble within the peripheral curve. The left lens was also lid attached but rode slightly high and slightly temporal, central pattern was even, pupil coverage was marginal, and there was good tear exchange. The lenses were dispensed with a plan to increase the OAD and steepen the peripheral curves of the left lens if it continued to decenter at follow up.
At the two week follow up, RT presented wearing lenses and was up to 13 hours of wearing time with the maximum being 15 hours. Lenses had been on for one hour that morning. The patient still reported that the left lens was moving around but stated that his vision was acceptable. Entering VA was 20/20 OD and 20/25 OS with an plano OR in each eye. RT showed mild injection and trace meibomian gland stasis. Otherwise, the anterior segment was unremarkable. The right lens centered well with good coverage and movement with a parallel central pattern. The left lens continued to lid attach excessively and showed significant movement with the blink. However, the central pattern was even and the pupil was fully covered. A CN bevel was added in office in hopes of decreasing the lid attachment. Binocular vision testing showed orthophoric posture at distance and 8 XP at near. Stereo showed +Randot forms and 50 seconds of arc. Even after modification, the patient continued to report left lens awareness. Therefore, a larger diameter was ordered.
|BV Power:||+5.50 back toric|
blend #2 & -2.00 carrier
RT returned for dispensing in January 2001. Visual acuity was 20/20 OD and 20/25 OS. OR was -0.25 OD and pl OS. The habitual left lens continued to ride high. When the new lens was inserted, there was less superior temporal decentration and the patient reported improved comfort and acuity. SLE showed inferior conjunctival staining consistent with the habitual lens dropping. Acuity with the new left lens was 20/20-2. The lens was dispensed and the patient was asked to return in two weeks.
At the two week follow up, RT had broken his right lens and presented wearing glasses. A spare lens was ordered and successfully dispensed the following week. At a one month follow up in March 2001, RT returned wearing his lenses with acuities of 20/20 OD and 20/30+OS. However, the left lens had a small scratch and was coated. The left lens was polished and vision improved to 20/20-2. RT was released for 6 month follow up.
RT was doing well with his contact lenses at his 6 month follow up. Vision remained good at 20/20 OD and 20/25 OS. RT continues to be followed annually. There have been no changes made to his RGPs and only minor changes to his spectacles.
Highly astigmatic patients may be labeled meridional amblyopes and may experience decreased vision with spectacles due to the marked difference in magnification between the two meridians.1,10
Some may experience symptoms of aniseikonia including blur, distortion, and asthenopia. GP toric lenses may provide an excellent alternative with improved vision in these cases. The selection of an appropriate GP toric lens is related to both the refractive error and corneal shape. Three options exist in GP torics: front, back and bi-toric lenses. Front toric lenses are indicated in patients with fairly spherical corneas but high refractive cylinder and are seldom used today due to greater availability and stability of soft toric lenses for these patients.11
However, for those with significant corneal astigmatism the selection of a back or a bi-toric lens is indicated and may be challenging to the general practitioner. The rule of thumb is that if the refractive cylinder is less than or equal to the corneal cylinder a bi-toric lens may provide better acuity. If there is morde refractive corneal cylinder than corneal astigmatism a back toric is indicated. A back toric lens will provide more cylinder power than exists in the base curve due to induced cylinder. The induced cylinder results from the index of refraction of the lens material.9,12,13
The spherical front surface may provide an advantage in its interaction of the lids over a toric front surface. However when the cornea and refractive cylinders are very close, a bi-toric lens with spherical power effect (SPE) design is most appropriate.12,14
SPE lenses are designed with a base curve span in diopters equal to the span in refractive power of the lens. This lens will function on the eye very much like a spherical lens without induced cylinder upon rotation. Again, this may provide more stable acuities when indicated. However, when there is significantly more or less refractive cylinder, a cylinder power effect (CPE) lens may be needed.12,14 Having the laboratory dot the lens in its flattest meridian so that rotation can be assessed may be necessary in these instances. This will help in problem solving if the patient experiences vision fluctuation. Should this occur, it may be necessary to truncate the lens to stabilize rotation.7
When selecting a GP with back surface toricity, one of the most common fitting philosophies utilizes the Mandel Moore fitting guide.12,15 For the flat meridian, the base curve is selected on K to 0.50D flat. In the steep meridian, the base curve is significantly flatter that the K reading. The more corneal cylinder the flatter the lens is fit in the steep meridian. Ultimately, most base curves spans are between two-thirds and three-quarters of the total corneal cylinder. RT’s Mandel Moore empirical results and his diagnostic fitting results were markedly similar.
