It was just after dawn and Yuen So-choi stirred in his sleep. When he opened his eyes, the angelic face of his 11-month-old daughter met his gaze. Yee-ting, who just two hours earlier had been rushed to hospital for a seemingly deteriorating case of cold and fever, now seemed better. Her father, seeing she had been fed medicine by her mother, felt pleasantly assured and dosed off again. That was 6am - the last time the 28-year-old engineer saw his daughter's contented face, the final moment of the family's tranquil being. At 8am, Yee-ting began panting. 'Her eyes rolled into her head,' said Mr Yuen. As he drove her to St Teresa's Hospital - where the family had been just four hours earlier, and had been sent away after a doctor diagnosed tracheitis and prescribed some oral medication - the girl's body went rigid, her arms and legs extended as spasms rocked her body. Within minutes, the fit was over. As the couple carried her out of the car, cold air hit her face. The girl looked relaxed again although her eyes were still shut. About 10am, the paediatrician in charge at the St Teresa's, a private hospital, attended to Yee-ting. He diagnosed meningitis. At 12.56pm, after a cardiac arrest, Yee-ting died. That was February 11, 1994. Mr Yuen thought it would be the worst day of his life. It wasn't. Almost two years later, on a similar chilly winter morning, Mr Yuen relived the nightmare that has hollow echoes of his daughter's death. This time, it was his second child Chun-hin's life that ended prematurely and in circumstances that started as unsuspiciously. The six-month-old boy was rushed to the Prince of Wales Hospital when he refused food, had a cough, runny nose, and low-grade fever - nothing desperate but enough to alarm the young couple who had experienced the anguish that the death of one child unleashed. Chun-hin was admitted on January 13, 1996. He too died a few hours after admission after a cardiac arrest. This is the story of something dreaded by every parent; producing two children that do not live to life's full. What was worse for this couple was the fact that when they buried not one but two of their children, it was not certain what they died of. What could have caused two young lives to end so quickly and so unexpectedly? The pregnancies were uneventful and their births, and upbringings, smooth. There were simply no signs of the tragedies to come. 'I saw the statue of St Mary in the hospital and I prayed to her for my daughter's life,' recalled Mr Yuen. 'When she died, I was very angry. I wondered about the doctors. It was heartbreaking to lose her.' Mr Yuen said that coping with the what, how, and whys once was difficult enough but twice over was almost unbearable. 'When death took my son, I thought I would leap from the Prince of Wales Hospital. Then I saw my wife was sobbing. I had to be strong for her.' When Yee-ting died, the post mortem report suggested the girl suffered from a little-understood Reye's syndrome, a rare, acute and sometimes fatal disease of childhood that usually follows a viral infection, and is marked by recurrent vomiting, distinctive changes in the liver, acute brain swelling and seizures. The couple believed the chances of a recurrence was non-existent. They went ahead and had their second child, hoping all would be well. All's well sometimes does end well - but it was only after the couple had lost two children that the doctors come up with a firm diagnosis. The investigation would not have reached a conclusion had it not been for the perseverance of professionals at the Prince of Wales, the Great Ormond Street Hospital in London and Amsterdam University Hospital. Between them, the experts discovered the children had died of carnitine transport defect, a rare genetic disease affecting less than 40 children worldwide. Yesterday an inquest returned a verdict of death by natural causes on Yee-ting. Mr Yuen said without this information, the family would have remained in limbo. Mrs Yuen was 20 weeks pregnant when Chun-hin died. Should she continue with the pregnancy? Would the couple lose their third child? Without a firm diagnosis, they would be living in fear. 'We had considered abortion. We did not want our hearts to break for the third time. When Chun-hin died, I thought fate was playing a strange game on us. The whole thing was mind-numbing. But the doctors said we had a good chance of bearing a normal child.' 