Oct. 13 (HealthDay News) -- Increased survival of extremely pre-term infants has led to a greater number of babies with vision problems caused by abnormal development of blood vessels in the retina, a new study finds.

Swedish researchers looked at data on 506 extremely pre-term infants (born before 27 weeks of gestation) who survived until their first eye examination and found that 368 (72.7 percent) had retinopathy of prematurity -- 37.9 percent with mild cases and 34.8 percent with severe cases. Only 99 (19.6 percent) of the infants were treated for the condition.

Gestational age was a more significant risk factor for retinopathy of prematurity than birth weight.

"The incidence was reduced from 100 percent in the five infants born at 22 weeks' gestation to 56 percent in those born at 26 completed weeks. In addition, the risk of retinopathy of prematurity declined by 50 percent for each week of gestational age at birth in the cohort," wrote Dr. Dordi Austeng, of University Hospital in Uppsala, Sweden, and Trondheim University Hospital, Norway, and colleagues.

The study appears in the October issue of the Archives of Ophthalmology.

Previous research has found much lower rates of severe retinopathy of prematurity than the 34.8 percent noted in the new study: 25.5 percent in a Belgian study and 16 percent in an Austrian study.

"The higher incidence of retinopathy of prematurity in the present study may be because of the higher proportion of infants born in the earliest weeks of gestation [i.e., 11.5 percent of infants in weeks 22 to 23 vs. 0 to 6 percent in other studies]," Austeng and colleagues wrote. "These extremely premature infants, who previously did not survive, are probably especially vulnerable and prone to develop complications such as retinopathy of prematurity."

SOURCE: JAMA/Archives journals, news release, Oct. 12, 2009