1 Department of Ophthalmology, St. Thomas' Hospital, Lambeth Palace Road, London, SE1 7EH, UK.
2 King's College, London, WC2R 2LS, UK.
3 Department of Optometry and Visual Science, School of Health and Psychological Sciences, City, University of London, Northampton Square, London, EC1V 0HB, UK.
4 Department of Ophthalmology, St. Thomas' Hospital, Lambeth Palace Road, London, SE1 7EH, UK. david.obrart@gstt.nhs.uk.