NEUROIMAGING / NEUROSONOLOGY

Neurosonology and the brain death diagnosis

Created 29/11/2021, last revision 06/11/2023

Introduction

  • brain death and cerebral circulatory arrest (CCA) are not identical conditions
    • brain death is a clinical diagnosis characterized by the irreversible loss of all brain functions, including those of the brainstem
    • several mechanisms lead to brain death:  increase in intracranial pressure (ICP), subsequent loss of cerebral autoregulation, ↓ cerebral blood flow, and ultimately, cerebral circulatory arrest
    • there may be a delay of several hours between the onset of circulatory arrest and clinically detectable loss of brain function
  • TCD/TCCD is a non-invasive bedside technique that allows for monitoring or frequent checks in neurocritically ill patients
  • a decrease in intracerebral flow can be detected, progressing to cerebral circulatory arrest
TCCD

Intracranial flow patterns

  • the increase of intracranial pressure (ICP) and subsequent decrease of cerebral perfusion pressure (CPP) result in progressive changes in the waveform of the Doppler spectra  → Hemodynamics notes

decrease of flow velocities

increase of pulsatile and resistance indexes (resistant flow)

PI and RI rise since ICP > 15 mm Hg
rough ICP formula = (10.927* PI) -1.284 [Brandi, 2010]
PI >3 = CBF < 20% of normal values

a gradual decrease of end-diastolic velocity (EDV) down to 0 cm/s (ICP = diastolic blood pressure)
there is no circulatory arrest at this stage as the brain is still perfused during systole

biphasic, reverberating flow
net flow = anterograde-retrograde component of Vmean is < 4 cm/s (some report < 10 cm/s), the cerebral perfusion has ceased
Brain death - alternate flow

systolic spikes
flow with very short duration during systole, PSV < 50 cm/s, duration <  200 msec,  no hemodynamic significance
Brain death - systolic spikes


absent flow
may serve as a sign of brain circulatory arrest if the examiner has previously documented intracranial flow,  thus ruling out a poor insonation window

Examination procedure

  • the examination should be performed by a trained physician experienced in ultrasound technique and familiar with the pathophysiology of brain death
  • a 2MHz diagnostic probe is used
  • basic approaches include:
    • transtemporal window
    • transforaminal window (via the foramen magnum)
    • transorbital window
  • proof of cerebral circulation arrest – requires registration of typical flow patterns (either oscillations or systolic spikes) in multiple intracranial arteries (MCA, ACA, PCA, terminal ICA, VA, and BA) is required – at least in the MCA bilaterally and in the basilar artery
    • evaluation of the ICA alone is not appropriate (possible shunt to ECA without brain perfusion may occur) (Freitas, 2003)
    • assessment of the BA is essential to avoid false-positive results
    • a pathological flow pattern can also be detected in the extracranial arteries (CCA, ICA, VA) or the ophthalmic artery (Blanco, 2020)
  • proof of the irreversibility of cerebral circulatory arrest:
    • an examination showing the aforementioned flow pattern ≥ 30 minutes  OR
    • repeated examinations separated by intervals of ≥ 30 minutes
      • transitory patterns compatible with cerebral circulatory arrest have been described in patients with SAH rebleeding and after cardiac arrest; a 30-minute 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 undergone large craniectomy or have skull fractures, persistent diastolic flow may be observed (similar to what occurs in children less than 1 year of age in whom the fontanelles are still open)
  • in such cases, other diagnostic modalities like cerebral angiography or radionuclide imaging may be necessary to confirm the diagnosis of brain death

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Neurosonology and the brain death diagnosis
link: https://www.stroke-manual.com/neurosonology-brain-death/