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INTRODUCTION AND EPIDEMIOLOGY
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Bacterial meningitis is a life-threatening emergency that affects 1.38 out of 100,000 people, with a case fatality rate of 14.3%.1 Although the incidence of bacterial meningitis has declined significantly since the initiation of vaccination programs, the disease is still prevalent and associated with significant morbidity and mortality.2,3,4 In the United States, the most common causes of bacterial meningitis are Streptococcus pneumoniae (58.0%), group B Streptococcus (18.1%), Neisseria meningitidis (13.9%), Haemophilus influenzae (6.7%), and Listeria monocytogenes (3.4%).1 Escherichia coli in the neonatal population and Mycobacterium tuberculosis in immunocompromised hosts are also important considerations.5
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Organisms enter the cerebrospinal fluid either through hematogenous or direct contiguous spread. In hematogenous spread, bacteria colonize the upper airway and invade the bloodstream, gradually making their way to the subarachnoid space. The subcapsular components of S. pneumoniae, H. influenzae type b, and N. meningitides induce an inflammatory cascade, and leukocyte toxins cause cellular swelling and inflammation of the brain and meninges.6 Blood–brain barrier permeability increases, allowing protein and water to enter and leading to vasogenic edema. Cerebrospinal fluid drainage is inhibited by reduced absorption of the arachnoid granules with resultant obstruction and hydrocephalus, and cerebrospinal fluid is forced into the periventricular parenchyma causing interstitial edema. Disruption of cell membrane homeostasis causes cytotoxic edema. As the brain and meninges rest in a fixed-volume skull, this leads to an elevation in intracranial pressure. Vasculitis decreases cerebral blood flow and can cause ischemia and thrombosis. Additionally, neurons are directly injured by free radicals from granulocytes and endothelial cells.7
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In direct contiguous spread, organisms gain entry into the cerebrospinal fluid from adjacent infections such as sinusitis, brain abscess, or otitis media. Organisms can also enter directly with penetrating traumatic injury, through congenital defects, or during neurosurgical procedures. In these cases, the organisms and their pathophysiologic effects vary.
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Important risk factors for bacterial meningitis are listed in Table 174-1.
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The presentation of fever, headache, stiff neck, and altered mental status is commonly seen in patients with bacterial meningitis. Although most patients have at least two of four of these symptoms, their absence does not exclude meningitis. Headache is the most common symptom and is seen in more than 85% of patients. Fever is the second most common symptom.7 Seizures and focal neurologic deficits are seen in 25% to ...