The general practitioner then referred Ho to Dr K.W. Liu at the Prince of Wales Hospital's department of medicine and therapeutics, who has a special interest in cognitive impairment.

Liu and his team of experts saw that Ho had some signs of dementia but the symptoms were not typical of Alzheimer's disease, which accounts for 34 per cent of young-onset dementia cases (those that start before 65 years of age). Ho was a non-drinker, so alcohol-related causes were unlikely. To find the cause, Ho ran a battery of tests.

First, Liu had Ho do cognitive assessment tests including the mini-mental state examination to determine the degree of cognitive impairment. Ho appeared to have lower than normal cognitive abilities for his age and educational background. He also had difficulty drawing a clock showing 10 minutes past 10 and drawing a cube, which was very unusual given his job as a tailor. It revealed Ho's poor visual-spatial ability in determining and navigating space.

Liu investigated possible physical causes such as sleep apnoea, thyroid disorders, nutritional deficiencies, HIV/Aids and even venereal diseases such as syphilis, which could cause cognitive impairment. Ho did not suffer any of these conditions.

Liu's team used a CT scan to check for brain lesions but only found that the back part of Ho's right brain had shrunk a little. It was not enough to explain the dementia.

Next, an MRI scan and angiogram were used to check for any additional structural or vascular lesions, as 18 per cent of young-onset dementia cases are caused by a stroke. This can cause part of the brain to die. Ho's scans came back clean.

Next up was the single photon emission computed tomography (Spect) scan, which shows the amount of blood flow in the right and left sides of the brain. If there is a difference in the amount of blood flow in any part of the brain, it could help pinpoint the problem area.

Finally, Liu made some headway in the investigations. Ho's Spect scan showed that his occipital lobe in the rear part of the brain had abnormally low blood flow, and there was a marked decrease in metabolic activity in that area. This part of the brain - the parieto-occipital lobe - houses one's visual-spatial abilities and a problem in that area was consistent with the type of cognitive impairment that Ho suffered. But Liu had another important scan to conduct.

In patients with Alzheimer's disease, deposits of an abnormal protein called a beta-amyloid are often found in the brain. Scientists think that these deposits likely coincide with the areas that suffer impairment. Liu conducted a positron emission tomography (PET) scan, which showed that Ho had the deposits in the part of the brain that interprets what we see.

Liu could finally put a name to Ho's troubles - posterior cortical atrophy, a neurodegenerative disease that is considered an atypical form of Alzheimer's disease. There is no cure, but drugs called cholinesterase inhibitors may slow the rate at which the body naturally destroys neurotransmitters that carry messages between his brain cells. They would hopefully retard the progress of the disease.

When Liu broke the news to Ho, the patient heaved a sigh of relief. He had a diagnosis and a name to his suffering. He was also vindicated, as his inability to work lay in a physiological cause, and not a failing in his character or personal ethos.

Ho was given the cholinesterase-inhibitor therapy, and it has helped improve his cognitive abilities for now. While Ho no longer works, Liu reports that he seems to have found a new lease on life and is pursuing new interests and taking exercise.

He has gained muscles, and his wife reports that he seems much happier than before.

With the knowledge that dementia will eventually exact a terrible toll as it slowly robs Ho of his cognitive and even physical abilities, he and his wife have chosen to seize the day and enjoy the lucid years while they last.