Aims There is a paucity of literature concerning intractable diplopia. The aims of this study were to determine the incidence of intractable diplopia in the UK, identify the causes and any associated risk factors, establish how cases are managed and if the treatment is successful and tolerated.

Methods A 1-year prospective observational study was undertaken via the British Ophthalmological Surveillance Unit (BOSU). This involved implementation of a reporting mechanism, which then triggered distribution of an incident questionnaire to explore clinical details concerning each case and a follow-up questionnaire 6 months later to explore how the case had been managed.

Results The incidence of intractable diplopia was 53 cases per year. The most common preceding events were strabismus surgery (32%), no known preceding event, that is,spontaneous (25%), severe head trauma (8%), cataract surgery (6%) and vitrectomy (6%). In the at-risk age group of 7 years and above, the incidence of intractable diplopia following strabismus surgery is 1 in 494 (95% CI; 1 in 296 to 790) cases. A total of nine different treatments were used in the management, with many patients receiving between two and four different methods. The overall success rate was poor, but most effective were opaque intraocular lenses (IOLs) (86%) and opaque contact lenses (50%).

Conclusion Intractable diplopia is a relatively rare but important condition. The main risk factor is a pre-existing strabismus, and careful counselling is needed when planning surgical correction in patients with no demonstrable binocular function. Treatment success of intractable diplopia is high when using opaque IOLs, although with additional risk, but is often disappointing via other methods where it can be difficult to eradicate the diplopia successfully.