What is stroke

The World Health Organisation has defined stroke as: “A neurological deficit of cerebrovascular cause that persists beyond 24 hours or is interrupted by death within 24 hours.”

Obstruction of the cerebral blood vessels and deprivation of oxygen, glucose and other essential nutrients to the supplied vascular territory, leads to cell death, through liquefactive necrosis. This is because brain tissue stops functioning if deprived of oxygen for more than 60 to 90 seconds. If circulation is not re-established within three hours, irreversible damage will be done to the affected area of the brain, causing it to lose function and can lead to visual impairment, inability to comprehend or formulate speech, move one or multiple limbs and other physical, cognitive and emotional problems resulting in acquired disability. (Figure 1)

Figure 1 Post stroke disabilities (Genetech 2014)

Two major types of stroke are: ∙ Ischaemic sroke (Figure 2)

∙ Haemorrhagic stroke (Figure 3)

87% Of strokes are ischemic, the rest are hemorrhagic.

Figure 2 Ischaemic Stroke (Heart and stroke foundation of Canada, 2008)	Figure 3 Haemorrhagic Stroke (Heart and stroke foundation of Canada, 2008)

Explanations why ischaemic stroke can occur include:

• Thrombosis - thrombus forms locally, obstructing blood vessels within the brain, either veins or arteries.

• Systemic hypoperfusion – such as in shock , for example hypovolemic, which results from inadequate blood volume for the continuation of satisfactory cardiac output, blood pressure, and tissue perfusion.

• Cerebral embolism - embolus forms in any blood vessel within the body and travels to the brain.

• Cryptogenic stroke – obscure or of unknown origin, without any obvious explanations, occurs in 30-40% of ischaemic strokes.

H

• Cerebral venous sinus thrombosis

• Arteriovenous malformation

• Bleeding within a tumour

• Amyloid angiopathy

aemorrhagic strokes are intracerebral bleeds and can be caused by:

• Penetrating head trauma

• Depressed skull fractures

• Rupture of an aneurysm

• Acceleration-deceleration trauma

Stroke risk factors can be fixed or modifiable and include: (Colledge, Walker and Ralston., 2010, p. 1181)

• A

  • High blood pressure

  • Heart disease (atrial fibrillation, heart failure)

  • Diabetes mellitus

  • Hyperlipidaemia

  • Smoking

  • Excess alcohol consumption (≥2/day)

  • Polycythaemia

  • Oral contraceptives

  • Social deprivation

  ge

• Gender (male > female, except in the very young and very old)

• Race (Afro-Carribean > Asian > European)

• Heredity

• High fibrinogen

• Previous vascular event, e.g. MI, stroke or peripheral embolism

• Menopause

Kazuhiko et al., (2011) also suggest that infection with serotype k Streptococcus mutants expressing collagen-binding protein is also a potential risk factor for haemorrhagic stroke.

Molecular mechanisms of neuronal ischaemia and infarction are depicted in Figure 4 and occur in the following order:

1. Reduction of blood flow reduces supply of oxygen and glucose, hence ATP and H⁺ions.

2. Failure of energy-dependent membrane ionic pumps, leads to cytotoxic cerebral oedema and membrane depolarisation, which allows calcium entry and release of glutamate.

3. Calcium enters cells via glutamate-gated channels and activates destructive intracellular enzymes, destroying intracellular organelles and cell membrane with release of free radicals.

4. Free fatty acid release activates pro-coagulant pathways, which exacerbate local ischaemia.

5. Glial cells take up H⁺ions and thus cannot take up extracellular glutamate and suffer cell death.

6. The release of inflammatory mediators by microglia and astrocytes lead to necrosis of all cell types in the whole arterial territory.