Although transcranial sonography is not yet an established diagnostic modality for dementia screening or differential diagnosis of Alzheimer's disease (AD) from vascular dementia (VaD), intracranial hemodynamic assessment may provide crucial information about the association between cognitive deterioration and vascular risk factors. We conducted a systematic narrative review of available literature through MEDLINE and EMBASE search to identify all available data about the evaluation of VaD patients with transcranial Doppler, and to discuss further the vascular disorders of the cerebral circulation in patients with vascular cognitive impairment. According to the available literature data to date, VaD patients were found to have lower mean flow velocity values in four studies (indicating cerebral hypoperfusion), higher pulsatility indices in three studies (indicating increased downstream vascular resistance), and more severe impairment of cerebrovascular reactivity in five studies (indicating exhausted vasodilatory reserve) compared to AD patients and controls. Microembolic signals were also found to be significantly more common in patients with VaD or AD compared to their age- and gender-matched controls, suggesting that asymptomatic microembolism, apart for being only marker of VaD, could presumably be involved in the genesis of dementia, and in the overlap between VaD and AD. Further studies with larger and carefully selected groups are required to eliminate potential confounders and to set specific cut-off values for the aforementioned hemodynamic parameters in demented patients and dementia subtypes.
Keywords: Cerebral hemodynamics; multi-infarct dementia; transcranial Doppler; transcranial ultrasound; vascular dementia.