Involvement is usually symmetric, but the degree of involvement may be asymmetric [2] (Fig. 15.2a–h).

Signal alterations can also occur to a lesser extent in the basal ganglia, posterior temporal and frontal lobes, corona radiate, brain stem, and cerebellum [6, 7] (Fig. 15.3a–e).

Evaluation with diffusion-weighted images of patients with PRES show increased diffusion with elevated values of ADC due to the presence of vasogenic edema. Nevertheless, there have been described cases of coexistence of vasogenic with cytotoxic edema [6].

Even though most reports have underlined a white matter predilection, Casey et al. in a recent study showed that cortical lesions are more common than what is previously thought [7, 9].

Patients with seizures rarely had involvement of cortical matter; patients with autoimmune disease were more likely to have cerebellar involvement; patients with sepsis or active infections were more likely to have cortical involvement [10].

Comparing pregnant and nonpregnant patients, despite some clinical significant differences in pregnant women, headache and visual disturbance are more common than in nonpregnant patients [11]; no other significant differences are reported about the topographic distribution of cerebral lesion. In both group, common areas involved by the lesions are occipital and parietal lobes, followed by frontal and temporal regions [10].

Pathophysiology of PRES is still incompletely understood.

One proposed mechanism is that hypertension induced potentially reversible vasospasm, especially of posterior border zone territories, characterized by cytotoxic edema which can possibly evolve to ischemic lesions if not successfully treated [6]. Occasional angiographic evaluation of women with preeclampsia/eclampsia documented widespread intracranial vessel vasospasm [12]. On the other hand, several considerations are in contrast with this hypothesis. If we accept arteriolar vasospasm and distal ischemia as the principal alteration, the acute lowering of mean arterial pressure should further reduce perfusion to these regions; however, most patients respond well to antihypertensive therapy and ischemic infarctions are fairly rare [6].

An alternative theory, which has been best characterized in preeclampsia, eclampsia, and sepsis, implicates endothelial dysfunction [10].

The third and now widely accepted hypothesis considers the altered status of cerebral perfusion autoregulation.

Brain perfusion is maintained constant by an autoregulatory system consisting of a myogenic and a neurogenic response (sympathetic innervation). The transient disruption of the cerebral autoregulation with forced dilatation of arterioles causes leakage of serum through capillary walls into the cerebral interstitium [2, 6, 8].

The functionality of the autoregulation can be expressed as the autoregulatory index (ARI), with 0 being absent and 9 perfect cerebral autoregulation. This ARI has been shown to be lower in preeclampsia when compared with normotensive control subjects [14].

The principal posterior distribution of white matter alterations has been attributed to the sympathetic innervation of the cerebral vessel which accounts for the neurogenic response of cerebral autoregulation to hypertension.

This distribution has an anteroposterior gradient with a relative reduced innervation of the posterior cerebral arterial circulation. During acute elevation of blood pressure, posterior brain regions may be particularly susceptible to breakthrough of autoregulation.

It has been proposed that the fluid accumulation often observed during pregnancy, particularly in the third trimester, may accentuate the increase in development of vascular endothelial permeability in the brain. An increase in permeability can promote under hypertension [8].

Cortical blindness is a rare and dramatic complication of preeclampsia. Petechial hemorrhages and focal cerebral edema in the occipital cortex secondary to ischemic injury and vasogenic edema causing increased capillary permeability are considered to be important factors for the pathogenesis of cortical blindness in preeclampsia [15].

PRES has been less commonly described in the setting of autoimmune disease. The distinctive role of autoimmune disease is often clouded by concurrent hypertension, renal disease, and the use of immunosuppressants [10]. In autoimmune disorders, PRES is in part caused by endothelial dysfunction due to an inflammatory cytokine response that leads to autoimmune-mediated disruption of the endothelial aquaporin 4 water channels [10].

It is known that a woman who develops preeclampsia has a higher risk of developing cardiovascular disease later in life, as hypertension, ischemic, and hemorrhagic stroke. This condition leads formerly preeclamptic women to have cerebral white matter lesions more often and more severely than control women with normotensive pregnancies. Preeclampsia appears to be an independent risk factors for white matter lesions, and this was predominant in women with early-onset preeclampsia [16].

Another study assessed that an episode of PRES may play an additional role in increased prevalence of cerebral white matter lesions, visible in frontal lobes rather than in occipital lobes. Severe vasogenic edema has been suggested to potentially progress to such an extent that regional perfusion decreases, leading to areas of cytotoxic edema, infarction, and development of white matter lesions in the long term. In the same study, the authors reported that white matter lesions are more common in women with prior pregnancies complicated by preeclampsia or eclampsia compared with parous women in a control group [16].

Other two pathologic conditions are reported in pregnancy-related vascular encephalopathy: HELLP (hemolysis-elevated liver enzymes-low platelets) syndrome and RCVS (reversible cerebral vasoconstriction syndrome) [18]. The first is associated with strong epigastralgia, hepatic failure, disseminated intravascular coagulation, and intracerebral hemorrhage. The syndrome is caused by low placental gene expression of nitric oxide. Generally, MR imaging findings of HELLP syndrome may overlap with those of PRES (cerebral edema), with prevalent involvement of basal ganglia and brain stem [19]. Lacunar lesions are frequent [18].

Typical symptom of RCVS is a “thunderclap” headache due to multifocal or segmental cerebrovascular spasm, visible on MR angiography, which resolved within 12 weeks. Parenchymal lesions are frequently located in areas of the anterior circulation [18].