A man with obsessive compulsive disorder (OCD) flew into a rage when he saw his father using the same piece of tissue to wipe his mouth as well as his nose.

Foo Shun Xiang, 37, hit Mr Foo Sah Leong, 68, with a 1.2m-long aluminium pole, and continued beating the older man even after he started bleeding.

When his mother, Madam Chow Ah Lee, 65, tried to intervene, Foo used the same pole to hit her multiple times until it broke. The court heard that her injuries included three fractured ribs.

After the attack, the jobless man told his parents to take a shower as they were covered in blood.

Foo pleaded guilty in court yesterday to one count of using the pole to cause grievous hurt to Madam Chow. A second assault charge involving his father will be considered during sentencing.

Deputy Public Prosecutor Esther Wong said Foo, who was diagnosed with OCD in 2009, fears dirt and contamination. Despite this, he refused to take his prescribed medication regularly.

Foo's parents were afraid of their son as he was liable to turn violent at any moment. From 2009 until this year, he has assaulted them on at least four occasions.

These incidents, however, were not reported to the police as Foo had threatened to hit his parents if they did so.

He was at home with them in their Chai Chee Drive flat at around 11pm on Jan 28 this year when he saw his father wiping his mouth and nose with the same piece of tissue paper.

After assaulting his parents, he threw the pole down a rubbish chute when it broke.

His mother complained of severe pain in her left rib cage at around 5pm the next day, and his parents went to the Singapore General Hospital.

DPP Wong said: "Before they left for the hospital, he raised his voice and threateningly told them to tell their doctors that they sustained their injuries from being attacked by dark-skinned foreigners who were robbing them.

"He admitted he wanted his parents to lie on his behalf so that no police reports would be made against him."

Foo's parents did as they were told but the truth emerged after a nurse who attended to Madam Chow lodged a police report on Jan 31 about the purported robbery.

At first, Madam Chow told police on Feb 1 that robbers had attacked her. She finally admitted the next day that she had lied to protect her son.

Court documents did not state if any action would be taken against her for giving false information. Foo, who is now in remand, is expected to be sentenced next Tuesday.