Fixation preference testing or a binocular fixation pattern is the mainstay for diagnosis and treatment of preverbal children with Strabismic amblyopia.1'2 This test is based on the assumption that nonamblyopic patients with strabismus will alternate fixation well with either eye during binocular viewing. Conversely, patients with amblyopia will show strong fixation preference for the sound eye and will not hold fixation with the amblyopic eye.3,4 During this study, we aimed to reevaluate the reliability of fixation preference testing for the diagnosis and treatment of strabismic amblyopia.
PATIENTS AND METHODS
Between January 2003 and December 2004, a total of 80 patients with strabismus were included in this study. Of the 80 patients, 30 were diagnosed as having partially accommodative esotropia, 15 as having congenital exotropia, 15 as having primary acquired esotropia, 15 as having congenital esotropia, and 5 as having hypertropia (Table 1). All patients underwent full ophthalmic and orthoptic examination.
Patients with structural ocular abnormalities or anisometropia of more than 1 D in any meridian were excluded. Visual acuity of patients in this study was determined using Snellen ?-charts with patients wearing proper spectacle correction. Patients with a visual acuity difference of two or more Snellen lines between the two eyes were defined as having amblyopia.
Evaluation of fixation was performed by having the patient fixate with both eyes open on an accommodative target. The fixating eye was temporarily occluded to force fixation to the nonpreferred eye. When fixation was established with the nonpreferred eye, the occluder was removed, and by observing the period during which fixation was maintained, fixation preference was classified into four grades: (O) no fixation preference (patient freely alternates fixation); (1) mild fixation preference (patient prefers one eye, but still can hold fixation by the other eye through a blink); (2) moderate fixation preference (patient prefers one eye, but still can hold fixation by the other eye up to a blink); (3) strong fixation preference (patient prefers one eye and cannot hold fixation by the other eye). Amblyopia was defined as unmaintained fixation to binocular fixation pattern testing.
Visual acuity levels of the two eyes that were measured with the ?-chart and estimated with the fixation preference testing method were then compared. All tests were performed by the same author.
Of the 80 patients with strabismus who were studied, 50 were boys and 30 were girls, with a median age of 5.3 years (range, 3 to 8.3 years). Among the patients examined, 60 had a strong fixation preference (grade 3), 10 had a moderate to mild fixation preference (grades 1 and 2), and 10 patients freely alternated fixation (grade O) [Table 2].
Of the 60 patients with strong fixation, 10 (16%) had amblyopia, whereas the remaining 50 had no significant difference in visual acuity between the two eyes. For patients with moderate or mild fixation preference, five (50%) had no amblyopia and five were amblyopic. Patients who freely alternated fixation had equal vision in both eyes (Table 3).
Of the total of 80 patients in this study, 70 (87%) were diagnosed as having amblyopia using fixation preference testing, whereas only 15 (18%) patients were truly amblyopic using Snellen Echarts.
The results of this simple study showed that fixation preference testing was not reliable in diagnosing amblyopia because 55 of the 70 (80%) patients who were diagnosed as having amblyopia by this test were nonamblyopic (pseudoamblyopia) according to Snellen ?-chart testing. On the other hand, fixation preference testing was reliable for excluding amblyopia because patients with alternate fixation were nonamblyopic according to Snellen E-chart examination.
Zipf,5 in 1976, wrote a landmark piece on the reliability of fixation preference testing. He found that fixation preference testing was reliable for diagnosing amblyopia in patients with large-angle deviations. To date, there is no other simple standard test to help general or pediatrie ophthalmologists with rapid evaluation of visual function in infants and younger children. Fixation preference testing became a part of the standard evaluation protocol of ophthalmologists.6
The current study showed that fixation preference testing is unreliable for the diagnosis, and consequently, for the treatment of strabismic amblyopia. This agrees with the findings of other studies.7'8 Additionally, this test may be confusing, misleading, and even medically hazardous:
The testing can be confusing. In preverbal children, a uniocular strong fixation preference means deep amblyopia and requires patch treatment for the preferred eye until a shift to equal fixation preference occurs. This concept had led many general ophthalmologists to order patching for the preferred eye of verbal children in the presence of a strong fixation preference, even if the child has good Snellen visual acuity in both eyes. This is done to alternate fixation preference, which may never happen.
The testing can be misleading. Strong fixation preference may be present in the absence of amblyopia, and consequently, in preverbal children, the presence of a strong fixation preference could be misleading if used as the basis for a decision to start or continue amblyopia treatment.
This testing can be medically hazardous. Treatment of strabismic amblyopia on the basis of fixation pattern only could result in the long-term misdiagnosis and treatment of pseudoamblyopia. This might create amblyopia where none was present before. This also may delay the onset of strabismus surgery, with poorer sensory outcome.9
From my point of view, the strong fixation preference recorded in many patients in this study could be an exaggerated form of eye dominance in which the brain, for unknown reasons, uses only one eye and neglects the other eye as if it does not exist. This results in a high rate of pseudoamblyopia (a strong fixation preference without amblyopia) and makes fixation preference testing unreliable for the diagnosis of strabismic amblyopia.
Measurement of visual acuity is necessary for the diagnosis and treatment of strabismic amblyopia to guarantee successful long-term outcome of strabismus surgery.10 The Snellen ?-chart, Landolt rings, and Alien figures can be used with cooperative children. Visual acuity usually cannot be assessed with these tests before 4 years. For children younger than 1 year, we rarely need to evaluate or treat strabismic amblyopia before surgery, and fixation preference has no role. For children 1 to 4 years, we can use fixation preference as a diagnostic test for amblyopia to start a scheduled regimen of patch treatment if there was a strong fixation preference. Different studies showed that patients with strabismic amblyopia require a shorter duration of occlusion to equalize vision than patients with other types of amblyopia, and visual outcome was statistically significantly related to the age at initiation of treatment.11 Therefore, for such children, it will be easy to commit to a patch treatment strategy, after which we can proceed to corrective strabismus surgery without the need to reevaluate visual acuity using this unreliable method of fixation preference testing. Such studies are needed to overcome the dilemma of interpreting fixation preference testing results.
In conclusion, fixation preference testing is not appropriate for the evaluation of vision and treatment of patients with strabismic amblyopia. It should not be overvalued for that purpose, and better testing methods are required.
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