Ординатура / Офтальмология / Английские материалы / Practical Ophthalmology A Manual for the Beginning Ophthalmology Residents 4th edition_Wilson_1996

OO Chapter 5: Retraction

Cycloplegia *

V; Binocular balance is achieved simply by the monocular findings when full cycloplegia has been used. It is important to ascertain that cycloplegia is complete in both eyes if this method is to be used. Residual accommodation can be determined by asking the patient to look at a distance target while you neutralize the reflex (the standard way of performing retinoscopy), and then asking the patient to look directly at

'' the retinoscope light while you note if the reflex is moving more

"with" your sweep. If additional "with" movement is noted, accommodation has not been fully reduced.

Near Point and Reading Add

,trol lever on the front that allows you to adjust the front tilt so that the

'lb select the appropriate reading add, follow the steps outlined below:

1.Determine the accommodation requirements for the near vision task. For example, reading at 40 cm would require 2.50 diopters of accommodation (1/0.4 m = +2.50 D).

2.Measure the accommodative amplitude independently for each eye. A simple way to do this is to have the patient fixate on a reading target (eg, a 20/25 line of print held at 40 cm) with each eye. Stimulate accommodation by placing successively stronger minus spheres until the print blurs. Then relax accommodation by using successively stronger plus lenses. The print will clear, but as successively stronger plus lenses are added, the onset of blurring will again be noted. The sum of the two lenses used to reach the blur points is a measure of accommodative amplitude. For example, if the patient accepted -3.00 D to blur (stimulate accommodation) and +2.50 D to blur (relax accommodation), the amplitude would be 5.50 D.

3.From the measured accommodative amplitude, allow one half to be held in reserve and consider one half to be the available

accommodation. Other techniques for more precise measurement of accommodative amplitudes and for measurements of binocular amplitude of accommodation are beyond the scope of this text, but in general a consideration of both the NPA and the patient's age will give a good prediction of the add that will be needed. Average accommodative amplitudes for different ages are summarized in Table 5.4.

4.Subtract the patient's available acconiTnodation (step 3) from the total amount of accommodation required for the task at hand

(step 1) for the tentative power of the add that will be needed.

5.Add this extra plus power (the reading add) to the sphere of the distance refractive correction already in the refractor and again

determine the patient's near visual acuity. If you are using a trial frame, make sure you have placed the distance sphere lens in the appropriate slot on the back of the frame, and the cylindrical lens in the front, as you would normally do for determining the distance correction. Put the appropriate plus lens as an additional lens /;; front of the lens combination already in place.

6. By moving the reading material closer to and then further away from the patient, determine that the add allows the patient an adequate range lor seeing near work clearly.

88 Chapter 5: Refraction

Table 5.4 Average Accommodative Amplitudes for Different Ages

*Counting back from age 40, accommodation increases by 1 D for each 4 years. Beyond age 40, the decrease in accommodation is somewhat more rapid. From age 48 on, 0.5 D is lost every 4 years.Thus, one can recall the entire table by remembering the amplitudes at age 40 and age 48.

Guidelines for Prescribing Glasses

• ; ) , lb help you avoid many common problems and get started with proper prescribing habits, this section presents general guidelines and rips on prescribing tor myopia, hyperopia, and astigmatism as well as for presbvopia. Consult the detailed texts on this topic listed at the end of this chapter lor more complete discussions than can be supplied here.

General Prescribing Guidelines

•Don't prescribe minor changes (less than 0.25 D).

•Specify vertex distance for lenses with power of greater than or

•Check the final eyeglass prescription using the duochrome test.

•Put the prescription in a trial frame and let the patient walk

around in the waiting room if you think the prescription might be questionable, or if this is a first prescription, or if there is a change from the previous prescription of more than 0.50 D.

•Check and recheck your prescription for accuracy. Errors are often made when copying the prescription from the refractor or trial frame into the medical record and onto a prescription pad.

Prescriptions for Myopia

The typical presentation of uncorrected myopia is blurred distance vision with good near vision. Patients often state that they need to squint to see objects far away.

•In general, give the manifest refraction for best acuity and do not overcorrect. Young myopic patients will often prefer more minus power in the manifest refraction than they need for best acuity, because the additional minus enhances the contrast of the dark test letters on the light chart background. During the subjective refraction, to help prevent overcorrection, ask the patient if the letters are actually clearer and if detail is more easily seen, or if the letters are just darker and smaller.

•Use the results of cycloplegic retinoscopy and cycloplegic refraction to avoid overcorrecting.

•For adult myopic patients over the age of 40, make sure that increasing the minus power does not induce presbyopic symp-

uncorrected astigmatism may also complain of a bifrontal headache with prolonged near work. Observe the following prescribing guidelines in cases of astigmatism:

• For children under the age of about 4, prescribe the full astigmatic correction and advise full-time wear. In general, if more than 1.00 D of cylinder is detected, glasses are indicated.

•Be cautious about changing the axis of the astigmatic correction, especially in adults, because such changes are often poorly tolerated. Asking the patient to walk around for a few minutes wearing the prescription in a trial frame is helpful in this situation.

•Ascertain that any new astigmatic correction produces significant improvement in both distance and near acuity.

•Be wary of introducing new astigmatic correction in adults, as it may produce intolerable distortions for the patient even if measurable acuity is improved.

•Advise patients that there may be a period of adjustment to the new correction.

