Like any organ, the brain has many arteries and veins - giving it the potential to bleed (haemorrhage). Bleeds happen when blood escapes the vessels. A common cause of vessel damage in the brain is an external trauma, such as falling and hitting one's head. A severe trauma will cause a bleed in the best of brains; however, even minor traumas can cause a significant bleed if the vessels are susceptible to begin with. 

Vessels become more susceptible to bleed as we age, that is unavoidable. However, certain things predispose us to a higher risk. These include high blood pressure, high cholesterol, diabetes, obesity, smoking....all of the big hits your GP nags you about. For a number of reasons, some patients require blood thinning medication - this will make them more susceptible to bleeding (often a necessary risk to take).

Following a head trauma, ED doctors will follow two trains of thought. If the patient has minimal risk of a bleed, has no neurological symptoms and the impact was seemingly minor - they may decide not to scan the brain, and instead keep a close eye on them for a period of observation. 
However, if the patient has any neurological symptoms or has a high risk of bleeding - they will order a CT Brain scan. Fresh blood will appear white on a CT scan - making it rather easy to detect. The case will be likely be discussed with a neurosurgeon, who will review the images and decide if surgery is likely to be needed/beneficial to the patient's outcome. A period of observation and follow up imaging (to ensure the bleed has stabilised) may be required. 

Considering a bleed can occur in any region, the neurological symptoms can vary wildly. Brain bleeds are instead largely categorised by their anatomical location. Let's discuss three different types - your doctor may use these terms. 

An extradural haemorrhage involves a bleed that happens outside the dura mater. The dura mater is the outer layer of the meninges, essentially a membrane that surround the brain. The dura mater knits into the inside of the skull in various sections called 'suture lines'. As a result, the bleed will collect in a portion and appear as a convex, lemon shape on the CT scan. These types of bleeds are common after a single, substantial trauma.

A subdural haemorrhage involves a bleed that happens between the dura and arachnoid layers of the meninges. This layers contains a number of small vessels called 'bridging veins' that are quite sensitive. Subdural haemorrhages often occur after repeated falls with head trauma.  On a CT scan, they have a concave, half-moon shape as the blood is not closed in by the suture lines. 

A subarachnoid haemorrhage is a bleed, you guessed it, under the arachnoid mater of the meninges. The most common cause of this bleed isn't an external trauma - it involves an aneurysm (section of inappropriately widened blood artery) that bursts. A subarachnoid haemorrhage is very serious, and usually presents with a 'thunderclap headache' - described as the worst headache one can experience. 

Brain bleeds vary massively in their severity, following head trauma or any concerns involving the brain - it's really best to get checked out. If a bleed leads to a lack of blood supply to the brain causing cell death - this is called a hemorrhagic stroke. We will cover more about strokes in our 'Stroke' section.