Sometimes pain in the arms and legs mimics arthritis or bone infection, in which case leukemia may not be suspected. Rarely patients may present with respiratory difficulties as a consequence of a mediastinal mass (resulting from lymph nodes swelling) ( Fig. 5.6 ). Occasionally, in both ALL and AML patients, especially AML subtypes M4 and M5, may present with central nervous system (CNS) involvement by the leukemia cells. This usually results from infiltration of meninges (lining of the brain), and contamination of the fluid which circulates around the brain and spinal cord [the cerebrospinal fluid (CSF)]. Rarely the CNS involvement is actually due to the very large number of leukemic cells (called hyperleukocytosis ) within the brain circulation causing the blood flow to be sluggish. Both ALL and AML can also involve other organs, for example, testicular leukemic infiltration is often seen in boys with ALL.