|DAH from GPA or CPE/ESRD?
A woman presented with thunderclap headache and had recurrent seizures during initial evaluation. A differential diagnosis was formulated and it included PRES (posterior reversible encephalopathy syndrome) with a PTP of about 20%. Subarachnoid hemorrhage was excluded with CT and LP and the PTP of PRES rose to about 40% (since it occupied some of the probability space previously occupied by SAH once the latter was excluded.) The subsequent MRI images were consistent with PRES. Nonetheless, a vascular MRI was ordered to "exclude the possibility of cerebral vasculitis". The problems are twofold. First, the probability of PRES is now on the order of 70% if the sensitivity and specificity of MRI are on the order of 80%, and it is 85% if sensitivity and specificity are each 90%. (Go ahead and plug some numbers into the calculator on the sidebar of the blog.) This probability meets or exceeds the probability threshold to both consider the diagnosis made, and to take action based on it. In this case inaction and supportive care are indicated. Even if a vascular MRI were consistent with cerebral vasculitis, which has a PTP an order of magnitude or more less than PRES, the diagnosis is still PRES. The truth is not in the test, the truth is in the rationally considered diagnostic process of which the test is one part.