[Intermittent exotropia]. / Les strabismes divergents intermittents.

Intermittent exotropia (IXT) is the most common type of divergent strabismus. It is the consequence of passive mechanisms due to the anatomy of the globes and orbits or due to active innervational mechanisms, resulting in divergence of the visual axes, which is compensated by fusional convergence. Intermittent insufficiency in this compensation gives this form of exotropia its intermittent nature. The most common symptoms of IXT are closure of one eye, asthenopia and diplopia, but they are often absent. The clinical classification of IXT (according to Burian) is based on the difference between the distant and the near angles of deviation. It defines 4 types: true divergence excess (at distance), pseudo-divergence excess, the basic form (distance and near angles are equal) and convergence insufficiency (near angle greater than distance angle). One of the main difficulties in examination of IXT is neutralizing the fusional convergence in order to classify the strabismus. For this purpose, the monocular occlusion test, a near addition, or a prism adaptation test can be used. IXT is also characterised by the quality of control of the deviation by the patient, which is taken in account for therapeutic decision. Tools for measurement of this control have recently been developed and are not commonly used. The natural history of IXT is not well understood. Treatment relies mainly on optical correction, binocular visual training therapy and surgery, but their indications are not well defined, nor are outcomes analysis criteria. In the case of surgery, it aims to treat the maximum measured distance angle; the medium- and long-term angular results of surgery are often disappointing, although it probably improves control of the strabismus in most cases.

Proposing a new scoring system in intermittent exotropia: towards a better assessment of control.

OBJECTIVE: There is a clinical need for a valid and reliable assessment system to quantify the severity and measure the progression of intermittent exotropia. We have developed a new scoring system (Look And Cover, then Ten seconds of Observation Scale for Exotropia [LACTOSE]) based on the examinee's potential to regain the phoria for both distance and near fixation. The test takes only 1 minute to complete. The aim of this study was to evaluate the reliability and validity of LACTOSE. METHODS: The LACTOSE was created by incorporating both distance and near score (0-4 each) into a total score ranging from 0 to 8. A total of 235 consecutive patients with intermittent exotropia were prospectively evaluated using this new scoring system. Inter-rater and test-retest reliability was assessed from a cohort of 10 patients who were video recorded while undergoing the scoring testing and then scored by 10 trained ophthalmologists independently. Construct validity was studied by comparison with clinical variables indicating disease severity. RESULTS: Inter-rater reliability was good for both distance and near (intraclass correlation coefficient [ICC] = 0.729 and 0.818). Test-retest reliability was excellent for both distance and near (ICC = 0.849 and 0.727). Median scores for distance, near, and total were 3, 1, and 4, respectively. A broad representation of the total scores was significantly linked to the age of onset and the amount of exotropia (r2 = 0.194, p = 0.003 and r2 = 0.159, p < 0.001). CONCLUSIONS: We found that our newly developed scoring system is a reliable and valid tool for assessing the severity of intermittent exotropia. This new measure can be implemented easily and efficiently across diverse clinical settings.

Classifying medial rectus muscle attachment in consecutive exotropia.

PURPOSE: To evaluate interexaminer agreement in classifying medial rectus muscle attachment in patients with consecutive exotropia. METHODS: A series of intraoperative photographs of 26 eyes in 25 patients who underwent surgery for consecutive exotropia were retrospectively studied. Two examiners independently classified the medial rectus attachment as either normal, stretched scar, slipped muscle, or lost muscle. Agreement between examiners was evaluated using the weighted kappa (κ) statistic, and causes of disagreement were assessed. RESULTS: Agreement was found in 15 of 26 eyes (58%), signifying "moderate" agreement (κ = 0.41). Approximately two-thirds of the disagreements, 7 of 11 eyes (64%), were between stretched scar and slipped muscle, with characteristics of each entity being present in the same muscle. CONCLUSIONS: The clinical distinction between stretched scar and slipped muscle appears to be obscure. We propose that they should be considered a single entity, which could be referred to as "abnormal scleral attachment."

Strabismus.

Defining the type of strabismus creates a framework for work-up and management. Comitant esotropia is most commonly a childhood condition treated with glasses and surgery. Comitant exotropia is often a childhood condition that may require surgical correction. Microvascular disease is the most common cause of ocular cranial nerve palsies in adult patients.

