Unclogging blockages in retinal veins

If a vein in the retina gets blocked, it's not just your eyesight that's at risk: the pressure can be so painful that the whole eye has to be removed. Now researchers at Stanford University in California believe they may be able to treat the problem with a new instrument that uses a plasma to fire tiny bullets of blockage-dissolving drugs into a clogged vein.

Like the blockages in blood vessels that cause heart attacks, "occlusions" in retinal veins are one of the most common retinal problems-apart from those caused by diabetes-says Rodney Grey, a surgeon at the Bristol Eye Hospital.

The tiny vessels that supply blood to the retina drain into a single vein. "If this central vein becomes blocked, you get a build-up in blood pressure in all the other, smaller veins," says John Forrester, an ophthalmologist at the University of Aberdeen. The pressure makes the small veins rupture and leak, flooding the inner cavity of the eye with blood. But that's just the beginning of the trouble. "Because of the blockage, blood can't flow into the eye, so the cells in the retina begin to die off-they don't get enough oxygen," says Forrester. And once the retinal cells start dying, they release growth factors to encourage new veins to grow-but they grow in the iris instead of the retina. "It's a crazy, misplaced attempt by the body to fix things," says Forrester. These larger iris veins obscure vision and exert painful pressure in the eye.

To treat the condition, Stanford researchers Daniel Palanker and Dan Fletcher have developed a needle-shaped probe they say could fire drugs right at the site of the blockage and reopen the vein. The hollow needle is 1.5 millimetres in diameter and tapers to a point just 50 micrometres across. Inside the needle, a fine wire electrode surrounded by a metal sheath is bathed in electrically conductive saline solution containing a drug. Palanker found that when he applied 1000 volts between the electrode and metal sheath, an ultra-hot plasma formed at the electrode tip, producing a rapidly expanding vapour bubble. By alternating the voltage, he could make the bubble expand and collapse within a millionth of a second. "Each time the bubble expands, it ejects a pulse of saline," says Palanker.

The pair plan to mix a drug called a tissue plasminogen activator into the saline solution within the probe. "If you can get the tip of the probe near the blocked vein, you could shoot the drug into the vein, where it will dissolve the blockage," Palanker says, adding that the hole made by the bullet will heal quickly.

The probe has advantages over surgical needles, says Palanker. "These veins are very thin. If your hand is trembling slightly, you'll end up causing far more damage." But his new probe will never touch the vulnerable vein, so mistakes aren't disastrous.

"There's no new treatment for this at the moment, so any new ideas are good news," says Forrester. "But like treating a heart attack, you'd have to get going quickly to save sight." It needs to be done in a matter of hours-but that's not impossible, he says. Grey says: "The key question is: can they get enough drug into the vein to have a good effect?"

Palanker now plans to test how good the plasma-powered bullets are at cutting tissue during microsurgery. While lasers tend to damage surrounding tissues by creating bubbles in local body fluids, the pure saline jets might make even finer cuts without stressing nearby tissue.

Author: Ian Sample

New Scientist issue: 28th April 2001

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