NEW DELHI (AFP) - The parents of an 18-month girl in India whose head has swollen to more than double its normal size reacted with joy on Monday after a top private hospital offered to fly her to New Delhi for tests.

Roona Begum, who suffers from hydrocephalus, a rare but treatable disorder that causes fluid to build up on the brain, was discovered last week living with her parents who are too poor to pay for treatment.

Her condition has resulted in her head swelling to a circumference of 91cm, putting pressure on her brain and making it impossible for her to sit upright or crawl on the ground.

The publication of pictures taken by an AFP photographer in remote Tripura state in northeast India last Friday led numerous well-wishers to step forward offering donations, while a website has been set up to collect money for her.

Leading Indian neurosurgeon Sandeep Vaishya, who is the head of neurosurgery at a hospital for the private Fortis Healthcare group near the capital, pledged to examine the girl and see if surgery was possible.

"Fortis will fly her down and while we will have to do an MRI to check the condition of her brain, I am hopeful that we will be able to carry out a surgery and relieve the pressure on the poor child's brain," Dr Vaishya told AFP.

The group, which operates 76 hospitals in 11 countries, has a charitable foundation that carries out surgery free of charge.

Roona's 18-year-old father, Mr Abdul Rahman, who lives in a mud hut with his family in the village of Jirania Khola, told AFP he was "very thankful for everyone's generosity".

He added: "We have no money and I don't know how we can ever repay them for what they are doing."

Her elated 25-year-old mother Fatema Khatun said: "I am so grateful. I have no words to express my happiness."

Local doctors had told the family to take the newborn to a private hospital in a big Indian city but the costs were too high for Mr Rahman, an illiterate labourer who earns 150 rupees (S$3.40) a day working in a brick plant.

The US government's National Institute of Neurological Disorders and Stroke estimates about one in every 500 children suffers from hydrocephalus.

The most common treatment involves the surgical insertion of a mechanism to drain cerebrospinal fluid away from the brain and towards another part of the body where it can be easily absorbed into the bloodstream.

Extreme cases like Roona's are very rare, according to Dr Vaishya.