[1] Torbay Hosp, Dept Ophthalmol, Torquay TQ2 7AA, England
[2] Royal Devon & Exeter Hosp, W England Eye Unit, Exeter EX2 5DW, Devon, England
[3] Musgrove Pk Hosp, Dept Ophthalmol, Taunton, Somerset, England
[4] Univ Plymouth, Dept Math & Stat, Plymouth PL4 8AA, Devon, England