The symptoms can appear very suddenly and end in irreversible blindness. Dr Saleyha Ahsan met a group of people who've been affected and explains what to look out for.
"My mother in law lost a lot of weight, she was very low mood, very painful scalp, jaw pain - and when I say painful scalp, brushing her hair became impossible," says Amanda Bartlett.
"Within the four hours that we were in eye casualty, towards the end she reached round her chair and grabbed my hand and said 'Amanda, I can't see anything.' And she lost her sight that afternoon."
