By Dr. Eef van der Worp
A 61-year-old patient with moderate myopia comes in asking about bifocal contact lenses. He mentions that he is working on the computer very frequently and reads a lot in fine print.
What options do we have?
Hydrogel lenses do not provide the same optimal visual quality that GP lenses will give. In general: if superior vision is required, GP lenses are (or should be) advised as a first lens of choice. The major advantage that GP lenses have over most hydrogel contact lenses, intra ocular lenses and other refractive surgery procedures is that they are capable of alternating (often used as a synonym for translating) and use different optics for different tasks, very similar to spectacle glasses. This is also true for the so called simultaneous GP lens designs, and the main reason why simultaneous GP lenses work while simultaneous soft lenses are limited in visual performance.
For GP lenses, the most critical question that should be answered is whether a simultaneous or an alternating design is desired by the patient. This can be decided based upon several variables, but visual requirement is the number one consideration. The basic decision is fairly simple: if good and stable vision is required for far and or for near, a translating system is the first method of choice. The goal is to create a situation where upon down gaze most of the reading section (not necessarily all of it) is in front of the pupil while in primary gaze the pupil is minimally bothered by the near segment. They are particular suitable for presbyopes with reading additions over +1.50D and/or those who require excellent near vision performance. This is the only lens design option available that can create reading additions up to +3.50. They are also fairly pupil independent, as long as the overall diameter is increased accordingly.
Usually we ask the patient to choose ‘two out of three’. We can almost guarantee that we can fix them with the two most desired distances with the currently available lenses: near, intermediate or far. The third distance we can strive for, but can’t be promised. This way we know exactly what to aim for, and at least as important: the patient knows that it is not the ultimate magic lens that will solve all their problems instantaneously.
In this particular case the patient tells us he understands the limitations, but still wants us to do try to aim for correcting all three distances, although that near and intermediate are his main concerns.
Typically, two lens designs are best in providing “three-out-of three.” The first one would be one of the different trifocal lens designs that are available and the other one would be a triangle shape reading segment with intermediate zone for computer use. All of these can work if:
- Pupil size is limited
- The lens is positioned centrally on the cornea in primary gaze
Pupil size is relatively small in our particular subject, which is not unusual in elderly patients. Therefore we decide to try a trifocal lens design for this patient.
Central keratometry values are:
The trifocal lens on the right eye centers well on the cornea, translates optimal and reading is fine immediately with this lens. However, on the left eye, a low riding lens is seen with a toric (with-the-rule) fluorescein pattern (see below).
This despite the fact that the central corneal astigmatism seems to be moderate (2D). However, if we look at the topography map it can be seen that the astigmatism actually increases towards the periphery. While in the central 3mm zone the corneal astigmatism is registered as 2.17D, in the central 7mm this is 3.75D. The latter should be enough to consider a back toric lens design.
Corneal toricity may increase or decrease towards the periphery and thus influence fitting characteristics. Since GP lenses rest mostly peripherally, this influence should not be neglected either in GP lenses. Central cornea astigmatism is easier to deal with than limbal to limbal corneal astigmatism when fitting GP lenses. Corneal topographers can aid in assessing the degree of peripheral astigmatism. Researchers at the University of Brisbane, Australia looked at peripheral corneal astigmatism and found that in 38% of cases showed a spherical central cornea with a spherical periphery. In 21% of cases a toric cornea was found with a stable astigmatism towards the periphery. However, 15% showed a spherical central with a toric toricity, 22% a toric center with a decrease of astigmatism towards the periphery and in 4% the toricity increased towards the periphery. Although this was a small sample size, it clearly shows that different types of corneal astigmatism can be present. In additional to normal (GP and hydrogel) lens fitting, this is extremely important in orthokeratology practice and in bi- and multifocal lens fitting. In these cases it can predict the risk on lens decentration, and therefore it influences lens fit to a large degree.
For our patient, a new, back toric trifocal lens design is ordered for the left eye and the fluorescein pattern with this lens is much more equally distributed and follows the shape of the cornea much more closely. The lens centers perfectly on the cornea.
The use of corneal topographers can help us tremendously deciding on the lens design that is required, while avoiding the sometimes long, expensive and exhausting trial and error process. Peripheral corneal astigmatism is something to consider with every lens fit, but especially with bifocal lens designs. Good lens centration is usually required or desired, and this can be achieved easily with a full or peripheral back toric bifocal lens design.
By Dr. Jon Kendall
Hx: A 33 year old male has insurance now, his contacts are either dirty or worn-out. He has no problems currently, he just lost the left contact about a week ago, and now wears the OD only. He has no idea how old the prescription is.
ClHx: No idea of the brand he uses, he replaces them twice a year and uses Optifree as his primary solution. He does enzyme the contacts, if he find “those,” pills or drops. He has been wearing soft lenses for about 20 years.
Entering VAs were 20/50 OD and 20/400 @ five feet OS. The right contact looked good (but soiled) on the eye, and there were no external findings, other than some central staining on both corneas, with the right one being the more obvious.
Refraction and acuities were:
OD: -4.50 20/50
OS: -1.50 -0.50 x 80 20/40
Over-refraction on his contact did not improve VA for the OD.
The corneal mires were both distorted on Keratometry (topography was not available).
Both were approximately 43.50/43.00 @ 180
Verifying the softies for power they both appeared to be about -4.25.
I grabbed a pair of diagnostic GP lenses, spheres, that with fluorescein pattern indicated a decent fit and a resultant acuity of 20/25 for either eye. I used the information to design him a pair of standard curve GPs, while letting him wear the diagnostic lenses.
7.94 9.6 -4.75 for both eyes.
On follow-up the patient reported improved vision in both eyes, but noticed problems removing the left contact. Fluorescein pattern revealed a good fit for both eyes, but both were riding slightly
nasal, and there were some air bubbles under the left lens. Even thought the patient’s manifest refraction was stabilizing, as were his visual acuities (20/25=), and the patient was happy, I wanted to see if I could improve the fit and finish by fitting the pt with a back surface aspheric design. I find that with a distorted cornea, the aspheric designs usually gives a good fit. Being designed to fit slightly steeper, they tend to centralize the lens a little better than standard curves.
The patient tried the aspheric contacts and found them quite comfortable, but I still could not completely eliminate the small couple of air bubbles under the central portion of the left lens. With proprietary curves on many of our lab’s aspherics it makes it difficult to make small adjustments for a true custom fit. In this case, I ordered a lens with a smaller OZ, secondary curves about .7mm flatter than the BC, standard peripheral curves, and had the lab blend them heavily with tools of curvature about half way between the BC and SC.
The patient didn’t notice much difference with these new contacts, but they centered even better, which helps in the rehabilitation of the pt’s cornea (assuming no underlying disease) and the air bubbles were eliminated.
I could have had the pt wear spectacles until his corneas stabilized, but I feel I gave him the best acuity, the fastest I could, with an immediate refit of a GP.
Let’s say his corneas didn’t do well with no contact lens wear, and the distortion remained. It’s possible this pt may have keratoconus or PMD, and his corneal distortion is just part of his condition. By the immediate fitting of a GP we did this pt a real service. Unfortunately after five years of GP wear he was lost to follow-up.