NEUROIMAGING / NEUROSONOLOGY

Neurosonology and the brain death diagnosis

Created 05.10.2019, last update 29.11.2021

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 CCA and clinically detected loss of brain function
  • TCD/TCCD is a noninvasive bedside technique, allowing monitoring or frequent controls in neurocritically ill patients
  • it can detect a decrease in intracerebral flow, progressing up to 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 trained physicians 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 perfusion of the brain) (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 rebleeding after SAH, and after cardiac arrest, 30 minutes interval suffices 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, however, that 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

  • the presence of large craniectomy or skull fractures can show persistent diastolic flow in cases with clinical brain death (similar to what occurs in children less than 1-year-old in whom fontanelles are still open)
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