Efficacy of diagnostic monocular occlusion in revealing the maximum angle of exodeviation.

AIMS: To determine the efficacy of diagnostic monocular occlusion in revealing the maximum angle of exodeviation compared with repeated measurements taken during three or more consecutive examinations in the outpatient clinic. METHODS: We retrospectively analysed 185 patients with intermittent exotropia in an institutional referral centre. The angle of exodeviation was measured at distance and near fixation on three or more consecutive examinations in the outpatient clinic. Then 1 day of diagnostic monocular occlusion was performed and the angle of exodeviation was measured. RESULTS: After diagnostic monocular occlusion, the mean angle of deviation at distance (23.5 prism dioptres (PD)) and near fixation (23.5 PD) was significantly smaller compared with the average maximum angle of deviation before occlusion at distance (27.0 PD) and near fixation (25.2) (p=0.001, 0.022). However, 26 patients (14.1%) showed an increase of ≥5 PD in their distant angle after occlusion and 57 patients (30.8%) showed an increase of ≥5 PD in their near angle of deviation. After occlusion, 39.1% (9/23) of divergence excess (DE)-type, 20.0% (3/15) of convergence insufficiency (CI)-type and 2.7% (4/147) of basic-type exotropia were converted to other types. Patients with hyperopia were more likely to show a significant increase of ≥5 PD during near fixation. CONCLUSIONS: Diagnostic monocular occlusion could be useful in patients with DE-type or CI-type exotropia and with hyperopia. In other cases, however, it has a limited role in determining the maximum angle of exodeviation compared with multiple examinations.

Comitant horizontal strabismus in South African black and mixed race children--a clinic-based study.

PURPOSE: To examine the types of comitant horizontal strabismus in children belonging to the two largest ethnic groups of the Western Cape, South Africa. METHODS: Retrospective review of 2371 children presenting with comitant horizontal strabismus to a single, large, pediatric tertiary referral center in Cape Town, South Africa, between 1997 and 2007. RESULTS: 81% of children were of mixed race and 19% were black. The overall esotropia:exotropia ratio was 73:27. For black children this was 85:15 and for mixed race, 71:29. The overwhelming majority of black children had congenital esotropias (74%), whereas accommodative esotropia was rare (9%). In mixed race children, esotopias were more equally divided between congenital (46%) and accommodative types (35%). Exotropias were uncommon in black children, and were mainly constant (61%). Intermittent exotropias predominated in mixed race children (55%). CONCLUSION: Congenital esotropia remains common in South African black children, and of the few who presented with exotropia, most had constant type. Accommodative esotropias were more prevalent in South African mixed race children, but in smaller proportions than in other studies. Exotropias were mostly intermittent in this ethnic group, and showed a similar profile to that in white American children.

Classification and misclassification of sensory monofixation in intermittent exotropia.

PURPOSE: The course of intermittent exotropia and response to surgery may depend on whether there is underlying monofixation. The purpose of this study was to report the prevalence of sensory monofixation in intermittent exotropia using different stereotests and to determine the risk of misclassifying monofixation based on a single administration of each test. DESIGN: Retrospective case review of children with intermittent exotropia. METHODS: Forty-four children were identified for whom Preschool Randot, Frisby, and Titmus stereoacuity were measured at a single examination. Ninety-two children were identified with near stereoacuity measured on 2 sequential visits (Preschool Randot, n = 73; Frisby, n = 66; and Titmus, n = 40). Monofixation was defined as stereoacuity worse than previously published age-referenced normal thresholds, bifixation was defined as at least 40 arc seconds, and uncertain was defined as within normal range for age but worse than 40 arc seconds. RESULTS: In children measured by all 3 tests on the same visit, sensory monofixation occurred in 36% using Preschool Randot, in 48% using Titmus, and in 55% using Frisby (P > .1 for each comparison). There was poor agreement between Frisby and Preschool Randot when classifying monofixation in individual patients (P = .05). In children measured on sequential visits, misclassification occurred in 5% with Preschool Randot, in 13% with Titmus, and in 23% with Frisby (Preschool Randot vs Frisby, P = .005). CONCLUSIONS: Classification of monofixation depends on the stereotest used. Regardless of the stereotest, there is a risk of misclassifying monofixation on a single assessment. Potential misclassification needs to be considered in clinical practice and in study design.

The fixation target influences the near deviation and AC/A ratio in intermittent exotropia.

