A rare disease advocacy group on Wednesday urged the government to remove the age cap on funding treatment for a spinal cord disease that causes serious muscle weakness among patients and may leave them paralysed.

Rare Disease Hong Kong said currently, only those aged 25 or below will get government funding to buy medicine for spinal muscular atrophy (SMA) treatment for life, which could cost millions of dollars per year.

"If you applied [for the funding] before turning 25, you can receive subsidies even when you’re 25, 35, and beyond. However, if you’ve already over 25 years old, you are not eligible to apply," the group's president Tsang Kin-ping said.

He called for funding for all SMA patients.

Dong Dong, an assistant professor from the Chinese University of Hong Kong, said authorities might have set the age cap based on clinical trials of the drugs, which targeted the younger population.

"We understand that the government needs to have certain guidelines in terms of who should receive how many resources from the society or from the government," she said.

"However, the age limit, based on our understanding, is purely based on the clinical trials... If we can borrow the evidence from other countries, for example in mainland China, in the US, in Japan, in many countries, all SMA patients, regardless of their age groups... their results [after taking the drugs] are very optimistic."

Dong said although the drugs may not reverse the deterioration of body functions, they can slow down the process.

The concern group also urged the government to offer additional support for patients to undergo regular physiotherapy treatment, saying it's crucial for maintaining their muscle function.