Meningitis

Meningitis is inflammation of the meninges, the membranes covering the brain and spinal cord, together with the cerebrospinal fluid in the subarachnoid space they enclose. It is the prototypical diffuse central-nervous-system infection and is defined by inflammation of the leptomeninges rather than by a focal collection of pus.

Definition

Meningitis is inflammation of the meninges and the cerebrospinal fluid in the subarachnoid space, most often caused by bacterial, viral, fungal, or other infection, and less commonly by non-infective inflammatory processes.

Scope

This entry describes meningitis as an inflammatory process of the meninges and cerebrospinal fluid: its infective and non-infective causes, how it differs from focal intracranial infections, the central role of cerebrospinal fluid examination in its recognition, and its relevance to neurosurgical practice, including post-procedural meningitis. It is a reference overview and does not provide antimicrobial protocols.

Core questions

• What distinguishes meningitis, as a diffuse meningeal inflammation, from focal intracranial infections?
• How does examination of the cerebrospinal fluid characterise meningitis and point to its cause?
• What are the principal infective and non-infective causes of meningeal inflammation?
• How does meningitis arise as a complication of neurosurgical procedures and implanted devices?

Key concepts

• Inflammation of the leptomeninges
• Subarachnoid space and cerebrospinal fluid
• Cerebrospinal fluid examination (cell count, protein, glucose, microbiology)
• Community-acquired versus healthcare-associated meningitis
• Aseptic (non-bacterial) meningitis
• Raised intracranial pressure as a complication

Mechanisms

Organisms reach the meninges most often by haematogenous spread after colonisation of the nasopharynx, and sometimes by contiguous extension from adjacent infected structures or by direct introduction through trauma, surgery, or cerebrospinal fluid devices. Once within the subarachnoid space, multiplication of organisms and the host inflammatory response alter the cerebrospinal fluid and irritate the meninges, producing the characteristic clinical and laboratory picture. The inflammation can impair cerebrospinal fluid absorption and raise intracranial pressure, and in severe cases provoke vascular and parenchymal complications. Examination of the cerebrospinal fluid is the central means of characterising the process and inferring its cause.

Clinical relevance

Meningitis is relevant to neurosurgery both as a medical emergency and as a recognised complication of cranial and spinal procedures and of cerebrospinal fluid devices, where it overlaps with healthcare-associated ventriculitis and meningitis. Understanding how it is defined and recognised supports critical appraisal of the evidence and guidelines. This entry describes the entity and is not a basis for individual diagnostic or treatment decisions.

Epidemiology

Acute bacterial meningitis is a major global cause of central-nervous-system morbidity and mortality, with the predominant organisms varying by age, geography, vaccination coverage, and host factors. Healthcare-associated meningitis associated with neurosurgical procedures and cerebrospinal fluid drainage represents a distinct and increasingly recognised subset addressed in dedicated guidelines.

Seminal works

• mcgill-2016
• vandebeek-2016
• tunkel-2017

Frequently asked questions

How does meningitis differ from a brain abscess?

Meningitis is a diffuse inflammation of the meninges and cerebrospinal fluid, whereas a brain abscess is a focal, encapsulated collection of pus within the brain parenchyma.

Why is examination of the cerebrospinal fluid central to meningitis?

Because the inflammation involves the subarachnoid space, the cerebrospinal fluid reflects the process directly; its cell count, protein, glucose, and microbiology characterise the inflammation and point toward its cause.