PURPOSE: Divergence excess intermittent exotropia is thought to comprise 2 types of deviation, simulated and true. It is believed that simulated divergence excess is akin to a basic deviation but is controlled at near through accommodative and adaptive fusional vergences. While fixation target detail is known to influence accommodation and subsequently deviation size, no previous study has investigated the effect of the fixation target on the AC/A ratio in intermittent exotropia and its influence on the deviation's classification. METHOD: Twenty-five participants with intermittent exotropia underwent near and distance measurement before and after 45 minutes of occlusion. The near angle was also measured through +3.00 D lenses, and using 2 different sized targets, an N60-equivalent "butterfly" picture and N5 print. The gradient AC/A ratio was calculated for each target. RESULTS: There was a significant difference between the measurements using the 2 targets, t(24) = 8.3, p < or = <0.001. On average, the near angle was 8.8(Delta) greater using the N5 print. This also resulted in a significant difference for the AC/A ratio, t(24) = -8.4, p < 0.001, the mean with the target being 3.6(Delta):1D, as compared to 6.6(Delta):1D with the N5 print. CONCLUSIONS: Through careful control of accommodation by ensuring relaxation with plus lenses to clear N5 print, we revealed increases in the AC/A ratio and unmasked deviations that would otherwise have been considered to be characteristic of true divergence excess. Simulated and true deviations are possibly part of a continuum and clinical delineation may be influenced by testing artefact.

[Clinical classification of variable angle exotrophy]. / Clasificación clínica de la exotropía de ángulo variable.

BACKGROUND: Variable exotropia is determined by a series of mechanisms that provide different clinical characteristics to outline individual therapy. There is no specific categorization that can identify such an entity. A clinical classification is proposed that includes different clinical features in accordance with the mechanisms involved in the variability genesis of such exotropias. METHODS: Patients with a diagnosis of variable angle exotropia without previous surgery were included. A complete ophthalmic and strabismus exploration was performed. Variability characteristics were studied and grouped by entries in the classification proposed. RESULTS: Ninety patients were examined from the Pediatric Ophthalmology and Strabismus Unit of the General Hospital of Mexico O.D., with a diagnosis of variable angle exotropia. The following forms of presentations were found: secondary variable angle exotropia due to neurological alteration in 32 cases (35.5%), secondary variable angle exotropia due to monocular bad vision in 19 cases (21.2%), dissociated horizontal deviation in 14 cases (15.5%), mixed variable angle exotropia in 22 cases (4.5%) and primary variable angle exotropia in 3 cases (3.33%). CONCLUSIONS: This classification allows categorization of all patients of variable angle exotropia into proposed categories. The most common mechanism responsible for variability is neurological alteration. Primary variable angle exotropias are rare and are present in a minimum percentage.

An office-based scale for assessing control in intermittent exotropia.

INTRODUCTION: Although intermittent exotropia may deteriorate with time, there are no widely accepted criteria for measuring progression in this disorder. The purpose of this study was to prospectively evaluate a new scale for assessing the level of control in children with intermittent exotropia. METHODS: Thirty consecutive pediatric patients (< 14 years) with intermittent exotropia were prospectively evaluated from July 1, 2004 through June 30, 2005 using a new scale to assess the level of control for both distance and near fixation. The distance score (0 to 5) was combined with the near score (0 to 5) to yield an overall control score from 0 to 10. RESULTS: The 30 patients were examined at a median age of 72 months (range, 15 months to 13 years). The level of control at distance was worse than or equal to the near level of control in all 30 patients. The control scores ranged from 0 to 5 for distance and 0 to 4 for near, with an overall control score ranging from 0 to 8 (median of 3). CONCLUSIONS: This new scale for assessing control in children with intermittent exotropia can be easily applied in the office setting and characterizes the wide range of control in this disorder.

Conversion of intermittent exotropia types subsequent to part-time occlusion therapy and its sustainability.

