Former prime minister Gordon Brown has described the dramatic moment he feared he would lose his sight completely.
Mr Brown had been left blind in one eye and suffered a loss of vision in the other after a blow to the head in a teenage rugby match.
In an extract from his memoir, My Life, Our Time, he describes how in Number 10, four decades on, he suffered a sudden deterioration in his good eye.
"When I woke up in Downing Street one Monday in September (2009), I knew something was very wrong. My vision was foggy," he writes.
"That morning, I was to visit the City Academy in Hackney to speak about our education reform agenda.
"I kept the engagement, doing all I could to disguise the fact that I could see very little - discarding the prepared notes and speaking extemporaneously."
As soon as the event was over, Mr Brown was driven to the consulting room of a prominent surgeon at the Moorfields Eye Hospital in London.
"To my shock, in examining my right eye, he discovered that the retina was torn in two places and said that an operation was urgently needed. He generously agreed to operate that Sunday," he writes.
On his way out, Mr Brown asked if an old friend, Hector Chawla, who had treated him in the past, could be invited to give a second opinion.
He saw him the day the operation was due to take place.
"I was already prepared for surgery when he examined me and said he was convinced that the tears had not happened in the past few days. They were not new but longstanding," Mr Brown writes.
"His advice was blunt. There was no point in operating unless the sight deteriorated further. Laser surgery in my case was more of a risk than it was worth."
Mr Brown - who did not even let on to Cabinet colleagues what he was going through - says he feels "lucky beyond words" that the retina has continued to hold.
"Even if I felt fate had dealt me a hand I would not have chosen, my time in and out of hospital - and the fight for my eyesight - gave me a perspective that I still feel helps me to be more understanding of difficulties facing others in a far worse position than me," he writes.