Traumatic subarachnoid hemorrhage

Traumatic subarachnoid haemorrhage (tSAH) is a common injury, and trauma is the most common cause of subarachnoid haemorrhage (SAH). 

Epidemiology

Traumatic subarachnoid haemorrhage occurs in ~35% (range 11-60%) of traumatic brain injuries ¹. 

Pathology

Traumatic subarachnoid haemorrhage is mostmore commonly seen in the cerebral sulci than in the Sylvian fissure and basal CSF cisterns ¹. When in the basal cisterns, it has an affinity for the quadrigeminal cistern and ambient cistern ². tSAH is also commonly seen adjacent to skull fractures and cerebral contusions ³. 

The exact mechanism of tSAH remains uncertain although it is clear that a number of aetiologies exist and these will determine, at least to a degree, the distribution of blood. Causes of tSAH include ⁴:

• direct extravasation of blood from an adjacent cerebral contusion
• arterial dissection
• direct damage to small veins or arteries
• sudden increase in intravascular pressures leading to rupture

Radiographic features

CT

CT of the brain is almost always the first scan obtained in the setting of trauma, often as part of a CT panscan. 

Although the sensitivity of CT to the presence of subarachnoid blood is strongly influenced by both the amount of blood and the time since the haemorrhage, in the setting of trauma scans are almost always obtained early, often mere minutes or hours from head injury, making even small amounts of blood readily visible. 

The distribution and amount of blood varies greatly depending on the underlying mechanism (see above) and from patient to patient.

Often a small amount of blood is seen filling a few sulci, sometimes with an adjacent cerebral contusion. Small amounts of blood can also sometimes be appreciated pooling in the interpeduncular fossa, appearing as a small hyperdense triangle, or within the occipital horns of the lateral ventricles.

Occasionally, and worrying for an underlying arterial dissection or an aneurysmal haemorrhage that preceded trauma, larger amounts of blood may be seen around the circle of Wilis and within the posterior fossa. 

Treatment and prognosis

Traumatic subarachnoid haemorrhage has a better prognosis than aneurysmal SAH ². 

Complications

• cerebral vasospasm
• communicating hydrocephalus ²

Differential diagnosis

It can be difficult to delineate from tSAH, particularly as in many instances the cause of head trauma may have been spontaneous subarachnoid (e.g. while driving). 

The distribution of blood, particularly if closely related to cerebral contusions can suggest traumatic aetiology, whereas extensive blood around the circle of Willis should prompt arterial imaging to exclude an aneurysm. 

Distinguishing between aneurysmal subarachnoid haemorrhage and traumatic non-aneurysmal subarachnoid haemorrhage is not always possible, and the trauma may have in reality been precipitated by a spontaneous aneurysmal haemorrhage (e.g. while driving).

That having been said, there are helpful features in suggesting that subarachnoid haemorrhage is the result of trauma, rather than the reason for trauma. These features include: 

1. documented (witnessed) trauma not being preceded by a headache or loss of consciousness or seizure
2. subarachnoid blood being relatively minor and associated with cerebral contusions
3. subarachnoid blood located over the convexity of the brain rather than around the circle of Willis or posterior fossa
4. location of subarachnoid blood deep to scalp haematoma or in a contrecoup distribution