To try to fully understand what a stroke is, why it happens, and how it should be treated, we need to first of all understand how the brain normally works.
Anatomy of the brain
The brain is the control centre of the body. The brain dictates how we walk, talk, swallow, breathe, and feel, to name just a few of its jobs.
The largest part of the brain is known as the cerebrum. Divided into the left and right cerebral hemispheres, the cerebrum consists of grey matter and white matter; grey matter forms the most outer surface of the cerebral hemispheres and is known as the cerebral cortex. The cerebrum has ridges and grooves which are known as the sulci and gyri respectively. Dividing the two cerebral hemispheres is a large sulcus known as the longitudinal fissure. Everyone is said to have a dominant and non-dominant cerebral hemisphere i.e., the right or left.
Each cerebral hemisphere is divided into four lobes, which have specific functions:
- Frontal lobe: intelligence, personality, mood, behaviour, and language.
- Parietal lobe: language and calculation (dominant hemisphere), visuospatial function (non-dominant hemisphere).
- Temporal lobe: memory, language, and hearing.
- Occipital lobe: vision
Below the occipital and temporal lobes lies the cerebellum. The cerebellum can be divided based on its functionality:
- Cerebro-cerebellum: planning movements, visually guided movements
- Spino-cerebellum: proprioception, regulates body movements.
- Vestibulo-cerebellum: balance and ocular reflexes.
Connecting the cerebrum, cerebellum, and spinal cord, is the brainstem. The brainstem is comprised of the midbrain, pons, and medulla oblongata. This area of the brain, whilst only small, is responsible for critical bodily processes such as:
- Heart rate
The brainstem is also where the cranial nerves emerge, and where the central nervous system and sleep cycle are regulated. The brainstem is crucial for conveying motor and sensory pathways from the brain the body, and from the body to the brain.
The brain is supplied with blood via the vertebral arteries and the internal carotid arteries in the neck.
Once in the skull, the internal carotid arteries branch to the:
- Ophthalmic artery
- Posterior communication artery (PCA)
- Anterior choroidal artery
- Anterior cerebral artery
- Middle cerebral artery (MCA)
Once in the skull, the vertebral arteries branch to the:
- Meningeal branch
- Spinal arteries
- Basilar artery
The internal carotid and vertebral branches then branch further, linking together to form the Circle of Willis:
- Anterior cerebral arteries
- Internal carotid arteries
- Posterior cerebral arteries
- Anterior communicating artery (ACA)
- Posterior communicating artery (PCA)
Each blood vessel supplies a specific part of the brain, and therefore brain function is dependent on blood vessel competency. Consequently, damage/blockage in different parts of the cerebral circulation will present with different symptoms according to the location.
Venous blood of the brain drains into the dural venous sinuses (pools of blood in the brain), which then drain into the internal jugular veins. The brain has two sinuses, known as the cavernous sinuses, that lie beneath the eye sockets. The cavernous sinuses have arteries and nerves of the eye and face which pass through them. The cavernous sinuses receive blood from veins draining the eye and different parts of the brain.
What is a Stroke?
NICE defines a stroke (cerebrovascular incident) as:
“A clinical syndrome of presumed vascular origin characterized by rapidly developing signs of focal or global disturbance of cerebral functions which last longer than 24 hours or leads to death.”
i.e., there is an acute onset of brain dysfunction, presumed to be due to a disturbance to the blood supply to the brain, resulting in symptoms which last longer than 24 hours.
In contrast, a transient ischaemia attack (TIA) is when there is neurological deficit lasting less than 24 hours, with no evidence of an acute infarction. Hence, a TIA can also be known as a “mini stroke”.
The majority of strokes are ischaemic i.e., there is reduced blood flow to the brain. However, some strokes can be haemorrhagic. In both instances, a stroke can be severely disabling with lifelong implications.
As a stroke is a medical emergency, it is critical that the symptoms are identified promptly, and medical attention is sought. Using the FAST test can be used to identify the most common signs of a stroke:
F – facial weakness e.g., facial drooping
A – arm weakness. This can also present as leg weakness, usually one-sided.
S – speech problems e.g., slurred speech, incomprehensible speech, or the individual not understanding what is spoken to them
T – time (call 999 asap)
What is a Haemorrhagic Stroke?
