Posterior reversible encephalopathy syndrome (PRES), also known as reversible posterior leukoencephalopathy syndrome (RPLS), is a rare condition in which parts of the brain are affected by swelling, usually as a result of an underlying cause. Someone with PRES may experience headaches, changes in vision, and seizures, with some developing other neurological symptoms such as confusion or weakness of one or more limbs. The name of the condition includes the word "posterior" because it predominantly, though not exclusively, affects the back of the brain (the parietal and occipital lobes). Common underlying causes are severely elevated blood pressure, kidney failure, severe infections, certain medications, some autoimmune diseases, and pre-eclampsia. The diagnosis is usually made by a brain scan (MRI), in which areas of swelling can be identified. The treatment for PRES is supportive: removal of the cause or causes and treatment of any of the complications, such as anticonvulsants for seizures. PRES may be complicated by intracranial hemorrhage, but this is relatively rare. The majority of people recover fully, although some may experience some residual symptoms. PRES was first described in 1996.
Signs and symptoms PRES usually has an acute onset. Most people with PRES experience headaches and seizures; many also experience visual changes, confusion, drowsiness, weakness of the arm and/or leg on one side of the body (hemiplegia), difficulty speaking, or, more rarely, other neurological symptoms. Some people with PRES may experience coma. The visual changes in PRES may include hemianopsia (inability to see the left or right part of the visual field), blurred vision, lack of visual awareness on one side, visual hallucinations, and cortical blindness. Seizures occur in about two thirds of cases with seizures being the initial symptom in about 50% of cases. In children seizures may be seen in up to 90% of cases of PRES. If seizures occur they may be focal or generalized. About 18% of people who have seizures develop status epilepticus, where seizures are not controllable with simple measures.
Causes Causes that may contribute to the development of PRES are: immunosuppression (especially for organ transplantation, e.g. with tacrolimus), severe infection and/or sepsis, chemotherapy, autoimmune disease, and pre-eclampsia. High blood pressure is often present. Similarly, the majority of people with PRES have an impaired kidney function, and 21% are receiving regular hemodialysis. In PRES related to medications, there may be an interval of weeks to months between the initiation of the treatment and the development of PRES. After a hematopoietic stem cell transplantation (bone marrow transplant) the risk of PRES is approximately 8%, whereas the risk is lower (0.4-6%) after a solid organ transplant. The following autoimmune conditions have been found to be associated with PRES: thrombotic thrombocytopenic purpura (TTP), primary sclerosing cholangitis (PSC), rheumatoid arthritis (RA), Sjögren syndrome, polyarteritis nodosa (PAN), systemic sclerosis, systemic lupus erythematosus (SLE), granulomatosis with polyangiitis (GPA), Crohn's disease and neuromyelitis optica (NMO), as well as hemolytic-uremic syndrome (HUS). A number of other associations have also been reported, including some other groups of medications, blood transfusion, elevated calcium levels, decreased magnesium levels, postpartum cerebral angiopathy, and drugs of abuse (cocaine and amphetamine). It has been suggested that PRES is identical or closely related with hypertensive encephalopathy, the presence of neurological symptoms in those with a hypertensive emergency.
Mechanism The precise mechanism of PRES is not fully understood, it is considered to be related to a problem with the blood vessels of the brain. There are several theories as to why these blood vessels may become inappropriately permeable and allow the surrounding brain tissue to become swollen. The "vasogenic" theory posits that elevated blood pressure overcomes the normal capability of blood vessels in the brain to maintain a normal cerebral blood flow. The excessive pressure damages the endothelial layer and the blood–brain barrier, leading to swelling (edema). The predilection toward the posterior brain may be explained by the reduced density of sympathetic innervation in the posterior circulation compared to the anterior circulation (thus a reduced adaptive capacity to fluctuations or elevations in blood pressure). The "vasogenic" theory seems to explain the almost 50% of cases of PRES where there had been severely elevated blood pressure. It is also called the "breakthrough" theory, or the "hyperperfusion theory". This theory does not explain the edema in many other cases where the blood pressure has been normal or even low; in fact, the edema tends to be more severe in those without abnormally elevated blood pressure. In PRES secondary to other causes, the blood vessel damage has been attributed to other mechanisms. The "cytotoxic" theory suggests that it is direct cell damage by toxins (usually medications) that precipitates the edema. The "immunogenic" theory suggests a role for the immune system (specifically T cells). Some consider the cytotoxic and immunogenic theories together as a single "toxic" theory. There appears to be a role of cytokines in causing endothelial dysfunction. Finally, according to the "neuropeptide/cerebral vasoconstriction" theory, some specific substances (endothelin 1, thromboxane A2) trigger spasm of the blood vessels with resultant vessel wall damage and edema. The latter hypothesis is supported by the frequent finding of diffuse blood vessel spasms (vasoconstriction) in many people with PRES, and the evidence for decreased perfusion, although the spasm may also be a consequence of the blood vessel damage rather than the cause. Some, therefore, include the vasospasm in the "toxic" theory. It is considered likely that these multiple mechanisms all potentially play a role in the development of PRES.
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