Histological confirmation is the gold standard for the diagnosis of PACNS. As mentioned before, brain biopsy is important not only to confirm the diagnosis, but also for the exclusion of a number of other conditions which may mimic vasculitis. Vasculo-pathy due to hypertension and atherosclerosis, multiple sclerosis or its variants, sarcoidosis, primary brain lymphoma and other lymphoprolifer-ative diseases, primary or metastatic brain tumours, and infectious encephalitis such as neurocysticerco-sis, cytomegalovirus, herpes simplex, fungal infections, progressive multifocal leukoencephalopathy and Creuzfeldt-Jacob disease were some of the diagnoses found in patients referred for a brain biopsy with a suspected clinical diagnosis of PACNS (Table 20.1).8,15,16
The biopsy site is selected on the basis of an abnormality on MRI. The biopsy from enhancing lesions improves the sensitivity of the procedure.6 In the absence of any focal abnormality, the biopsy should be taken from the anterior tip of the nondominant temporal lobe. It is also essential to obtain tissue samples for staining and culture purposes.6 The biopsied sample should include the lepto-meninges, together with cortical and subcortical tissues, in order to increase the diagnostic sensitiv-ity.8'11'22-35 In a recent pathology study, however, parenchymal involvement was found to be more frequent than leptomeningeal involvement, contradicting some earlier reports, and it was also shown in this study that there was no significant difference in the diagnostic yield between open and stereotactic biopsies.15
The typical biopsy reveals segmental inflammation of small arteries and arterioles, intimal proliferation and fibrosis, with sparing of the media, and, in some cases, multinucleate giant Langerhan's cells.9,11 Although PACNS is considered to be a diffuse disease, it is segmental in vascular involvement; hence a false-negative rate of at least 25% can be expected, but correlating the site of biopsy with neuroimaging findings may improve the histo-pathological diagnosis. False-positive biopsy results, although rare, have also been reported, which make it a necessity to interpret the biopsy in the light of the entire clinical picture, CSF and imaging findings of the patient.6
The morbidity of brain biopsy in patients with suspected vasculitis was found to be 3.3%, a much lower rate than the significant side-effects associated with immunosuppressive therapy.8 As up to 50% of patients referred for a brain biopsy, with a presumptive diagnosis of PACNS, are found to have other diagnoses,8,15 some necessitating totally different treatment modalities, histopathological confirmation is highly recommended in most cases of suspected PACNS.
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