Bacterial Meningitis I: Introduction

Bacterial meningitis is an acute bacterial infection involving the leptomeninges and cerebrospinal fluid within the subarachnoid space. The infection particularly affects the pia mater and arachnoid mater, along with the ventricles. If untreated, it can lead to significant neurological complications or death.

In adults, the main pathogens are Streptococcus pneumoniae and Neisseria meningitidis. Listeria monocytogenes is more common in older adults and immunocompromised individuals. Other important causes include Haemophilus influenzae, Mycobacterium tuberculosis, and non-typhoidal Salmonella, depending on risk factors and immune status.

Many causative bacteria, including meningococcus, pneumococcus, and Haemophilus influenzae, asymptomatically colonize the nasopharynx and spread via respiratory droplets or secretions. Group B streptococcus colonizes the gastrointestinal or genital tract and transmits from mother to infant during childbirth. These transmission routes vary by pathogen and population.

Early clinical features include fever, severe headache, and neck stiffness. Systemic features also include chills and tachycardia. Meningeal signs include photophobia and sensitivity to light. As inflammation progresses, altered mental status and focal neurological deficits such as hemiparesis may develop.

The Kernig sign is elicited when the hip is flexed in a supine patient, causing pain in the lower back or posterior thigh due to meningeal irritation. The Brudzinski sign is present when passive neck flexion leads to involuntary hip and knee flexion, which lowers traction on the irritated meninges, indicating meningeal inflammation.

Meningococcal meningitis may present with a petechial or purpuric rash. A severe complication, purpura fulminans, involves disseminated intravascular coagulation, skin necrosis, and multi-organ failure, which can lead to death. Raised increased intracranial pressure can cause projectile vomiting in children and other neurological complications.

Predisposing factors include otitis media, sinusitis, pneumonia, and immunosuppression. Age and immune status significantly influence which pathogens cause infection. Neonates face particular risk from Group B streptococcus, while elderly and immunocompromised individuals are more susceptible to Listeria monocytogenes and other opportunistic organisms.