Low CSF Pressure Headache is caused by an internal spinal fluid leak and may range from obvious and disabling to subtle and nagging. The brain normally sits inside a "bag" filled with spinal fluid which extends down from the skull into the spine. The "bag" consists of membranes called the meninges (as in meningitis). The fluid helps to circulate waste products out of the brain and provides cushioning and support. When the pressure of this fluid is too low – typically when there is a small leak somewhere in the meninges – the brain may "sag" downward when the patient is upright, stretching the meninges and nerves lining the brain and causing pain.