Subdural hemorrhage (summary)

• this is a basic article for medical students and non-radiologists

Subdural haemorrhage (SDH) is a collection of blood between the dura and the arachnoid layers of the meninges. They are common (15% of all head injuries) and can occur in any age range. There is usually a history of head trauma. Prognosis tends to depend on the extent of the bleed and associated mass effect.

Epidemiology

Subdural haemorrhages can occur across many ages, but with different mechanisms at different ages:

• children: non-accidental injury¹
• adults: high energy trauma, e.g. road traffic collisions
• elderly: falls (there may not be a clear history of trauma)

Clinical presentation

Acute subdurals are usually associated with head injury, particularly in younger patients who may have evidence of head injury, e.g. extradural haemorrhage, cerebral contusions. There may be underlying abnormalities such as arteriovenous malformations in spontaneous bleeds.

Subacute or chronic subdural haemorrhages can present in the elderly as confusion or with vague neurological change. SDH is a classic cause of pseudo-dementia and there may only be a history of minor head injury (sometimes there is no history of head trauma)².

Pathology

The bleed in subdural haemorrhages usually comes from tearing of "bridging veins" found in the subdural space³. These veins can be subject to shearing or tearing when the head is subject to a change in velocity (as in head injury). Tearing of these veins may occur with less force in the elderly where the effect of cerebral atrophy can leave these stretched veins more vulnerable to damage².

CSF may leak into the subdural space if the underlying arachnoid mater is damaged. A small proportion of subdurals can rebleed as veins are stretched and subsequently damaged.

Radiographic features

Subdural haemorrhages are typically unilateral (85%) ⁴ and follow a crescent-like distribution around the periphery of the brain. They are not limited by sutures and can fill dural reflections (falx cerebri, tentorium). 

CT

Non-contrast CT is usually enough to make the diagnosis but contrast can be used in challenging cases, particularly with the different appearances of these bleeds over time.

Acute

Acute subdural haemorrhages are hyperdense collections. While many are uniformly hyperdense, up to half can have mixed appearances as (hypodense) unclotted blood, serum of CSF is involved in the collection.

Acute bleeds may be isodense and therefore difficult to identify in patients with anaemia or coagulopathies (including warfarinisation) where it fails to clot. The clotting process causes increased density and failure to clot therefore results in an isodense collection.

Subacute

In the first few weeks after a bleed, the clot and proteins are broken down making the collection appear hypo- or isodense. Here, indirect signs of a subdural collection are important to identify, e.g. a gap between the cerebral sulci and skull, mass effect or an apparent ill-defined, thickened cortex.

Chronic

Over time, these collections can resolve and leave few products behind with a hypodense region on CT with density similar to CSF².

MRI

MRI is used to assess the underlying parenchyma. The signal of blood in the subdural haemorrhage change with time and MRI can be used to age a collection. 

Treatment and prognosis

Subdural haemorrhages usually merit discussion with neurosurgical services. The treatment can depend on the neurological impact⁵ of the mass effect and size of the bleed.

Small chronic SDHs may be incidental findings and can be observed with repeated scans.

Correction of abnormal coagulation can be performed prior to neurosurgical intervention with haematologist advice. Evacuation of the bleed is usually required in symptomatic cases but can carry high morbidity and mortality. Acute SDHs may need urgent craniotomy to control the clot. Symptomatic subacute or chronic clots can be managed with burr-hole drainage as the older clots can be more easily removed from smaller spaces.

Differential diagnosis

• subdural empyema: similar appearance on CT but usually febrile and generally unwell, can enhance with contrast with evidence of abscess or underlying infarction
• extradural haemorrhage: difficult if small to differentiate, EDH are bi-convex rather than crescentic, EDH limited by sutures, SDH by dural reflections, usually clear history of head trauma/skull fracture
• subdural hygroma: very similar appearance of CT, same density as CSF, no evidence of previous haemorrhage
• enlarged subarachnoid space (due to cerebral atrophy or changes in infancy)
• motion artefact

More information

• subdural haemorrhage (main article)