Balanced - Lens Selection for the Monocular and Amblyopic
By Rialeigh Yoder, ABOC-AC
This course will help opticians understand what it means when the patient hands them a prescription that indicates "balance" for one eye. We will review lens options and other factors when designing and selecting a balance lens or occluded lens. Honing these advanced skills will allow the optician to create outcomes that often exceed patient expectations aesthetically and functionally.
Upon completion of this program, the participant should be able to:
- Explain the criteria when final inspecting a job with a balance lens.
- Specify a balance lens power to correct for physical abnormalities in the amblyopic eye.
- Describe potential solutions when the amblyopic eye has some acuity to provide better lifestyle adaptation for the patient.
- Use frosted lenses correctly as prescribed.
Rialeigh Yoder, ABO-AC, is the Optical Manager of Sea Eye Care in Norfolk, Virginia. She is a graduate of Durham Tech's Opticianry program and has worked in the optical field for 15 years in positions for a busy ophthalmology/optometry group to small private practice. She also gives new life to vintage frames and does repairs through Stumpy Lake Optical.
This course is approved for one (1) hour of CE credit by the American Board of Opticianry (ABO) for Technical Ophthalmic Level 2. Course # STWJHI072-2
This course will help opticians understand what it means when the patient hands them a prescription that indicates “balance” for one eye. We will review lens options and other factors when designing and selecting a balance lens or occluded lens. Honing these advanced skills will allow the optician to create outcomes that often exceed patient expectations aesthetically and functionally.
What is a "balance" lens? A balanced eyeglass prescription is used for cosmetic purposes more than corrective purposes to make both eyes look similar behind the lenses even though one eye has no vision and therefore no correction. When the prescriber writes "balance" on the prescription, it indicates that the eye with a balance lens has impaired vision or no vision. The balance lens should be made with equivalent power, thickness and magnification to cosmetically balance the eye's appearance behind the lens relative to the corrected eye.
When most opticians see a "balance" Rx, they simply check the box on the order form that says "balance" and get on with their day. But this need not be the case! When the optician offers options that produce better outcomes than the patient ever expected, they can be superheroes.
The balance lens power will be roughly equal to the power of the prescribed correction in the companion eye. When the companion eye has spherocylindrical correction, then the spherical equivalent power is used in the balance lens. Use the following formula to find the spherical equivalent: SPHERICAL EQUIVALENT = CYLINDER POWER + SPHERE POWER/2
The option "balance lens" is there to make the lab and optician's job easy and create an aesthetically pleasing result for the patient. The weight of the balance lens should result in a comfortable fitting triangle for the patient. The frame should evenly distribute the weight between the ears and nose for optimal comfort. A too heavy or even too light lens on one side will be off balance.
Typically, a balance lens (without specifications like the ones explained below) cost less from the lab and are filed with a different code, V2700, when creating an insurance claim for the patient, if applicable. And very important: If a balance lens is prescribed, the order should include impact-resistant material such as Trivex or polycarbonate. Impact resistance lens material is necessary to protect the better functioning eye for the preservation of the remaining vision. Patients should also be encouraged to wear their glasses full time for this protection.
In many cases, physical abnormalities of the eye and orbital appearance are present when a balance lens is prescribed. When designing a balance lens for these patients, we can create a "wow" factor. The design and power of the balance lens can be adjusted for better symmetrical appearance of the patient's features behind their lenses, thus increasing patient confidence and satisfaction.
Adjustments to the lens are based on magnification and prism displacement. Magnification is a product of power and shape factors. The power factor is based upon the lens power and the nodal distance. Increasing the back vertex distance aka BVD (which increases the nodal distance) increases magnification, and inversely reducing the back vertex distance reduces magnification. The shape factor is comprised of the base curve, lens thickness and refractive index. By increasing the base curve or the lens center thickness increases magnification and vice versa. In these situations, we will keep the vertex and lens material the same for the symmetry of the eyeglass' appearance. Power is the variable we will change when designing these lenses without incurring costs over a standard pair of glasses. The provider can also use the magnification change along one lens meridian. Manipulating these magnification shifts makes some areas of the orbit appear larger and others smaller. It can also adjust the angle at which it seems the eye sits. An aspheric lens design will decrease the overall magnification and works particularly well for patients with a high plus power as they want to avoid exaggerating the appearance of irregularities. An increase in lens thickness or base curve without a change in power will also create a small amount of magnification.
