Subdurals happen in the potential (or traumatically created, there is no consensus to what truly happens) space between the dura and the arachnoid. Radiologists find this space occasionally under fluoroscopy and note that when fluid is injected here it ascends against gravity. Secondly, once it is found (or created) it probably becomes a permanent defect making it more difficult to find the subarachnoid space, either at the time or subsequently. It can happen if the needle pokes through the dura or if the epidural catheter is threaded or/and migrates there. In one series of over 2000 epidurals, the incidence was 0.82%. Because it is tough to diagnose, the true number is likely higher. Subdural injection may be the cause of failed spinals despite free aspiration of fliud. (A few theories as to why: Arachnoid defects allow flow of CFS into the subdural space? Bevel changes directions and puts some medicine in both compartments? A previous pass has damaged the dura and allowed CSF to enter subdurally?)
Classic presentation is delayed and gradual. It usually occurs when your epidural seems to have gone perfectly well. Onset is in 10-30 minutes and starts with an unexpectedly extensive sensory block after negative test dose/aspiration. Motor block is typically minimal, though it can be extensive. Why? The subdural space is thought to be bigger posteriorly and the anesthetic tracks there, minimizing anterior (motor) block. Anteriolateral sympathetic fibers are hit, causing hypotension that is more than expected from epidural placement but less than a total spinal, and it is relatively easy to treat with fluids and small doses of ephedrine/phenyleprine. If the subdural injection continues to track cephalad, it can pool intracranially and cause slow-onset dyspnea and loss of consciouness.
Subdural Injection: Characteristics and Symptoms
- Gradual/delayed onset (10-30 mins)
- Extensive sensory block with minimal motor block
- Hypotension (more than with epidural, less than with spinal)
- Worst case scenario, intracranial tracking leads to dyspnea and LOC
- Clive B Collier Accidental subdural injection during attempted lumbar epidural block may present as a failed or inadequate block: radiographic evidence. Reg Anesth Pain Med: 2004, 29(1);45-51