Definitive diagnosis: Demonstration of typical histopathology in brain tissue and also in tonsil in vCJD: Spongiform vacuolation, proliferation of astrocytes and microglial cells and neuronal loss with lack of inflammatory response or detection of pathological form of prion proteins in brain tissue by immunohistochemistry or immunoblot assay are the definitive diagnosis of prion disease. However, the disadvantage is collection of brain biopsy which is rarely done antemortem. Thus, the definitive diagnosis is almost limited as post mortem diagnosis.
Suggestive diagnosis: The following findings are highly sensitive but can be seen in other neuronal disorders.
Magnetic resonance imaging (MRI): High signal in caudate, putamen and cortex is 80% sensitive and highly specific in sporadic CJD. Pulvinar sign (symmetric hyperintensity of posterior thalamus in comparison to anterior putamen) is characteristic of vCJD.
Electroencephalogram (EEG): Characteristic periodic triphasic sharp wave complex is characteristic of sCJD but also seen in other conditions. However, not a feature of vCJD where it may be seen only rarely in terminal cases.
CSF: Increased level of 14-3-3 protein band in CSF is more commonly found in sCJD whereas it is positive only in 50% vCJD cases.
Newer non-invasive methods Ultrasensitive ELISA: This is used for detection of prion protein in blood sample. It can detect very low concentration of protein (10-10 dilution). However, helpful only in vCJD and not sCJD or other prion diseases.
Protein misfolding cyclic amplification (PMCA): The test is in vitro amplification of prion protein based on the principle of prion protein replication mechanism.
Sample containing normal prion protein (PrPc) is mixed with test sample. Amplification will occur in test sample containing pathological form of prion protein and protein aggregates will be formed. These protease resistant prion proteins will be detected by immunoblotting after proteinase K treatment.
Conventionally the detection methods of these protein aggregates are immunoblot method or ELISA which are less sensitive. Detection by surround optical fiber immunoassay (SOFIA) has been shown to be much more sensitive. However, the major disadvantage is use of brain homogenate as source of normal prion protein.
Real-time quaking-induced conversion (RT QuIC): Principle of the test is same as PMCA. However, the test is easier than PMCA. It is carried out in 96 well plate, detection of prion aggregates is done by fluorescent plate reader and uses recombinant PrPc instead of brain homogenate.
Sensitivity, and specificity of CSF sample are 77–97% and 99–100%, respectively.
Nasal/olfactory mucosa: 97% sensitivity and 100% specificity.
Sterilization of prion contaminated instruments: Figure 1 describes the WHO recommended sterilization method for prion contaminated instruments.
Surfaces and reusable heat sensitive contaminated items are decontaminated by soaking in 2N NaOH for 1 hour.
All prion contaminate disposable materials, instruments and wastes should be disposed by incineration.
Fig1. WHO recommended sterilization method for prion contaminated instruments