Prism displacement is based on the power of the prism and how far it is placed away from the eye. Increasing vertex distance will increase the power of prism compensations applied to the lens. The image will be displaced in the direction of the prism apex opposite of the prism base direction (Fig. 1).
We can utilize these two optical concepts to customize a balance lens for the patient. It is essential to consider the added weight or thickness as we make these adjustments. We want to achieve the best aesthetics while keeping the lens balanced for comfort. An excellent tool for this is Zeiss' 3D view of the lens pair in the Visustore before ordering. All variables can be added to the rendering so the optician can visualize the lens outcome before ordering.
How to use this information? The easiest way to deter-
mine cosmetic corrections is through real-life trial and
error. Place the patient in front of a mirror while sitting
down. Sit in front of the patient and move loose trial lens-
es over the amblyopic eye using the above table as a guide-
line. Add, subtract or move power as necessary until you
and the patient are satisfied with the results. The power
changes you determined should be applied to the distance
prescription in the eye that has a correction. For example,
if a +1.00 trial lens over an amblyopic eye provides the correct amount of magnification, and the correct-
ed eye is a -2.00, then the balance lens should be
ordered as a -1.00. This is a system, and everything
is in relation to the corrected eye.
RX: OD BALANCE
OS -3.00 SPH
PD 32.5/29 HT 14.5
VA: NLP (No light perception)
This patient has a right eye that appears slightly smaller, and the outside corner is tilted downward in comparison to the left eye. They have currently lost their glasses. After trial lenses, it is determined that with no correction on the left eye, the right eye matches the left in appearance with a +3.00-sphere lens and a -2.00 cylinder lens oriented at axis 130 (Fig. 4).
With the prescription, the
-3.00 OS lens is going to make
the OS appear smaller. This
must be taken into consideration. What was found using
the trial lenses should be added
to the prescription for the OS.
What is ordered:
OD Pl -2.00 x 130
OS -3.00 SPH
The patient is requesting a lined bifocal because it has worked well in the past, and they are not a heavy computer user. The add power stays the same to maintain the ledge appearance of the segment. The frame selection allows for the plastic to cover any edge thickness differences between the eyes, and the lenses will be similar enough in weight. If this was a thinner metal frame, the request could be made for a 0.5 mm greater center thickness OD to better match the overall thickness between OD and OS.
You can practice the ability to compensate for facial irregularities by finding a picture online with an irregular face. Use trial lenses to manipulate the image's appearance until the result looks more even and aesthetically pleasing. It is fun to practice, and when the time comes when you need this skill, it will make you less awkward while in front of the patient.
Frame selection is imperative when designing cosmetic-altering vision correction systems. Generally, the optician should guide the patient to choose a smaller frame size. The small size is for two reasons. The first is to minimize any lens thickness disparities. The second is to use the frame as another form of camouflage to distract from unwanted features. If eyebrow asymmetry is a concern, pick a frame that covers the eyebrows but gives the illusion of symmetrical eyebrows. Frames with a round shape can also be used when the desire is to create a more dramatic shift in eye position or tilt. More power or prism difference between the eyes is less noticeable in a small round frame compared to a large square frame. The exception to the smaller frame size is if the glasses use a tint, mirror or lens frost to conceal some of the outer adnexa. Ensure the frame or lens will conceal the areas of concern in these cases.
A plastic frame does a great job of hiding lens edge thickness. A neutral color will avoid drawing attention to the eyes. There are frame factors that are the most "ideal," and there are your patient's wants. These two factors do not always align. It is a delicate prioritization between an optimal frame shape and a style the patient enjoys and feels comfortable with wearing. Do not automatically say no to a frame style if you can see that a patient is excited about it. The best way to control the outcome and have the patient end up with a frame that looks good and will suit the optical needs is to present frames to the patient instead of having them peruse the boards without guidance. Have the patient sit at the dispensing table. You, the optician, then curate a selection of frames that match their style but is also an option based on the type of lenses for their unique situation. In an ideal world, this is how it works for every patient, not just the ones with amblyopia or monocular vision.
