Meningitis

Meningitis is inflammation of the meninges, the membranes that surround the brain and spinal cord, and of the cerebrospinal fluid in the subarachnoid space. Within bacterial infections, acute bacterial meningitis is a medical emergency caused most often by Streptococcus pneumoniae and Neisseria meningitidis, and it is characterized by a rapidly evolving syndrome of fever, headache, neck stiffness, and altered consciousness.

Definition

Meningitis is inflammation of the meninges and cerebrospinal fluid in the subarachnoid space; acute bacterial meningitis is the form caused by bacterial invasion of this space, producing an acute purulent inflammatory response and the clinical syndrome of meningeal irritation and central nervous system dysfunction (van de Beek, 2006).

Scope

The entry covers meningitis as a clinical entity with emphasis on its acute bacterial form, including the principal causative organisms, the pathophysiology of subarachnoid-space infection, the role of cerebrospinal fluid examination in defining the syndrome, and the disease burden. It is a reference overview and does not provide antibiotic or steroid regimens or individualized care.

Core questions

• What does inflammation of the meninges involve, and how is bacterial meningitis distinguished from other causes?
• Which bacteria most commonly cause acute community-acquired meningitis, and how does age affect the spectrum?
• How does bacterial invasion of the subarachnoid space produce neurological injury?
• What role does cerebrospinal fluid examination play in characterizing the syndrome?

Key concepts

• Meninges and the subarachnoid space
• Acute bacterial versus aseptic meningitis
• Streptococcus pneumoniae and Neisseria meningitidis
• Cerebrospinal fluid pleocytosis
• Meningeal irritation (neck stiffness)
• Blood-brain barrier and inflammatory injury
• Conjugate vaccines and prevention

Mechanisms

Bacteria typically reach the meninges by hematogenous spread after nasopharyngeal colonization and bloodstream invasion, or by direct extension from a contiguous focus. Once in the subarachnoid space, where host defenses are limited, bacteria multiply and trigger an intense inflammatory response that disrupts the blood-brain barrier, raises intracranial pressure, impairs cerebral blood flow, and injures neural tissue (van de Beek, 2016). This inflammatory cascade, rather than the organism alone, drives much of the neurological damage, which is reflected in the purulent, neutrophil-rich cerebrospinal fluid characteristic of the bacterial form (van de Beek, 2006).

Clinical relevance

Acute bacterial meningitis is a time-critical syndrome with substantial mortality and risk of neurological sequelae, and cerebrospinal fluid examination is central to recognizing and characterizing it. This entry describes how the entity is defined and reasoned about as a reference; it is not a basis for diagnosis or for individual treatment decisions.

Epidemiology

The leading causes of community-acquired bacterial meningitis in adults are Streptococcus pneumoniae and Neisseria meningitidis, while the pathogen distribution differs in neonates and changes across the lifespan (van de Beek, 2006). The introduction of conjugate vaccines against Haemophilus influenzae type b, pneumococcus, and meningococcus has markedly reduced incidence in many settings, though meningitis remains a significant cause of death and disability and contributes to the burden of bacterial resistance (van de Beek, 2016; Murray, 2022).

History

Bacterial meningitis was almost uniformly fatal before antimicrobial therapy. The later development and widespread use of conjugate vaccines transformed its epidemiology by sharply reducing disease caused by Haemophilus influenzae type b and, subsequently, by pneumococcal and meningococcal serotypes, shifting the burden toward non-vaccine causes and older age groups (van de Beek, 2016).

Debates

What is the role of adjunctive corticosteroids in bacterial meningitis?

Because much of the injury in bacterial meningitis is inflammation-mediated, adjunctive corticosteroids have been studied to reduce harm; their benefit appears to depend on the causative organism and the clinical setting, and the evidence has been debated across populations.

Seminal works

• vandebeek-2006
• vandebeek-2016
• tunkel-2004

Frequently asked questions

What are the most common bacterial causes of meningitis in adults?

In community-acquired bacterial meningitis in adults, Streptococcus pneumoniae and Neisseria meningitidis are the most common causes; the spectrum differs in neonates and the very elderly, where other organisms such as group B streptococcus and Listeria become more important.

Why is bacterial meningitis considered an emergency?

Bacterial invasion of the subarachnoid space provokes an intense inflammatory response that can rapidly raise intracranial pressure and injure the brain, so the syndrome can progress quickly and carries substantial risk of death and lasting neurological sequelae.