Possible pitfalls in the diagnostic of progressive multifocal leukoencephalopathy

The most accurate diagnosis of clinically suspected progressive multifocal leukoencephalopathy (PML) is made by neuronavigated needle brain biopsy and microscopic examination of the specimen confirming typical morphological features of the disease and, additionally, using immunohistochemistry (IHC) for detection of early viral proteins of the etiologic agent - polyoma virus JC (JCV). Due to the small biopsy volume, this approach can sometimes fail to confirm the clinical diagnosis of PML, as demonstrated by the presented clinical case. To check the reliability of using only IHC, we additionally tested 6 archival cases from our institute using IHC, in-situ hybridization (ISH) and real-time polymerase chain reaction (PCR). In the presented case, both biopsy and autopsy material were tested, in three archival cases only biopsy material and in the remaining cases post-mortem brain tissue was available. IHC (Anti-SV40 T antigen, mAb Pab416) was negative in 3 samples, in another 3 fewer than 10 cells per one ×20 microscopic field were positive. In our study, ISH proved to be a more sensitive method for JCV detection than IHC, being positive in all cases. Out of 7 tested specimens, realtime PCR failed to confirm the presence of JCV in 1 specimen, which was the oldest brain autopsy of an AIDS patient. Our study demonstrated that, especially when confronted with borderline clinical suspicion of PML and when only a small biopsy specimen is available, a combination of at least two different methods for JCV detection should be considered, preferably IHC with one of the available molecular methods.