PURPOSE: To evaluate the effects of part-time occlusion therapy on types of intermittent exotropia and sustainability of converted types. METHODS: Forty-four and 26 children with basic-type and convergence-insufficiency-type intermittent exotropia, respectively, were evaluated in this study. Upon initial examination, we obtained both distant and near deviating angles using prism cover tests, after correcting for refractive errors. We conducted occlusion of the nondeviating eye for 3 months at 3 h/day and assessed the changes in types of intermittent exotropia. We also observed the changes of deviating angles and sustainability of types after 3 months of cessation of part-time occlusion in patients who did not undergo surgery. RESULTS: Preocclusion deviating angles (mean +/- SD) were determined to be 27.1+/-7.46 prism diopters (PD) on distant measurements and 30.6+/-7.92 PD on near measurements. After 3 months of occlusion, the deviating angles were 25.9+/-9.10 PD on distant measurements and 21.4+/-11.00 PD on near measurements, corresponding to a significant reduction (p=0.005 and p<0.001, respectively). Fourteen patients (32%) suffering from basic type of intermittent exotropia converted to the pseudodivergence excess type. In patients suffering from the basic type who exhibited no changes in type, 9 patients (20%) exhibited reductions on both near and distant angle measurements. Among the convergence insufficiency type of patients, 18 (69%) converted to basic type and 2 patients (7%) converted to the pseudodivergence excess type. In the 15 patients who did not undergo surgery, the converted types were maintained in 6 patients, though the other 9 patients showed regression to the prepatching types after cessation of patching for 3 months. CONCLUSION: Part-time occlusion therapy resulted in the conversion of the basic and convergence insufficiency types to pseudodivergence excess and basic types in more than half of the intermittent exotropes. Future studies on correlation between type conversion and surgical outcome would be necessary.

[Antinomic horizontal deviations: esodeviation associated with exodeviation. Clinical features and surgery]. / Déviations horizontales antinomiques: association d'une exodéviation et d'une ésodéviation. Aspects cliniques et chirurgie.

INTRODUCTION: Antinomic deviations are defined as a basic exoposition of the eyes associated with innervational convergence excess leading to an esoposition. PATIENTS AND METHODS: Thirty-four patients with both exodeviation and esodeviation were studied. Surgery consisted in lateral recti (LR) recess with fadenoperation on the medial recti. RESULTS: Four types of antinomic deviations were found: 1- Primitive exodeviation with an excess of physiological accommodative vergences; 2- Consecutive exodeviation due to surgical overcorrection of an accommodative esotropia; 3- Infantile exodeviation with fixation in adduction, DHD +/- convergence excess; 4- Consecutive infantile exodeviation: overcorrection of a congenital esotropia. CONCLUSION: Although fadenoperation on the medial recti may seem paradoxical in exodeviations, it is the most logical procedure to deal with convergence excess and fixation in adduction prevalence. Adjustable sutures for the lateral recti recess appear safer and more predictible in these difficult cases.

Incidence and types of childhood exotropia: a population-based study.

OBJECTIVE: To determine the incidence and types of childhood exotropia in a defined population. DESIGN: Retrospective, population-based cohort. PARTICIPANTS: All pediatric (<19 years old) residents of Olmsted County, Minnesota diagnosed with an exodeviation (>or=10 prism diopters) from January 1, 1985 through December 31, 1994. METHODS: The medical records of all potential patients identified by the resources of the Rochester Epidemiology Project were reviewed. MAIN OUTCOME MEASURES: Incidence and types of childhood exotropia. RESULTS: Two hundred five cases of childhood exotropia were identified during the 10-year period, yielding an annual age- and gender-adjusted incidence of 64.1 (95% confidence interval: 55.2-72.9)/100,000 patients younger than 19 years. This rate corresponds to a prevalence of approximately 1.0% of all children younger than 11 years, with a significant decrease in the incidence during the second decade of life (P<0.001). Eighty-six percent of the children had intermittent exotropia, convergence insufficiency, or an exotropia in the setting of an abnormal central nervous system. CONCLUSIONS: The incidence of childhood exotropia from this population-based study is comparable to the prevalence rates in prior reports. Exotropia is most prevalent during the first decade of life, with intermittent exotropia and convergence insufficiency occurring most frequently.

Clinical features and surgical treatment of A-pattern exotropia.

