Lumbar Drain: A small flexible tube placed in the cerebrospinal fluid filled thecal sac of the lower back (lumbar area). The tube drains the fluid that surrounds the brain and spinal cord which relieves pressure. It may be used to drain and assess cerebrospinal fluid (CSF) for many different reasons. A lumbar drain may be inserted to allow healing at the operative site by decreasing CSF volume and pressure. A lumbar drain is usually inserted if a CSF leak persists with bed rest or if CSF is seen in the surgical field.