Subarachnoid hemorrhage

Definitions & Key takeaways

A subarachnoid hemorrhage (SAH) is bleeding into the subarachnoid space, the area between the arachnoid membrane and the pia mater surrounding the brain. This may occur spontaneously, usually from a ruptured cerebral aneurysm, or may result from a head injury.

Symptoms of subarachnoid hemorrhage can include sudden, severe headache; nausea and vomiting, confusion, or reduced level of consciousness. Diagnosis is usually done with a CT or MRI that shows blood in the subarachnoid space and blood in a lumbar puncture. Treatment requires prompt surgery to stop the bleeding and prevent further damage. Medications may also be prescribed to reduce swelling and control seizures.

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Introduction0:00–0:47

There are two main types of stroke: a hemorrhagic stroke, which occurs when an artery ruptures and bleeds within the brain, and an ischemic stroke, which occurs when an artery gets blocked.
Hemorrhagic strokes can be further split into two types, an intracerebral hemorrhage which is when bleeding occurs within the cerebrum, and a subarachnoid hemorrhage which is when bleeding occurs between the pia mater and arachnoid mater of the meninges - the inner and middle layers that wrap around the brain.
We’ll focus on subarachnoid hemorrhage, which can quickly lead to death if they’re left untreated. Subarachnoid hemorrhages can lead to a pool of blood under the arachnoid mater that increases the intracranial pressure and prevents more blood from flowing into the brain.

Anatomy0:47–3:58

Ok - let’s start with three protective layers of the brain called meninges. The inner layer of the meninges is the pia mater, the middle layer is the arachnoid mater, and the outer layer is the dura mater.
Between the arachnoid mater and the pia mater is the subarachnoid space, which houses cerebrospinal fluid, or CSF. CSF is a clear, watery liquid which is pumped around the spinal cord and brain, cushioning them from impact and bathing them in nutrients.
This space is also where the arteries that supplies the brain travel, and it is the location of the blood brain barrier where CSF and the vascular system can exchange nutrients.
The brain has a few regions - the most obvious is the cerebrum, which is divided into two cerebral hemispheres, each of which has a cortex - an outer region - divided into four lobes including the frontal lobe, parietal lobe, temporal lobe, and the occipital lobe.
There are also a number of additional structures - including the cerebellum, which is down below, as well as the brainstem which connects to the spinal cord.
The right cerebrum controls muscles on the left side of your body and vice versa. The frontal lobe controls movement, and executive function, which is our ability to make decisions.
The parietal lobe processes sensory information, which lets us locate exactly where we are physically and guides movements in a three-dimensional space.
The temporal lobe plays a role in hearing, smell, and memory, as well as visual recognition of faces and languages. Finally, there’s the occipital lobe which is primarily responsible for vision.
Within the cortex are deeper structures like the internal capsule, which is like a highway that allows information to flow through neurons that are going to and from the cerebral cortex.
There’s also the basal ganglia, which helps controls smooth movement and cognitive function, along with the cerebellum. The cerebellum also helps with muscle coordination and balance.
And finally, there’s the brainstem, which plays a vital role in functions like heart rate, blood pressure, breathing, intestinal motility, and consciousness.
The brain receives blood from the left and right internal carotid arteries, as well as the left and right vertebral arteries, which come together to form the basilar artery.
The internal carotid arteries turn into the left and right middle cerebral arteries which serve the lateral portions of the frontal, parietal, and temporal lobes of the brain.
Each of the internal carotid arteries also give off branches called the anterior cerebral arteries which serve the medial portion of the frontal and parietal lobes and connect with one another with a short little connecting blood vessel called the anterior communicating artery.
Meanwhile, the vertebral arteries and basilar artery give off branches to supply the cerebellum and the brainstem. In addition, the basilar artery divides to become the right and left posterior cerebral artery which mainly serve the occipital lobe and some of the temporal lobe as well as the thalamus.
Finally, the internal carotid arteries each give off a branch called the posterior communicating artery which attaches to the posterior arteries on each side.
So together, the main arteries and the communicating arteries complete what’s called the Circle of Willis - a ring where blood can circulate from one side to the other in case of a blockage.

