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Contact Lens Over-Refraction

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Title: Contact Lens Over-Refraction
Authors: Noell Acord, OD; David Meyer, OD, FAAO
Date: 6/30/22
Keywords/Main Subjects: Over-refraction, phoropter, spherocylindrical over-refraction, contact lens rotation, vertex power

Description of Case: This paper outlines the definition of an over-refraction and its clinical uses. It discussed spherical vs spherocylindrical over-refraction and practical applications to this knowledge. Multifocal over-refractions are covered along with vertex calculations. This outline provides important concepts for improved contact lens patient care.

Contact Lens Over-Refraction

Common practice for contact lens fittings and follow-ups is to perform an over-refraction (OR) over the contact lens to reach a maximum visual outcome. Common decisions regarding over-refraction are whether to use the phoropter or not, or whether to perform a spherical over-refraction (SOR) or spherocylindrical (SCOR). Special considerations may be taken for different kinds of contact lenses.

Loose Lens vs. Phoropter

Contact lens over-refraction is commonly completed with loose lenses in free space. In cases where a patient is happy and seeing well, the most efficient mode of over-refraction is with loose lenses or a flipper containing various low-powered loose lenses. If a patient is being corrected for distance-only in both eyes, each eye can be over-refracted individually, starting with showing more plus over the uncovered eye. If the patient prefers the plus or does not notice any change in vision, the plus over-refraction is added. This is continued until the maximum plus for maximum visual acuity (MPMVA) is achieved. The same is done with the following eye. The results are then shown binocularly to ensure acceptance of refractive change and binocular comfort.

In cases where vision is not optimal, or a SCOR is expected, an over-refraction using the phoropter is more efficient than flipping through many loose lenses. An example where an over-refraction using a phoropter would save significant time over loose lenses is in the case of a new specialty lens fit where a high-power OR may be anticipated. From an exam flow standpoint, using the phoropter for an OR makes more sense as retinoscopy is often performed in the phoropter over the diagnostic lenses to cut down on refraction time. However, it is important to note that the phoropter may induce proximal accommodation and often gives less of a “real-world sense” compared to viewing objects in free space. After an over-refraction in the phoropter, it is advised to confirm such findings in free space with loose lenses in a trial frame both monocularly and binocularly. Depending on the patients’ presenting symptoms, taking the trial frame outside of the exam room so the patient can look out a window or look at a target they were struggling with may potentially prevent another follow-up visit.

Spherical vs. Spherocylindrical Over-Refraction

Most contact lens over-refractions will be spherical (or nearly spherical). Cases where a SCOR may be appropriate include:

1. Over a scleral or gas-permeable (GP) lens where BCVA is not achieved with spherical power only
2. Over a scleral or gas-permeable lens if it is the intention to change from a spherical to a toric lens
3. In rare cases, over soft contact lenses.

If a SCOR is found over a spherical powered scleral or GP lens, this power can often be added in the form of front surface toric (FST) power. Front surface toric power is not available for hybrid lenses, thus only spherical equivalent power can be added to the lens itself, or spherocylindrical specs can be worn over the contact lenses.

In the case where a soft contact lens, spherical or toric, is not achieving expected visual outcomes, it is most appropriate to recheck manifest refraction and to assess the fit of the contact lens for lens rotation and stability. After these steps have been checked, if vision is still not optimal, an SCOR may be done. An example where this may be helpful is the case of a patient with a low amount of astigmatism in their spectacle refraction (i.e. 0.50 DC).  These patients are often put into spherical lenses of the appropriate spherical equivalent as most toric lenses do not correct for lower than 0.75D astigmatism. In a visually sensitive patient that is unhappy in spherical lenses, a SCOR may elicit a positive response with slight cylindrical over-correction, and this patient may do better in a toric lens design. Most visual issues with soft-toric contact lenses can be fixed by adjusting for lens rotation and switching to a lens design with greater on-eye stability. These changes tend to be more useful in finding the source of the vision problem than doing a SCOR over the soft contact lenses.

If a SCOR is found over a toric lens of any kind, an oblique cross-cylinder calculation must be performed if the over-refraction power is to be added to the power of the existing lens. The exception to this is if the axis found in the OR matches the axis in the wearing lens and the lens shows no rotation. In this case, the OR can be directly added to the existing power. Clinically, the most efficient way to determine a cross-cylinder OR is to use an oblique cross cylinder calculator¹. In theory, a lensometry technique can also be used for cross-cylinder calculations. To do this, place the sphere, cylinder, and axis powers of the diagnostic lens into a trial frame. Next, place the resultant SCOR on top, and then use the lensometer to read the new lens power. However, this technique is not commonly done.

