The second way is by relaxing accommodation. With the target distance fixed at 0.33 m, measure the phoria before and after interposing +3.00 D spheres. The phoria difference divided by 3 is the AC/A ratio.
An abnormal AC/A ratio can place stress on the patient’s fusional mechanisms at one distance or another, causing asthenopia or manifest strabismus. Abnormal AC/A ratios should be accounted for when prescribing corrective lenses.
Parks MM. Vergences. In: Tasman W, Jaeger EA, eds. Duane’s Clinical Ophthalmology [CD-ROM]. Vol 1. Philadelphia: Lippincott Williams & Wilkins; 2006:chap 7.
Effect of Spectacle and Contact Lens Correction on Accommodation and Convergence
Both accommodation and convergence requirements differ between contact lenses and spectacle lenses. The effects become more noticeable as the power of the correction increases.
Let us first consider accommodative requirements. Recall that because of vertex distance considerations, particularly with high-power corrections, the dioptric power of the distance correction in the spectacle plane is different from that in the contact lens plane: for a near object held at a constant distance, the amount that an eye needs to accommodate depends on the location of the refractive correction relative to the cornea. Patients with myopia must accommodate more for a given near object when wearing contact lenses than when wearing glasses. For example, patients in their early 40s with myopia who switch from single-vision glasses to contact lenses may suddenly experience presbyopic symptoms. The reverse is true with patients with hyperopia; spectacle correction requires more accommodation for a given near object than does contact lens correction. Patients with spectacle-corrected high myopia, when presbyopic, need only weak bifocal add power or none at all. For example, a patient with high myopia who wears –20.00 D glasses needs to accommodate only approximately 1.00 D to see an object at 33 cm.
Now let us consider convergence requirements and refractive correction. Because contact lenses move with the eyes and spectacles do not, different amounts of convergence are required for viewing near objects. Spectacle correction gives a myopic patient a base-in prism effect when converging and thus reduces the patient’s requirement for convergence. (Fortunately, this reduction parallels the lessened requirement for accommodation.) In contrast, a patient with spectacle-corrected hyperopia encounters a base-out prism effect that increases the requirement for convergence. This effect is beneficial in the correction of residual esotropia at near in patients with hyperopia and accommodative esotropia. These effects may be the source of a patient’s symptoms on switching between glasses and contact lenses. (See also Chapter 4.)
Prescribing Multifocal Lenses
A multifocal lens has 2 or more refractive elements. The power of each segment is prescribed separately.
Determining the Add Power of a Bifocal Lens
The information necessary to prescribe bifocal lenses includes (1) an accurate baseline refraction,
(2) the accommodative amplitude, and (3) the patient’s social or occupational activities that require near-vision correction (eg, reading, sewing, or computer use).
Measuring accommodative amplitude
Any of the following tests can provide useful information for determining the accommodative amplitude: (1) the near point of accommodation with accurate distance refractive correction in place,
(2) the accommodative rule (eg, with a Prince rule), (3) the use of plus and minus spheres at near distance until the fixation target blurs. Binocular amplitude of accommodation is normally greater than the measurement for either eye alone by 0.50–1.00 D.
Near point of accommodation A practical method for measuring the near point of accommodation is to have the patient fixate on a near target (usually small print such as 5-point or Jaeger 2 type print) and move the test card toward the eye until the print blurs. If the eye is emmetropic (or rendered emmetropic by proper refractive correction), then the far point of the eye is at infinity and the near point can be converted into diopters of amplitude.
This method is subject to certain errors, including the apparent increased amplitude resulting from angular magnification of the letters as they approach the eye. In addition, if the eye is ametropic and not corrected for distance, the near point of accommodation cannot be converted into diopters of amplitude. In the following examples, each eye has 3 D of accommodative amplitude:
A person with emmetropia would have a near point of 33 cm and a far point at optical infinity.
A patient with an uncorrected 3.00 D of myopia would have a near point at 16.7 cm because at the far point of 33 cm, no accommodation is needed.
A patient with an uncorrected 3.00 D of hyperopia would have a near point at infinity because all of the available accommodation is needed to overcome the hyperopia.
Accommodative rule Amplitude of accommodation can be measured with a device such as a Prince rule (Fig 3-24), which combines a reading card with a ruler calibrated in centimeters and diopters. Placing a +3.00 D lens before the emmetropic (or accurately corrected ametropic) eye places the far point of accommodation at 33 cm, and the near point is also brought closer by a corresponding 3.00 D. The amplitude is then determined by subtraction of the far point (in diopters) from the near point (in diopters).
Figure 3-24 Prince rule. (Courtesy of Tommy Korn, MD.)
Method of spheres Amplitude of accommodation may also be measured by having the patient fixate on a reading target at 40 cm. Accommodation is stimulated by the placement of successively stronger minus spheres before the eye until the print blurs; accommodation is then relaxed by the use of successively stronger plus lenses until blurring begins. The difference between the 2 lenses is a measure of accommodative amplitude. For example, if the patient accepts –3.00 D to blur (stimulus to accommodation) and +2.50 D to blur (relaxation of accommodation), the amplitude is 5.50 D.
Range of accommodation
Determining the range of accommodation, like measuring the amplitude of accommodation, is valuable in ensuring that the prescribed bifocal add power meets the patient’s visual needs. The range of accommodation measures the useful range of clear vision when a given lens is employed. For this purpose, a measuring tape, meter stick, or accommodation rule may be used.
Selecting an add power
Determine the amount of accommodation required for the patient’s near-vision tasks. For example, reading at 40 cm would require 2.50 D of accommodation. From the patient’s measured accommodative amplitude, allow one-half to be held in reserve. This reserve allows for some comfortable movement should the patient move the reading material either closer or farther away
from the optimal reading distance. For instance, if the patient has 2.00 D of accommodation, 1.00 D may be comfortably contributed by the patient. (Some patients may use more than one-half of their available accommodation with comfort.) Subtract the patient’s available accommodation (1.00 D) from the total amount of accommodation required (2.50 D); the difference (1.50 D) is the approximate additional plus-lens power (add) needed.
Place a lens with this add power in front of the distance refractive correction, and measure the range of accommodation (near point to far point of accommodation in centimeters). Does this range adequately meet the requirements of the patient’s near-vision activities? If the accommodative range is too close, reduce the add power in increments of 0.25 D until the range is appropriate for the patient’s requirement. Because binocular accommodative amplitude is usually 0.50–1.00 D greater than the monocular measurement, using the binocular measurement generally guards against prescribing an add power that is too high.
Types of Bifocal Lenses
Most bifocal lenses currently dispensed are 1-piece lenses that are made by generating the different refracting surfaces on a single lens blank (Fig 3-25). One-piece round segment bifocal lenses have their segment on the concave surface. One-piece molded plastic bifocal lenses are available in various shapes, including (1) round top with button on convex surface, (2) flat top with button on convex surface, and (3) Franklin (executive) style with split bifocal.
