Presentation
Patient with spontaneous subdural collections found on CT and suspected intracranial hypotension
Patient Data
The upper head was imaged for 10 minutes before, during and after gadolinium administration with a T1-weighted 3D spoiled gradient-echo sequence at a temporal resolution of 8 seconds. The contrast agent slides down the outer membrane of the subdural effusions accumulating in the posterior aspect of the collections. This is better appreciated on the left side while scrolling through the images to advance in time.
On T2 and FLAIR, subdural collections appear hyperintense along both cerebral convexities.
Axial T1-weighted images acquired around 10 minutes after contrast administration show enhancement of the posterior aspect of the subdural effusions.
Coronal reconstruction of T1-weighted MPRAGE sequence acquired around 15 minutes after contrast administration shows enhancement of the posterior aspect of the subdural effusions.
Case Discussion
Despite its name, the "subdural space" is not subdural and it is not a naturally occurring space. The classic view is that blood or fluid accumulates in a preexisting (or potential) space called "subdural space", however electron microscopy evaluation of the meninges 1 demonstrates that there is no naturally occurring space at the dura-arachnoid junction. The so-called "subdural space" appears within one of the dural layers (dural border cell layer) due to pathological/traumatic processes. The "subdural space" is therefore intradural and collections or hematomas in this location would be better described as "dural dissections".
When a "subdural" hematoma or effusion occurs, the newly created subdural space is in direct contact with the wide extracellular spaces that characterize the dural border cell layer. This configuration explains the free diffusion of contrast from the outer membrane into the collection seen on the T1 dynamic images presented. It is important to emphasize that the subdural space is not a potential space where two layers can be separated without tissue disruption like in the case of the pleura 1. The formation of a "subdural space" implies tissue damage. Some studies have investigated the rate of enhancement of chronic subdural hematomas and subdural effusions 2,3.
Subdural effusions (without old blood) seem to enhance faster than chronic subdural hematomas and recent subdural hematomas tend to enhance faster than older ones 3. These findings suggest that the etiology and chronology of the pathological processes disrupting the dural border cell layer have an impact on the vessel density, vessel size or vessel permeability of the dura resulting in different rates of enhancement of the "subdural collections".
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