Pinpoint cold saves sight, not acuity.

Since its beginnings early in the baby boom, the plastic oxygen tent that serves as a womb for premature babies has saved many lives-but often at the cost of the infants' eyesight.

Now, doctors who can save a preemie weighing just 1 pound can sometimes preserve the infant's vision, though it may never be 20/20, says Earl A.

Palmer of the Oregon Health Sciences University in Portland in the April Archives of Ophthalmology.

The key is cryotherapy-pinpoint applications of extreme cold-to the retina.

The link between oxygen and blindness was recognized in the 1940s. At that time, doctors thought excess oxygen was the only culprit. They now know better. Excess oxygen halts the growth of blood vessels in the eye. As preemies are weaned from the oxygen, cells in the retina react by issuing a chemical that triggers rapid blood vessel growth. The new, weak-walled vessels leak blood, prying the retina from its bed. This causes severe loss of vision in 60 percent of the smallest preemies.

Doctors use cryotherapy to destroy tissue bordering the retina, thereby limiting the creation of new vessels. In this way, a baby's vision can sometimes be saved.

The researchers studied 234 children in 25 medical centers. They treated just one eye in each child with cryotherapy to find out whether treated eyes fare better than untreated ones. Thus researchers could 'make sure each patient gets the best treatment, not knowing which it would turn out to be,' says Palmer. At age 5, 32 percent of the children had lost all sight in the treated eye; 48 percent had lost all vision in the untreated eye. On the other hand, 13 percent of the children who retained sight in the treated eye had 20/40 vision or better, whereas 17 percent of the children with sight in the untreated eye had 20/40 vision or better.

Anthony Adamis of Harvard Medical School in Boston says that unless methods improve, saving premature babies means having more blind children. 'The incidence is rising,' he notes.