CNS leukemia is more severe. Invasion of the central nervous system by leukemia cells is a specific extramedullary infiltrative form of leukemia called CNS leukemia. Because the blood-brain barrier prevents most drugs from reaching effective concentrations in the cerebrospinal fluid, almost all types of leukemia are at risk for developing CNS leukemia, and when it occurs it becomes one of the major obstacles to leukemia treatment. CNS leukemia presents with headache, nausea, vomiting, irritability, and if it infiltrates the brain parenchyma, it may manifest as seizures and lethargy. If leukemia patients have symptoms of central nervous system abnormalities, they should inform their doctors promptly.