In adults with anisometropic amblyopia, the doctor might under correct the more ametropic eye to avoid ocular discomfort. Working closely with the prescribing doctor is vital when the patient has acuity in the amblyopic eye. There are times when a balance lens is used because the anisometropia between the eyes is so great that fully correcting one of the eyes would lead to such an image size disparity the patient would not adapt. Image size disparity is especially a concern in multifocal lenses where prism imbalance and image jump in the near portion of the lens causes diplopia. Sometimes the prescriber will reduce the correction in one eye to better balance the monocular vision or leave the one eye uncorrected entirely. When the patient has some acuity in the "bad" eye, and anisometropia is present, the prescriber will weigh giving the patient the best possible acuity with the possibility of inducing aniseikonia. You can either try correcting the patient fully in the lower acuity eye or lower the corrected visual acuity in that eye by bringing the prescription closer to the better eye. Binocular balancing gives up some clarity to produce a more comfortable vision correction system.
When a prescription comes across the dispensing desk with one eye marked as balance, or the power difference between the two eyes is so great that imbalance concerns surface in your mind, the place to start is to gather as much information about the patient as possible and then contact the prescribing doctor. When balance is prescribed, you need complete information from the doctor such as best-corrected visual acuity, if the balance is permanent, and if any occlusion is needed. On the other hand, if a patient presents you with a prescription with an imbalance, you will want to check with the doctor before filling a prescription. Before bringing the imbalance to the doctor's attention, it is helpful to have the patient's adaptation history, if available, along with the following:
- Best-corrected acuities of both eyes individually
- Age and activity level of patient
- Lifestyle priorities of patient
- Multiple pairs or one pair of glasses
- Aesthetic concerns If you have ordered a balance lens, use the following checklist for visual inspection of glasses:
- Nasal thickness allows for uniform nosepad adjustment on a metal frame, photochromic, tint or polarization matches in color and density.
- Anti-reflective treatment and mirror options match.
- Any visible segment is mirrored in position between the amblyopic eye and corrected eye.
- Lens mounting should create a laterally balanced product in weight and appearance with no unwanted differences in magnification, base curve or lens thickness between eyes.
Frosted and occluded lenses: There are three main reasons for lens frosting and occlusion--intractable diplopia which can only be cured by occluding one eye. In intractable diplopia (double vision) cases cannot be treated with prism since prism cannot create the necessary binocular fusion. The diplopia can be temporary or permanent. The second is when the patient wishes to conceal a physical abnormality of an eye with no vision. The third reason is occlusion therapy to treat amblyopia. Amblyopia causes decreased vision due to abnormal visual development in infancy or childhood. It can result from strabismus, anisometropia or visual deprivation. When a child's brain receives different images from each eye, due to unequal focus or eye misalignment, their brain circumvents double vision by suppressing, or ignoring, the image coming from the weaker eye. Rather than see two disparate images in the case of eye misalignment or try to fuse a clear image and a blurry image in the case of unequal focus, the brain can suppress vision from one eye. The result is normal vision development in the preferred eye, while the suppressed eye stops developing and can even lose vision, if not treated. The treatment consists of penalisation of the healthy eye to make the weak or "lazy" eye work harder. There are different levels of occlusion with the most extreme being patching applied directly to the eye blocking vision and all light from reaching the healthy eye to strengthen the weak eye in amblyopia. The use of occlusion lenses is to blur the vision of the better seeing eye and augment the vison in the amblyopic eye. The eye can be penalised for near, distance or all distances and is often combined with cycloplegia. In the past it was believed that amblyopia could not be corrected if not corrected by age 7. Now it has been shown that amblyopia can be treated in older children and teens up to age 17. Nearly half (47 percent) of the participants ages 13 to 17 who had never been treated before for amblyopia were successfully treated. (Scheiman MM, Hertle RW, Beck RW, et al. Randomized trial of treatment of amblyopia in children aged 7 to 17 years. Arch Ophthalmol 2005 Apr;123(4):437-47) According to this article Part-time occlusion therapy for amblyopia in older children, "… treatment of mild to moderate amblyopia, as little as two hours/day of patching may be adequate in the 7-12 years age group while in severe amblyopia, six hours and full-time occlusion are more effective than two hours occlusion therapy in this age group." (PubMed Central PMC) Most often, amblyopia occurs in one eye, the brain comes to rely on the stronger eye for visual input and can completely suppress the weaker eye over time.
