Subdural haemorrhage

A subdural haemorrhage (SDH) is a collection of blood accumulating in the potential space between the dura and arachnoid mater of the meninges around the brain.

Epidemiology

Subdural haematomas are seen in all age-groups although aetiology will vary^4-5: 

• infants - non-accidental injury
• young adults - motor vehicle accidents 
• elderly - falls (although a definite history of trauma may be lacking)

They are present in 10 to 20% of all head trauma cases and occur in up to 30% of fatal injuries.  

Clinical presentation

Acute subdural usually present in the setting of head trauma. This is especially the case in young patients, where they commonly co-exist with cerebral contusions. 

Most patients (65-80%) present with severely depressed conscious state, and pupillary abnormalities may be seen in 30-50% of cases ⁵. 

Occasionally spontaneous acute subdural haematomas are seen with an underlying abnormality (e.g. dural arteriovenous fistula).

Clinical presentation of subacute / chronic subdurals in the elderly is often vague and is one of the classic causes of a pseudo-dementia. A history of head trauma is often absent or very minor. 

Pathophysiology

Subdural haemorrhages are believed to be due to stretching and tearing of bridging cortical veins as they cross the subdural space to drain into an adjacent dural sinus. These veins rupture due of shearing forces when there is a sudden change in the velocity of the head. The arachnoid may also be torn, creating a mixture of blood and CSF in the subdural space.

10 to 30% of chronic subdural haematomas show evidence of repeated haemorrhage. Rebleeding usually occurs from rupture of stretched cortical veins as they cross the enlarged fluid-filled subdural space or from the vascularized neomembrane on the outer (calvarial) side of the fluid collection.

Subdural haematomas are interposed between the dura and arachnoid. Typically crescent-shaped, they are usually more extensive than extradural hematomas. In contrast to extradural haemorrhage, SDH is not limited by sutures, but are limited by dural reflections, such as the falx cerebri, tentorium, and falx cerebelli. 

Radiographic features

Overall 85% of subdural haematomas are unilateral.Common sites for subdural hematomas are fronto-parietal convexities and the middle cranial fossa. Isolated inter-hemispheric/ parafalcine subdural hematomas are seen more frequently in children, and are common in cases of non-accidental trauma. 

In the vast majority of cases, CT scans are sufficient to make the diagnosis and manage these patients. Contrast is sometimes helpful if there is concern of a subdural empyema, of the presence of a small iso-dense subdural, or to try and distinguish enlargement of extra-axial CSF space from a chronic subdural haematoma. 

CT

The appearance of SDHs on CT varies with clot age and organization. 

Hyper-acute

In most instances patients are not imaged in the hyper-acute phase (first hour or so), but on occasion when this is performed they appear relatively iso-dense to the adjacent cortex, with a swirled appearance do to mixture of clot, serum and ongoing unclotted blood ⁴. There is often a degree of underlying cerebral swelling (especially in young patients where head trauma is often more severe) which accentuates the mass effect created by the collection ⁴. 

Acute

The classic appearance of an acute subdural haematoma is a crescent-shaped homogeneously hyperdense extra-axial collection that spreads diffusely over the affected hemisphere.  As the clot starts to retract the density increases typically to > 50 - 60 HU and is thus hyperdense relative to cortex ⁴. 

Up to 40% of SDHs have mixed hyper / hypodense areas that reflect unclotted blood, serum extruded during clot retraction, or CSF within the subdural haematoma due to arachnoid laceration.

Rarely, acute SDHs may be nearly isodense with the adjacent cerebral cortex. This occurs with coagulopathies or severe anemia when the hemoglobin concentration drops to 8 to 10 g/dl. Patients with deficient coagulation can also demonstrate a hematocrit fluid-fluid level as the blood does not form a clot and red cells have time to drift dependently ⁴. 

Subacute

As the clot ages and protein degradation occurs, the density starts to drop. At some point between 3 and 21 days (typically 10 - 14 days) the density will drop to ~30HU and become iso-dense to the adjacent cortex, making identification potentially tricky, especially if subdural collections are bilateral ⁴. They key to identification is visualising a number of indirect signs, including: 

• CSF filled sulci do not reach the skull but rather fade out into the subdural
• mass effect including sulcal distortion and midline shift
• apparent thickening of the cortex

Chronic

Eventually the subdural collection becomes hypodense and can reach~0HU and be iso-dense to CSF, and mimic subdural hygromas. 

Acute on chronic

Acute on chronic subdural hematomas refer to a second episode of acute haemorrhage into a pre-existing chronic subdural hematoma. It typically appears as a hypodense collection with a haematocrit level (located posteriorly). A similar appearance can be seen in patients with clotting disorders or on anticoagulants ⁴. 

Treatment and prognosis

Treatment depends primarily on the amount of mass effect and neurological impairment caused by the collection, and thus correlates with the size of the subdural haemorrhage. 

Small (so-called 'smear subdurals') collections, especially those which are chronic, which are not causing symptoms can be observed with serial CT scans. 

Symptomatic collections need to be surgically evacuated. In the acute setting this should be performed rapidly (within 4 hours) ³ and usually requires a craniotomy as the clot is not easily evacuated via burr-holes. 

Symptomatic subacute / chronic subdural haematomas are often treated via one or more burr-holes as the blood clot has liquefied and can be washed out more easily. The compressed brain can take some time to re-expand, and subdural collections may re-accumulate. 

Although subdural haematomas are often thought of as relatively benign entities it should be noted that the mortality in acute subdural haematomas requiring surgery is actually very high (50 - 90%), particularly in patients who are anticoagulated, and that only 20% fully recover ^2-3,5.  

Differential diagnosis

General imaging differential considerations include ^1-4:

• prominent subarachnoid space due to cerebral atrophy or benign enlargement of the subarachnoid space in infancy
  • can look similar
  • lack of mass effect
  • vessels course through the space rather than displaced towards the brain
  • CT contrast therefor helps (in adults) by delineating the vessels as well as demonstrating an enhancing 'capsule' of a subdural haematoma / empyema
  • ultrasound and MRI are useful in infancy
• sub-dural empyema
  • similar appearance on non-contrast scans
  • different clinical context (patients usually unwell and febrile)
  • prominent marginal enhancement
  • associated cerebral abscess / infarction
• extra-dural haemorrhage
  • sometimes difficult to differentiate if small
  • bi-convex in shape (lentiform) rather than crescentic 
  • limited by sutures
  • may displace dural venous sinuses
  • usually associated with fractures
• subdural hygroma
  • on CT can be indistinguishable from a chronic subdural haematoma
  • exactly CSF density
  • no evidence of prior haemorrhage
• motion artefact