Kushner BJ. Diagnosis and Treatment of Exotropia With a High Accommodation Convergence–Accommodation Ratio. Arch Ophthalmol. 1999;117(2):221-224. doi:10.1001/archopht.117.2.221
Copyright 1999 American Medical Association. All Rights Reserved. Applicable FARS/DFARS Restrictions Apply to Government Use.1999
Patients with exotropia often have a slow-to-dissipate fusional mechanism at near, which masks the true near deviation. Consequently, determination of the accommodation convergence–accommodation (AC/A) ratio in patients with exotropia must be based on near measurements obtained after prolonged monocular occlusion (typically 1 hour). When determined in that manner, the presence of a high AC/A ratio before surgery in an exotropic patient has been reported to be predictive of an esotropia at near after surgery.
To investigate the diagnosis and management of exotropia with a high AC/A ratio.
Three hundred four consecutive patients with exotropia were studied. In addition to the usual measurements, measurements were obtained at near after 1 hour of monocular occlusion, with and without additional +3.00-diopter lenses. Also, a gradient AC/A ratio was obtained by using additional minus lenses at distance fixation.
One hundred fifty-four (50.7%) of 304 patients would have been thought to have a high AC/A ratio if that diagnosis was based on measurements obtained before prolonged monocular occlusion. In fact, only 22 patients (7.2%) actually had a high AC/A ratio; 132 patients (43.4%) had a pseudo–high AC/A ratio. Six of 22 patients with a high AC/A ratio underwent surgery to correct the exotropia. The presence of a high AC/A ratio before surgery had sensitivity, specificity, and positive and negative predictive values of 100% for predicting a postoperative esotropia at near associated with a high AC/A ratio. The remaining 16 patients with high AC/A ratios were treated with overcorrecting minus lens therapy (including a bifocal). Ten of them have been followed up to at least 18 years of age, by which time 9 have shown normalization of the AC/A ratio.
Near measurements used to calculate the AC/A ratio in exotropic patients must be made after prolonged monocular occlusion. Otherwise, many patients with a pseudo–high AC/A ratio will be thought to have a true high AC/A ratio. The presence of a high AC/A ratio is infrequent in patients with esotropia, but it is highly predictive of a postoperative esotropia at near fixation.
ASSESSMENT OF the ratio between accommodation and accommodative convergence (AC/A ratio) is important in treating patients with strabismus. Classically, the AC/A ratio can be measured in 1 of 2 manners. The gradient method involves measuring the change in angle of alignment, while the amount of accommodation is altered by the use of either minus lenses at distance fixation or plus lenses at near fixation. The heterophoria method involves comparing distance alignment to near alignment and factoring in the effect of interpupillary distance on the amount of convergence that is needed at near.1- 4 For most esotropic patients, the difference between the distance and near deviation is a function of the AC/A ratio. For many patients with exotropia, there is another powerful factor that affects the difference between the distance and near deviation. In 1952, Scobee5 pointed out that many patients with exotropia have a slow-to-dissipate fusional mechanism at near that will mask the true deviation at 0.33 m. This mechanism has been labeled tenacious proximal fusion (TPF).6,7 Approximately 1 hour of monocular occlusion has been recommended to suspend TPF and to permit the full exotropic deviation at near to become manifest.6- 9 It has been stated6,7,10 that TPF will contaminate the calculation of the AC/A ratio in patients with exotropia if the determination is made using any near measurements obtained before the suspension of TPF with 1 hour of monocular occlusion.
