Non-granulomatous inflammatory lesions of CNS: Approach to diagnosis

Infections constitute an important and common category of diseases, particularly in less developed countries. Infections present with a broad spectrum of clinical and radiologic features dictated by the cell and tissue tropism and host response elicited, posing a considerable diagnostic challenge. Early diagnosis and treatment are crucial in preventing mortality and morbidity. Recourse is often made to biopsy for ascertaining the diagnosis, and hence the pathologist plays a vital role in patient management. Therefore, knowledge of the histopathologic changes is necessary to recognize the histological changes and guide the diagnostic workup and management. Each microbial agent elicits a distinctive pattern of inflammatory tissue response, which can serve as a clue to the etiological agent. Based on the causative organism, microbial, and host factors, the inflammatory response may be acute or chronic, necrotic or non-necrotic. The inflammation can be of varied patterns - lymphohistiocytic, granulomatous, inflammatory demyelinating, fibrosing, or showing minimal inflammation. The pattern of necrosis also differs based on the causative organism. Typically, pyogenic bacteria are associated with suppurative inflammation, tuberculosis with caseous granulomatous, and fungi with suppurative granulomatous inflammation. Viral infections are associated with lymphohistiocytic non-necrotizing inflammation and, based on cell tropism, can cause demyelination (e.g., JCV) and/or viral inclusions. Parasitic infections (protozoal or metazoal) display a broad spectrum of inflammatory changes that overlap with other types of infections. This review briefly describes pathological patterns and associated pathogens and provides an algorithmic approach based on pattern recognition that may be useful for the practicing pathologist.