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Watershed Infarcts


Learning objectives

  • What is a watershed infarct
  • Causes of watershed infarct
  • Investigations

Introduction

A watershed infarct is a stroke caused by a drop in circulating pressure and or volume that results in critical ischaemia or infarction between territories. Classically between MCA and ACA or MCA and PCA. The actual blood stream blockage/restriction site can be located far away from the infarct.

Aetiology

How to explain. If you and friends go out for dinner to a restaurant at a table for 20 people and the waiter places 3 bottles of wine at each end then as everyone pours a glass and passes it on everyone should get a drink. If however on another night you can only afford 1 bottle at each end of the table then those in the centre will find that the bottle is empty by the time it gets there. This is the watershed area and can in the brain be a line of infarction. It only happens when there is a generaised reduction in blood flow and the areas at the edges of any territory will get the least. One cause may be global drop in BP during sepsis or other causes of shock or during surgery or an occluded or critically stenotic ICA which supplies ACA and MCA and so this is where infarction is worst.

Epidemiology

Causes 5-10% of infarcts.

Images

Aetiology

  • Arterial stenosis - may be multiple
  • A Drop in systemic blood pressures
  • A Drop in oxygen carriage - hypoxia, anaemia
  • May be a role for microembolism
  • All or some of these

Locations

  • Between ACA and MCA: in frontal cortex extending from anterior horn to the cortex
  • Between MCA and PCA: in parieto-occipital region extending from posterior horn to the cortex
  • Parallel parafalcine stripes in subcortical white matter at the vertex suggests profound hypoperfusion

Clinical

  • May be episodic, fluctuating, or progressive weakness of the hand
  • May be occasionally associated upper limb shaking

Investigations

  • FBC, U&E, ABG if needed
  • CT/MRI can define infarct
  • Carotid duplex for ICA stenosis
  • CTA for intracranial stenosis
  • Exclude causes of hypotension e.g. MI ECG/Troponin, PE Dimer/CTPA

Management

  • Identify cause if possible
  • Caution with lowering BP
  • Surgical referral for ICA stenosis

References

Last updated: 15/11/2018

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