What is a watershed infarct?

Watershed Infarct: Definition and Characteristics

A watershed infarct is an ischemic lesion that occurs at the border zones between territories of two major cerebral arteries, resulting from either hemodynamic compromise or embolic mechanisms, and typically presenting with characteristic neurological deficits depending on the affected region.

Types of Watershed Infarcts

• External (Cortical) Watershed Infarcts: Located at the junction between superficial territories of major cerebral arteries (anterior/middle or middle/posterior cerebral arteries) 1
• Internal (Subcortical) Watershed Infarcts: Located between the superficial and deep territories of the middle cerebral artery or in periventricular white matter regions 1, 2

Pathophysiology

Hemodynamic Mechanism

• Internal watershed infarcts are primarily caused by hemodynamic compromise due to severe stenosis in large arteries or acute hypotensive events 2, 3
• Decreased cerebral perfusion in distal regions of vascular territories makes these areas vulnerable to ischemia 2
• Bilateral common carotid artery occlusion or stenosis can induce watershed infarcts through hemodynamic changes, as demonstrated in experimental models 3

Embolic Mechanism

• External watershed infarcts are more commonly associated with embolism 2, 4
• Emboli may lodge at the distal junction between arterial territories where blood flow is reduced 4
• Cardioembolism is the most common etiologic subtype in patients with external watershed infarcts 4

Clinical Presentation

• Neurological deficits vary depending on the location of the watershed infarct 5
• Common presentations include:
  • Syncope at onset (observed in 37% of cases) 5
  • Focal limb shaking (observed in 12% of cases) 5
  • Weakness or sensory changes in affected vascular territories 6
  • Cognitive impairment when affecting multiple regions 1

Risk Factors and Associated Conditions

• Internal carotid artery occlusion or severe stenosis (75% of cases) 5
• Hemodynamically significant cardiac disease 5
• Increased hematocrit 5
• Acute hypotension or shock 5
• Hypertension (more common in patients with internal watershed infarcts) 4
• Large-artery atherosclerosis (significantly associated with internal watershed infarcts) 4

Diagnostic Evaluation

• Brain Imaging:

  • MRI with diffusion-weighted imaging is the preferred modality to identify watershed infarcts 2
  • CT scanning can also demonstrate the characteristic pattern of infarction 5
  • Typical findings include infarcts in border zones between arterial territories 1

• Vascular Imaging:

  • Assessment of carotid and cerebral arteries to identify stenosis or occlusion 4
  • Evaluation of the circle of Willis for atherosclerotic changes 1

• Cardiac Evaluation:

  • Assessment for cardioembolic sources, especially in external watershed infarcts 4
  • Evaluation for hemodynamically significant cardiac conditions 5

Prognostic Implications

• Multiple small internal watershed infarcts may be independent predictors of subsequent ischemic stroke 2
• Patients with small-vessel or lacunar infarcts generally have better survival rates compared to those with other stroke subtypes 1
• Watershed infarcts constitute approximately 10% of all brain infarcts 2

Management Considerations

• Treatment should address the underlying mechanism:
  • For hemodynamic watershed infarcts: optimization of cerebral perfusion and management of carotid stenosis 1
  • For embolic watershed infarcts: appropriate anticoagulation or antiplatelet therapy based on embolic source 4
• Blood pressure management is crucial, with careful consideration of targets to prevent hypoperfusion in patients with arterial stenosis 1
• In patients with intracranial arterial stenosis, aggressive medical treatment targeting systolic blood pressure <140 mmHg for nondiabetics and <130 mmHg for diabetics has shown benefit in reducing stroke recurrence 1

Common Pitfalls and Caveats

• Distinguishing between hemodynamic and embolic mechanisms can be challenging but is important for appropriate management 2
• Watershed infarcts may be misdiagnosed as other stroke subtypes if border zone territories are not carefully evaluated 6
• Multiple advanced imaging techniques (diffusion and perfusion MRI, CT, PET, transcranial Doppler) may be needed to identify the pathophysiologic process and guide management 2
• Uncommon causes such as vasculitis, hypercoagulability, and aneurysms should be investigated in cryptogenic cases 4