Non-fixating Eye Research Articles

A new approach to measure interocular suppression in amblyopia and strabismus

Interocular suppression is commonly estimated by the contrast “balance point” between the eyes in individuals with amblyopia, in which the depth of suppression is defined as the increased contrast in the amblyopic eye that is perceptually matched to the fellow eye. However, this method may not be suitable for individuals with strabismus who have normal or even better contrast sensitivity in the non-fixating eye. In this study, I introduced a new approach that can be used for measuring interocular suppression for both amblyopia and strabismus without balancing contrasts between the eyes. The method consists of the following components:•The stimuli have equal contrasts in both eyes.•The percepts of objects seen monocularly and binocularly are compared.•Interocular suppression is defined as the percept of object in the amblyopic eye is suppressed when an object is presented in the fellow eye.

Dissociated vertical deviation: Simplified

Abstract Dissociated vertical deviation (DVD) is not a well-understood ocular motility disorder. It is defined as an intermittent anomaly of a non-fixing eye consisting of upward excursion, ex-cyclotorsion, and lateral deviation. It can present as latent or manifest deviation. It is usually bilateral, asymmetrical, and often associated with latent nystagmus and infantile esotropia. In unilateral cases, it can be associated with amblyopia. Binocular vision is usually absent in these patients. There are huge variations in the measurements of DVD, as it becomes visible only with one eye covered, hence making quantification of its magnitude difficult. Therefore, choosing an adequate management option is difficult. Management options include both conservative as well as surgical treatment. Conservative management is done with spectacles correction to encourage fusion. Surgical management is indicated in large, frequent DVD or with anomalous head posture. Surgical options commonly adopted are superior rectus recession with or without posterior fixation sutures. Different surgical approaches to DVD are currently being used but no recommended guidelines are available, hence is based on the amount of deviation and experience of a surgeon.

Clinical Management of Monofixation Syndrome due to Microtropia

BACKGROUND: Monofixation syndrome is a binocular vision disorder characterized by peripheral fusion with a central suppression of one eye due to smallangle strabismus (microtropia) or anisometropic amblyopia. Reduced stereopsis and acuity are noted in the non-fixating eye. Although often left untreated due to good cosmetic appearance, central suppression and reduced stereopsis can affect visual performance, comfort, and depth perception. CASE REPORT: EJ, a 17-year-old Caucasian female was previously treated for amblyopia of the left eye (OS) with glasses and patching since age 8 with no improvement to vision. EJ experienced the following symptoms: blurred vision at near when reading, blurred vision at far distances, eye pain and fatigue, difficulty copying from the board, difficulty seeing at night while driving, and poor depth perception. Entering uncorrected visual acuity was 20/20-2 OD and 20/50-2 OS with Snellen Letter Chart. Near uncorrected acuity was 20/20 OD and 20/200 OS with a reduced Snellen chart at 40 cm. Microtropia of 2 to 12 prism diopters of esotropia, eccentric fixation on visuoscopy, and central suppression on various tests were measured. EJ was diagnosed with monofixation syndrome, monocular esotropia of the left eye, strabismic amblyopia of the left eye, suppression of binocular vision (OS), and fusion with defective stereopsis. The patient completed 19 sessions of in-office vision therapy with the practice of home reinforcement activities between therapy sessions. Visual performance was reassessed after 10 and 19 sessions of vision training with improvements noted in acuity, accommodation, oculomotor accuracy and speed, eye alignment, fusional vergence skills, and stereopsis. CONCLUSION: Optometric vision therapy can decrease suppression, improve central fusional ability, visual acuity, accommodative accuracy, and stereopsis in a patient with monofixation syndrome.

Interocular suppression in primary visual cortex in strabismus: impact of staggering the presentation of stimuli to the eyes.