PURPOSE: To investigate the clinical characteristics and determine the effective surgical managements of A-pattern exotropia. METHODS: Thirty-two patients with A-pattern exotropia underwent superior oblique muscle weakening procedures, medial rectus resection or (and) lateral rectus recession. Pre- and post-operative eye position, deviation angle, superior oblique function and binocular vision were examined and analyzed in the cases. RESULTS: Overaction of the superior oblique muscles (31/32) and underaction of the medial rectus muscle (20/32) were presented in the cases. Postoperatively, a satisfactory ocular alignment was obtained in 28 cases (87.5%), and the A-pattern was corrected in 31 cases (96.9%). Four cases got binocular vision after surgery. CONCLUSIONS: As one of the most common forms of A and V patterns, A-pattern exotropia showed clinical characteristics of superior oblique muscle overaction and medial rectus muscle underacion, which should be the primary factors in the etiology of A-pattern exotropia, and superior oblique weakening procedures combined with horizontal surgery should be an effective approach to A-pattern exotropia treatment.

Some clinical characteristics of V-pattern exotropia and surgical outcome after bilateral recession of the inferior oblique muscle: a retrospective study of 22 consecutive patients and a comparison with V-pattern esotropia.

BACKGROUND AND PURPOSE: A V-pattern exotropia (XT) with bilateral overaction of the inferior obliques (IO) and/or underaction of the superior obliques (SO) is a common finding. The clinical characteristics of this condition in a large series are not available in the scientific literature. Also, data is lacking about the surgical outcome of graded bilateral IO recession. Lastly, a comparison of the pre- and postoperative findings of a V-pattern exotropia population with those of a V-pattern esotropia population is also lacking in the literature. SUBJECTS AND METHODS: Twenty-two consecutive patients without complicating factors were fully evaluated and submitted to bilateral graded recessions of the IO; 19 were also operated for a horizontal binocular alignment imbalance. RESULTS: Preoperative findings: The distribution of V patterns showed 77.2% in the range of 15 to 25 prism diopters (PD) and 22.8% in the range 26 to 35 PD. An overaction of the IO was present bilaterally in 86.4% of the patients and unilaterally in 4.5%. Bilateral underaction of the SO was observed in 4.5% and unilateral in 13.6%. Bilateral overaction of the SO was seen in 18.2% and unilateral in 31.8%. Elevation in adduction was observed bilaterally in 13.6% and unilaterally in 31.8%. A vertical deviation was seen in 50.0% and a marked tendency of it to vanish or diminish in the up- and down positions, as well as at near, was observed. After surgery, 72.7% had less than 15 PD of V pattern or less than 10 PD of A pattern; 36.4% had the V pattern fully corrected. Surgery eliminated a vertical imbalance in 72.7% of the patients, reduced in 9.1%, left unchanged in 9.1% and increased in 9.1% Of the 11 patients devoid of vertical deviation, 72.7% remained so and in 27.3% a vertical deviation was created. Binocularity: There was an improvement of the fusional status with surgery, (ascertained with the Worth Four Dot Test and the major amblyoscope measurement). CONCLUSION: All cases were in the range of 15-35 PD preop'. Overaction of the IO was a prominent finding. Underaction of the SO was less prevalent as compared with a V-ET population. Overaction of the SO was observed bilaterally in 18.2% and unilaterally in 31.8%, a finding never detected in a V-ET population. A vertical imbalance was observed in one-half of the cases. A good outcome (collapse of the V pattern) was obtained with bilateral graded recession of the IO, This procedure is relatively safe concerning the vertical alignment.

Common forms of childhood exotropia.

OBJECTIVE: To determine the most common forms of childhood exotropia. DESIGN: Retrospective, consecutive, observational case series. PARTICIPANTS: All exotropic children (with >/=10 prism diopters) younger than 19 years from a predominantly rural Appalachian region evaluated from August 1, 1995 through July 31, 2001. METHODS: Demographic and clinical data were collected on all patients. MAIN OUTCOME MEASURES: The relative proportion of the various forms of childhood exotropia. RESULTS: Two hundred thirty-five consecutive children without prior surgical treatment were evaluated for exotropia. Of the 235 study children, the specific forms of exotropia diagnosed and numbers were as follows: intermittent exotropia, 112 (47.7%); exotropia associated with congenital or acquired abnormalities of the central nervous system (CNS), 50 (21.3%); convergence insufficiency, 27 (11.5%); sensory exotropia, 24 (10.2%); paralytic exotropia, 5 (2.1%); congenital exotropia, 4 (1.7%); neonatal exotropia that resolved after 4 months of age, 3 (1.3%), whereas the remaining 10 (4.3%) had an undetermined form of exodeviation. CONCLUSIONS: Intermittent exotropia was the most common form of divergent strabismus in this population. Exotropia associated with an abnormal CNS, convergence insufficiency, and sensory exotropia were also relatively common, whereas the congenital, paralytic, and late-resolving neonatal forms were uncommon.

