Background and Objectives: Until now, case reports after accidental subdural injection during attempted epidural block have usually described extensive neuraxial blocks with a delayed onset, after low doses of local anesthetic, with a characteristic radiographic appearance on contrast injection. Our radiographic investigation of atypical “epidural” blocks has revealed that subdural injection may go unrecognized clinically and may be a cause of inadequate blocks. The mechanism is explored.

Case Reports: A radiographic study of 35 cases of atypical or inadequate blocks for cesarean delivery has unexpectedly revealed four instances of subdural contrast injection. On imaging, only localized posterior spread of subdural contrast was detected in these 4 cases, unlike the more widespread distribution previously reported after extensive neuraxial blocks. Recent findings on the ultrastructure of the subdural space are reviewed and related to the new clinical findings, which include inadequate blocks and pain developing on postoperative reinjections.

Conclusions: Accidental subdural injection may now be added to the list of causes of failed or inadequate “epidural” block. Clinicians should be aware of the diagnosis of a possible subdural injection, if a poor quality block with restricted spread and slow onset is associated with pain on postoperative reinjection of the catheter.