In most patients with intermittent exotropia, the distance deviation initially exceeds the near deviation.6,7,11- 13 If one considers a hypothetical patient:
Distance exotropia=35 prism diopters (Δ)
Near exotropia=05′ Δ
Near exotropia after monocular occlusion=35′ Δ (was masked by TPF)
If the AC/A ratio in this patient is calculated by comparing the distance and near deviation before monocular occlusion (heterophoria method), the AC/A ratio will seem to be high by definition. This patient, however, may actually have a normal AC/A ratio because TPF is masking a near deviation that equals the distance deviation. Such a patient has been described as having a pseudo–high AC/A ratio by the heterophoria method.6,7 Many such patients will also manifest an increase in near deviation if they are tested with +3.00 lenses at near before monocular occlusion.6,7,14 If one again considers the hypothetical patient:
Distance exotropia=35 Δ
Near exotropia=0′ Δ
Near exotropia with +3.00 dipoters (D) before monocular occlusion=35′-40′ Δ
Near exotropia after monocular occlusion and +3.00 D=35′-40′ Δ
Even with a normal AC/A ratio, the addition of +3.00 lenses at near (before monocular occlusion) would be expected to add between 4 and 10 Δ to the true near angle of exotropia (35 Δ) because of a relaxation of normal accommodative convergence. This results in the near deviation exceeding the amount to which TPF has been accustomed to masking, and the entire deviation becomes manifest. When this patient is first measured at near after 1 hour of monocular occlusion, the true larger near deviation of 35′ Δ becomes manifest. When +3.00 lenses are subsequently used for obtaining a near measurement, the additional deviation is only 0 to 5 Δ, revealing an AC/A ratio that is low to normal. Similarly, a repeated measurement of the gradient AC/A ratio in the distance typically reveals that it is qualitatively unchanged after monocular occlusion. Also, in such patients, the AC/A ratio, calculated using the gradient method at distance and near (after monocular occlusion), is qualitatively the same. Such a patient is described as having a pseudo–high AC/A ratio by the gradient method.6,7 In addition, the presence of nonaccommodative proximal convergence can result in a near deviation that is smaller than the distance deviation.3 Proximal convergence will thus also result in a pseudo–high AC/A ratio if it is determined using the heterophoria method (even after prolonged monocular occlusion). The gradient method must be used to document that the AC/A ratio may in fact be normal in this circumstance.
It is important to identify exotropic patients who truly have high AC/A ratios for the following reason. In 1988, I reported6 that exotropic patients who show a high AC/A ratio when that diagnosis is made after 1 hour of monocular occlusion have a high likelihood of maintaining their high AC/A ratio after surgery. If their distance angle of strabismus is successfully eliminated with surgery, they predictably develop and maintain an esotropia at near associated with a high AC/A ratio. They need to use a bifocal after surgery to maintain good alignment at near. This is in contrast to most exotropic patients, who do not show a consecutive near esotropia even if their distance exotropia greatly exceeds their near exotropia before surgery.15 I have recommended that such patients be treated optically with overcorrecting minus lenses. Because of their high AC/A ratio, they experience a large decrease in deviation with a relatively small amount of overcorrecting minus lens power. Unlike most intermittent esotropes receiving overcorrecting minus lens therapy, however, these patients manifest a persistent esotropia at near when wearing overcorrecting minus lenses; they also need a bifocal.6,7,16
The purpose of this study is to report my experience with diagnosis and treatment of exotropic patients with a high AC/A ratio, to report the prevalence of that entity, and to present its long-term outcome with conservative management.
The routine I have used since 1982 for measuring cooperative patients with intermittent exotropia has been reported previously.6,7,16 In addition to prism and cover test measurements at 6.00 and 0.33 m, the following measurements and calculations are performed: (1) A gradient AC/A ratio is obtained by repeating the measurement at 6.00 m with an additional −1.50- or −2.00-D sphere added to each spectacle lens, or in a trial frame while fixation is maintained on a 20/40 Snellen optotype. The gradient AC/A ratio is then calculated by dividing the decrease in the exotropia by either 1.5 or 2, depending on which minus lens power was used.(2) An additional measurement at 0.33 m is made after 1 hour of monocular occlusion of the habitually deviating eye to determine if TPF is present. During the testing to obtain this measurement, the patient is not permitted to regain binocular fusion until measuring is completed, as has been described previously.9 The AC/A ratio using the heterophoria method is then calculated using the near measurement obtained after monocular occlusion using the standard formula:
AC/A=ipd+exotropia at 6.00 m−exotropia at 0.33 m/3, where ipd is the interpupillary distance in centimeters. (3) Another postocclusion measurement at 0.33 m is obtained with an additional +3.00-D sphere over each eye before allowing the patient to regain binocular fusion. The gradient AC/A ratio at 0.33 m is then calculated by dividing the difference between the near measurement obtained after monocular occlusion and that obtained with +3.00-D lenses at near after monocular occlusion by 3. In addition to the previously mentioned measurements, 1 was obtained at 0.33 m before monocular occlusion. As previously stated, I do not believe that this measurement is of any practical utility. It was obtained, however, as part of this clinical investigation to help understand the relationship between accommodation and convergence in patients with exotropia.
This report consists of 304 consecutive patients with intermittent exotropia who were measured in the above-described manner. These patients have been described previously as part of 2 different published series,6,7 and their defining characteristics are in those reports. This series was limited to patients who were sufficiently cooperative to permit the measurements described above, had an exotropia that was intermittent at 6 or 0.33 m, and had not undergone previous strabismus surgery.
