Chapter 174: Central Nervous System and Spinal Infections

CENTRAL NERVOUS SYSTEM INFECTIONS

BACTERIAL MENINGITIS

INTRODUCTION AND EPIDEMIOLOGY

Bacterial meningitis is a life-threatening emergency that affects about 1.2 millions individuals worldwide each year.¹ The incidence and major pathogens vary by region, country, age group, and immunization status.^2-4 For example, although meningococcal meningitis occurs worldwide in epidemic and sporadic form, the highest incidence is in the meningitis belt of sub-Saharan Africa, from Senegal to Ethiopia.⁵ 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%).⁶ Escherichia coli in the neonatal population and Mycobacterium tuberculosis in immunocompromised hosts are also important considerations.⁷

PATHOPHYSIOLOGY

Organisms enter the cerebrospinal fluid (CSF) 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. meningitidis induce an inflammatory cascade, and leukocyte toxins cause cellular swelling and inflammation of the brain and meninges.⁸ Blood–brain barrier permeability increases, allowing protein and water to enter and leading to vasogenic edema. CSF drainage is inhibited by reduced absorption of the arachnoid granules with resultant obstruction and hydrocephalus, and CSF 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.⁹

In direct contiguous spread, organisms gain entry into the CSF 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.

Important risk factors for bacterial meningitis are listed in Table 174-1.

CLINICAL FEATURES

The presentation of fever, headache, stiff neck, and altered mental status is commonly seen in patients with bacterial meningitis. Most patients have at least two of four of these symptoms, but their absence does not exclude meningitis. Headache is the most common symptom and is seen in more than 85% of patients.⁹ Fever is the second ...