Previous authors have suggested that patching might disrupt fusion, leading to a sensory deviation, and that atropine by altering accommodative vergence, also might lead to strabismus.2-5
In our 2-year prospective study we found that amblyopia treatment is associated infrequently with the development of significant strabismus. Although 14% of children measured with orthotropia at baseline developed a microstrabismus in this clinical trial, only 5 (3%) were measured with a heterotropia of more than 8 Δ. We found similarly low rates for both atropine and patching initial treatment groups.
In this trial, the decision to perform surgery was not part of the protocol but was made by the investigator. When strabismus surgery was performed, it was almost exclusively performed on children with deviations >8Δ at enrollment, and not on patients who developed strabismus during their time on treatment.
Some patients with strabismus at baseline were measured to have an improvement (heterotropia >8Δ to microtropia or orthotropia; microtropia to orthotropia) in alignment without strabismus surgery. More than one-third of patients with a microtropia at baseline had no deviation measured after amblyopia therapy. Even more surprising was the chance of a patient with a heterotropia greater than 8Δ improving without strabismus surgery. Nearly one-half of patients reduced their angle to 8Δ or less, and 14% improved to orthotropia. This improvement was seen irrespective of treatment group assignment or presence of esotropia or exotropia at baseline. Our data show a trend to greater alignment improvement in those patients with more improvement in visual acuity. It seems likely that the treatment-related increase in acuity was associated with an improvement in fusion that led to an improved control of the strabismus.
Some patients with no strabismus on enrollment had a microtropia documented at the 2-year examination. This change occurred in 14% of patients with similar frequency in both atropine and patching groups. It is possible that some of these children might have had a microstrabismus at enrollment that was obscured by the reduced visual acuity or cooperation of the young child.
There are several limitations to our data. We measured the deviations with the simultaneous prism and cover test, but did not collect the exact size of the ocular deviations. Therefore, we are unable to report whether there were changes in alignment within the group of patients with deviations greater than 8 Δ between baseline and outcome. In addition, it is possible the improvement in alignment associated with amblyopia therapy was experienced primarily by children with moderate size deviations, rather than by children with larger deviations. It is also likely that patients with larger strabismus angles would undergo strabismus surgery. Therefore, the patients who completed the study without strabismus surgery might have had smaller angles of strabismus at baseline and been more likely to resolve or improve to a microstrabismus.
A second limitation is associated with spectacle correction. Although the investigators were instructed to prescribe full cycloplegic refraction determined hyper-metropic correction for their esotropic patients at baseline, they were free to adjust spectacle power at their discretion at subsequent visits. A change in power of the spectacles could be responsible for changes in ocular alignment. In an exploratory analysis we found no evidence of a spectacle adjustment in which plus power was increased for the esotropic patients with improved alignment or decreased for the esotropic patients with worsened alignment in an effort to maintain control of the alignment with the spectacles.
Third, because nearly every strabismic patient (90%) enrolled in this study had esotropia, the impact of amblyopia therapy on children with exotropia is unknown. Also, changes of alignment from orthotropia to microstrabismus or the reverse could be the result of variability of the measurement. Such changes in classification (eg, orthotropia to microtropia, or microtropia to heterotropia) should be viewed cautiously. Larger changes with a shift of at least 8Δ from orthotropia to heterotropia greater than 8Δ or the reverse would be less prone to measurement variability.
Finally, these data on alignment change do not show a causal relationship between amblyopia therapy and motor alignment, nor do they demonstrate whether the better time for strabismus surgery is during amblyopia treatment or once treatment is concluded. A study with an untreated control group would be needed to answer the former question and a clinical trial needed to answer the latter question. These motor alignment outcomes could represent the natural history in the setting of current amblyopia treatment. This study also lacks a baseline measure of binocular vision; hence, we are unable to correlate a change in alignment to a change in binocular vision with amblyopia treatment.
In conclusion, children who undergo amblyopia treatment with patching or atropine have similar low rates of deterioration and improvement of their ocular alignment. Whether this change in alignment is caused by prescribed amblyopia therapy is not known. However, parents of children with no strabismus or microstrabismus should be forewarned that there is a chance of a new or worse strabismus developing during treatment, but parents of children with strabismus may also be informed that there is also a chance of improvement.