When the doctor recommends occlusion, here is a list of the information you need:
- Is the occlusion permanent, temporary
- How much light is to be blocked
- Is all or part of the lens to be occluded
While most of us have used tape to occlude lenses, there is a better choice that won't potentially damage the AR coating, the press-on Bangerter Occlusion Foils or Bernell Cling Patch. Both are a press-on style occlusion and similar to Fresnel prism and can be applied in office. The material is cut to fit the back surface of the spectacle lens and applied using water. They are also removable but are more durable and will not cause damage to an anti-reflection coating.
Both options come in various levels of occlusion, from 20/20 to light perception only. If the use of occlusion is temporary, the most transparent level possible should be used. The goal is to stop the diplopia but obtain the most natural look while retaining light perception in the occluded eye to help with peripheral tasks. Metal frames with a traditional eyewire screw are the easiest to apply these types of press-on films.
If a tropia is present, using occlusion past 20/300 can lead to a greater tropia in the occluded eye. The best course of action is always to check with the prescribing doctor to verify the level of occlusion for the patient.
There are instances where a doctor will prescribe the occlusion only to occur at the nasal or inferior portion of the lens. You can cut the above water-applied options to fit this application or use magic tape.
Permanent lens frosting: If the lens frosting is to be a permanent solution, the frosting can be accomplished directly on the lens. Permanent frosting is an option that the optician can order from most labs. Typically, it is done by skipping the fining polish during the lens surfacing. If using a patient's current pair of glasses, a skilled optician can also do frosting in the office with simple supplies:
- Place the lens front surface down on a microfiber cloth on a steady surface.
- On the back side of the lens begin with wet sandpaper at a grit of 200 to 400 lightly rubbing in small circles.
- Frequently dip the sandpaper into more water.
- Continuing in small even circles, rub the back of the lens until it looks homogenous.
- Water lubricates the process. If there is too much resistance, then add more water.
- Use finer and finer sandpaper (going higher in number) with water until the lens looks evenly translucent and frosted.
- Be mindful that lens frosting will appear more opaque as it dries.
*If the occlusion is only to be applied on a specific portion of the lens; it can be masked using simple blue painter's tape during the frosting process.
If a tint is going to be used in conjunction with a frosted lens, be aware that the tint will appear more vibrant or brighter on the frosted lens. It is difficult to obtain a balanced appearance with tints on a pair of glasses where one lens is frosted; the dispenser should manage patient expectations. For sunglasses, a polarized lens can be frosted while maintaining a more cohesive appearance in the pair of glasses. The superior cosmetic appearance is due to the polarization film being laminated in the lens and not subject to the mechanical changes that frosting produces.
Mirrors are a great way to also conceal lens frosting and any facial features the patient wishes to reduce noticeability. A light gold flash mirror works wonders and looks good even indoors. The optician can use this in conjunction with designing a lens power for cosmetics or a frosted lens.
Long-term patching in adults: Some patients use patching instead of frosting to conceal a more significant part of the eye. Patches can be either attached to an adjustable or elastic band and sit directly on the eye, or they are made to fit onto the spectacle frame itself. The optician should consider the patient's preferred patch type during the frame selection. Make sure the patch works well with the frame and that there is enough clearance to provide a comfortable patch fit.
