Mr Lawrence Yong, 53, was buying lunch when he had difficulty taking money out of his wallet.
The muscles in his right arm had stiffened suddenly and he could not move it.
The hawker shot him a dirty look.
"It was like he thought I had ordered the food but didn't want to pay," Mr Yong recalled.
This sudden freezing of muscles is a symptom of Parkinson's disease, which he has had for about 12 years.
He also gets the more common symptom of tremors in his right hand and arm, which get worse when he is under stress, causing him to quit his engineering job two years ago.
He is on four types of medication, which he has to take three times a day. A two-hour delay would bring on intense tremors.
The disease is caused by degeneration in the brain that interferes with motor transmission signals.
Over time, it results in trembling in the hands, arms, legs and face, stiffness of the limbs and body, slow movement and poor balance and coordination.
Patients in severe cases have difficulty swallowing, chewing and speaking, and may also have urinary problems, constipation and disrupted sleep.
Dementia is also more prevalent among such patients.
At the cremation service of former Prime Minister Lee Kuan Yew on March 29, his daughter Dr Lee Wei Ling revealed that he had Parkinson's disease during the last three years.
Mr Lee died on March 23 at the age of 91, after being ill with severe pneumonia. He had been warded at the Singapore General Hospital (SGH) since Feb 5.
The Republic's ageing population has seen a rise in the number of people with Parkinson's disease, and this group is likely to grow as it is the most common neurodegenerative disease among the elderly.
The Parkinson Society Singapore says there are 6,000 to 8,000 people with the disease here.
Associate Professor Louis Tan of the National Neuroscience Institute (NNI) said it usually takes more than 12 years for a patient to progress to the severe stage of the illness. Most patients are over 60 years old, though it can affect those as young as 40.
"Unfortunately, to date, there are no proven therapies that can help prevent the disease from happening or progressing," he said.
But there are medications that can counter the effects of the disease.
Dr K. Puvanendran, a neurologist who practised for more than 40 years at SGH before moving to private practice, said the effectiveness of the medications could wane after four to five years, and the disease could then progress very rapidly.
Despite taking medication, Mr Leong Piew Seng, 65, went from not being able to write properly to using a wheelchair in just five years. Now, he also has incontinence and difficulty swallowing.
Dr John Thomas, a neurosurgeon at Mount Elizabeth Novena Hospital and a visiting consultant at SGH, said surgery could bring about immediate improvement that could last for several years.
In Europe, many opt for this in the earlier stages of the disease, when greater physical changes occur, such as a shuffling gait and lack of balance, and the risk of falls increases, he said.
Said Prof Tan: "Surgery is a proven effective means of improving the symptoms and movement of patients with Parkinson's disease. However, we recommend surgery only when medical therapies are not able to sufficiently control the patient's condition."
NNI's medical director Associate Professor Ng Wai Hoe noted that only patients who remain responsive to the medication L-Dopa will benefit, with more than nine in 10 showing improvement following surgery.
The improvement can be quite dramatic - from a patient having difficulty walking and poor balance, to walking like a normal person.
Prof Ng added that surgery is safe and the benefits long-lasting, but its effectiveness will decline as the disease progresses.
Deep brain stimulation (DBS) surgery involves putting two electrodes into the brain to help regulate the signals. The electrodes are attached by wire to a battery set in the patient's chest.
It is expensive and can cost $90,000 or more in private practice. A basic battery lasts about five years and replacing it costs $25,000. The implant alone costs $36,000.
Prof Ng said the NNI does about one such operation a month, adding: "All patients who have been referred to us for DBS and are suitable for surgery have never been denied subsidy."
This article was first published on April 12, 2015.
Get a copy of The Straits Times or go to straitstimes.com for more stories.