Diplopia (double vision) in strabismus is prevented by suppression of the image emanating from one eye. In a recent study conducted in two macaques raised with exotropia (an outward ocular deviation) but having normal acuity in each eye, simultaneous display of stimuli to each eye did not induce suppression in V1 neurons. Puzzled by this negative result, we have modified our protocol to display stimuli in a staggered sequence, rather than simultaneously. Additional recordings were made in the same two macaques, following two paradigms. In trial type 1, the receptive field in one eye was stimulated with a sine-wave grating while the other eye was occluded. After 5 s, the occluder was removed and the neuron was stimulated for another 5 s. The effect of uncovering the eye, which potentially exposed the animal to diplopia, was quantified by the peripheral retinal interaction index (PRII). In trial type 2, the receptive field in the fixating eye was stimulated with a grating during binocular viewing. After 5 s, a second grating appeared in the receptive field of the nonfixating eye. The impact of the second grating, which had the potential to generate visual confusion, was quantified by the receptive field interaction index (RFII). For 82 units, the mean PRII was 0.48 ± 0.05 (0.50 = no suppression) and the mean RFII was 0.46 ± 0.08 (0.50 = no suppression). These values suggest mild suppression, but the modest decline in spike rate registered during the second epoch of visual stimulation might have been due to neuronal adaptation, rather than interocular suppression. In a few instances neurons showed unequivocal suppression, but overall, these recordings did not support the contention that staggered stimulus presentation is more effective than simultaneous stimulus presentation at evoking interocular suppression in V1 neurons.NEW & NOTEWORTHY In strabismus, double vision is prevented by interocular suppression. It has been reported that inhibition of neuronal firing in the primary visual cortex occurs only when stimuli are presented sequentially, rather than simultaneously. However, these recordings in alert macaques raised with exotropia showed, with rare exceptions, little evidence to support the concept that staggered stimulus presentation is more effective at inducing interocular suppression of V1 neurons.

Inferior Oblique Muscle "Overaction" Caused by Inferior Oblique Muscle Shortening, Not by Hypertonicity.

To determine whether fusional vergence adaptation in patients who can fuse in at least some gaze positions can cause curvature of the non-fixing eye movement paths of patients with apparent "overaction" or "underaction" of the oblique muscles, yielding possibly erroneous evidence of hypertonicity as the cause of the overaction or underaction. The authors retrospectively studied Lancaster red-green plots of patients with oblique muscle "overaction/underaction." If fusion was present, the plot had usually been repeated after monocular occlusion for at least 30 minutes. Fundus torsion and fusion status were also recorded. After a patch test in patients displaying fusion, the non-fixing eye's movement path became more linear. Although it has been argued that true overaction of the oblique muscles would show curved eye movement paths on side gazes, in the study patients displaying fusion in at least some directions of gaze, the eye movement paths became more linear after patch testing, favoring the more mechanical explanation. Illustrated cases were consistent with the hypothesis that short inferior oblique muscles simply hold the globes in extorted positions, and the appearance of inferior oblique muscle "overaction" arises from the eyes' following their extorted movement paths on side gazes, not from hypertonicity of the inferior oblique muscle in the adducting eye. [J Pediatr Ophthalmol Strabismus. 2022;59(1):28-34.].

Monocular Diplopia in Idiopathic Intracranial Hypertension: A Case Report and Literature Review.

Diplopia is the double vision of a single object, and can be binocular or monocular. Binocular diplopia is caused by the misalignment of the visual axes, with images falling on the fovea of the fixating eye and on the extra-foveal retina of the non-fixating eye, as a consequence of both neurological (i.e., oculomotor nerve palsies, ocular myopathies, neuromuscular junction disorders) and ophthalmic disorders (i.e., decompensation of a pre-existing strabismus). In contrast, monocular diplopia is generally explained by intraocular pathology (i.e., refractive errors, ocular media abnormalities, dry eyes), causing the image of a single object to fall, at the same time, on the fovea and on the extra-foveal retina of the same eye. We report the case of a 22-year-old woman presenting with acute-onset monocular diplopia. The diagnosis of idiopathic intracranial hypertension (IIH) was based on the presence of papilloedema and elevated cerebrospinal fluid (CSF) pressure. Monocular diplopia resolved after CSF subtraction. We describe a case of monocular diplopia as a presenting symptom of IIH, and discuss diagnostic issues of this possibly underestimated symptom in neurology clinical practice. Careful ophthalmic and neuro-ophthalmic examination can identify clinical features of diplopia, and drive diagnosis and treatment. Monocular diplopia is mostly an ophthalmological condition but can occur in a number of neurological diseases.Idiopathic intracranial hypertension can present with monocular diplopia.Differential diagnoses of diplopia in neurology and ophthalmology settings need to account for headache disorders.