If the patient has had enucleation, the orbit of the removed eye tends to appear smaller and hollower. A decrease in bone density is associated with enucleation and radiation treatment, if necessary, after the enucleation. Some post-enucleation patients will have orbital implants that do a good job of mimicking the movement and appearance of the missing eye. Others may only have a closed lid or obvious missing anatomy. Be sensitive to the patient. Ensure they understand that cosmetic concerns are not trivial, and you are there to help. A more common but subdued issue that is similar is an eye that is on prostaglandin analogs for glaucoma and has subsequent periorbital fat atrophy. It may be necessary to make the appearance of that eye larger.
The following are two case studies. The first illustrates lifestyle influences on patient preference, and the second illustrates cosmetic occlusion:
Example 1: Fifty-nine-year-old male with post retinal detachment and scleral buckle in the OS while the
OD is normal. The full correction and corrected acuities are as follows:
OD -2.50 -0.25 x 097 BCVA 20/20
OS -5.50 -1.75 x160 20/40 with distortion
Add +2.50 OU
Before the retinal detachment, the patient was content in a premium progressive lens. He is a heavy reader and has a long commute to work. The first attempt with the new correction was the same premium progressive he had in the past. This resulted in strain and diplopia at near with “shadowing” around words. The next attempt was a balance lens of around -2.50 sphere OS. The patient reported that the strain and diplopia were gone, but he greatly missed his peripheral vision at distance while driving, even if it is reduced. The final solution was to make the OS lens not just a balance lens but a single vision lens with only distance correction. The solution gives the patient the peripheral vision he would like while driving but causes enough blur at near to force the patient to suppress the left eye vision while reading. This lens design works for this patient's lifestyle. Proper frame selection helped reduce the lens appearance difference between the two eyes. The patient left happy and will be a patient for life because he felt the optician custom made glasses specifically for him.
Another possible solution would be to make the patient a pair where the OS balance lens is the reading power in a single vision lens. This trade-off would not have addressed the patient's main concern for better peripheral vision while driving. The patient would have still missed out on the peripheral vision but may have liked the increased reading clarity, particularly in low light. The best way to test these possibilities is to correct the OD as normal and then remove any lenses in the glasses on the OS side. Then, using trial lenses in a Jannelli clip, try out some single vision powers to see what feels best for the patient. The goal is not always to find the best possible corrected acuity in both eyes but to find a solution that works for the patient's lifestyle and level of comfort.
Example 2: Thirty-one-year-old female has post-trauma injury to the left eye. Enucleation was performed, and the ophthalmologist fitted the patient with a prosthetic. The patient is not happy with the appearance of the prosthetic eye and wishes to do as much as possible to conceal the eye. You work together on frame selection and find something stylish that suits her face. Her right eye vision is 20/20, and she reports no glare or low-light visibility issues. After looking at lens and mirror demos in the office, as a team, the optician and patient decide to do an extra-dark photochromic lens (such as Transitions XTRActive or PhotoFusion Extra Grey) with a style flash rose-gold mirror. You talk to the patient and order the lenses without the left eye frosted to see if that provides enough concealment. You also assure the patient that you have options, and if more concealment of the damaged orbit is requested, you can frost the left lens in-office for her. This solution puts the patient at ease, and she knows the optician is listening to her needs in a non-judgmental tone considering cosmetics are a priority to the patient.
In conclusion, do what dispensing opticians do best, listen to your patient and ask questions such as:
- What tasks during the day are you having difficulty doing?
- What is your goal for your glasses and your vision?
- Can we solve your issues with only one pair of glasses, or do we need to talk about a system of eyewear to provide a complete solution?
- How do you want your glasses to make you look?
- Is anything bothering you about your glasses or vision currently?
Addressing the cosmetic issues will increase the patient's confidence and feelings of satisfaction when wearing their glasses. Work closely with the prescribing doctor when creating eyewear for a patient with partial or no vision in one eye. Remind yourself that vision happens in the brain, and it is personal. One size does not fit all. You can develop the skills to fashion eyewear that help the monocular and amblyopic patient look good in and feel good about their glasses. Hopefully, this article gives you more tools to better serve the community that trusts you with their vision and comfort.