|  Figure 1:(a) CT scan brain showing bilateral cerebellar edema; (b) MRI after 2 weeks showing complete resolution of cerebellar edema

DISCUSSION

The reversible syndrome of altered mental status, headache, seizures, visual disturbances, and hypertension has been described in literature with different terminologies like hypertensive encephalopathy, reversible posterior leukoencephalopathy, and reversible occipitoparietal leukoencephalopathy.[2]

This syndrome has been recognized without hypertension or with only mild increase in blood pressure; there is usually no accompanying destructive process of the white matter; and neuroradiologically, the involvement is not strictly restricted to parieto-occipital regions. Hence, PRES is more acceptable and popular terminology.[3,4]

PRES is seen exclusively in the setting of a significant systemic process or condition, including sudden hypertension, bone marrow or solid organ transplantation (specially on cyclosporine immunosuppression), infection/sepsis/shock, toxemia of pregnancy, autoimmune disease, and during cancer chemotherapy (e.g., l-asparaginase, prednisolone, high-dose cytarabine, etc.). A case series from India revealed induction chemotherapy, hypertension, high-dose cytarabine, and cyclosporine as the triggers for PRES in 13 leukemic patients.[5]

Cerebral hypoperfusion and cytotoxic edema secondary to underlying systemic condition or drugs seems to be the principle pathophysiologic process leading to development of encephalopathy. Hypertension may have a modulating effect to this process.[3] An obvious risk factor in our patient was induction chemotherapy for ALL, but he also had one record of sudden onset hypertension at the time of neurotoxicity, which might have modulating effect. Furthermore, despite receiving high-dose cytarabine and repeat induction cycle subsequently, PRES did not recur, probably due to absence of hypertension during subsequent therapy.

Although MRI is the most sensitive imaging test to diagnose PRES, it is not necessary for the diagnosis if CT is suggestive. CT usually shows bilateral hypodensities in the posterior white matter, with some involvement of the overlying cortex. On MRI, these lesions appear hyperintense on T2-weighted images, and are usually hypointense or isointense on diffusion-weighted images, with an increase of the apparent diffusion coefficient, indicating vasogenic edema. Mckinney et al. reported two cases of completely unilateral involvement in their series, which is difficult to distinguish from infarction.[6] The calcarine and paramedian occipital-lobe structures are usually spared, a fact that distinguishes PRES from bilateral infarction of the posterior-cerebral-artery territory.[4] The posterior part of the brain is more frequently involved due to poor sympathetic innervation of the vertebrobasilar system.

Fugate et al. found the parieto-occipital regions as the most commonly involved site (94%) on MRI in a large prospective study, followed by the frontal lobe (77%), temporal lobe (64%), and cerebellum (53%). Cerebellar involvement was significantly more frequent in patients with a history of autoimmunity, and patients with sepsis were more likely to have cortical involvement.[7] Although parieto-occipital involvement was found in 99% of the cases, McKinney et al. noted higher incidence of atypical distributions (posterior frontal, 78.9%; temporal, 68.4%; thalamus, 30.3%; cerebellum, 34.2%; brainstem, 18.4%; and basal ganglia, 11.8%) than commonly perceived. Only one patient lacked parieto-occipital edema, but this patient had severe brainstem, thalamic, and deep white matter edema.[6] “Atypical” cases with only frontal, temporal, or brain stem lesions have been reported in literature. Only one case of PRES with isolated cerebellar involvement has been reported in the English literature, ours being the second.[1]

Although the reversibility of the clinicoradiological features is most characteristic, it should be noted that prolonged seizures, hypertension, or both may result in permanent neurologic deficits and cerebral infarction. Hence, PRES must be managed with good supportive care including antihypertensive and antiepileptic medications to prevent any irreversible damage. A follow-up scan in a period of 1 to 2 weeks is needed to document the reversibility.[8]

CONCLUSION

Awareness of atypical patterns and variations in distribution of imaging abnormalities is important to recognize PRES more accurately, to avoid expensive or potentially invasive work-ups for other primary cerebral diseases.

Footnotes