Bilateral hypotropic dissociated vertical deviation: a case report\xa0

BackgroundHypotropic dissociated vertical deviation (DVD) is a special form of strabismus characterized by a slow drift of the non-fixating eye when the other eye is fixating on a target. In contrast to hypertropic DVD, which is common, hypotropic DVD is exceedingly rare and seldom reported in previous literature. In this case, we report the clinical features of a rare case of a Syrian child with bilateral hypotropic DVD accompanied by manifest latent nystagmus and intermittent exotropia.Case presentationA 4-year-old Syrian Arab girl presented with intermittent exotropia of both eyes since the age of 7 months, without any prior treatment. The fixation was alternating. She had manifest latent nystagmus in both eyes and anomalous head posture. She had bilateral hypotropic DVD in both eyes which only appeared when covering each eye. The patient underwent bilateral lateral rectus recession with posterior fixation and bilateral inferior oblique recession. Three months after surgery, she was orthophoric in the primary gaze position with a normal head posture. No alteration of the appearance of the hypotropic DVD was observed after the surgery.ConclusionThis is a rare case of hypotropic DVD showing bilateral hypotropic DVD with different characteristics from those previously reported cases (bilateral hypotropic DVD with intermittent exotropia, dissociated horizontal deviation, manifest latent nystagmus, and bilateral inferior oblique overaction). The hypotropic DVD only appeared when covering each eye, and thus there was no need for surgery. Moreover, the inferior oblique recession did not seem to negatively affect the appearance of the eyes.

Supramaximal Recession and Resection Surgery in Large-Angle Strabismus

Purpose:To assess the postoperative surgical outcomes and the changes in deviation in patients treated by supramaximal recession and resection (R&R) of rectus muscles to correct the large-angle exotropia and esotropia.Methods:This study was a prospective interventional case series, and patients with esotropia of ≥65 prism diopter (PD) or exotropia of ≥60 PD who had undergone supermaximal R&R in non-fixating eye with lower vision in unilateral strabismus or bilateral medial rectus (BMR) recession up to 8.5 mm for esotropia and bilateral lateral rectus (BLR) recession up to 12.5 mm for exotropia in bilateral strabismus were enrolled. Successful results were achieved if postoperation deviation was <10 PD for esotropia and <15 PD for exotropia during their final examinations.Results:A total of 131 cases (48 patients with esotropia and 83 patients with exotropia) were included. The mean ages of the patients with esotropia and exotropia were 16.83 ± 15.06 and 23.19 ± 11.29 years, respectively. The mean preoperative esodeviations for bilateral and unilateral surgeries were 69.5 ± 6.5 and 80.7 ± 10.3, respectively, and these values for bilateral and unilateral exodeviations surgeries were 67.3 ± 7.6 and 74.2 ± 12.1, respectively. The overall successful outcomes were achieved in 50% of the esotropic patients and 79.5% of the exotropic patients. At final follow-up examination, no patient had diplopia on lateral gazes.Conclusion:Based on our surgical results, it is possible to consider monocular recession-resection surgery in non-fixating eye (with poor vision) or BLR and BMR recession in both eyes as a viable option for surgical treatment of large angles deviations.

Role of a Standardized Prism Under Cover Test in the Assessment of Dissociated Vertical Deviation

ABSTRACTBackground: Dissociated vertical deviation (DVD) is commonly measured using a prism and alternate cover test (PACT), but some providers use a prism under cover test (PUCT). The aim of this study was to compare a standardized PUCT measurement with a PACT measurement, for assessing the magnitude of DVD.Methods: Thirty-six patients with a clinical diagnosis of DVD underwent measurement of the angle of deviation with the PACT, fixing with the habitually fixing eye, and with PUCT, fixing both right and left eyes. The PUCT was standardized, using a 10-second cover for each prism magnitude, until the deviation was neutralized. The magnitude of hyperdeviation by PACT and PUCT was compared for the non-fixing eye, using paired non-parametric tests. The frequency of discrepancies more than 4 prism diopters (PD) between PACT and PUCT was calculated.Results: The magnitude of hyperdeviation was greater when measured with PUCT (range 8PD hypodeviation to 20PD hyperdeviation) vs. PACT (18PD hypodeviation to 25PD hyperdeviation) with a median difference of 4.5PD (range –5PD to 21PD); P < 0.0001. Eighteen (50%) of 36 measurements elicited >4PD hyperdeviation (or >4PD less hypodeviation) by PUCT than by PACT.Conclusions: A standardized 10-second PUCT yields greater values than a prism and alternate cover test in the majority of patients with DVD, providing better quantification of the severity of DVD, which may be important for management decisions.

