Changes in Cerebral Blood Flow (CBF) in Prone and Sitting Position Before and After GSP-block and Before and After Epidural Blood-patch: A Substudy of NCT03652714

Study Population: Subjects who have consented to participate in GSPB-2018 will be approached for participation in this sub-study. Subjects who have consented to participate in GSPB-2018 are not required to participate in this sub-study. Inclusion Criteria: * Age \> 18 years * Patients with postdural puncture headache defined as moderate to severe postural headache (VAS \>= 30mm) after lumbar puncture or accidental dural puncture during epidural insertion with an onset within 3 days following the puncture. * Lack of headache remission within 1 day after dural puncture and after treatment with fluids, caffeine and paracetamol * Patients who have given their written informed consent for participation in the study and substudy after fully understanding the protocol content and limitations. Exclusion Criteria: * Patients who cannot cooperate to the study * Patients who does not understand or speak Danish * Allergy to the drugs used in the study * Has taken opioids within 12 hours prior to intervention * No possible transcranial window on transcranial doppler. If not possible to find a useable window to get measurement the patient will be excluded and replaced with another patient.