MAKE A ‘NOTICEABLE DIFFERENCE’

What is JND and why is it so useful?

By Sue Corwin, CO, COMT, Jacksonville, FL

During refractometry, patients need to have a significant enough change in the lenses we present to them to notice a difference in their vision.

To determine the correct degree of change, consider using the “Just Noticeable Difference” (JND) rule: Show an amount of change equal to the denominator of the acuity, in decimal form (divided by 100). So, if the patient’s vision is 20/50, show a 0.50-D change. Some “rounding” might be necessary if the acuity denominator number does not represent a lens that you have in your trial lens set or refractor heads. For example, if a patient’s vision is 20/20, show a change of 0.25 D sphere. See Table 1 for additional examples.

The takeaway: Don’t bother showing a patient with 20/50 vision a quarter of a diopter change. It can frustrate you and the patient when he or she can’t tell the difference.

Be kind to yourself and to your patients by using the JND rule.

SURGICAL TREATMENT OF PRESBYOPIA

Patients who desire freedom from reading glasses have options

Presbyopia, or the loss of near accommodation, is a condition caused by the hardening of the crystalline lens. This common vision disorder comes with aging and is not preventable or reversible. Therefore, patients looking to move away from readers are left to consider surgical solutions.

Here is an update on the available options.

Kamra inlay

Kamra (CorneaGen) is a removable corneal inlay for the correction of presbyopia and blurry near vision. FDA approved in 2015, this 3.8-mm diameter disc with a central 1.6-mm aperture is made of opaque polyvinylidene fluoride. The device improves near vision by increasing depth of focus, which is achieved by creating a pinhole effect in the non-dominant eye. It is implanted within the cornea in a femtosecond laser-created pocket.

The ideal patient for this inlay has a manifest refraction spherical equivalent of +0.50 D to -0.75 D with >0.75 cylinder, and a near correction of +1.00 D to +2.50 D, according to the FDA. Central positioning of the device is an important factor in the success of the procedure, according to a 2012 study in Journal of Cataract and Refractive Surgery by Seyeddain et al. The inlay can be combined with both LASIK and PRK. If the inlay is removed, vision generally returned to the level the patient had prior to implantation of the device during clinical trials.

Intraocular lenses

For presbyopic patients undergoing cataract surgery, several multifocal, defractive, and accommodating IOLs are available.

Options include the AcrySof IQ ReStor +2.5 D and +3.0 D (Alcon), the Tecnis Multifocal and Symfony (J&J Vision) and Bausch + Lomb’s Crystalens and Trulign. These lenses expand a patient’s range of focus through different designs. For example, the ReStor +2.5 D’s Activefocus optical design features a central portion that is 100% dedicated to distance vision. Alternatively, the Tecnis multifocal features nonapodized defractive steps or rings on the posterior surface of the lens starting close to the optic’s center that continue toward the periphery. The Crystalens is an accommodating, single-optic lens with flexible hinges at the plate style haptic that facilitate anterior movement of the lens and help patients to focus on objects at all distances. The lenses have all demonstrated various levels of spectacle independence, patient satisfaction, ease of implantation and long-term capsular bag stability.

Keep in mind that these lenses require significant consultation to effectively match the patient’s needs with the appropriate option. Also, patients need to be aware of the potential for glare and halos, particularly when driving at night.

Refractive lens exchange

The approach with refractive lens exchange is similar to cataract surgery in that in involves the removal of the natural, crystalline lens. The difference is that the surgery is performed on a clear lens, which is then replaced with an IOL. Also, this permanent correction avoids the need for cataract surgery later in life.

As with cataract surgery, patient discussions should take place prior to the procedures to match the technology with his or her vision needs and to set proper expectations about outcomes and potential complications.

Monovision

With this well-established refractive procedure, the dominant eye is corrected for far vision and the nondominant eye for near. This is achieved via corneal refractive surgery or monofocal implant. However, this surgical procedure comes with limitations, including reduced stereopsis, blurred vision at night, and poor intermediate vision.

In the pipeline

The Presbia Flexivue Microlens is a corneal inlay that uses refractive add power and does not require the removal of any eye tissue. This will potentially allow surgeons to remove and replace it if the presbyopia advances and the patient requires a new prescription. This inlay has not yet received FDA approval.

Another procedure, presbyLASIK, uses an excimer laser to create zones on the cornea similar to the design of a multifocal lens. PresbyLASIK is an investigational procedure and has not been approved by the FDA.