He has breathlessness and his blood count is low. His kidneys are also not functioning properly, a doctor said.
Karunakaran was admitted to a private hospital here following fever Dec 10. The doctors are contemplating if he needs to be put on a ventilator.
The medical bulletin said the 92-year-old veteran's condition has "worsened". His son, daughter and Congress party supporters are by his side.
Karunakaran has been admitted to hospital on more than two occasions this year. In November last year, he spent more than a week in the hospital.
Karunakaran has been remaining indoors of late and rarely attends public functions.