After passing the epidural catheter through the Ligamentum Flavum we expect the catheter to end up in the epidural space (good) or in the subdural spaces (less good). That is a working model, but according to a study just published in Anaesthesia and Intensive Care, it might not be the whole truth.
The authors (Collier, Reina et al) suggest the existence of a fourth space.
The possibility was raised in a prior study, by the same authors, where they took a closer look at patients with ‘failed epidurals’.
In some of these patients, radiological studies showed restricted spread of contrast injected into the failed epidurals. This striking compartmentalisation (see image) didn’t look at all like any spread you would expect from epidural or subdural injections.
The fourth space
The authors realized this could represent ‘a fourth space’, more precisely located within the dura.
Next, the authors obtained samples of the epidural sac from cadavers and analyzed them.
After preparing the samples the authors could, in some of them, clearly identify anomalous separations of the laminae that make up the dura. The separations could expand into compartments, the intradural spaces.
They also documented how an epidural catheter can fit into one of these intradural spaces.
Check out the awesome electron microscope images on the right. It shows an 20G epidural catheter passing through one of these intradural space.
This could be the answer to some epidural fails that we anaesthetists otherwise struggle to explain.