The Tokyo attacks in retrospect: sarin leads to memory loss. (Science Selections).

In the wake of the 11 September 2001 terrorist attacks on New York City and Washington, D.C., the threat posed by chemical and biological weapons has instantaneously evolved from hypothetical nightmare to clear and present danger. Under this new set of circumstances, any addition to the body of scientific knowledge about the health effects of chemical or biological agents is particularly timely. In this month's issue, a group of Japanese researchers led by Yuji Nishiwaki report the results of their investigation into the long-term physical and psychiatric effects of acute poisoning by sarin, a deadly military nerve gas [EHP 109:1169-1173].

On 20 March 1995, members of the Aum Shinrikyo religious cult released sarin in the Tokyo subway, using umbrellas to puncture newspaper-wrapped bags of the gas as they left the trains. Twelve people were killed in the incident, and more than 5,500 required emergency medical treatment. Although several studies have looked at the acute clinical manifestations of sarin poisoning, the authors felt there was an urgent need to perform an epidemiologic study of the chronic effects of sublethal exposures to sarin.

The team examined rescue workers and police officers who had been dispatched to the scene and were exposed to sarin in the course of performing their duties--a group of subjects with similar occupational, socioeconomic, and educational backgrounds. The study included 56 exposed subjects from the Tokyo fire and police departments, who were subdivided into high- and low-exposure groups, and 52 nonexposed subjects of similar backgrounds from the same departments. The research was conducted three years after the exposure.

To assess neurobehavioral effects, the investigators administered five tests designed to measure psychomotor function and memory function. A significant causal relationship was discovered between exposure to sarin and memory disturbance. The exposed group performed less well, in a dose-effect manner, than the control group in the digit span memory test. In this test, the subject is asked to memorize a series of digits displayed on a computer screen at 1-second intervals and then enter the digits into the computer in the correct order within 10 seconds. Later, the subject is asked to enter the digits in reverse order. It was this backward digit portion of the test that uncovered significant memory loss in the exposed subjects. Other tests suggested other exposure-related memory effects, but the results were not statistically significant.

Subjects were also given psychometric tests to assess traumatic stress symptoms, in order to examine whether there was any correlation between psychologic stress and chronic physiologic effects. No such correlation was discovered.

The authors conclude that their findings suggest causality between the sarin attack and memory disturbance, although the mechanism behind that disturbance is unclear. They recommend further study of the link between sarin exposure and memory loss.