Botulinum toxin as an adjunct to monocular recession-resection surgery for large-angle sensory strabismus

To evaluate the result of using intraoperative botulinum toxin A (BTA) as an adjunct tomonocular recession-resection surgery in the management of large-angle sensory strabismus. The medical records of patients diagnosed with sensory strabismus with constant large-angle strabismus were reviewed retrospectively to identify those who underwent monocular recession-resection surgery combined with 5 units of BTA injection into the recessed muscle of the nonfixating eye. Surgical outcome was considered successful if the final deviation was within the range of 0Δ-10Δ. A total of 13 patients (mean age, 31.04±18.5years) were included, 8 with exotropia and 5 with esotropia. The mean follow-up period was 52.77±10.9months. The mean preoperative deviation was 66Δ ± 16Δ in the esodeviation group and 56Δ ± 5Δ in the exodeviation group. The final postoperative mean deviation was 6Δ ± 7Δ in the esodeviation group and 6Δ ± 8Δ in the exodeviation group. There were 7 patients (87.5%) with final deviation of ≤10Δ in the exodeviaton group and 4 (80%) in the esodeviation group. Adjunctive usage of BTA with conventional surgery appears to enhance surgical outcomes by increasing the amount of expected correction. The combined use of BTA with monocular recession-resection rectus muscle surgery may be a good alternative in the treatment of large-angle sensory strabismus with the advantage of less risk of potential complications compared to supramaximal surgery or surgery in the "good" eye.

Long-term stability of postoperative deviation in sensory strabismus: surgery of the dominant or nondominant eye

The nonfixing eye (NFE) is typically operated to improve sensory strabismus (SS). It is believed that surgery on the fixing eye (FE) would provide more long-term stability. To evaluate the surgical success, stability and complications of monocular surgery in FE versus NFE in SS.

DVD—a conceptual, clinical, and surgical overview

Dissociated vertical deviation (DVD) is a slow, disconjugate hypertropic deviation of a nonfixating eye. It is usually bilateral, asymmetrical, and often associated with congenital esotropia. The deviating eye elevates, abducts, and excyclotorts. This type of strabismus is often variable, making measurement and clinical quantification difficult. Specific knowledge of the mechanisms and characteristics of the dissociated deviation are required for proper assessment and effective treatment. There is currently no consensus on the mechanisms and pathophysiology of DVD. In this workshop, participants discuss the characteristics and most current methods for assessing and quantifying the deviation and explore the potential etiologies, clinical characteristics, and indications for surgical intervention and nonsurgical management of DVD.

Eye Fixation in Patients with Dissociated Vertical Diviation

Purpose: To clinically determine the proportion of people with a fixed dissociating eye in patients with dissociated vertical deviation (DVD). Methods: Medical records of 86 patients diagnosed with DVD in our department of ophthalmology from 2001 to November 2011 were retrospectively analyzed. Results: Of the 71 patients, 25 (35.21%) showed a higher degree of dissociation in the fixating eye, 26 (36.62%) had higher dissociation in the non-fixating eye; 20 of 25 patients with a higher level of dissociation in the fixating eye and 22 of 26 patients with a higher level of dissociation in the non-fixating eye received surgical correction for DVD, such as superior rectus muscle recession or inferior oblique muscle anterior transposition. Conclusions: In DVD, the proportion of patients showing a higher level of dissociation in the fixating eye was significant. Patients with DVD require thorough evaluation; measurement of the level of dissociation and determination of the fixating eye is beneficial for management and surgical intervention. J Korean Ophthalmol Soc 2014;55(6):882-886

