1 Central Middlesex Hospital, Ophthalmology Department, London North West University Healthcare NHS Trust, Acton Ln, Park Royal, London, NW10 7NS, UK. elizabeth.yang@nhs.net.
2 Northwick Park Hospital, Anaesthetics & Intensive Care Medicine, London North West University Healthcare NHS Trust, Watford Rd, Harrow, HA1 3UJ, UK.
3 Central Middlesex Hospital, Ophthalmology Department, London North West University Healthcare NHS Trust, Acton Ln, Park Royal, London, NW10 7NS, UK.