Lens rotation must be accounted for when determining new lens power. The acronym LARS (left-add/right-subtract) is used to help compensate for lens rotation. This rule is used with the assumption that the final lens will rotate the same amount as the diagnostic lens being evaluated. If the lens is rotated LEFT (clockwise), then ADD the amount of rotation to the axis of the subjective refraction. If the lens is rotated RIGHT (counter-clockwise), then SUBTRACT the amount of rotation.

For example, you have a patient with spectacle refraction of -1.00 – 0.75 x 180 in the right eye, wearing a soft toric lens of the same prescription.  If the lens is rotated 10˚  to the left, you will add 10 to the spectacle Rx axis for a final contact lens power of -1.00 -0.75 x 010.

Another example:

• You perform a SCOR over a scleral lens on the right eye with front surface toric power of -3.00 -1.25 x 180.
• You find an over-refraction of: plano -0.75 x 150.
• The lens is rotated 15˚ to the right.
• Should you wish to incorporate the over-refraction, use an online cross cylinder calculator. You will need to enter the current contact lens power, the over-refraction, and the degree and direction of rotation. The resultant contact lens power to order will be calculated for you.
• In this case, the final lens power would be -3.25 -1.50 x 165.

Multifocal Lens Over-Refraction 

Loose lens over-refraction should be used over multifocal contact lenses to avoid inducing pupillary or accommodative changes that can be elicited behind a phoropter. Additionally, refracting under binocular conditions allows for the most natural environment. For multifocals, it is particularly important to find the highest plus power for clear distance vision, as any additional plus added to the distance power will also help near vision. More plus may be found in the OR than expected for various reasons. This commonly occurs if the patient was over-minused in the spectacle refraction or the CL power was not vertexed properly. Hyperopes may accept more plus in a contact lens than expected. Keep in mind that certain multifocal lens design characteristics may provide more distance minus power than expected based on vertexed spectacle refraction. This has been shown to be the case in some of the newer, front-surface, center-near aspheric multifocal designs.²

After the most plus distance OR is found, the “best balance point OR” is used to help solve remaining visual complaints. Patient complaints should guide this approach. For example, if complaints are at distance, begin by introducing small amounts of minus in 0.25D steps over the dominant eye. If issues remain, trial adding minus to the non-dominant eye. If the patient is still unhappy, try reducing the add in the dominant eye. If the patients’ complaints are at near,  follow a similar approach by adding plus to the non-dominant eye and move to the dominant eye if necessary before adjusting the add power. As always, refer to the specific manufacturer fitting guide for troubleshooting.

As mentioned above, for multifocal fits, visual performance in binocular conditions is emphasized, thus over-refraction is best done binocularly. Monocular acuity, however, gives an idea of what each eye is contributing to at each distance. Thus, monocular OR is helpful in troubleshooting.

Vertex Power

Vertex power should always be kept in mind, especially for new contact lens fits. Vertexing spectacle refractions prior to selecting a new contact lens power will save unnecessary time in the over-refraction later on. A true “over-refraction” will rarely need to be vertexed, except in cases of an OR over a new diagnostic specialty lens fit. At follow-ups for specialty lens visits and for soft lenses, the over-refraction should be much closer to optimal and thus not high enough to need to be vertexed. Any meridian over +/- 4.00D should be vertexed.

Online vertex calculators are available, or a clinical estimation as follows can often be used:

The exact vertex calculation is:

Fv = F/1-dF

Fv is the vertex power, F is power in one specified meridian, and d is the vertex distance in mm.

1. SpecialEyesqc. https://calculators.specialeyesqc.com/over-refraction-calculator.php
2. Quinn, Thomas. “Prescribing for Presbyopia Over-Refracting Over Multifocal Contact Lenses.” Clspectrum.com, May 2013, https://www.clspectrum.com/issues/2013/may-2013/prescribing-for-presbyopia.

Identifier: Moran_CORE_126071

Copyright: Noell Acord, OD; David Meyer, OD, FAAO, ©2022. For further information regarding the rights to this collection, please visit: http://morancore.utah.edu/terms-of-use/