Subdural Hemorrhage and Posttraumatic Hygroma

Key Words

subdural hematoma, subdural hygroma, intracranial hemorrhage

Introduction

The accurate age determination of a subdural hemorrhage is one of the most common and basic assessments in the setting of head trauma. On computed tomography (CT), the classic descriptions of blood products within the subdural space relate to density changes which evolve over time. These changes reflect the evolution from acute blood to clot formation, clot retraction, clot lysis, and eventual resorption. Based on the density of the subdural collection, subdural hematomas (SDHs) are classically subdivided into acute, subacute, and chronic SDHs. Although the process of estimation is generally straightforward in everyday clinical practice, several variations must be taken into account to avoid confusion. This confusion may be ameliorated by focusing first on the relevant anatomy and then on the different types of subdural collections, including both SDHs and subdural hygromas.

An SDH is a typically crescent shaped extraaxial collection of blood within the innermost layer of the dura, designated the dural border cell layer ( Fig. 2.1 ).

The fact that SDHs form within the innermost layer of the dura is of crucial importance for a conceptual understanding of the different types of subdural collections. This is because there is a rich venous plexus within this layer ( Fig. 2.2 ). The small caliber of these vascular structures is beyond the resolution of our current imaging. Although there is still much that is unknown about its function, this venous plexus is thought to play a role in cerebrospinal fluid (CSF) resorption into the venous system.

Subdural Hematoma Evolution: Overview

At its most basic, there are two types of traumatic subdural collections: SDH and subdural hygroma. An acute SDH represents acute blood products with or without clot formation. On CT imaging, an acute SDH often presents as a hyperdense subdural collection ( Fig. 2.3 ).

A subdural hygroma is the accumulation of clear or xanthochromic CSF within the subdural space. An acute subdural hygroma results from the acute accumulation of CSF within the dural border cell layer. This can result from an acute tear in both the arachnoid and the dural border cell layer, resulting in communication of these two spaces. Alternatively, this can also result from the acute impairment of CSF resorption (as often seen in the setting of subarachnoid hemorrhage), affecting the intradural venous plexus along the inner layer of the dura. On CT imaging, an acute subdural hygroma exists when a CSF isodense or nearly isodense subdural collection accumulates acutely ( Fig. 2.4 ).

Of course, the presence of a subdural hygroma and an SDH is not mutually exclusive. Varying degrees and combinations of clotted blood, unclotted blood, bloody CSF, and clear CSF can therefore be present within an acute subdural collection ( Fig. 2.5 ).

These varying degrees and combinations of clot, blood, and bloody CSF are what lead to the marked heterogeneity of patient imaging presentations ( Fig. 2.6 ).

The variable concentrations of either blood or CSF within a specific area of the acute subdural collection lead to different fluid properties and therefore different fluid behavior as time elapses. In other words, many portions of these subdural collections are not simply “blood” or “hematoma.” It should be now readily apparent why the imaging characteristics of these collections generally do not conform to the magnetic resonance imaging (MRI) stages of hematoma evolution so firmly established for parenchymal hematomas ( Fig. 2.7 ).

Both SDHs and subdural hygromas can be either acute or chronic. SDHs are classified into acute, subacute, or chronic categories, depending on the amount of time elapsed since the time of injury. As previously noted, this determination is classically based on the density of the collection. At its most basic, the CT density of a simple SDH depends on the time interval between the bleeding episode and imaging ( Fig. 2.8 ).