Patients

The subjects of our observational study were consecutive patients younger than 15 years who visited our clinic between September 2009 and January 2010. Patients with basic intermittent exotropia according to the Burian classification and normal controls with orthotropia or exophoria of less than 10 prism diopters (Δ) were included. Strabismus was evaluated by the same pediatric ophthalmologist (J.-M.H.). We noted patient characteristics, including age, gender, best-corrected visual acuity (BCVA), refractive error, deviation at distance and near, type of exotropia according to the Burian classification, constancy of deviation (intermittent or constant), fixation dominance, and stereoacuity.

The patients were divided into 3 groups: (1) basic intermittent exotropia with equal dominance, (2) basic intermittent exotropia with a dominant eye, and (3) orthotropia or exophoria of less than 10 Δ. Patients were characterized as having basic intermittent exotropia if they met the following criteria : (1) normal accommodative convergence or accommodation ratio determined by the gradient method, (2) difference between distance and near exotropia of 10 Δ or less measured before monocular occlusion, and (3) lack of tenacious proximal fusion or strong proximal convergence after monocular occlusion for 1 hour. Patients were characterized as having intermittent exotropia if deviation was manifest intermittently at either distance or near; consequently, patients with constant exotropia at distance but intermittent exotropia at near were included. Eye dominance was determined on the basis of the results of a repeated cover–uncover test alternatively on each eye more than 3 times, which invariably revealed a fixation shift to a dominant fixating eye after uncovering the occluder during the tropic phase of exotropia. Patients were excluded if they had amblyopia, anisometropia, refractive errors exceeding −6.00 diopters (D; myopia) or +3.00 D (hyperopia), astigmatism of more than 1.50 D, constant exotropia at near, A- or V-pattern strabismus, dissociated vertical deviation, vertical deviation of more than 5 Δ, a history of strabismus surgery, paralytic or restrictive exotropia, ocular disease other than strabismus, chromosomal anomalies, or systemic disorders such as congenital anomalies or neurologic disorders. Patients with BCVA worse than 20/20 in either eye also were excluded. Anisometropia was defined as a spherical or cylindrical difference of more than 1.50 D between the eyes. Amblyopia was defined as a difference of 2 lines or more between monocular BCVA. An A pattern was defined as an increase of 10 Δ or more in exodeviation at downgaze compared with upgaze, and V pattern was defined as an increase of 15 Δ or more of exodeviation at upgaze compared with downgaze.

Ophthalmologic Examination

All patients underwent a complete ophthalmologic examination. Prism and alternate cover testing were performed using accommodative targets for fixation at 1/3 and 6 m. An additional near measurement was obtained after 1 hour of monocular occlusion of the habitually deviating eye, and another postocclusion near measurement was obtained using an additional +3.00-D sphere over each eye before allowing the patient to regain binocular fusion. Constant deviation was defined as divergence for 100% of waking hours; the constancy of deviation was assessed by the same ophthalmologist (J.-M.H.). Patients with hyperopia were given spectacles with lenses of approximately +1.00 to +1.50 D less than the full cycloplegic hyperopic refraction. Refractive errors were determined using cycloplegic refraction with 1% cyclopentolate hydrochloride and were reported as spherical equivalent values. Sensory status was evaluated using the Randot stereoacuity test (Stereo Optical Co, Inc, Chicago, Illinois, USA). A stereoacuity of 100 arc seconds or better, measured with the Randot stereoacuity test, was defined as good.

Correction of Distant Refractive Errors

The patients wore full or partial correction spectacles to achieve BCVA that was 20/20 or better in both eyes. A distant autorefractor measurement after correction was obtained with the Shin-Nippon NVision-K 5001 autorefractor. The patients were positioned such that the tested eye viewed a distant target directly aligned along the visual axis of the eye. The patients were asked to observe a distant spotlight monocularly. Five consecutive autorefractor readings were obtained for each eye, and the final autorefractor prescription was the average of these readings (power in increments of 0.12 D). The patients proceeded to the next examination if their corrected spherical and cylindrical refractive errors were within 0.50 D (−0.50 to +0.50 D). If either spherical of cylindrical refractive errors were more than 0.50 D, spectacles were prescribed according to their cycloplegic refractive errors. These patients came back 1 month later to continue with the examination wearing their newly prescribed spectacles, of which the corrected distant refractive errors were within 0.50 D.

Measurement of the Accommodative Response

Accommodative response was measured while the patient was wearing distance correction spectacles. The patient was asked to fix his or her gaze on a detailed near target (a high-contrast Maltese cross) located 33 cm away; this induced a 3.00-D demand of accommodation. The patients were instructed to look at the center of the cross and to keep the image clear. The accommodative response of each eye was measured using a Shin-Nippon NVision-K 5001 autorefractor during binocular and monocular accommodation. Binocular viewing was assessed first to maintain fusion during measurements, and the left and right eyes were examined in random order. The spherical equivalent of the accommodative response was subtracted from the 3.00-D demand, yielding an accommodative lag if positive or an accommodative lead if negative.

Main Outcome Measures

Primary outcome measures included the accommodative lag or lead of each eye in the binocular fusional state and during monocular occlusion. Accommodative responses were compared between fellow eyes within groups. Secondary outcome measures were evaluated on the basis of the asymmetry of the accommodative response between both eyes, which was defined as the difference in the amount of accommodative response between fellow eyes; the accommodative responses of the right eye or the dominant eye were subtracted from those of the left eye or the nondominant eye.

Statistical Analysis

Statistical analyses were performed using SPSS software for Windows version 15.0 (SPSS, Inc, Chicago, Illinois, USA). The paired-samples t test was used to compare characteristics between fellow eyes within the groups. The Pearson chi-square test, Fisher exact test, and one-way analysis of variance were used to compare the accommodative response and asymmetry of fellow eyes between groups. P values of < .05 were considered statistically significant.