Three-muscle surgery for very large-angle constant exotropia

To report the outcomes of surgery on three horizontal muscles for very large-angle constant exotropia. In this prospective case series, consecutive adult patients with primary constant exotropia with angles ≥120(Δ) underwent strabismus surgery on three horizontal muscles. Surgery consisted of lateral rectus recession of 10-15 mm on the fixing eye in addition to lateral rectus recession of 9-13 mm and medial rectus resection of 6.5-9 mm on the nonfixing eye. The mean follow-up period was >6 months. Success was defined as horizontal eso- or exodeviations of ≤10(Δ). A total of 23 patients were included. The mean age at surgery was 30.4 ± 9.3 years (range, 16-52). The mean preoperative near exodeviation was 128(Δ) ± 9.8(Δ); the mean preoperative distance exodeviation, 130(Δ) ± 10.4(Δ). The mean follow-up period was 8.1 ± 1.4 months. At the last follow-up, 19 of the 23 cases (83%) were successfully aligned. At final follow-up examination, the mean postoperative exodeviation was 5(Δ) ± 4.2(Δ) (near) and 5(Δ) ± 4.3(Δ) (distance); no patient had diplopia on lateral gazes. In this patient cohort, surgery on three-muscle surgery for very large-angle exotropia successfully restored alignment in primary gaze in over 80% of cases without inducing symptomatic abduction deficits.

DVD—a conceptual, clinical and surgical overview

Dissociated vertical deviation is recognized by clinicians as a slow, dissociated hypertropic deviation of a nonfixating eye. It is usually bilateral, asymmetrical, and often associated with congenital esotropia. The deviating eye elevates, abducts, and excyclotorts. There is no unanimous agreement on the mechanism and pathophysiology of DVD.

Relationship of Hypertropia and Excyclotorsion in Superior Oblique Palsy

PurposeTo evaluate the correlation between hypertropia and excyclotorsion in acquired superior oblique palsy (SOP).MethodsThirty-one patients with acquired unilateral SOP were recruited for this study. The torsional angle of each patient was assessed via one objective method (fundus photography) and two subjective methods (double Maddox rod test and major amblyoscope). The patient population was divided into two groups (concordance group, n = 19 and discordance group, n = 12) according to the correspondence between the hypertropic eye (paralytic eye) and the more extorted eye (non-fixating eye), which was evaluated by fundus photography.ResultsThe mean value of objective torsion was 5.09° ± 3.84°. The subjective excyclotorsion degrees were 5.18° ± 4.11° and 3.65° ± 1.93° as measured by double Maddox rod test and major amblyoscope, respectively. Hypertropia and the excyclotorsional angle did not differ significantly between the groups (p = 0.257). Although no correlation was found in the discordance group, the concordance group showed a significant and positive correlation between hypertropia and excyclotorsion (p = 0.011).ConclusionsTorsional deviation was not related to hypertropia. However, in the concordance patients in whom the hypertropic eye showed excyclotorsion, a significant positive correlation was found between hypertropia and excyclotorsion.

Design and efficacy of surgery for horizontal idiopathic nystagmus with abnormal head posture and strabismus

The design and efficacy of surgery for horizontal idiopathic nystagmus (HIN) with abnormal head posture and strabismus were investigated. Different surgical procedures were selected according to the angle of head turn in 44 cases of HIN with abnormal head posture and strabismus. For patients with a head turn of 15° or less, the Anderson procedure was used; the yoke muscles were recessed upon slow-phase. For patients with a head turn between 15° and 25°, the surgery was designed as a Kestenbaum 5-4-4-5 procedure. For patients with a head turn of 25° or more, the surgery was designed as a Parks 5-8-6-7 procedure. The surgery to correct the abnormal head posture was performed on the fixating eye while that to correct the deviation was then performed on the non-fixating eye at the same time. The amount of surgery of the horizontal rectus muscles on the non-fixating eye was sum of the angle of head turn and the degree of deviation, which was calculated as follows: recession/resection amount of medial and lateral rectis / 2×5 =angle of head turn ± degree of deviation. The results showed as follows: (1) Visual acuity: the visual acuity in the primary ocular position increased two lines or more in 35 patients, accounting for 79.55%. Nine patients had no or only one-line improvement, accounting for 20.45% of the entire study population; (2) The degree of deviation in the primary ocular position: 37 cases had a normal primary ocular position or the degree of deviation ≤ 8(δ) after surgery, accounting for 84.09%. Six patients had a residual degree of deviation of 8(δ)-15(δ), accounting for 13.64%. One patient had a residual degree of deviation >20(δ), accounting for 2.27% of the patients examined; (3) Abnormal head posture: 34 patients had a normal head posture or a head turn of less than 5°, accounting for 72.27%. Eight patients had a residual head turn of 5°-15°, accounting for 18.18%. Two patients had a head turn of 15°-25°, accounting for 4.55%. It was concluded that different surgical procedures based on the angle of head turn and the relationship between deviation and null zone can eliminate anomalous head posture, correct deviation, and improve vision acuity in the primary ocular position.