For the purpose of this report, an AC/A ratio is defined as being high, with the gradient method for distance or near if it was greater than 6:1. With the heterophoria method, the AC/A ratio is defined as being high if it was greater than 8:1 to 9:1 (depending on the interpupillary distance). These latter numbers correspond to a distance deviation exceeding the near deviation by 10 Δ or more. If patients would have been considered to have a high AC/A ratio with the heterophoria method based on the near measurement before monocular occlusion but had a normal AC/A ratio if the postocclusion near measurement was used, they were diagnosed as having a pseudo–high AC/A ratio (heterophoria method). If patients would have been considered to have a high AC/A ratio with the gradient method based on the near measurement with +3.00 D before monocular occlusion but had a normal AC/A ratio if the postocclusion +3.00 near measurement was used, they were diagnosed as having a pseudo–high AC/A ratio (gradient method). Patients with a pseudo–high AC/A ratio (gradient method) are a subset of patients with a pseudo–high AC/A ratio (heterophoria method).
It has been previously reported7,17 that many exotropic patients with a truly high AC/A ratio will show a small esophoria if tested at a near fixation distance (0.17 or 0.08 m). This is in contrast to most patients with exotropia, for whom such a near measurement is closer than their near point of convergence. The typical exotropic patient breaks down to manifest a larger exotropia when measured at such a close range. Consequently, most patients in this series also had a measurement obtained at 0.17 or 0.08 m if it seemed from the other measurement that they might have a high AC/A ratio.
Table 1 gives descriptive variables of the 304 study patients. The presence of a pseudo–high AC/A ratio was a common finding in patients with intermittent exotropia. Forty-four percent of the 304 patients would be classified as having a high AC/A ratio using the heterophoria method, and 34% using the gradient method at near if those diagnoses were based on near measurements obtained before monocular occlusion. Only 22 (7.2%) of 304 patients actually had a high AC/A ratio. They all met the previously mentioned criteria of having a high AC/A ratio with the gradient method at distance fixation, the gradient method at near fixation after monocular occlusion, and the heterophoria method after monocular occlusion. In all patients, the AC/A ratio was qualitatively the same with all 3 methods of testing. No correlation was found between the size of the deviation at 6.00 m and the magnitude of the AC/A ratio. In addition, 18 patients had a near measurement of less than 10 Δ (before monocular occlusion), and 16 of those 18 patients showed an esophoria when tested at 0.17 or 0.08 m.
Of 304 patients reported in this series, 245 have undergone strabismus surgery. The surgery was based on the deviation at 6.00 m; the surgical dose and technique have been described previously.7 Six of 22 patients with a true high AC/A ratio exotropia were included in that surgical group. All 6 of these patients developed an esotropia at near associated with a high AC/A ratio after surgery. Five of them needed to wear a bifocal; the sixth patient was able to control a moderate-sized esophoria without a bifocal. These 6 were the only patients of the 245 undergoing surgery who developed an esotropia at near associated with a high AC/A ratio after surgery. Thus, in this series, the diagnostic criteria stated above had a sensitivity, specificity, positive predictive value, and negative predictive value of 100% for determining which patients would develop an esotropia at near associated with a high AC/A ratio after surgery for exotropia.
Of the 16 patients with a high AC/A ratio who did not undergo surgery, overcorrecting minus lens therapy was successful in all cases in controlling the distance deviation. The deviation at 6.00 m in these patients ranged from 15 to 35 Δ. Typically, myopic patients were given spectacles that incorporated 1 to 1.5 D more minus power than their cycloplegic refraction. Usually, hyperopic patients were given spectacles so that a spherical equivalent of the lens before the dominant eye was between −1 and −1.5 D. The power of the lens before the nondominant eye was chosen to make the amount of accommodation equal in the 2 eyes. Although using this amount of overcorrecting minus lens power would not be expected to reduce a substantial exotropia to orthotropia, it seems to decrease the deviation sufficiently to permit better control over an intermittent deviation. All 16 patients treated with overcorrecting minus lens therapy showed a persistent esotropia at near with the overcorrecting minus lenses in place, and needed a bifocal as part of their management. There were no obvious differences characterizing the 6 patients who underwent surgery from those who received optical management. Four of them were operated on before I was aware of the importance of a true high AC/A ratio before surgery in predicting a postoperative esotropia with a persistent high AC/A ratio. The other 2 patients were treated unsuccessfully with spectacles before surgery and elected to undergo surgery with the understanding that a bifocal would be needed afterward.