In the case of RT, he required a different design for each eye. The right eye had corneal cylinder similar to refractive cylinder and thus an SPE lens was ordered. On initial inspection a similar design might be considered for the left eye given the equivalent cylinder in the spectacles and on the cornea, however vertex conversion results in markedly more cylinder at the corneal plane. Thus, a back toric lens was more appropriate. The back toric also was preferable given the increased mass and more anterior center of gravity. This allowed a more even front surface and lenticulation for improved interaction with the lid.
RT’s case was further complicated by his latent hyperopia, making lens selection more challenging and requiring additional minus in a second lens for the right eye. For the left eye, the increased cylinder and increased plus power resulted in the need for two significant adjustments. By increasing the base curve cylinder span, the GP was better aligned to the cornea and translated less. By increasing the OAD, there was a larger area of lenticulation allowing better lid interaction and a more stable fit.13,16
Highly astigmatic patients like RT are well served by GP toric lenses. These designs provide the potential for clearer more stable vision.2,7,8,9 Some patients with high astigmatism may be labeled as amblyopic due to problems actually related to aniseikonia from their glasses.10 Many of these problems can be eliminated with a well fit GP toric lens. RT is an excellent example of a patient who experiences improved visual acuity and depth perception by the use of the proper visual correction. In the case of this astigmat, a back surface GP toric.
References for Case 1
- Guyton DL. Prescribing cylinders: the problem of distortion. Surv Ophthalmol 1977 Nov-Dec;22(3):177-88
- Maltzman BA. Management of astigmatism with toric contact lenses. Int Ophthalmol Clin 1983; 23:33-56.
- Hall, DK, Ward JA, Edmondson W. Spectacles and custom toric hydrogel contact lenses: a comparison of vision. J Am Optom Assoc 1997; 65(11): 783-787.
- De Brabander J, Brinkman CJJ, Nuyts RMMA, et al. Clinical Evaluation of a custom –made Toric Soft Lens. Contact Lens & Anterior Eye 2006; 23: 22-28.
- Molinari JF, Boggess EA, Caplan L. Clinical Evaluation of Boston IV® Custom Toric Contact Lenses. J Am Optom Assoc 1986; 57(3): 216-218.
- Lee VW. The LADAR6000: results in highly myopic and highly astigmatic eyes. J Refract Surg. 2006 Nov;22(9):S980-2.
- Korb DR. A preliminary report on toric contact lenses. The Optometric Weekly 1960 Dec: 2501-2505.
- Molinari JF. High degrees of astigmatism: are hydrogels the answer. J Am Optom Assoc. 1986; 57(3): 216-218.
- Ellerbrock V.J. The Role of Toric Surfaces in Contact Lens Practice. American Journal of Optometry and Archives of American Academy of Optometry 1963; 439-446.
- Harvey EM. Amblyopia in astigmatic children: patterns of deficits. Vision Res 2007 Feb ;47(3):315-26.
- Gilbert ML, Kastl PR. Front Toric Gas Permeable Contact Lens Fitting for Residual Astigmatism. Contact Lenses 1988; 14(2): 73-74.
- Blackmore K, Bachand N, Bennett ES, Gas Permeable toric use and applications: Survey of Section on Cornea and Contact Lens Diplomats of the American Academy of Optometry. Optometry 2006; 77: 17-22.
- Goldberg, Joe B. Clinical Application of Toric Base Curve Contact Lenses. The Optometric Weekly 1962: 1911 – 1915.
- Kame RT, Hayashida JK. A simplified approach to bitoric gas permeable lens fitting. ICLC 1988; 15(2):53-58.
- Mandell RB, Moore CL. A bitoric lens guide that really is simple. Spectrum 1988 Nov; 83-86.
- Kastl, Peter R. Correction of astigmatism with rigid gas permeable lenses. Ophthalmol Clin N Am 2003; 16: 359-363.
By Dr. Ron Watanabe
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