What Causes a Stroke?

A stroke occurs when an inadequate supply of oxygen-rich blood reaches a specific area of the brain. There are two main types of stroke: ischemic and hemorrhagic.

 

Ischemic Strokes

Ischemic strokes are more common than hemorrhagic strokes and can occur due to numerous causes. Regardless of the specific cause, ischemic strokes occur when the blood flow to the brain is blocked. When the blood flow is blocked, oxygenated blood fails to reach a specific brain region or regions, which can cause the brain cells in those regions to die. Symptoms correspond to the region or regions involved. For example, if the stroke occurs in an area responsible for controlling the movement of your left leg, you can end up with weakness or paralysis in the left leg. If the stroke occurs in an area responsible for speech recognition, you can end up with problems understanding language, and so on. Strokes can also be fatal (and often are).

The causes of ischemic stroke include anything that blocks the flow of blood to the brain. These include:

 

Thrombosis – Thrombosis is the formation of a blood clot inside of a blood vessel. Thrombosis is due to either large vessel disease or small vessel disease. Large vessel diseases can include:

1. Atherosclerosis, also called arteriosclerotic vascular disease (ASVD), is when there is thickening of arterial walls due to the accumulation of calcium and fatty materials.
2. Vasoconstriction is when the muscular walls of blood vessels contract, resulting in the narrowing of the blood vessels.
3. Aortic, carotid, or vertebral artery dissection occurs when there is a tear in the inner lining of an artery.
4. Inflammatory diseases of the blood vessel wall can also lead to ischemic stroke. These can include conditions such as giant cell arteritis, vasculitis, Takayasu arteritis, and others.
5. Non-inflammatory vasculopathies are disorders of the blood vessels not caused by inflammation.
6. Moyamoya syndrome is a condition in which certain arteries of the brain are constricted.
7. Fibromuscular dysplasia is a condition in which there is abnormal growth in the wall of an artery.

 

Embolism – Embolism is the sudden interruption of blood flow due to an embolus (a mass that blocks a blood vessel).

 

Systemic hypoperfusion ­– Systemic hypoperfusion is when there is a reduction in blood flow to all parts of the body.

 

Cerebral venous sinus thrombosis (CVST) – CVST is when there is a blood clot (thrombosis) in the dural venous sinuses that drain blood from the brain.

 

Hemorrhagic Strokes

A hemorrhage is when blood escapes from the blood vessel due to a rupture. When blood comes in contact with brain tissue, it results in the death of that brain tissue. There are two main types of hemorrhages (specifically intracranial hemorrhages) that can result in a hemorrhagic stroke.

1. Intra-axial hemorrhage is blood inside the brain.
2. Extra-axial hemorrhage is blood inside the skull but outside of the brain.

Most hemorrhagic strokes have distinct symptoms associated with them.

 

Causes of Stroke

 

The most common causes of stroke include atherosclerosis, blood clots, neurological failures, and alterations in the glandular system. I described atherosclerosis and blood clots above, so let’s talk about neurological failures and alterations in the glandular system.

 

Neurological Failures

The brain has two hemispheres (right and left). In these two hemispheres are two areas known as the nucleus tractus solitarii (NTS) and the intermediolateral nucleus (IML) and they contribute to autonomic functioning including the regulation of blood vessel diameter.

 

If you have ‘short circuits’ in the brain (neurological misfiring), this can lead to loss of regulation and control of the NTS and the IML. 1 2 As these regions become disinhibited, they go haywire and this can lead to fluid accumulation in the blood vessels. As the IML keeps firing, or overfiring, you keep increasing sympathetic division activation, which causes increases in blood pressure and in circulation of inflammatory chemicals, such as adrenaline and cortisol. As a result, blood vessels become inflamed. If these chemicals are consistently secreted, then veins become weak. The pressure keeps building and they enlarge and they bulge (aneurysm). The bulging of the blood vessel leads to bursting of the blood vessel (hemorrhage). Once the blood vessels burst, you have a hemorrhagic stroke.

 

If the blood vessels constrict due to a lesion in the mesencephalon, then over time (months or years) this over-constriction of the blood vessels disables the blood from going through the capillaries and the blood vessels die, leading to death of brain tissue. 3 4 5

 

A quarter of all ischemic strokes are caused by lacunar infarcts (lacunar stroke). 6 These are caused by occlusion (blockage of blood vessel), usually from blood clots. However, there are other causes of ischemic strokes. These may be due to congenital defects in the Circle of Willis (the veins and arteries that supply the brain). Intracranial dural arteriovenous fistulas, which are vascular anomalies in the connection between veins and the dura mater that normally drain the brain, can lead to ischemic strokes.

 

Alterations in the Glandular System

Another condition that may lead to stroke is diabetes. 7 The glandular system has a direct effect on the blood vessels. High insulin surges are a major insult to the blood vessels. What happens is that when insulin spikes, this means the brain is telling the body that it is not able to absorb sugar. The brain responds by telling the pancreas to increase insulin production, because insulin carries sugar into cells. When the body cannot absorb glucose (sugar), the insulin level will continue to increase. As the glucose cannot be absorbed, it remains in the blood and the glucose level spikes, leading to an inflammatory reaction. This is known as diabetes.

 

Diabetes causes inflammation and inflammation weakens blood vessels. The inflammation leads to the muscles of the blood vessels being destroyed. As the muscles of the blood vessels are destroyed, the blood vessels lose their tone. Repetitive insults on the blood vessels cause them to bulge and the bulging ends up causing a stroke.

 

References

1. Zanutto B, Valentinuzzi M, Segura E. Neural set point for the control of arterial pressure: role of the nucleus tractus solitarius. BioMedical Engineering OnLine 2010, 9:4 doi:10.1186/1475-925X-9-4
2. Badoer E. Hypothalamic paraventricular nucleus and cardiovascular regulation. Clin Exp Pharmacol Physiol. 2001 Jan-Feb;28(1-2):95-9.
3. Taoufik E, Probert L. Ischemic neuronal damage. Curr Pharm Des. 2008;14(33):3565-73.
4. Martin LJ et al. Neurodegeneration in excitotoxicity, global cerebral ischemia, and target deprivation: A perspective on the contributions of apoptosis and necrosis. Brain Res Bull. 1998 Jul 1;46(4):281-309.
5. Lipton P. Ischemic cell death in brain neurons. Physiol Rev. 1999 Oct;79(4):1431-568.
6. Wardlaw J M. What causes lacunar stroke? J Neurol Neurosurg Psychiatry 2005;76:617-619 doi:10.1136/jnnp.2004.039982
7. Kisialiou A et al. Risk factors and acute ischemic stroke subtypes. J Neurol Sci. 2014 Jan 15. pii: S0022-510X(14)00036-7. doi: 10.1016/j.jns.2014.01.014.