Double-masked randomised controlled trial of an amblyopia treatment
Prof Bruce Evans
Amblyopia has a prevalence of 1-4% and is the leading cause of monocular visual loss in the age group 20-70 years (Simons, 1996). Since AD 900 (Thabit Ibn Qurrah, 900), amblyopia has been treated by occluding the eye with better acuity. Although the lack of randomised controlled trials (RCTs) has been criticised (Moseley et al., 1995), this form of treatment is widely accepted clinically as long as the patient is treated within the so-called “sensitive period” or “critical period” of relatively high neural plasticity (Nelson, 1989).
Mallett (1983) claimed that amblyopia can be treated outside the sensitive period, in adults of up to at least 65 years. Other authors have also argued that relatively brief periods of active stimulation may be more effective than passive occlusion during everyday life (Wick et al., 1992). Mallett (1983) also claimed “moderate success” for anisometropic amblyopes.
Unfortunately, there have been no randomised controlled trials (RCTs) of the Mallett treatment. The whole subject of amblyopia treatment has recently become the subject of considerable controversy (Snowdon & Stewart-Brown, 1997) and this increases the importance of investigating Mallett’s claims. RCTs of another form of active amblyopia therapy, the CAM stimulator (Banks et al., 1978), showed that initial promising clinical results (Campbell et al., 1978) were attributable to practice and placebo effects (Tytla and Labow-Daily, 1981; Lennerstrand and Samuelson, 1983; Nyman et al., 1983).
Evans et al. (1999) carried out a clinical audit of Mallett’s IPS treatment for amblyopia. The mean improvement was two lines of the Snellen chart and 100% of this improvement had occurred after 5 treatment sessions.
The aim of this study is to compare the Mallett IPS treatment with a placebo, using a randomised double-masked protocol. Patients meeting strict diagnostic criteria for amblyopia are randomly allocated to an experimental and a control group. Subjects are treated for 6 weeks and, a week after the final treatment, acuities are measured but by a researcher who does not know which treatment the subjects have received.