(a) Significance in diagnosis and treatment The convergence accommodation to convergence (CA/C) ratio is still not commonly assessed in the clinical situation. Determination of the CA/C ratio is important in analysis of optometric data. The CA/C finding is sometimes an important characteristic in the final determination of the diagnosis. It may also play a key role when one determines appropriate management. For example, divergence excess and other cases of high exophoria at distance may benefit from the use of added minus lenses. Analysis of the CA/C ratio helps in this determination.

(b) Clinical determination of the CA/C ratio To measure the CA/C ratio clinically, one has to use either a blur-free target or pinholes to eliminate blur as a stimulus. There is still no widely accepted method for determining the CA/C ratio. One possible approach is to use a target called the Wesson DOG (difference of gaussian) card (26) along with dynamic retinoscopy. To use this technique, ask the patient to view this target at four different distances as you perform retinoscopy. You can determine the amount of accommodation with different vergence levels.

(c) Stimulus versus response CA/C Unlike the accommodative system, in which there may be a significant difference between the stimulus and response, the vergence stimulus and vergence response are generally identical. There is, therefore, no need to differentiate between a stimulus and response CA/C ratio (27).

(a) Amplitude versus facility Smooth vergence testing is the most common method used for assessing the amplitude of the fusional vergence response for both positive and negative fusional vergence. The blur finding is a measure of the amount of fusional vergence free of accommodation. The break indicates the amount of fusional vergence and accommodative vergence. The recovery finding provides information about the patient’s ability to regain single binocular vision after diplopia occurs. Although smooth vergence testing provides important information about the amplitude of fusional vergence, studies (6) have shown that it is possible to have normal fusional amplitudes and still have a problem referred to as fusion vergence dysfunction. Additional testing must be performed to assess fusional facility.

(b) Reliability Rouse et al. (21) reported only fair intraexaminer reliability, both within and between sessions using the von Graefe smooth vergence testing procedure in children aged 10 to 11 years. Their results suggest that differences up to 12Δ occur with follow-up visits even without intervention. Thus, when evaluating the effects of treatment such as vision therapy, a change of greater than 12Δ is needed to be confident that the change is real and not the result of measurement variability.

(c) Smooth versus step vergence Smooth and step vergence testing are both designed to evaluate fusional vergence amplitude. The primary value of step vergence testing is that it is administered outside the phoropter. This is an important advantage when examining young children. Before the age of 8 or 9, children tire quickly and may move around, making testing with a phoropter difficult. Because it is impossible to see the child’s eyes behind the phoropter, the clinician cannot be sure whether the patient is responding appropriately. Studies (1,2) have demonstrated that expected findings are different for smooth versus step vergence. Two studies have also compared fusional vergence ranges with rotary prism (smooth) versus step vergence with a prism bar (28,29). Antona et al. (28) compared phoropter rotary prism vergence ranges with phoropter prism bar fusional vergence ranges for 61 optometry students in Spain. The results suggested that the two tests should not be used interchangeably. Goss and Becker (29) did a similar study and also concluded that fusional vergence ranges determined by prism bars out of the phoropter cannot be used interchangeably with those determined by phoropter rotary prisms for the purpose of follow-up on individual patients or for the purpose of comparison with norms. Thus, clinicians should use one method or the other in the initial examination and when following the patient’s progress, reevaluate using the same method.

1. Purpose Step vergence is a method of evaluating fusional vergence amplitude outside the phoropter.

2. Important issues Testing is done outside the phoropter. When a young child is evaluated who is either very active or not responding reliably, step vergence testing represents a useful alternative. The child’s eyes can be seen because testing is done with a prism bar, and the test becomes more objective. Instead of relying on the patient’s responses, the examiner can observe when the child loses binocularity.

3. Expected values The expected values have been determined to be different for adults and children (1,2). Table 1.4 lists the break and recovery values for positive and negative fusional vergence testing for both children and adults.