Patient positioning is key when measuring opening pressure (OP) of cerebrospinal fluid (CSF) during lumbar puncture.
OP is only accurate when measured while the patient is completely horizontal and relaxed, in the lateral decubitus position (i.e. no neck flexion or extension, legs extended, no valsalva). In order to achieve this, you may need to carefully place patient in a lateral decubitus position if they are initially sitting upright prior to dural puncture and/or be sure to have patient straighten their legs (i.e. abort fetal position) once ready to measure OP.
Strictly speaking, normal range of CSF pressure is 8 to 21 cm, but obesity can increase it up to 25 cm and still be considered normal. Thus, while the significance of measurements between 20 and 25 cm in obese patients may be unclear, levels above 25 cm are always abnormal.
