'He would run nonstop for 20 minutes, dashing up and down the house, and he is nervous and excited in crowded environments like shopping malls and supermarkets,' Yang says.

'His teacher at school called me several times telling me his behaviour in classroom is dangerous to other children.'

Nathan is among a significant number of Hong Kong children with ADHD - the Hong Kong Association for ADHD estimates there are more than 40,000 local children suffering from the condition.

According to Australian child psychologist Louise Porter, the prevalence worldwide is alarming. Three to 5 per cent of school-age children in Australia, New Zealand and the US have been diagnosed, and 8 per cent in Japan, which is 'a very high figure', she says.

Genetics and mothers' health during pregnancy are possible factors in the neuro-developmental problem, says Porter, who recently gave a lecture at the University of Hong Kong.

'The exact cause of the impairment is uncertain,' she says. 'Genetics is clearly implicated, as is mothers' health during pregnancy. Two known risk factors are stress and maternal smoking in the first half of pregnancy, when nicotine alters the foetus' serotonin production and brain-cell growth.

'The deficit is thought to result from immaturity in how the frontal parts of the brain function. This area of the brain is responsible for planning and impulse control.'

People with ADHD are broadly divided into two categories.

The first involves mainly inattentiveness. Symptoms include failure to pay attention to details, trouble with sustaining focus, being forgetful, easily distracted by external stimuli and often losing things necessary for activities, like books.

With the second, hyperactivity and impulsiveness are predominant characteristics: children usually fidget with hands or feet or squirm in their seat, run about excitedly or climb at inappropriate times, find it difficult to play quietly or wait, and talk excessively.

More recently, some experts have suggested a third category - the combined type where individuals show at least six symptoms of inattention and six of hyperactivity.

A child must display at least six of the symptoms for six months to be diagnosed with ADHD, Porter says, and symptoms should appear before the child turns seven.

To identify the condition, she first observes the child in a group and conducts IQ and medical tests to rule out other causes. But in early childhood, she says, 'it's hard to distinguish normal childhood exuberance from ADHD, which makes accurate diagnosis difficult'.

The disorder has far-reaching effects.

'Attention difficulties lead to reading impairments. Their reading failure leads to progressive declines in academic attainment. Children whose attention deficits give rise to social difficulties suffer emotional problems, including depression and anxiety. Their inappropriate social behaviour leads to acrimony at home and difficulties in establishing or sustaining friendships.'

And because ADHD children have poor impulse control, often engaging in excessive and repetitive actions, they may face rejection or bullying by other youngsters. Eleven-year-old Tecna, for instance, is ostracised by other kids in school because of her condition.

'She has symptoms like facial and vocal tics, which other students pick on,' says her father Michael, a civil servant. 'They think she's un-cool. She can't wait in a queue, and when she has something to say, she is unable to wait for her turn.

'She is loud, blunt and socially quite clumsy. She has no friends among her peers. Her classmates were told of her condition and are willing to help out, but she is left out of things on the playground.'

His daughter is lonely, Michael says, and poor concentration inevitably affects her academic performance.

'She is just scraping by. She abhors numbers. When she gets stumped by a maths problem, she cries and whines. Her ability to plan or prioritise is poor if not non-existent. When her mind is fixated on certain things, it's difficult to get her to switch focus to other stuff. She likes playing computer games and watching YouTube, but once she is into the groove of iPad or whatever, she can't shift to academic work.'

Michael and his wife try to be patient and tolerant, but admit Tecna's foibles can be maddening.

Given the high genetic component of ADHD, Porter says, many parents may share some of the symptoms and have difficulty dealing with their child.

'As a result of their children's demanding behaviour, parents often feel stressed and may discipline their children in ways that exacerbate their behavioural difficulties.

'Coercive discipline and negative parenting just make the behavioural problems worse. Parents should not punish children because their brains are not well.' Porter says medication should be the last resort as it might cause side-effects without bringing any permanent improvement.

'A common criticism is that drugs are over-prescribed and used as a first and only treatment. Naturally, drugs should never be the first treatment option. While around 75 per cent of those who are accurately diagnosed respond to an amphetamine, science is still uncertain about which children benefit most from medication and at what doses. Almost 40 per cent of individuals using medication experience side-effects including fatigue, confusion, insomnia, appetite suppression, nausea and headaches.'

Medication certainly didn't help Tecna, who was prescribed a drug after she was diagnosed at the age six, her father says.

'Instead of relying on medicine, we talk to her and try to calm her when she breaks down in tears. We have a timetable so that she knows what to do at what time. She does her violin practise and takes her breaks according to the schedule.'

Changes to the teaching environment and parenting techniques may help, Porter says. In classrooms, for instance, it can be useful to seat children with ADHD in the least distracting location. 'They need to be actively engaged in learning in contrast to passive listening. They need to receive simplified instruction in short bursts and be allocated extra time to complete tasks.'

She suggests parents can approach their children more as guides, focusing on teaching self-monitoring and emotional self-control, particularly anger management.

Tips from Nathan's school principal have been a great help to Margaret Yang in curbing her son's disruptive tendencies.

'The principal advised me to set house rules. I give him a red star when he completes daily tasks I assign him. A red star for being able to sit down to eat breakfast, walking to school properly without running, taking a nap without screaming and so on. I tell him: 'You have to control yourself if you want red stars'. He gets seven to eight red stars every day and tells me he wants 10 more stars tomorrow.'

A healthy diet without any processed food and quality time with Nathan has also helped check his aggressive behaviour. 'No candies and refined sugar. [It is] a diet heavy on fruit, vegetables and brown rice. Everything is home-cooked,' Yang says.

'As a working mum, I don't have much time to spend with him. In the past, I would check e-mail when talking to him after work. Now, I give him one hour of undivided attention every night. From 7pm to 8pm, I give him 100 per cent of my attention with no Blackberry, phone, internet or television. We talk about his day at school. The good bonding time gives him confidence in our relationship. Now, he's a bit calmer and can sit for 10 to 12 minutes to read a book.'