The Early vs Late Infantile Strabismus Surgery Study: do sources for bias exist in this non-randomised trial?

Background—The Early vs Late Infantile Strabismus Surgery Study Group investigates whether early or late surgery is preferable in infantile convergent strabismus, in a non-randomised, prospective, multicentre clinical trial. The current state of the study after end of recruitment is reported here, focusing on the question of possible sources for bias in this nonrandomised trial. Method—The prognostic factors were analysed at baseline in order to check for imbalances between the two treatment groups. Reasons for possible diVerences are discussed. Results—There is no evidence for clinically relevant inhomogeneities between the two groups concerning the distribution of the three prognostic factors spherical equivalent, degree of amblyopia, and limitation of abduction. The fourth prognostic factor, horizontal angle of squint, diVers significantly between the two groups. Conclusion—In the analysis of the final results we may have to account for diVerences in angle of squint at baseline by its inclusion as a covariate or by stratification. (Br J Ophthalmol 1998;82:934–938) Infantile convergent strabismus in most cases starts before the age of 6 months. The child will fail to develop regular binocular vision or lose it rapidly, suppressing the image of one eye. Some children will alternate with either eye, others will use one eye preferentially, resulting in loss of visual acuity in the other eye (amblyopia). Most but not all cases of amblyopia are reversible by patching the healthy eye. Infantile strabismus is very common: its incidence is about 1% in the general population in the first year of life. Therefore, strabismus operations for infantile strabismus are carried out frequently. In the United States, surgery for infantile strabismus is now generally performed at an early age (age 1–2), whereas in Europe many eye clinics still operate late (age 4–5). The main argument for early surgery is that binocular vision can be restored if the child is operated early enough. However, to what extent binocular vision can be restored has not been confirmed in a controlled study. Most reports concerning this controversy so far were uncontrolled studies or clinical observations. Additional arguments of the proponents of early surgery are that the parent-child relationship is not disturbed by the outer appearance of the child, the psychomotor development of the child is better when the eyes are straight, and an operation at a later age constitutes a greater psychic trauma