After studying this chapter, the student should be able to:
Recognize various types of neurologic infections based on localization.
Identify the common organisms and the clinical scenarios for these infections.
Discuss the pathology of these infections.
Neurologic infections remain an important cause of mortality and morbidity worldwide. Over 1.2 million cases of bacterial meningitis are estimated to occur worldwide each year. The case fatality rate for meningitis can be as high as 70% without treatment. The prevalence of central nervous system (CNS) infections varies based on geographic locations, with higher prevalence in developing areas and areas with a high incidence of human immunodeficiency virus (HIV) infection.
Neurologic infections can be caused by various pathogens, including bacteria, fungi, viruses, parasites, and prions. Common organisms and the clinical syndromes associated with each of them are discussed in this chapter. Although detailed discussion is beyond the scope of this chapter, one should be aware of emerging viruses that affect the nervous system, such as Zika virus, Nipah virus, dengue virus, and chikungunya virus.
Pathogens can affect various parts of the nervous system, but most often, they have a certain predilection (eg, varicella-zoster virus resides in the sensory ganglia). Diffuse infection of the meninges results in meningitis, whereas infection of the brain parenchyma results in encephalitis. Encephalitides occur either due to contiguous spread from nearby structures or due to hematogenous spread. Within the brain parenchyma, certain organisms have a predilection for white matter (eg, JC virus infecting oligodendrocytes). Abscesses can occur in epidural and subdural spaces as well as in the brain parenchyma.
Meningitis refers to an inflammatory reaction in the subarachnoid space. The clinical syndrome is that of headache, fever, and neck stiffness. Cerebrospinal fluid (CSF) shows evidence of an inflammatory reaction, as noted by elevated white blood cell counts and protein. Acute meningitis usually presents within hours to days, whereas by definition, chronic meningitis is longer than 4 weeks in duration.
Acute meningitis is most often infectious, caused by bacteria or a virus, with noninfectious etiologies in the differential diagnosis. As noted previously, the morbidity and mortality of bacterial meningitis remain high despite widespread availability of antibiotics. Early diagnosis and treatment are imperative to reduce morbidity and mortality.
Acute bacterial meningitis is seen in all age groups. With the extensive use of the Haemophilus influenzae type b (Hib) vaccine, the incidence of acute bacterial meningitis in the pediatric age group has been reduced. It is now more commonly seen in young and older adults. The most common community-acquired organisms are Streptococcus pneumoniae (pneumococcus), Neisseria meningitidis (meningococcus), Listeria monocytogenes, group B streptococci, and H influenzae.