Changes of saccadic eye movements in thyroid-associated ophthalmopathy

To establish whether or not the dynamics of saccadic eye movements are significantly changed in patients with different stages of thyroid-associated ophthalmopathy (TAO) and, subsequently, if analysis of saccades could serve as an additional diagnostic tool for early detection of inflammatory activity in TAO. Thirty-seven patients with TAO and 10 age- and gender-matched control subjects were investigated. The patients were divided into four groups according to the stage of the disease: (i) early mild disease (n = 10), (ii) early severe disease (n = 11), (iii) long-standing restrictive disease (n = 10) and (iv) proven auto-immune hyperthyroidism without any signs of TAO (n = 6). Horizontal and vertical saccades with amplitudes of 10°, 20°, 30° and 40° were recorded binocularly using the induction scleral search coil technique. The two main sequence constants V(max) and C were calculated for each eye. Repeated measurement analysis of variance was carried out to test for differences between different gaze directions, eyes and groups. In horizontal saccades, significant differences were found between groups but not between abduction and adduction. In vertical saccades, differences between groups and the interaction between groups and up- and down-gaze saccades were significant. Compared with the control group, analysis of the main sequence curves revealed larger differences in patients of group 3 and 4 than in those of group 1 and 2. Whereas in the control group down-gaze saccades were faster than up-gaze saccades, the opposite behaviour was found in all patient groups. The largest differences were detected among those patients who had no signs of TAO (group 4). In this study, significant saccade differences were detected in all patients with TAO. In contrast to our earlier studies where evaluation of multiple individual saccade parameters did not reveal significant differences, analysis of the main sequence constants and mathematical reconstruction of the main sequence curves turned out to be a sensitive technique for reliable detection of subtle ocular motility changes. Significant differences were detected even in patients with auto-immune thyroiditis where no clinical signs of TAO were apparent.

Development of motor fusion in patients with a history of strabismic amblyopia who are treated part-time with Bangerter foils

To document the development of motor fusion when patients with a history of strabismic amblyopia are treated part-time with Bangerter foils. This was a prospective interventional outcome study of consecutive patients with a history of strabismic amblyopia, horizontal strabismus (only) ≤20(∆), visual acuity of 20/60 or better in the nonfixating eye, and no motor fusion (as indicated by the absence of prism vergence) for 1 year before entry into the study. Subjects wore a 0.1 density Bangerter foil for 3-4 hours daily. Data on visual acuity, alignment, and motor fusion status were collected for a minimum of 2 years. Patients with motor fusion were then followed for a minimum of 18 months to assess the stability of their motor fusion status after the Bangerter foil was discontinued. Of the 46 patients meeting entry criteria (mean age, 5.3 ± 1.7 years) who completed follow-up, 28 (61%) developed motor fusion. Motor fusion was retained in all 17 patients who were followed after their foils were discontinued for a mean of 13.3 months. A child's motor fusion status is generally believed to be established during an early formative period of visual development. The development of motor fusion in many of our patients during the course of part-time Bangerter foil treatment suggests that improvements in motor fusion status can occur at a later age than previously believed.

The Prismatic Effect on Stereoacuity in Intermittent Exotropia

PurposeTo evaluate the effect of acrylic refractive prism and Fresnel membrane prism on stereoacuity in intermittent exotropia.Materials and MethodsStereoacuities of fifty-two patients (mean age, 12.4 years; range 6 to 45 years) with intermittent exotropia were measured using the Titmus and TNO stereotests, while they wore prisms of varying power on nonfixating eye or evenly on each eye.ResultsStereoacuities were significantly reduced with increasing prism power for both prisms, ranging from 8 to 25 prism dipotres. The effects on stereoacuity in single acrylic prism and single Fresnel prism were similar, whereas spilt Fresnel prisms reduced stereoacuity more than spilt acrylic prisms. Spilt prisms were found to have much less effect on stereoacuity than single prisms for both acrylic and Fresnel prisms.ConclusionThe use of acrylic refractive prism shared evenly on each eye would be optimal method to minimize the reduction of stereoacuity during the prismatic therapy for intermittent exotropia.