 Twenty-six physicians were hired to help Lenin during his final years; many of them were foreign and had been hired at great expense.Some suggested that his sickness could have been caused by metal oxidation from the bullets that were lodged in his body from the 1918 assassination attempt; in April 1922 he underwent a surgical operation to remove them.The symptoms continued after this, with Lenin's doctors unsure of the cause; some suggested that he was suffering from neurasthenia or cerebral arteriosclerosis; others believed that he had syphilis, an idea endorsed in a 2004 report by a team of neuroscientists, who suggested that this was later deliberately concealed by the government.