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Refraction Tips

Refraction Tips

Prescribing for Myopia

Myopia most often appears in youth, will typically increase 0.25 to 0.50 diopters per year, and level off somewhere around the age of 20. There are exceptions, however. These include progressive (degenerative) myopia and individuals who become more myopic due to excessive amounts of near work (accommodative spasm).

Night Myopia

Occasionally, a patient may complain of poor night vision although little or no myopia is detected. Increasing the patient's spectacle prescription by -0.25 to -0.50 over the ususal binocular endpoint (2 clicks in the green) will take care of the patient's complaints if they are suffering from night myopia. Care must be taken to rule out other causes of "night blindness" such as retinitis pigmentosa and other degenerative conditions affected the retinal photoreceptors.

Pseudo Myopia

Some patients who spend a considerable amount of time focused at near develop a spasm of accommodation. The patient will present with the typical complaint of myopia: blur with distance vision. Because the patient has difficulty relaxing accommodation at distance, they will accept minus lenses to clear their distance blur. These patients will refract as myopes even though they may be emmetropic or even hyperopic. Treatment of the condition includes the use of plus lenses at near and vision therapy designed to increase accommodative flexibility.

Prescribing Spectacles for Myopia

For many myopes, the spectacle correction will be equal to the binocular endpoint of the refraction (exceptions include pseudo myopia and night myopia). When changing a prescription, the doctor must decide whether or not to prescribe the full amount of the change. In general, with spherical myopic corrections, there is minimum adaptation required and the full prescription may be given.

Remember that spectacles for myopia will require a higher prescription than contact lenses for the same patient. This is very important for spectacle corrections greater or equal to -4.00D. A spectacle prescription above -4.00D should include the vertex distance at which the refraction was performed (write the VD alongside the refractive error on the prescription pad).

Prescribing for Hyperopia

Unlike myopia, symptoms for hyperopia will depend on the magnitude of the refractive error and the age of the patient. Since hyperopia can be compensated for by accommodation, the patient's accommodative status will effect the patient's ability to compensate for hyperopic refractive error. For younger (pre-presbyopic) patients, the symptoms of hyperopia will depend on the degree of refractive error and the amount of near work the patient is required to perform. Symptoms may include: headaches, eyestrain, intermitent blur at distance and/or near. Fatigue may be reported with reading. Vision may fluctuate throughout the day. As the patient ages and accommodative ability changes, the symptoms will include blur at both distance and near. For an absolute presbyope the blur will be constant. The amount of reduction in visual acuity will be similar to that of the uncorrected myope.

Like the myope, the hyperope who over-stimulates the accommodative system may suffer from a spasm of accommodation. The symptoms will mimic those of the uncorrected myope. The patient may accept minus lenses due to an inability to relax the accommodative system. These patients require plus lenses at near and vision therapy to relax the spasm of accommodation.

For younger patients, plus lens acceptance will determine how much of the refractive error is to be prescribed. It is unusual to prescribe the full amount of correction for pre-presbyopic hyperopes.

Prescribing for Astigmatism

When astigmatism occurs in conjunction with spherical refractive errors, the doctor must decide whether the patient's symptoms are due to the uncorrected spherical refractive error or the uncorrected astigmatism. Uncorrected astigmatism can cause symptoms of blurred vision, headaches, asthenopia, and nausea. The severity of the symptoms will vary with the magnitude of the astigmatism. Patients may report difficulty with reading. Patients may also report monocular double vision or the presence of ghost images.

Younger patients may be able to compensate for their uncorrected astigmastism by accommodating within the interval of Sturm. This may work if the amount of astigmatic error is low (0.50 to 0.75). This will become more difficult as the patient ages. If the patient has with-the-rule astigmatism, squinitng may also help to improve vision.

For astigmatic refractive errors in the absence of myopia or hyperopia, correction will generally be required for astigmatism exceeding 0.75D. For lower amounts, the patient's symptoms should be taken into account when prescribing. When astigmatism occurs in conjunction with myopia or hyperopia amounts as low as 0.25D may be prescribed.

When astigmatism is found during the refraction, the amount should be compared to the amount expected using Javal's Rule and the keratometry readings. Care should be taken when changing the patient's axis of correction for astigmatism. The greater the amount of the astigmatism, the more sensitive the patient will be to changes in the axis. Astigmatism correction may require adaptation. Your patient may report that vertical or horizontal surfaces appear curved rather than flat. This will resolve as the patient adapts to the correction.It is easier for most patients to adapt to corrections at axis 180 or 090. Corrections for oblique cylinder will be the hardest ot adapt to. Some doctors will bias the axis toward 180 or 090 when changing the axis or prescribing cylinder correction for the first time.

The Trial Frame

Although automated refraction units have gained popularity in optometric offices today, the trial frame remains the best method for more complex refraction cases (ie. post-refractive surgery and low-vision patients). A new prescription or a change in prescription must always be compared to the old glasses, preferably with the proposed change held over the old glasses. Unfortunately, many patients will reject proposed changes once they are back in their habitual eye and head posture with their old glasses in place. Thus, a trial frame offers the advantage of allowing the patient to sit comfortably as they normally would during the refraction, whereas refraction by phoropter is more restrictive. The trial frame also allows for more fine adjustments of vertex distance and cylinder powers in patients with low vision.

When you see a low vision patient for the first time, it is best to assume that whatever is in the old glasses may be incorrect. This is especially true if he or she lives in a nursing home where there is a greater chance of the patient having someone else's glasses. In other cases, patients may have had cataract surgery but continue to wear their old glasses.

With high astigmatic patients, begin by using a hand-held ±0.50 Jackson Cross cylinder lens for the cylinder power and axis check. This additional ±0.25 D in the cylinder enables the patient to more easily distinguish between the two lenses. When it is difficult to obtain a definite cylinder axis, you can let the patient select the axis in the trial frame by asking them to turn the cylinder knob themselves until the best vision is achieved.

Many low-vision patients become very nervous when vision out of one eye is completely occluded during the refraction. The clinician can help ease the patient by blurring the non-refracted eye rather than occluding it. Start with a +10D lens, and explain to the patient that their vision will purposely be blurred during this portion of the refraction.

Before dispensing the final lens prescription, have the patient walk around or watch television in the waiting area with the trial frame worn for at least ten minutes.

Using A +10D Occluder To Measure Strabismus

The standard method for measuring strabismus in cooperative patients is the alternate prism cover test. This test is performed utilizing an opaque black occluder to cover 1 eye at a time. It is sometimes useful, however, to observe the alignment or motion of the eye behind the occluder. For example, some patterns of nystagmus may change in an eye when it is occluded. Also, the three components of dissociated vertical divergence (elevation, excyclotorsion, and abduction) may only be evident under cover. Finally, latent ocular misalignments can be documented for the purposes of publication or oral presentation if the occluded eye is visible.

A high plus (+10 D) translucent occluder used in place of the standard occluder permits a view of the eye under cover while providing sufficient dissociation to bring out a latent deviation. with a modest degree of clarity. According to a research study performed by Buron J. Kushner, MD at the University of Wisconsin Hospital and Clinics, alternate prism cover testing and prism under cover testing utilizing the +10 D occluder yields results comparable to the standard opaque occluder. In addition, the magnification of the +10 D occluder enhances the ability to study subtle movements of the deviating eye. An overview of Kushner's study may be found in Archives of Ophthalmology. Vol. 118 No.8, August 2000.