NEUROIMAGING / NEUROSONOLOGY

Neurosonology and the brain death diagnosis

Created 29/11/2021, last revision 22/01/2023

Introduction

  • brain death and cerebral circulatory arrest (CCA) are not identical conditions
    • brain death is a clinical diagnosis characterized by the irreversible loss of brain functions
    • various mechanisms leading to brain death increase intracranial pressure (ICP) with subsequent loss in cerebral autoregulation ⇒ ↓ cerebral blood flow, and finally, cerebral circulatory arrest appears
    • there may be a lag of several hours between the development of circulatory arrest and clinically detected loss of brain function
  • TCD/TCCD is a noninvasive bedside technique, allowing monitoring or frequent controls in neurocritically ill patients
  • a decrease in intracerebral flow can be detected, progressing to the cerebral circulatory arrest
TCCD

Intracranial flow patterns

  • the increase of ICP and subsequent decrease of cerebral perfusion pressure (CPP) results in progressive changes in the Doppler spectra waveform → hemodynamics notes
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Examination procedure

  • the examination should be performed by a trained physician with experience in the ultrasound technique and familiarity with the pathophysiology of brain death
  • 2MHz diagnostic probe is used
  • basic approaches:
    • transtemporal window
    • transforaminal window (via foramen magnum)
    • transorbital window
  • proof of cerebral circulation arrest – registration of typical flow patterns (oscillations or systolic spikes) in multiple intracranial arteries (MCA, ACA, PCA, terminal ICA, VA, and BA) is required – at least in MCA bilaterally and in the basilar artery
    • evaluation of ICA alone is not appropriate (possible shunt to ECA without brain perfusion) (Freitas, 2003)
    • assessment of the BA is essential to avoid false-positive results
    • a pathological flow pattern can be detected in the extracranial arteries (CCA, ICA, VA) or ophthalmic artery as well (Blanco, 2020)
  • proof of irreversibility of cerebral circulatory arrest:
    • examination showing above stated flow pattern lasting ≥ 30 minutes  OR
    • repeated examination separated by ≥ 30 minutes
    • transitory patterns compatible with cerebral circulatory arrest have been described in patients with SAH rebleeding and after cardiac arrest; 30 minutes interval is sufficient to confirm the irreversibility of the pattern

MAP (Mean Arterial Pressure) should be > 60 mmHg and systolic BP > 90 mmHg during the whole ultrasound examination

 Poor acoustic window

  • use ultrasound contrast agents (up to 98% of patients can be examined this way)
  • use a different approach (incomplete examination should not be used as a confirmatory test of circulatory arrest)

    • transorbital (detect flow in the carotid siphon)
    • extracranial examination (indirect proof of circulatory arrest)

Skull defects

  • patients with clinical brain death who have had large craniectomy or skull fractures may have persistent diastolic flow (similar to what occurs in children less than 1-year-old in whom fontanelles are still open)
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Neurosonology and the brain death diagnosis
link: https://www.stroke-manual.com/neurosonology-brain-death/