Prescriptions for Presbyopia

The predominant symptom of uncorrected presbyopia is diificulrv reading at near. Patients with early presbyopia also may report that they need more light to read and that they can read well in the morning, but not at night after a long day of accommodative effort (close work). Patients experience difficulty reading fine print, such as a telephone directory, and threading a needle. Very early in presbyopia, patients note that their eyes are slow to focus on near print, and then slow to change focus as they look at more remote objects. The usual onset of presbyopia is approximately between 40 and 45 years of age, but presentation may be younger in undercorrected hyperopes and older in undercorrected myopes. The guidelines below can be useful in determining presbyopic correction.

•Myopic patients with refractive errors under-3.00 D may not complain of difficulty widi near tasks because they may routinely remove their glasses for reading. Remember to ask about this habit, because these patients may have symptoms with accom-

modative effort if they wear their correction full time.

•Various medications can exacerbate presbyopic symptoms. Some of these are barbiturates, tricyclic antidepressants, and antihistamines and decongestants (including nonprescription medications).

pter 3: Retraction

•Bifocals with an add of +0.75 D or less are rarely necessary, although they may be required for specific tasks, such as seeing a music stand.

•Alternatives for the presbyopic emmetropic patient are single-

vision readers, bifocals with piano tops, and half glasses.

•Alternatives for presbyopic myopic and presbyopic hyperopic patients are separate glasses for reading and distance, and bifocals. Myopic patients also can remove their glasses or use half glasses when reading.

Several bifocal and trifocal styles are illustrated in Figure 5.19. In general, the type and the placement of the bifocal or trifocal segment is best left up to the optician, who will discuss the various options with the patient. The patient should be encouraged to tell the optician about specific near vision requirements, such as the need to view a computer screen at a particular distance. Most patients have heard about "bifocals without a line," by which they may mean either blended bifocals or one of a number of the progressive addition lenses (see Figure 5.19 H). These lens styles have no visible demarcation line and not only are more cosmetically acceptable for many patients, but also eliminate much of the image jump, or displacement, that may occur as gaze is shifted from the distance to the near segment. AltJiough the patient may ask you for a prescription by brand name (eg, Varilux) the available progressive bifocals are comparable, and die decision for the one used is often left to the optician. Because the correct fitting of these lenses is crucial to successful use, the fitter's personal expertise may be as important as the choice of the add power itself. Blended bifocals are less expensive than progressive-addition lenses, but they have an area ot blur where the near and distance segments join. This is at approximately the same position that die patient might experience an image jump as fixation shifts from one segment of the bifocal to the other.

Trifocals may be useful for patients who have intermediate distance needs, such as computer use. Other special-need adds are sometimes used for particular purposes, such as reading music, reading blueprints, and viewing museum exhibits. Try to duplicate the special-needs situation when testing. It is useful to ask the patient to measure the distances for the tasks for which the glasses will be worn. This will allow you to determine a more appropriate add.

G H

Figure 5.19 Common multifocal lens styles. (A, B) Executive bifocal and trifocal. (C, D) Round-top bifocai and trifocal. (E, F) Flat-top ("D" segment) bifocal and trifocal. (G) Invisible, or blended, bifocai. (H) Progressive addition multifocal lens.

(1) Distance correction. (2) Corridor of increasing plus power. (3) Maximum plus power of near segment. (4) Zone of significant distortion. -'•>;; ; ';- •"•: •

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94 Chapter 5: Refraction

Optical Fitting Considerations

tacle lens is approximately 8° forward compared to the bottom of the spectacle lens.

Interpupillary Distance

More than 90% of adults will have interpupillary distances between 60 and 68 mm. This measurement is not exactly the same as the geometric interpupillary distance, which is of importance only when highpower lenses are being fit. If lenses over 4.00 D are decentered, prism will be induced, which may be sufficient to affect binocular viewing. Usually the nose is midway between the two pupils, but occasionally there is significant facial asymmetry. If this is the case, special fitting adjustments must be made. Instructions for measuring binocular interpupillary distance are presented in Clinical Protocol 9.2 at the end of Chapter 9 in this manual. More detailed discussion on IPD determination and optical centration measurements can be found in the reference books listed at the end of this chapter.

Pitfalls and Pointers

The examiner's eyes and the patient's eyes must be properly aligned with each other for retinoscopy. If they are not, the examiner may develop back or neck strain after repeated examinations. In addition, if the eyes are misaligned, aberrations from the patient's lens, glare, and extraneous reflections will interfere with the accuracy of retinoscopic measurements.

Remember that retinoscopic "with" motion indicates the need for plus lenses to be added, whereas "against" motion indicates the need for minus power to be added.

If an irregular reflex or a very dull reflex is noted in retinoscopy, suspect high myopia or hyperopia, keratoconus, corneal warpage syndrome, other surface irregularities, or a cataract.

Although retinoscopy and subjective refraction seem to be difficult techniques to perform, with practice they become manageable. Don't be discouraged if tfiey seem bewildering at first.

Suggested Resources

Appleton B. Clinical Optics. Ophthalmic Technical Skills Series.

Thorofare, NJ: Slack; 1990.

Bannon RE. Clinical Manual on Refraction With the Ultramatic Rx Master Phoroptor Refracting Instrument. Buffalo, NY: Reichert Ophthalmic Instruments, Inc; 1979.