Subdural hematohygromas are subdural hygromas into which bleeding has occurred. They are distinct from, and should not be confused with, acute on chronic subdural hematomas or with acute subdural hematomas with separation of blood into plasma and hematocrit.
Radiographic features
CT
Subdural hematohygromas are usually imaged with CT and have the following features:
presence of mass effect, sulcal effacement, and midline shift
high density (blood) dependently and water-density (CSF) anti-dependently
smooth and often sharp transition between low and high-density collections
absence of internal strand-like membranes paralleling the skull vault (indicating a chronic subdural hematoma)
Differential diagnosis
acute subdural hematoma: it is difficult to distinguish subdural hematohygromas from acute subdural hematomas where blood has settled and separated with a hematocrit effect (red blood cells inferiorly and plasma superiorly) unless there are prior studies demonstrating a pre-exisitng subdural hygroma
acute on chronic subdural hematoma: the key feature is the presence of membranes within the collection that result in multiple fluid-fluid levels or heterogeneity; care must be taken to not mistake acute on chronic hematomas with hyperacute hematomas with active bleeding