Magnetic resonance imaging of extradural blood patches: appearances from 30 min to 18 h.

We have used magnetic resonance imaging to examine five patients treated with extradural blood patches for persistent post lumbar puncture headache. Images were obtained between 30 min and 18 h after patching. Extradural blood patch injection produced a focal haematoma mass around the injection site which initially compressed the thecal sac and nerve roots. The main bulk of the extradural clot extended only three to five spinal segments from the injection site, although small amounts of blood spread more distally. Spread from the injection site was principally cephalad. Mass effect was present at 30 min and 3 h, but clot resolution had occurred by 7 h, leaving a thick layer of mature clot over the dorsal part of the thecal sac. Eighteen hours after injection only small widely distributed clots, adherent to the thecal sac, were demonstrated. Extensive leakage of blood from the injection site into the subcutaneous tissues was present in all patients.