Posterior reversible encephalopathy syndrome
Posterior reversible encephalopathy syndrome, also known as reversible posterior leukoencephalopathy syndrome, 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 headache, 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. 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 brain scan on 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. 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.
Signs and symptoms
PRES usually has an acute onset. Most people with PRES experience headaches and seizures; many also experience visual changes, confusion and drowsiness, weakness of the arm and/or leg on one side of the body, difficulty speaking, or more rarely other neurological symptoms. The visual changes in PRES may include hemianopsia, blurred vision, lack of visual awareness on one side, visual hallucinations, and cortical blindness.Seizures occur in about two thirds of cases. In children this is more common still, at 90%. If seizures occur they may be focal or generalized. A small proportion develops status epilepticus, where seizures are not controlled with simple measures.
Causes
Causes that may contribute to the development of PRES are: immunosuppression, 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 the risk of PRES is approximately 8%, whereas the risk is lower after a solid organ transplant.The following autoimmune conditions have been found to be associated with PRES: thrombotic thrombocytopenic purpura, primary sclerosing cholangitis, rheumatoid arthritis, Sjögren syndrome, polyarteritis nodosa, systemic sclerosis, systemic lupus erythematosus, granulomatosis with polyangiitis, Crohn's disease and neuromyelitis optica, as well as hemolytic-uremic syndrome. 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.
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
While the precise mechanism is 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 extremely high blood pressure overcomes the normal capabity 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. This predominantly affects the "posterior" parts of the brain which is more susceptible. 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 that precipitates the edema. The "immunogenic" theory suggests a role for the immune system. 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 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 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.
Diagnosis
There are no formal diagnostic criteria for PRES, but it has been proposed that PRES can be diagnosed if someone has developed acute neurological symptoms together with one or more known risk factors, typical appearance on brain imaging, and no other alternative diagnosis. Some consider that the abnormalities need to be shown to be reversible. If lumbar puncture is performed this may show increased protein levels but no white blood cells. Computed tomography scanning may be performed in the first instance; this may show low density white matter areas in the posterior lobes.The diagnosis is typically made with magnetic resonance imaging of the brain. The findings most characteristic for PRES are symmetrical hyperintensities on T2-weighed imaging in the parietal and occipital lobes; this pattern is present in more than half of all cases. FLAIR sequences can be better at showing these abnormalities. Some specific other rare patterns have been described: the superior frontal sulcus watershed pattern, a watershed pattern involving the entire hemisphere, and a central pattern with vasogenic oedema in the deep white matter, basal ganglia, thalami brainstem and pons. These distinct patterns do not generally correlate with the nature of the symptoms or their severity, although severe edema may suggest a poorer prognosis. If the appearances are not typical, other causes for the symptoms and the imaging abnormalities need to considered before PRES can be diagnosed conclusively. In many cases there is evidence of constriction of the blood vessels, suggesting a possible overlap with reversible cerebral vasoconstriction syndrome. Diffusion MRI may be used to identify areas of cytotoxic edema caused by poor blood flow but it is not clear if this prognostically relevant. Abnormal Diffusion MRI|ADC image|apparent diffusion coefficient is seen in about 20% of cases.
In 10-25% of cases of PRES there is evidence of hemorrhage on neuroimaging. Various types of hemorrhage may occur: hemorrhage into the brain tissue itself, sulcal subarachnoid hemorrhage, and microbleeds.
Treatment
There is no direct treatment for PRES, other than removing or treating any underlying cause. For instance, immunosuppressive medication may need to be withheld. 40% of all people with PRES are unwell enough to require intensive care unit admission for close observation and treatment of complications. Those with seizures are administered anticonvulsants.If there is a hypertensive emergency, the blood pressure is lowered by 20-30% using continuous infusion of a blood pressure lowering drug with close monitoring. Of the blood pressure lowering agents available, nitrates may need to be avoided as there is a concern that this may aggravate the PRES even while lowering the blood pressure. In PRES secondary to pre-eclampsia, magnesium sulfate may be administered.
Prognosis
With adequate treatment, 70-90% of people with PRES make a full recovery within hours to days. 8-17% of people with PRES die, although this is not always a direct consequence of the PRES. Of those who have residual symptoms after PRES, this is attributable largely to hemorrhage. Non-resolution of MRI abnormalities has been linked with poorer outcomes. If PRES was caused by pre-eclampsia the prognosis is better than in PRES due to other causes.Factors that predict poorer prognosis are the person's age, the level of C-reactive protein in the blood, altered mental state at the time of diagnosis, and altered markers of coagulation. People with diabetes may have a worse outcome, and abnormalities in the corpus callosum on MRI have been linked with worse prognosis. Some patterns on electroencephalography are also associated with a poorer outcome.
After an episode of PRES, even when it was associated with seizure activity, only a small proportion of people remains at risk of ongoing seizures and the majority can eventually discontinue anticonvulsant treatment.