The measurement of VMR by TCD has been shown to be a valuable noninvasive instrument for both practical clinical work and pathophysiological research in the field of stroke and cerebrovascular disease. Cerebral VMR reflects the compensatory dilatory capacity of cerebral arterioles to a dilatory stimulus. In the absence of major stenosis, an impaired cerebral VMR may reflect increased rigidity of the arteriolar walls. Decreased cerebral VMR has been observed in patients with hypertension and those with insulin-dependent diabetes mellitus [6]. Moreover, impaired cerebral VMR in young hypertensive subjects appears to improve after the initiation of antihypertensive treatment, suggesting that hypertensive microangiopathic changes could be, at least initially, reversible [7].
Tzourio et al. [8] using TCD in 628 individuals (39 % treated hypertensives) found that the degree of WML was associated with a reduction in CBF velocity.
However, until now, most studies have been performed in elderly people or have included treated hypertensive patients or patients with a history of cardiovascular disease, carotid stenosis, previous stroke, or diabetes, which could act as confounding factors. In order to establish cutoff values for cerebral hemodynamic parameters to define silent cerebral organ damage, longitudinal studies in homogeneous samples of middle-aged hypertensive patients will be necessary.
19.3 Cerebral SPECT
Cerebral SPECT with 99mTc-HMPAO allows semiquantitative evaluation of brain perfusion in humans. HMPAO (hexamethyl-propylene amine oxime) is a lipophilic molecule that crosses the blood–brain barrier and is converted within the brain cell into a hydrophilic form that remains stable for many hours and displays areas of increased or decreased perfusion. The striatum and thalamus are the brain areas most susceptible to lacunar thrombotic infarction, particularly in hypertensive patients, due to the vessel anatomy and vascular supply in these areas. Cerebral blood flow in hypertensive patients is maintained at the same level as in normotensive individuals by cerebral autoregulation, until significant arteriosclerosis and hypertensive vascular disease develop [9].
Cerebral SPECT is an accurate examination for the evaluation of regional CBF. The disadvantages are radiation hazard (not optimal for screening), poor spatial resolution, and the fact that the technique does not permit continuous registration.
The possible relationship between CBF and the existence of target organ damage at other levels in essential hypertensive patients is unclear. A relationship between left ventricular hypertrophy and a reduction in regional CBF–SPECT, especially in the striatum region, has been shown in asymptomatic middle-aged untreated essential hypertensive patients [10].
19.4 Summary
It remains unclear whether early functional cerebral alterations in hypertension are related to a higher risk of stroke or not. Optimal techniques for the detection of early silent brain damage should (a) be simple, relatively cheap, noninvasive, and easy to perform; (b) have excellent topographical resolution for the detection of regional abnormalities; and (c) have the capacity to assess therapeutic efficacy. The measurement of VMR by transcranial Doppler is a valuable noninvasive examination for both practical clinical work and pathophysiological research in the field of stroke and cerebrovascular disease. Cerebral VMR reflects increased rigidity of the arteriolar walls, and this abnormality has been observed in patients with hypertension. In addition, impaired cerebral VMR in young hypertensive subjects improves with antihypertensive treatment, suggesting that hypertensive microangiopathic changes could be, at least initially, reversible. Cerebral SPECT is an excellent technique for the evaluation of regional CBF, but the radiation hazard and poor spatial resolution means it is not recommended for the detection of early silent cerebral damage in hypertension in routine clinical practice.
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