Karen Lee-Johnston took her two-year-old daughter to Vision Express at Tesco Silverburn in Glasgow after noticing she had a dilated pupil in her right eye, was vomiting, very thirsty and had regressed to crawling due to poor balance.
Optometrist Aaron Spears spotted that Erika, known as Boo, was showing classic symptoms of a Craniopharyngioma tumour and made an emergency hospital referral.
Boo had further tests at the Royal Hospital for Children in Glasgow that day, including a CT scan which detected the condition, and an MRI scan to confirm the type and severity of the growth.
The MRI scan confirmed the growth was Craniopharyngioma - a rare brain tumour most commonly found in children, with just 30 cases per year in the UK.
Ms Lee-Johnston, 33, said: "We were waiting in a shared ward and a nurse told me she would look after Boo whilst I spoke to a doctor in a private room - at this point, I felt sick.
"There were a lot of nurses and doctors in this tiny room and I'm a straight up type of person, so I said, 'This isn't good news, is it?'. One of the neurosurgeons said, 'No'."
"The doctor told me that she most likely would have died if we had left it another one or two weeks.
"One in 20 million get this type of tumour and even though it's benign, the oncologist said it's in a malignant place."
The family were told that Boo had completely lost her vision in her right eye and she underwent surgery to try and save the sight in her left eye.
The widowed mother of three, whose husband died in 2017, said: "Thankfully, following the surgery her sight came back in both eyes - it was a miracle.
"People don't believe me when I explain what has happened to our family and I suppose it is quite extraordinary, but it could happen to anyone.
"Her future won't be easy but she's alive and has her sight - Aaron saved her life."
Boo is due to start her first cycle of chemotherapy shortly to reduce the tumour further and it is hoped that the chemotherapy cycles with proton beam therapy will give the little girl's head and face more time to grow, so she can have a less invasive form of tumour surgery - a route through her nose leaving no scar and less risk associated overall.
Following the eye test on March 26 and surgery two days later, Ms Lee-Johnston, from Glasgow, is sharing her daughter's story as part of National Eye Health Week which runs from September 23-29.
She urged others to pay attention to warning signs and have them checked.
Vision Express optometrist Aaron Spears said: "Boo was showing classic symptoms of a Craniopharyngioma tumour, which grows above the pituitary gland - a pea-sized gland that controls many vital functions.
"This explains why Boo was often unstable on her feet and was having optical issues. Thanks to the referral, she got her scans and the consultant phoned me back later to say I had made a good spot.
"Parents with any concerns for their children's sight should have it checked out at their local optician as soon as possible."