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| - The toxin believed to have poisoned Russian opposition figure Alexei Navalny is from a group of chemicals that range from insecticides to military-grade nerve agents, experts said Monday. Specialists treating the 44-year-old at the Charite hospital in Berlin say their tests have confirmed "poisoning with a substance from the group of cholinesterase inhibitors". This is a family of substances that includes drugs for Alzheimer's disease and certain insecticides, but also some of the most toxic known chemical weapons: nerve agents. Among them are sarin, VX and Novichok, which was used in the 2018 attempted assassination of Russian double agent Sergei Skripal and his daughter Yulia in Salisbury in England. It later killed a British woman in the same city. The specific substance involved in the Navalny poisoning "remains unknown," the hospital said. Cholinesterase inhibitors block the activity of an enzyme that sends messages from nerves to muscles, meaning that muscles "go into a sort of spasm", said Alastair Hay, professor of environmental toxicology at the University of Leeds. "All muscles are affected with the most crucial being those which affect breathing. As breathing is inhibited individuals may become unconscious. There are also direct effects of cholinesterase inhibitors on the brain," he said. In severe cases the victim may asphyxiate or suffer heart failure. Hay said symptoms would depend on how a person is exposed to the substance. If it is swallowed they could become nauseous at first and then as the toxin is absorbed they would have difficulty breathing and seeing. A sufficient dose would make them lose consciousness, he added. Navalny became so severely ill on a flight on Thursday that the plane had to make an emergency landing in the city of Omsk. Doctors in Russia said they found no trace of a toxin and he was eventually transferred to Germany on Saturday. His supporters believe he was poisoned by a substance in the cup of tea he drank at the airport. "If a cholinesterase inhibitor was used to poison Alexei Navalny it would leave a chemical signature that can be detected and confirmed," said Wayne Carter, an associate professor at the University of Nottingham. In a statement, the Charite hospital said it detected the effects of the poisoning -- the inhibition of cholinesterase -- through "multiple tests in independent laboratories". More tests are ongoing to determine the exact substance. The hospital said Navalny was being treated with the antidote atropine. This relieves symptoms by blocking acetylcholine -- a chemical transmitter that controls muscle contraction. Nerve agents attack the enzyme that controls acetylcholine, leading to an overproduction and muscle malfunction. With time, the body clears out the nerve agent and starts producing the acetylcholine-controlling enzyme itself. Hay said treating this kind of poisoning "can be very difficult and delays add to complications". He said patients are often kept in a coma as their bodies clear the toxins, which can take days and even weeks. Charite hospital said that the outlook for Navalny was uncertain, adding that it could not rule out the possibility of long-term effects "particularly those affecting the nervous system". In 2018, Skripal and his daughter escaped death after medical treatment. There have been several infamous poisonings of Kremlin critics in the past. Former Russian security service agent Alexander Litvinenko was fatally poisoned with radioactive polonium in a cup of tea in London. Russia refused to extradite chief suspect Andrei Lugovoi, who became a nationalist MP after the 2006 attack. Other opposition figures have suffered severe illnesses in Russia that they blamed on poisoning. klm/pvh
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