Cataract and A-bomb survivors

Friday, August 8, 2008, Chandigarh, India

K.S. Parthasarathy

On August 6, 1945, exactly 63 years ago, an atom bomb destroyed Hiroshima. Three days later, Nagasaki faced similar devastation. The Radiation Effects Research Foundation (RERF) at Hiroshima carries out several studies on the health status of A-bomb survivors. One such study is the effect of ionising radiation on the eye lens of the survivors.




The 63rd anniversary of the atomic bombing at Hiroshima was on August 6; that of Nagasaki is on August 9



Until now, the radiation protection specialists assumed that only high doses of radiation of two Gy cause cataracts, but new data from the A-bomb survivors suggest that the dose threshold for both minor opacities and vision limiting cataracts may be below one Gy (RERF Update Vol 19, Issue 1 2008). Gray (Gy) is a unit of radiation dose; it is equal to an energy absorption of one Joule per kilogramme.

“That important finding is causing major risk assessment groups to consider re-evaluating their guidelines for permissible occupational and medical exposures to the eye”, RERF scientists claimed.

Lens of the eye is like lens of a camera. Radiation causes partial opacity (cloudiness) of the eye lens. Symptoms of cataract usually appear after a latency period of several months (two to three years on average) following radiation exposure. Senile cataract which is common in old age advances with age; radiation cataract seldom does. Radiation cataract infrequently causes visual impairment.

Radiation cataract possibly has a “threshold”, a certain dose value below which no effect is observed.

How does radiation cataract develop? There is a transparent layer of epithelial cells on the interior frontal side of the capsule that covers the lens. This layer maintains the function of the lens by slowly growing toward the centre, achieved through cell division at the periphery (called the equator) of the lens (RERF Update, 2008). Because radiation is especially harmful to dividing cells, exposed cells at the equator are most prone to damage. The damaged cells move toward the rear of the lens before converging on the centre. Such cells prevent light from travelling straight forward, causing opacity.

Study of radiation-induced cataracts in A-bomb survivors started at the Atomic Bomb Casualty Commission (ABCC), the predecessor of the Radiation Effects Research Foundation. During 1963-64, scientists found a statistically significant radiation dose response for certain type of cataracts. Later study in 1978-80 gave similar results. Some analyses gave conflicting results.

Neriishi and coworkers concluded that there is radiation dose response for cataracts indicating a threshold below one Gy ( RERF Update, 2008).

The authors found similar evidence in other studies. A study of children exposed during the accident at the Chernobyl nuclear power station reported a subsequent excess of cataracts. A Swedish followup study of infants treated with radium indicated excess cataracts at a dose of one Gy to the eye. A NASA study of the health of 295 astronauts predicted that relatively low doses of space radiation might predispose the crew to an increased incidence and early appearance of cataracts (RERF update, 19, 1, 2008). A recent study of Chernobyl clean-up workers reported similar results.

RERF scientists thus quoted the International Commission on Radiological Protection: (ICRP,2007): “…the lens of the eye may be more radiosensitive than previously considered. In particular, among the atomic bomb survivors,…. and a group of children treated for skin hemangiomas….., there is evidence of excesses of ….. cataract at doses somewhat lower than expected. Whether ICRP may now lower the dose limits to the eye for radiation workers and members of the public or not is not clear.

The RERF researchers found out that stored lens images of A-bomb survivors have great potential in cataract research, for re-evaluation and followup of the cases and for standardisation of analyses in training new researchers.

RERF is starting a project to collect and store lens tissues removed from A-bomb survivors for future use. Such tissues may provide information on molecular biological changes in the formation of cataracts.

Cataract studies on children of A-bomb survivors conducted during 2002-2006 are being analysed to verify whether there are trans-generational effects of radiation on the eye lens. RERF started a glaucoma prevalence study in 2006; it will conclude by September 2008; the results may be published later.

Dr K.S.Parthasarathy is former Secretary, Atomic Energy Regulatory Board