Acute Ischemic Stroke: Pathophysiology, Cerebrovascular Anatomy, and Stroke Syndromes

Introduction

• Stroke is a debilitating disease and represents the second most common cause of death worldwide and the leading cause of serious long-term disability in adults.
• Globally, 60% of all strokes are due to ischemia.¹
• In the United States, the etiology of strokes include²
  • 87% are due to ischemia;
  • 10% are due to intracerebral hemorrhage (ICH); and
  • 3% are due to subarachnoid hemorrhage (SAH).
• AIS subtypes according to the mechanism of injury:³
  • 30% are due to cerebral small vessel disease;
  • 18% are due to large-artery atherosclerosis;
  • 20% are due to a cardiac source of emboli;
  • 30% are without an identified cause (cryptogenic infarcts); and
  • 2% are due to other determined causes, such as arterial dissection or hypercoagulable states.³
• Among the common causes of AIS, large-artery atherosclerosis carries the highest risk of early stroke recurrence, while cardioembolism portrays the highest mortality rate.
• Traditional vascular risk factors associated with common causes of AIS include⁴
  • hypertension
  • smoking
  • diabetes
  • hyperlipidemia
  • cardiovascular disease
  • obesity
  • physical inactivity
  • family history of stroke prior to age 65

Pathophysiology

• Cerebral tissue is highly dependent on consistent cerebral blood flow (CBF) for the delivery of oxygen and glucose.
  • Acute occlusion of an artery supplying the brain leads to cell injury (ischemia) and death (infarct) when CBF falls below 20mL/100g/minute.
  • CBF is directly affected by arterial pCO₂ levels. Increases in pCO₂ will increase CBF, whereas decreases in pCO2 will decrease CBF.⁴
• Cerebral vasculature adapts to provide a relatively constant CBF despite changes in systemic mean arterial pressure (MAP), referred to as autoregulation.⁵ This adaptive mechanism may be overwhelmed when extreme variances in MAPs occur (Figure 1).
• Loss of cerebrovascular autoregulation following ischemic injury further impairs the delivery of oxygen to at-risk tissue, leading to cell death.

Cerebrovascular Anatomy

• Focal and abrupt reduction in CBF rapidly leads to cell injury; however, rapid treatment can prevent progression to cell death.
• A fundamental knowledge of cerebral vasculature is essential in the recognition and management of AIS.
• Cerebrovascular anatomy originates at the aortic arch from which the brachiocephalic, left common carotid and left subclavian artery are derived (Figure 2).
  • The brachiocephalic artery forms the right common carotid and subclavian artery shortly after takeoff.
  • The vertebral arteries originate from the subclavian arteries.
  • The extracranial segments enter the transverse foramen at C6 level and ultimately pierce the dura in the vertebral canal, becoming the intracranial segments.
  • Common carotid arteries divide into external and internal carotid arteries at the level of C3-C4 vertebrae (Figure 3).

• The circle of Willis connects the anterior (carotid) and posterior (vertebrobasilar) circulation as the major intracranial arterial system (Figure 4).
  • The intracranial carotid artery bifurcates into the middle cerebral artery (MCA) and anterior cerebral artery (ACA).
    • Prior to bifurcation, the ICA provides smaller branches including the
      • ophthalmic artery;
      • posterior communicating artery (PCom); and
      • anterior choroidal artery.
  • The vertebral arteries form the basilar artery at the pontomedullary junction, which later bifurcates to form the right and left posterior cerebral arteries (PCA).
    • The posterior inferior cerebellar artery (PICA) is derived from the vertebral artery prior to the vertebrobasilar junction, while the anterior inferior cerebellar artery (AICA) is a branch of the basilar artery.
    • The superior cerebellar artery (SCA) originates immediately prior to the termination of the basilar artery.
  • The communicating arteries provide anastomosis between arterial branches:
    • Anterior communicating artery connects the bilateral ACAs.
    • Posterior communicating artery connects each MCA to the respective posterior cerebral artery (PCA).⁴

Stroke Syndromes

• The rapid identification of stroke syndromes can assist in timely treatment and improved outcomes.
• Cortical signs can assist in the identification of large vessel occlusion (LVO):
  • aphasia
  • gaze preference
  • visual field cut
  • neglect

• Posterior circulation stroke syndromes may be overlooked as they frequently present with nonspecific, nonlocalizing signs and symptoms, including nausea, vomiting, and dizziness.