Pathology3:58–5:39

Three things can cause a subarachnoid hemorrhage. The first and most common cause of a subarachnoid hemorrhage is a traumatic injury, like falling in the bathtub and hitting your head.
Arteries in the subarachnoid space are unsupported, and can easily break. The second cause is an aneurysm, which is when a blood vessel has weak walls and starts to bulge out to about one and a half times larger than its normal diameter.
The most common aneurysms in the brain are saccular cerebral aneurysms which have a characteristic rounded shape on one side of the artery and are also called berry aneurysms.
Most saccular cerebral aneurysms arise in the anterior half of the circle of Willis whereas only a few arise in the posterior half.
Some genetic disorders like Marfan syndrome cause a defect in the connective tissues of arteries and they can also predispose individuals to having saccular aneurysms.
Aneurysms can burst open when there’s an increase in intracranial pressure, like what you might feel if you’re moving a large sofa into the living room.
The third cause of a subarachnoid hemorrhage is an arteriovenous malformation. Normally, arteries and veins are connected by small leaky blood vessels called capillaries.
But in arteriovenous malformations, they are replaced with abnormally formed tangled blood vessels characterized by at least one direct connection between the artery and vein.
Over time these abnormal vessels can dilate and since veins aren’t used to dealing with high arterial pressures, they can rupture causing a subarachnoid hemorrhage.

Symptoms5:39–8:10

Regardless of the cause, once there’s a subarachnoidintracerebral hemorrhage, blood starts spewing out from a damaged blood vessel creating a pool of blood which increases pressure in the skull and puts direct pressure on nearby tissue cells and blood vessels.
It also means that less blood is flowing downstream to the cells that need it, which leaves the downstream tissue deprived of oxygen-rich blood.
Healthy tissue can die from both the direct pressure and the lack of oxygen within a few hours. Another complication is that blood vessels that are “bathing” in a pool of blood can start to intermittently vasoconstrict - which is called vasospasm.
If the vasospasm affects arteries in the circle of Willis, it will reduce the supply of blood flow to the brain, causing further ischemic injury.
Over time, blood in the subarachnoid space can irritate the meninges and cause inflammation which leads to scarring of the surrounding tissue.
The scar tissue can obstruct the normal outflow of cerebrospinal fluid, causing that fluid to build up and that dilates the ventricles at the center of the brain.
This is referred to as hydrocephalus. As the ventricles dilate, there’s also increased intracranial pressure, which compresses the brain tissue.
Stroke symptoms depend on the exact part of the brain that is affected. For example, an anterior or middle cerebral artery stroke can cause numbness and sudden muscle weakness.
If a stroke affects the Broca’s area, which is usually in the left frontal lobe, or Wernicke’s area, which is usually in the left temporal lobe, then it can cause slurred speech or difficulty understanding speech, respectively.
If there’s a posterior cerebral artery stroke, then it can affect vision. An acronym to remember some common stroke symptoms is FAST - Facial drooping, Arm weakness, Speech difficulties, and Time.
Time is obviously not a symptom but just a reminder to get help as quickly as possible to minimize cell injury and maximize the chance of a full recovery.
The classic symptoms of a subarachnoid hemorrhage is the sudden onset of a severe headache, called a thunderclap headache.
It’s often described as the worst headache a person as ever had. There’s also nuchal rigidity, or neck stiffness which is caused by the blood irritating the meninges.
Occasionally people can develop seizures and symptoms of increased intracranial pressure like vomiting, vision changes, and general confusion.

Diagnosis8:10–8:35

The diagnosis of a subarachnoid hemorrhage is usually made with a CT or MRI scan, which will shows blood pooling in the subarachnoid space around the damaged artery.
Classically a lumbar puncture will show red blood or yellowness from bilirubin called xanthochromia. That’s because there may be fresh blood around the spinal canal or old blood that’s being metabolized and broken down.

Treatment8:35–9:03

Treatment of a subarachnoid hemorrhage typically involves emergent surgery. An artery that’s bleeding can be surgically clipped to apply direct pressure to it.
Alternatively, a catheter can be used to place a coil in an aneurysm, which serves as a site for clot formation, which also seals up the aneurysm.
Sometimes medications like calcium channel blockers can be used to help prevent arterial vasospasm from setting in. All right, as a quick recap.

Recap9:03–9:36

A subarachnoid hemorrhage is a pooling of blood in the subarachnoid space. It can arise due to a traumatic injury like hitting your head in a bathtub, having a preexisting saccular aneurysm burst, or from a preexisting arteriovenous malformation that bursts.
Usually there’s a sudden and severe thunderclap headache that’s described as the worst headache of a person’s life. Diagnosis is usually done with a CT or MRI that shows blood in the subarachnoid space and blood in a lumbar puncture.
Treatment is done by surgical clipping or coiling.