In this text, we stress another use for the NRA and PRA tests. These tests can be used to indirectly analyze both accommodation and vergence. This is explained in detail in Chapter 2.

(a) Instructional set It is important to ask the patient to keep the target clear and single during these tests. Traditionally, the instructional set is, “As I add lenses in front of your eyes, keep these letters clear for as long as you can. Tell me when the letters are blurry.” We believe it is important to also ask the patient to report diplopia, because these tests also indirectly probe the ability to maintain fusion using positive and negative fusional vergence.

(b) High NRA finding A high NRA finding indicates that the patient has been overminused during the subjective.

(c) At what level should the PRA be discontinued? The maximum value that should be expected with the NRA is +2.50, for the reasons explained above. However, there is no consistent endpoint for the PRA. The endpoint for the PRA will vary depending on the patient’s amplitude of accommodation, AC/A ratio, and the negative fusional vergence. The following examples illustrate the variables that determine the endpoint for the PRA.

In the first patient, we would expect the patient to be able to keep the target single and clear until about −6.00. As we add minus lenses binocularly, the patient must accommodate to maintain clarity.

This is not a problem because the amplitude of accommodation is 12 D. At the same time, the patient must maintain single binocular vision. As the patient accommodates, the AC/A ratio causes convergence that must be counteracted using negative fusional vergence. For every Δ of accommodation, the patient must use 2 Δ of negative fusional vergence. Because patient 1 has 12 D of accommodation and 12 Δ of negative fusional vergence, he or she will be able to maintain clear single binocular vision until about -6.00 D. Using the same reasoning, the PRA endpoint will decrease as the AC/A increases, as demonstrated above for patients 2 and 3, who have higher AC/A ratios and lower negative fusional vergence ranges. Patient 4 has findings identical to patient 1, except that the amplitude of accommodation is only 2 D. Even though this patient has a low AC/A ratio and normal negative fusional vergence, blur would be expected at -2.00 because of the low amplitude of accommodation.

In contrast to the NRA, where the maximum expected endpoint is always +2.50, the maximum endpoint for the PRA varies with multiple factors. Because the primary objective of the NRA and PRA tests is to determine whether the two values are balanced, it makes sense to stop the PRA test after reaching a value of −2.50.

3. Expected values The expected values for NRA are +2.00, ±0.50; for PRA, the expected values are −2.37, ±1.00.

Although sensory fusion anomalies can be very dramatic in cases of strabismus, in cases of nonstrabismic binocular vision disorders, sensory anomalies are much less severe. Most patients with nonstrabismic binocular anomalies have normal or only mildly reduced stereopsis. Intermittent mild suppression is common in heterophoria, but is less intense and the size of the suppression scotoma smaller than in strabismus.

Although sensory status is not as significant an issue in heterophoria, the presence of suppression or loss of stereopsis is still important in determining the prognosis and sequence of treatment. In many cases, the presence of suppression can be determined by performing the binocular vision testing described earlier. During the NPC, near lateral phoria, and fusional vergence testing, patients may be unable to appreciate diplopia in spite of misalignment of the visual axes, indicating suppression.

1. Purpose The Worth four-dot test is a subjective test designed to evaluate the presence and size of the suppression scotoma. It is considered one of the most accurate methods of evaluating suppression (43).

(a) Determining the size of the suppression scotoma The size of the suppression scotoma can be determined by moving the Worth four-dot flashlight away from the patient. As the flashlight is moved away from the patient, the target subtends a smaller angle. For instance, at 33 cm, the target subtends an angle of approximately 4.5 degrees. At 1 m, the angle subtended is approximately 1.5 degrees. When performing the Worth four-dot test, the flashlight is initially held at 33 cm, and the patient, wearing red/ green glasses, is asked to report the number of dots seen. If the patient reports four dots, the clinician should slowly move the flashlight from 33 cm to about 1 m. If the patient reports four dots at 33 cm, but two or three dots at 1 m, a small suppression scotoma is present. If a three- or two dot-response is present, even at 33 cm, the suppression scotoma is larger. The size of the suppression scotoma is important because there is an inverse relationship between the size of the suppression scotoma and the level of stereopsis. As the suppression scotoma becomes larger, the stereopsis decreases (3).

(b) Determining the intensity or depth of the suppression It is important to evaluate the intensity of the suppression scotoma. It is possible to have a small suppression scotoma that is more intense and, therefore, more difficult to treat than a larger, less intense, suppression scotoma. To assess the depth of the suppression, the clinician can perform the Worth four-dot test with normal room illumination and again with the room lights turned off. Normal illumination simulates the patient’s normal visual conditions and is more likely to yield a suppression response. As the conditions are made artificial, the patient has more difficulty maintaining the suppression. The suppression is considered more intense, therefore, if it is present even with the room lights off.

3. Expected values The expected response with the Worth four-dot test is four dots at both 33 cm and 1 m.