China's top health authority said on Friday that it is closely watching the ongoing investigation into what caused serious eye injuries, including blindness, among dozens of patients who received eye surgery at two hospitals nine months ago.

The National Health and Family Planning Commission said in a statement it will continue to guide the two hospitals involved in providing treatment to the victims and helping them seek compensation.

Impurities contained in perflutren, a medical gas commonly used to fill the hollow eye during surgery, may be the culprit. But experts have been unable to identify the impurities because of testing limitations and an insufficient amount of remaining samples, the China Food and Drug Administration said in a statement on Thursday night.

The statement followed media reports that 18 patients who were blinded in one eye after eye surgery at Peking University Third Hospital in June are waiting for the results of the investigation and still seek treatment.

The CFDA said 71 patients suffered eye injuries last year after eye surgery in which the same batch of perflutren had been used at Peking University Third Hospital in Beijing and Affiliated Hospital of Nantong University in Nantong, Jiangsu province.

The gas had been supplied by Tianjin Jingming New Technological Development Co, in Tianjin.

The company recalled the remainder of that batch after reports of the injuries emerged in July, and it suspended the production and sale of perflutren, it said on its website.

The company said it has been cooperating in the investigation with drug regulators, hospitals and patients and will make public any results of the investigation.

Zheng Xueqian, a lawyer at China Health Law Society, told China National Radio that by law, victims of accidents involving medical products can seek compensation from the hospitals that performed the surgery, and the hospitals can seek compensation from suppliers.

Peking University Third Hospital said in a statement that it has filed a lawsuit against Tianjin Jingming New Technological Development Co and provided compensation to some of the 45 patients who suffered eye injuries after its operations.

Affiliated Hospital of Nantong University in Nantong also has sued the company, it said in a statement last week.

The two hospitals bought a total of 150 boxes of the suspect gas from the batch and used most of it in May and June. The 13 boxes that remained were sent to local labs for testing, the CFDA said.

Test results released in July showed that the quality of the batch was uneven and some of the gas was substandard, but the impurities in the gas could not be identified because of testing limitations and insufficient samples, it said.

The CFDA said products from the same batch were sold in 25 provinces, municipalities and autonomous regions in China, and that 621 boxes were used in 82 additional hospitals, but no other injuries were reported.

[email protected]