Of 16 patients with high AC/A ratio exotropia who were treated with overcorrecting minus lenses and a bifocal, 10 have been followed up to at least 18 years of age. All of them showed normalization of the AC/A ratio by 18 years and no longer need a bifocal as part of their management. Typically, normalization of the AC/A ratio began to occur at approximately 8 to 9 years of age and was completed by approximately 16 years of age. Seven of them no longer need overcorrecting minus lens therapy to control their deviation. Three of the 6 patients who had undergone surgery have been followed up to age 18 years. Two of the 3 showed normalization of their AC/A ratio by age 18 years and no longer need a bifocal.
Based on the new understanding of the role of TPF as a contaminant of near measurements in exotropic patients, one can measure the AC/A ratio in exotropes in any of the following manners: the gradient method in the distance using minus lenses, the gradient method at near using plus lenses after 1 hour of monocular occlusion, or the heterophoria method after 1 hour of monocular occlusion. If this latter test indicates a normal AC/A ratio, one can assume that the AC/A ratio is normal. If the heterophoria method after 1 hour of monocular occlusion suggests a high AC/A ratio, proximal convergence may be present, resulting in a pseudo–high AC/A ratio. A gradient method determination of the AC/A ratio must be performed to properly assess the AC/A ratio. When one measures exotropic patients in these manners, it turns out that most patients who seem to have a high AC/A ratio in fact have a normal AC/A ratio.
The typical patient with intermittent exotropia has a distance deviation that exceeds the near deviation. Based on this observation alone, one might think such a patient has a high AC/A ratio using the heterophoria method. There are clinical reasons to suspect that such a patient does not, in fact, have a high AC/A ratio. For most such patients, if one performs surgery in the form of lateral rectus recessions bilaterally, and if one is successful in obtaining good alignment at distance fixation, the patient does not manifest an esotropia at near after surgery.6,7,15 If the distance vs near differences in such patients were in fact a function of the AC/A relationship, one would thus have to conclude that the AC/A ratio was decreased as a result of recessing the lateral rectus muscles. This does not make sense. Also, if one treats such a patient with overcorrecting minus lens therapy, and if one is successful in decreasing or eliminating the distance deviation, these patients typically do not maintain an esotropia at near.16 It is similarly difficult to understand why overcorrecting minus lens therapy should decrease the AC/A ratio; the AC/A ratio is linear and would not be expected to change in this situation.1- 4
In light of these data, recommendations for treating exotropia associated with a high AC/A ratio should be based on the following considerations. Surgery can be performed to correct the distance deviation; however, the patient will have a high likelihood of needing a bifocal after surgery. Overcorrecting minus lens therapy has good likelihood of providing good control over the misalignment for distance; however, a bifocal will be necessary for control of a near esotropia. In many cases, the patients will outgrow the need for a bifocal by the end of adolescence. I am aware of successful treatment of these patients with bilateral lateral rectus recessions and simultaneous posterior fixation of the medial rectus muscles, but I have no personal experience with that treatment approach.18
It is important to identify the exotropic patient who truly has a high AC/A ratio because of their propensity for developing an esotropia at near associated with a high AC/A ratio after surgery. Proper identification can permit forewarning of the patient who is at risk for needing a bifocal after surgery. It also indicates patients who are good candidates for overcorrecting minus lens therapy. Because it also has been shown that a measurement after 1 hour of monocular occlusion may help determine the maximum distance deviation in patients with intermittent exotropia, the latter method for calculating the AC/A ratios may be more practical if it reveals a normal AC/A ratio.19 However, if the AC/A ratio seems high with the heterophoria method after 1 hour of monocular occlusion, the possible effect of proximal convergence must be ruled out by a gradient method determination. Also, if results with either test are equivocal, both should be used. The presence of an esophoria at 0.17 or 0.08 m should alert the examiner to the high likelihood of a high AC/A ratio being present; however, the absence of this finding does not rule out that possibility. This is a helpful, but not essential, diagnostic sign.
Accepted for publication October 13, 1998.
Supported by an unrestricted grant from Research to Prevent Blindness Inc, New York, NY, and the Wisconsin Lions Foundation, Stevens Point (to the Department of Ophthalmology and Visual Sciences, University of Wisconsin, Madison).
Corresponding author: Burton J. Kushner, MD, University of Wisconsin Hospital and Clinics, 2870 University Ave, Madison, WI 53705 (e-mail: email@example.com).