To the Editor:-The case report by Borum et al.  illustrates the difficulty in the differential diagnosis of headache in a parturient. Postpartum headache may be due to dural puncture, eclampsia, meningitis, or cortical vein thrombosis. Epidural blood patch is a recognized treatment for postdural puncture headache (PDPH). However, in the other conditions, injection of blood in the epidural space may cause complications.
Prolonged leakage of cerebrospinal fluid (CSF) may occur after a dural puncture. Before performing a blood patch, it is advisable to demonstrate this leakage. Cerebrospinal fluid rarely emerges from the needle when an epidural is performed in a patient with PDPH. In a series of 50 patients with PDPH, the CSF emerged from the epidural needle in only 8 patients (16%).  However, insertion of an epidural catheter and gentle aspiration revealed CSF in the epidural space of all these patients. 
The compliance of the dura depends on the volume and pressure of CSF contained within it. The CSF pressure is low in patients with PDPH.  The compliance of the dura decreases when the CSF pressure is low. Reduction in the compliance of the dura lowers the epidural pressure.  The epidural pressure gives a rough estimate of the prevailing CSF pressure.  The mean epidural pressure of parturients is approximately 15 cm H2O in the lateral posture.  However, it decreases in patients with PDPH. In 50 patients with PDPH the mean epidural pressure was 6.4 cm H2O .
Demonstration of free CSF in the epidural space and a low epidural pressure assists in the differential diagnosis of PDPH in the parturient. Before performing a blood patch, it is advisable to demonstrate free CSF in the epidural space and a low epidural pressure. Epidural pressure is easily measured using a 16-gauge (1.1 mm OD) epidural catheter as a simple manometer. [2,4] Injection of blood through a 16-gauge epidural catheter is easy and safe. [2,6]
J. L. Shah, M.D.
Department of Anaesthetics; City Hospital NHS Trust; Dudley Road; Birmingham, B18 7QH; England