2020–2021 BCSC Basic and Clinical Science Course™

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2020–2021 BCSC Basic and Clinical Science Course™

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3 Clinical Optics

Introduction: Quick-Start Guide to Optics and How to Refract

Part 2: How to Refract

Step 1: Perform the Preliminaries

Check Visual Acuity

We use eye charts consisting of optotypes (letters or pictures) to test visual acuity. If a patient’s visual acuity is known from a prior examination, start at that line of the eye chart. Otherwise, first cover the left eye, and ask the patient to read the 20/40 line on the eye chart with the right eye (Figure I-21A). Go up or down the eye chart to find the line your patient can just barely read. Then cover the right eye and repeat the process to measure the acuity of the left eye.

Figure I-21 Eye charts for testing visual acuity. A, ETDRS-type chart with Sloan letters; B, Allen Symbols (not recommended).

(Part A Courtesy of Precision Vision; part B courtesy of Vision Training Products.)

Many patients will balk if asked to “Read the smallest line you can.” You may get the response “But Doctor, I can’t read the smallest line.” Expect patients to slow down as they reach the line(s) they can just barely see. Checking visual acuity well takes more time than you would imagine. Think of it as an opportunity to observe your patient’s vision, much as a neurologist carefully watches a patient walk into the exam room. Encourage patients to guess at letters they can barely see.

It is traditional to perform ophthalmology examinations first for the right eye, then for the left eye. Get into this habit to minimize confusion as to which findings go with which eye. Notice systematic errors at the beginning or end of the lines of the eye chart, which might suggest a visual field defect. Craning of the neck may indicate an attempt to cheat in cases of amblyopia (“lazy eye”). “Searching” may suggest a central scotoma (blind spot). Discourage squinting!

Modern picture optotypes, such as the LEA symbols (circle, square, apple, and house) or the “HOTV” character set (see Figure 3-6D, E) are preferred over traditional picture optotypes (Allen symbols: telephone, hand, bird, car, birthday cake, and horse and rider); the latter remain distinguishable even with a great deal of blur and may be confusing (Figure I-21B). Few children recognize a dial telephone these days!

If visual acuity is subnormal, try a pinhole occluder, a device with 1 or more small apertures used to reduce the eye’s effective aperture (Figure I-22). Note: Best-refracted vision may be better than, worse than, or the same as pinhole-corrected vision! Failure to improve with a pinhole does not exclude refractive error and does not justify omitting the refraction.

Figure I-22 Pinhole occluder.

For example, pinhole vision is typically worse than best-corrected vision in cases of dense cataracts and in many cases of macular degeneration. Conversely, when looking through a pinhole occluder, patients with early, inhomogeneous cataracts may improve dramatically but may fail to achieve this acuity with spectacles. Using the single pinhole that is provided in the phoropter accessory wheel, a patient may have difficulty finding the eye chart.

If the patient has distance glasses, there is rarely any reason to check the visual acuity without them, unless the patient requests it or acuity with the existing glasses is very poor. A patient may inadvertently use reading glasses while attempting to see at distance!

Check visual acuity at near (ie, reading vision) routinely only in the following cases: potential presbyopes (with age-related loss of near vision), latent hyperopia suspects, patients (especially children) suspected of amblyopia, and patients with a near-vision or an asthenopic complaint.

Asthenopic complaints are various symptoms associated with accommodative effort—that is, with the ability of the eye to increase the optical power of the crystalline lens.

Asthenopic symptoms include fatigue, headache, “eyestrain,” double vision, fluctuating visual acuity, and lengthening of the clearest reading distance. They suggest hyperopic refractive errors, presbyopia, “accommodative spasm,” or the possibility that the patient’s existing glasses contain a stronger myopic correction (or a weaker hyperopic correction) than is necessary. These topics are discussed in greater detail in Chapter 4.

In amblyopia, near vision may be better than distance vision. However, many patients, especially children, seem to see better at near only because they are permitted to hold the near-vision testing card closer than the calibrated distance (typically 14 inches, or 40 cm). Such compensations are impossible at distance.

Patients over the age of 40 or so should routinely be checked for presbyopia. It is better to allow such patients to hold the near-card at their own preferred reading distance (with or without reading glasses, according to the patient’s typical reading habits), and record the smallest legible line, and the distance at which the patient chose to hold the near-card, than to enforce the standard reading distance. Use “Jaeger notation,” optotype size in “points,” or both to record the patient’s near vision.

Obtain a Visual History

As in any other patient care encounter, obtaining an accurate history is a vital initial step in clinical refraction. Understanding the patient’s visual complaints, if any, is often critical to providing a satisfactory refractive solution. For example, the patient may benefit from glasses suitable for specific activities, such as using a computer, playing a musical instrument, or going shopping. Prescribing glasses for a patient without visual complaints is rarely helpful.

Seek the following items if they are not volunteered:

decreased visual acuity: At distance? At near? Both?

eyestrain

headaches

presbyopia (difficulty reading at comfortable distance)

age

occupation (computer work, in particular)

hobbies (eg, music, sewing)

other ocular history (eg, amblyopia, trauma, glaucoma, prior eye surgery)

Refractive errors are only rarely the cause of headaches. Headaches that wake a patient from sleep, or occur early in the morning, are almost never due to refractive error.

You may be called on to design spectacles for an occupational or recreational application.

Beware of ophthalmic disorders that might limit best-corrected visual acuity.

Review Indications for Refraction

Which patients need to be refracted?

those with subnormal visual acuity. Remember, 20/25 is not normal visual acuity. Patients with acuity of 20/25 or worse should generally be refracted, unless a credible refraction documenting a stable best-corrected visual acuity of 20/25 or worse is already present in the medical record. Patients with acuity of 20/30 or worse should be refracted routinely

those with asthenopic symptoms

those with decreased visual acuity since their previous visit

postoperative patients (especially after cataract surgery)

Some cataract surgeons delay postoperative refractions until there has been sufficient healing to ensure a good visual result. This delay may prevent the detection of an early, vision-threatening complication.

Excerpted from BCSC 2020-2021 series : Section 3 - Clinical Optics. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.

3 Clinical Optics

Introduction: Quick-Start Guide to Optics and How to Refract

Part 2: How to Refract

Step 1: Perform the Preliminaries

Check Visual Acuity

Figure I-21 Eye charts for testing visual acuity. A, ETDRS-type chart with Sloan letters; B, Allen Symbols (not recommended).

Figure I-22 Pinhole occluder.

Obtain a Visual History

Review Indications for Refraction

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