'A year ago, we had another child. Tests showed he was unaffected,' said Mr Yuen. Dr Patrick Yuen, reader in paediatrics at the Prince of Wales Hospital, who was invited to give his opinion on Yee-ting's death, had doubts over her official cause of death. The clinical presentation, the laboratory finding of hypoglycaemia (low blood glucose) and the autopsy findings are compatible with Reye's syndrome. But Yee-ting's condition deteriorated far too rapidly. According to Mr Yuen's statement, Yee-ting seemed to be 'vigorous and playful' in the afternoon of February 10. Her condition deteriorated after 9pm. She died 15 hours later. 'It would have taken three to four days for the condition to deteriorate if it were Reye's. This aroused my suspicion,' said Dr Yuen. Dr Yuen knows the importance of arriving at a definitive diagnosis 'for the sake of the next child so that preventive measures can be taken and genetic counselling be given.' But there were many obstacles ahead. Experts at Great Ormond Street were consulted. They suggested Yee-ting could be suffering some form of inborn metabolic error. But to diagnose which inherited metabolic disease Yee-ting was suffering, live cells were required. There were none. Dr Yuen, meanwhile, arranged for more investigations to be performed on Chun-hin and his parents. He was still awaiting results of these tests when Chun-hin was admitted to the Prince of Wales Hospital. According to Charles Stanley, professor of paediatrics at the University of Pennsylvania School of Medicine who has written extensively on carnitine disorders, the kind of carnitine defect suffered by Yee-ting and Chun-hin is such a rare condition that it was 'certainly not something doctors would be thinking about very much' when they looked for clues. Additionally, the condition - plasma membrane carnitine transporter defect - could be difficult to diagnose, he said. '[And] the problem is you have to suspect it [before you can make a diagnosis].' The rare metabolic defect blocks the ability of cells to accumulate carnitine - an essential substance that allows the body to burn fat for energy - at a high enough level. Babies born with the defect however, may be in perfect health until they reach three or four years of age. Then, each one of them develops heart failure, said Professor Stanley. This is because the heart prefers to use fat as fuel. Without carnitine, the heart becomes weaker with age, and the problems posed by a weak heart invariably emerge when a child is three or four years old. The age of onset could be earlier, as in the case of Yee-ting and Chun-hin. Professor Stanley said that prolonged starvation was dangerous for asymptomatic patients. In the case of Chun-hin at least, a runny nose and low-grade fever had caused poor feeding. 'A period of fasting can cause your body to need to burn fat as fuel. With carnitine deficiency, you cannot do it so you become acutely ill. 'A body that cannot burn fat is like an aeroplane which cannot switch to the reserved fuel tank . . . When you run out of fuel, you cannot fly any further. You just crash.' Professor Stanley said about 25 patients with the rare hereditary defect had been reported in medical journals, and he was aware of a further 10 cases identified. Children with the condition - once they were diagnosed - responded quite well to treatment but 'it is quite easy for patients not to get diagnosed,' he said, adding that the diagnostic tests which required measuring the blood level of carnitine 'are rarely available' and 'not done in a standard hospital laboratory'. Dr Yuen doubts whether therapy would help patients under severe stress. Experts from the Netherlands who performed the tests on Chun-hin and the couple's third child, said that it had taken a while to inform the Prince of Wales Hospital about the patients because 'to perform all the experiments we need a lot of cells. Also, we had to repeat several experiments to confirm the diagnosis.' The tests, completed in October 1996, confirm Chun-hin as suffering from carnitine deficiency. As the condition is an autosomal recessive genetic defect, any of the Yuen's children have a 25 per cent risk of developing a serious birth defect, a 25 per cent chance of being unaffected, and a 50 per cent chance of being a carrier as both the parents are. Dr Yuen said that tests conducted in the Netherlands confirmed their third child is unaffected, but a carrier of the disease. 'Like his parents, he will be all right,' he said. 'This is what paediatrics is about. We cleared the doctors [who looked after Yee-ting] of any wrongdoing. We also put the parents' minds at ease. They know they have a normal child.' And after much darkness and struggling, Mr Yuen, who had to hold back tears when telling his story feels relived. 'We know what caused the children's deaths. But I don't think we will have a fourth child. There is always a chance that a child will be affected. The pressure will be too great.'