The intradural space: the fourth place to go astray during epidural block

BackgroundConsiderable uncertainty exists regarding accidental injection of local anaesthetic into the ‘subdural space’ during attempted epidural block. A whole range of clinical findings, from excessively high to failed blocks has been reported although many of these findings appear difficult to explain on the basis of our current knowledge of the anatomy. The existence of another, adjacent space, the intradural space, is postulated.MethodsOur study of atypical epidural blocks using contrast injection and radiographic screening has now obtained data on 130 patients, and results were reviewed retrospectively, searching for contrast flowing into the subdural region.ResultsRadiographic studies have revealed 10 patients with an unusual dense localised collection of contrast in a space previously unrecognised by anaesthetists. Clinical presentation was of inadequate neuraxial block, which could eventually be corrected by top-up doses, but with the possible risk of developing a high block. Late radiographic pictures revealed contrast escaping from the mass into the epidural, subdural or subarachnoid spaces.ConclusionsA review of electron microscopy studies suggested that a ‘secondary’ subdural space could be opened up by trauma in the distal layers of the dura. Our findings suggest that injection into this ‘intradural’ space can occur, resulting in an initially inadequate neuraxial block with limited spread. Further volumes of local anaesthetic can be expected to produce satisfactory block, probably as a result of escape to the epidural space. However, late spread to the subdural or subarachnoid space may occur.