[Clinical features and surgical treatment of A-pattern exotropia].

OBJECTIVE: To investigate the clinical features of A-pattern exotropia and to evaluate the efficacy of surgical treatment for the correction of A-pattern exotropia (A-XT). METHODS: Pre- and post-operative eye position, superior oblique muscle function, clinical features and binocular function were examined and analyzed in 32 patients with A-pattern exotropia. RESULTS: Overaction of the superior oblique (SO) (31/32) and insufficiency of medial rectus muscle (20/32) were commonly seen in the group of patients. The tenectomy of the SO and the resection of the medial rectus muscle were effective in most of A-pattern exotropia. 87.5% (28/35) of A-pattern exotropia in our study were successfully corrected. Four cases got binocular function after surgery. CONCLUSIONS: Based on the clinical features and the effects of surgical treatment, we conclude that the superior oblique muscle overaction and medial rectus muscle insufficiency are the primary factors in the etiology of A-pattern exotropia that is the most common form of A-V syndrome. Weakening the overaction of superior oblique muscle combined with the correction of exotropia is an effective way to correct A-pattern exotropia.

Lower eyelid medial canthal tendon laxity grading: an interobserver study of normal subjects.

OBJECTIVE: To describe a simple grading system for medial canthal tendon (MCT) laxity and measure its reproducibility. STUDY DESIGN: Observational case series and interobserver variability study. PARTICIPANTS: Fifty subjects (100 lower eyelids) without eyelid disease. The age range was 19 to 98 years. METHODS: Subjects without eyelid pathology or previous surgery were selected. Two ophthalmologists assessed the position of the inferior punctum in relation to the cornea with the patient in primary gaze. Two measurements were made, the first with the lower eyelid at rest and then with the lateral distraction test. Both observers were masked to the other observer's measurements. MAIN OUTCOME MEASURES: Kappa statistics to show the strength of agreement between the two observers for the resting and lateral distraction eyelid positions. RESULTS: High kappa values were found for resting position (agreement in 92 of 100 eyelids) and for lateral distraction testing (agreement in 85 of 100 eyelids). The range of resting positions was from position -1 to position 1. The range of laterally distracted positions was from position 0 to position 5. Overall, a very high level of agreement was achieved between observers. CONCLUSIONS: This grading system for MCT laxity is simple, easy to learn and reproducible, easier than measuring in millimeters, and has an advantage over merely saying the tendon is "lax" or "not lax." We propose this standardized grading system in the evaluation of patients with entropion and ectropion to identify those patients who may benefit from MCT stabilization/plication.

Medial rectus resection(s) with adjustable suture for intermittent exotropia of the convergence insufficiency type.

BACKGROUND: The surgical results for intermittent exotropia of the convergence insufficiency type have been reported to be of varying success. The purpose of this study is to evaluate the surgical results of medial rectus muscle (MR) resection(s) with adjustable suture for this condition. METHODS: Twenty-one consecutive patients with intermittent exotropia of the convergence insufficiency type were included in this retrospective study. All patients had a history of prolonged difficulties at near work unrelieved by nonsurgical treatment. Unilateral or bilateral MR resection(s) were done with the adjustable suture, which was tied at the first postoperative day. The target angle was an esotropia of 10 to 20 PD at distance and an esotropia of 5 to 10 PD at near. Postoperatively Fresnel prisms were used temporarily in patients manifesting a consecutive esotropia with diplopia at distance. Postoperative follow-up period was between 6 months and 24 months with a mean of 9.1 months. RESULTS: MR resection(s) with the adjustable suture reduced the mean exodeviation at near from 25.7 to 3 PD. It also reduced the exodeviation at distance from 11.4 to -2 PD (esodeviation). The mean near-distance difference was collapsed from 14.3 PD preoperatively to 5 PD postoperatively. CONCLUSIONS: MR resection(s) with adjustable suture combined with intentional postoperative aggressive overcorrection and the use of Fresnel prisms is useful in intermittent exotropia of the convergence insufficiency type. The intentional overcorrection during the immediate postoperative period at distance and near is required to prevent long-term undercorrection.