In instances of typical accelerated hypertension, by the time the neurologic manifestations appear, the hypertension has usually reached the malignant stage, with diastolic pressures above 125 mm Hg, retinal hemorrhages, exudates, papilledema, and evidence of renal and cardiac disease. However, instances of encephalopathy at lower pressures are common, especially if the rise in pressure has been abrupt (see below). If the rate of elevation is high enough, the syndrome may be seen with blood pressure considered to be close to the normal range. Presumably, these circumstances have to do with a change in the permeability of cerebral vessels that is part of the process in special cases such as eclampsia and the hemolysis, elevated liver enzymes, low platelet count (HELLP) syndrome that are mentioned further on. The term hypertensive encephalopathy should probably be reserved for the above syndrome and should not be used to refer to chronic recurrent headaches, dizziness, epileptic seizures, TIAs, or strokes, which may occur in association with elevated blood pressure.
Encephalopathy may complicate extreme hypertension from any cause (renal disease, renal artery stenosis, acute glomerulonephritis, acute toxemia, pheochromocytoma, Cushing syndrome), cocaine, or administration of drugs such as aminophylline or phenylephrine, but it occurs most often in patients with rapidly worsening “essential” hypertension.
In eclampsia, which from a neurologic perspective may be considered a special form of hypertensive encephalopathy, and in acute renal disease, particularly in children, encephalopathic symptoms may develop at blood pressure levels considerably lower than those of hypertensive encephalopathy of “essential” type. Eclamptic retinal and cerebral lesions are the same as those that complicate malignant nephrosclerosis; in both there is also failure of autoregulation of the cerebral arterioles. A discussion of eclamptic seizures can be found in the section on that subject in Chap. 15.
Hypertensive encephalopathy is marked by changes already alluded to on CT and MRI. The findings are of large areas of white matter signal change of edema, but their tendency to normalize over several weeks is remarkable. As summarized by Hauser and coworkers, the main feature is a bilateral increase in T2 signal intensity in the white matter on MRI and a corresponding reduced density on CT, usually concentrated in the posterior part of the hemispheres (see Fig. 33-35). Thus the condition is one of the causes of reversible posterior leukoencephalopathy. These imaging characteristics are a result of an accumulation of fluid, but—unlike the edema in trauma, neoplasm, or stroke—there is little or no mass effect and the water does not tend to course along white matter tracts such as the corpus callosum. In addition, scattered cortical lesions may occur in a watershed vascular distribution and probably correspond to small infarctions. These same findings in the white matter and cortex occur in eclampsia and have been seen in cases of diffuse vasospasm caused by sympathomimetic and serotonergic drugs, discussed further on.
Many cases are also accompanied by focal or diffuse reduction in the caliber of cerebral vessels, especially those on the surface of the cerebral convexities, a configuration that characterizes reversible vasoconstriction syndrome discussed below. Hypertensive encephalopathy or eclampsia may cause additional localized subarachnoid hemorrhage. Most such cases are not caused by the rupture of an intracranial aneurysm and are not as overwhelming as in aneurysmal hemorrhage; indeed, the headache associated with reversible cerebral vasoconstriction syndrome tends to be milder than with aneurysmal rupture and it may be absent. The bleeding, if it occurs, is mainly a feature that is appreciated on MRI examination, as described by Shah. The mechanism is obscure.