Observations on deliberate dural puncture with a Tuohy needle: pressure measurements.

Pressure recordings were made during passage of a Tuohy needle from the interspinous ligament to the subarachnoid space for lumbar drain insertion. Epidural space pressures were always positive. Negative pressures were seen only at the moment of entry into the subarachnoid space. These were artefactual and were caused by tenting of the dura by the blunt Tuohy needle. Use of a closed measurement system such as this facilitates the development of large transdural pressure gradients because of the inability of the epidural space pressure to equilibrate with atmospheric pressure. This may contribute to ease of dural puncture.