When a blood vessel supplying the brain ruptures, there is a disruption to the bloody supply and an increase in pressure within the cranium (due to the accumulation of blood). Known as a haemorrhagic stroke (or bleed on the brain), these can be categorised depending on where the bleeding occurs:
- Intracerebral haemorrhage – bleeding into the brain tissue (parenchyma)
- Subarachnoid haemorrhage – bleeding into the (subarachnoid) space between the brain and the skull
The haemorrhage initially causes injury through an increase in intracranial pressure (ICP) as a result of the accumulating blood (haematoma). Following this, there is inflammation, swelling, and disruption to the lining of the brain. Surrounding the haematoma, the brain tissue is hypoperfused.
Haemorrhagic strokes have an acute and progressive presentation, sometimes following head trauma, and classically present with:
- Sudden severe headache
- Neck stiffness
- Neurological signs
What is an Ischaemic Stroke?
An ischaemic stroke is when a blood vessel supplying blood to the brain becomes blocked (occluded). An ischaemic stroke can be defined as either:
- Thrombotic – blood vessels of the cerebral circulation directly stop blood flow to the brain. This is achieved either because the blood vessel has become so narrowed and hardened (atherosclerotic) that blood can no longer pass through the lumen, or a blood clot forms and blocks the lumen.
- Embolic – a blood clot from another part of the body travels to the brain and blocks a blood vessel in the cerebral circulation.
The blocked blood vessel stops oxygenated blood from travelling to the tissues of the brain (ischaemia), which leads to necrosis (tissue death). Specific areas of brain necrosis will result in specific signs and symptoms; the location of the occlusion determines which specific brain functions are affected.
Classical presentation of an ischaemic stroke is with sudden, one-sided numbness/weakness in the face, arm, or leg. It is important to rule out a haemorrhagic stroke.
As previously mentioned, when different parts of the brain are affected by stroke the individual will present with signs and symptoms according to what brain function is impeded; this results from affliction to specific blood vessels that supply the brain. Accordingly, various syndromes can be described. These include:
- Medial medullary syndrome
- Lateral medullary syndrome
- Locked-in syndrome
- Gerstmann syndrome
- MCA syndrome
- PCA syndrome
- ACA syndrome
Common Stroke Terminology Explained
- Homonymous hemianopia – loss of specific visual fields
- Apraxia – inability to perform a learned movement
- Hemineglect – the person is not aware of one side of space e.g., they will eat half a plate of food, completely unaware the other half exists
- Contralateral – the opposite side to the cerebral hemisphere affected
- Ipsilateral – the same side as the cerebral hemisphere affected
- Nystagmus – rapid, uncontrolled, repetitive eye movements
- Ataxia – reduced coordination and balance
- Dysarthria – difficulty articulating speech
- Agraphia – difficulty writing
- Acalculia – difficulty processing numbers
- Receptive aphasia (Wernicke’s aphasia) – difficulty finding the correct words to speak.
- Expressive aphasia (Broca’s aphasia) – difficulty expressing the correct words.
- Chorea – involuntary, irregular, and unpredictable movements
- Hemiparesis – one-sided paralysis
Cavernous Sinus Thrombosis
A blood clot may form in the cavernous sinuses following a facial infection, sinusitis, cellulitis around the eye, sore throat (pharyngitis), an eye infection, or facial trauma. Once the thrombus has formed within the cavernous sinus, the facial and ophthalmic veins have reduced drainage; hence oedema around the eyes and reduced eye movements. Unlike venous systems elsewhere in the body, the sinuses in the brain do not have valves; the lack of valves allows the thrombus to travel elsewhere in the brain. If the thrombus has formed in the presence of infection (septic), the infection is likely to also spread, leading to meningitis, brain abscess, or an empyema.
The most common presentation of cavernous sinus thrombosis is that of fever, severe headache, swelling around the eyes, and reduced eye movements.
A subdural haematoma is a collection of blood in the subdural space i.e., between the lining of the skull and the brain. A subdural haematoma may or may not involve damage to the brain tissue, and can be classified as acute, subacute, or chronic.
Usually, arteries and veins are intercepted by a capillary network. In some individuals, the capillary network is absent, the arteries directly feed into the veins. This leads to a tangle of abnormal blood vessels known as an arteriovenous malformation. These most commonly occur in the brain and are associated with abnormal brain tissue. Arteriovenous malformations are weaker than normal blood vessels and are therefore susceptible to aneurysm and haemorrhage.