A patient who underwent transplant surgery using retinal cells derived from so-called iPS cells from another person has suffered a swollen retina, the team that carried out the world's first clinical trial of the procedure said Tuesday.

It is the first time a patient has developed a serious adverse reaction during the clinical research to assess the feasibility of using the artificially derived induced pluripotent stem cells, said the researchers from Kobe City Medical Center General Hospital and the government-backed Riken institute.

Doctors operated on the patient in his 70s on Monday to remove pre-retinal membrane, deemed to be the cause of the retinal edema. The team was unable to improve the symptoms despite the administration of steroid and anti-vascular endothelial growth factor medication.