1 Department of Ophthalmology, Gloucestershire Hospitals NHS Foundation Trust, Cheltenham, UK.
2 Department of Ophthalmology, Gloucestershire Hospitals NHS Foundation Trust, Cheltenham, UK; International Centre for Eye Health, London School of Hygiene and Tropical Medicine, London, UK.
3 Department of Ophthalmology, Gloucestershire Hospitals NHS Foundation Trust, Cheltenham, UK; School of Health Professions (Faculty of Health), Plymouth, UK. Electronic address: andrew.mcnaught@nhs.net.