We read with great interest the letter written by Zeeman et al, who reaffirmed posterior reversible encephalopathy syndrome (PRES) was a core component of the pathogenesis and diffusion-weighted imaging (DWI) together with apparent diffusion coefficient (ADC) maps should be added to the standard neuroimaging protocol in eclamptic patients. Those statements are of great significance for better understanding PRES, but some might be misleading.

In pregnant women who present altered mental status/coma or other neurological symptoms, neuroimaging examination should be regarded as the first choice. Pregnancy-related vascular encephalopathy could be divided into 3 entities: PRES, hemolysis-elevated liver enzymes-low platelets (HELLP) syndrome, and reversible cerebral vasoconstriction syndrome (RCVS). Magnetic resonance imaging (MRI) findings of PRES may overlap with those of HELLP syndrome, in which lesions were more frequently observed in brainstem, basal ganglia, and thalamus than cerebral hemispheres. RCVS is characterized by recurrent thunderclap headaches, seizures, strokes, and nonaneurysmal subarachnoid haemorrhage. RCVS could be associated with PRES in approximately 10% of cases regardless of etiology. RCVS seems to be associated with constriction and/or dilation of large or medium arteries while PRES does at the level of distal arterioles and capillary. An overlap between these 2 syndromes representing a continuum in them. Postpartum cerebral angiopathy is another ill-characterized RCVS, usually occurring within 30-day duration of uncomplicated pregnancy and delivery.

The diagnosis of PRES mainly relies on clinical symptoms and typical radiologic findings. DWI is sensitive for differentiating cytotoxic edema from vasogenic edema as well as ADC maps, which present reduced signal intensity for ischemia and cytotoxic edema. The prevalence of a cytotoxic edema in PRES is <1-17.3%, which is usually accented for surrounding larger areas of vasogenic edema. In practice, ADC mapping is helpful in certain cases in which clinical diagnosis is unclear or difficult to determine. Typically, reversibility of any MRI findings after treatment could confirm the diagnosis of PRES in these patients. Both DWI and ADC features might respond after proper treatment. However, it should be emphasized that MRI is only a confirmatory tool in those patients suspected of PRES, but not a mandatory diagnostic tool. Moreover, currently no imaging examination is considered as the gold standard for diagnosis or evaluation in PRES. Although DWI can successfully predict evolution course of brain edema, the value of DWI as well as ADC maps in predicting clinical outcome warrant further investigation.