Bacterial Meningitis

Detailed Overview

The classic triad of symptoms includes fever, nuchal rigidity (stiff neck), and altered mental status, though all three are present in less than 50% of adult cases. The most common causative organisms vary by age, with Streptococcus pneumoniae and Neisseria meningitidis predominating in adults. Diagnosis hinges on prompt lumbar puncture for cerebrospinal fluid (CSF) analysis, demonstrating neutrophilic pleocytosis, low glucose, and high protein. Given the high mortality rate and potential for severe neurological sequelae (e.g., hearing loss, cognitive impairment), empirical broad-spectrum antibiotics and adjunctive corticosteroids must be administered immediately upon suspicion, even before CSF results are obtained.

Epidemiology & Demographics

In the United States, the incidence is approximately 1-2 cases per 100,000 population annually. Rates are highest in infants <1 year of age. Neisseria meningitidis outbreaks occasionally occur in crowded settings such as college dormitories and military barracks. Mortality ranges from 10% to 20% despite appropriate antimicrobial therapy.

Etiological Mechanism

Common pathogens vary by age group. Neonates (<1 month): Streptococcus agalactiae (Group B Strep), Escherichia coli, Listeria monocytogenes. Children & Adults: Streptococcus pneumoniae, Neisseria meningitidis. Older adults (>50 years) and immunocompromised: S. pneumoniae, N. meningitidis, L. monocytogenes.

Primary Causes

Direct bacterial invasion of the subarachnoid space. This usually occurs via hematogenous spread following nasopharyngeal colonization, or less commonly, through direct spread from contiguous infections (e.g., otitis media, sinusitis) or penetrating head trauma.

• Age: Infants <1 year and adults >50 years are at highest risk.
• Immunocompromise: Conditions such as HIV, asplenia (increases risk of encapsulated organisms like pneumococcus), or complement deficiencies.
• Crowded Living Conditions: Dormitories and military barracks facilitate respiratory transmission of Neisseria meningitidis.
• Cranial Defects or Trauma: Basilar skull fractures leading to CSF leaks provide a direct portal of entry for nasopharyngeal flora.

Pathogens colonize the nasopharynx, evade host mucosal defenses (e.g., via IgA proteases), and enter the bloodstream. They cross the blood-brain barrier via tight junctions of the choroid plexus or cerebral capillaries. Within the subarachnoid space, rapid bacterial multiplication occurs due to poor localized humoral immunity. Bacterial lysis (spontaneous or antibiotic-induced) releases potent inflammatory mediators (teichoic acid, lipopolysaccharide), triggering a massive cascade of cytokines (TNF-alpha, IL-1). This results in increased blood-brain barrier permeability, vasogenic edema, loss of cerebral autoregulation, and significantly increased intracranial pressure (ICP), ultimately leading to neuronal ischemia and injury.

Characteristic Clinical Presentation

• Headache: Severe, generalized, and unrelenting headache caused by meningeal irritation.
• Fever: Acute onset of high fever, often > 39°C (102.2°F).
• Stiff Neck (Nuchal Rigidity): Pain and resistance to passive flexion of the neck.
• Photophobia: Extreme sensitivity to light due to meningeal inflammation involving cranial nerves.

Physical Examination Signs

• Kernig Sign (pain on passive extension of the knee with the hip flexed)
• Brudzinski Sign (involuntary flexion of the hips and knees when the neck is passively flexed)
• Petechial/Purpuric Rash (classic for N. meningitidis)
• Altered Sensorium (GCS < 14)

• Sensorineural Hearing Loss: Occurs in up to 30% of cases, particularly with S. pneumoniae, due to inflammation of the eighth cranial nerve.
• Seizures: Present in 20-30% of patients due to cortical irritability or focal infarction.
• Cerebrovascular Complications: Arterial or venous sinus thrombosis leading to ischemic stroke.
• Subdural Empyema or Effusion: Collection of infected or sterile fluid between the dura and arachnoid layers.

Diagnostic Criteria & Guidelines

Diagnosis is confirmed by Lumbar Puncture (LP) showing typical CSF characteristics and positive Gram stain or culture. A CT head prior to LP is indicated only if there are papilledema, focal neurologic deficits, altered mental status, immunocompromise, or a history of CNS disease. Empiric therapy should NOT be delayed for neuroimaging.

Differential Diagnosis

• Viral Meningitis/Encephalitis (e.g., HSV)
• Subarachnoid Hemorrhage
• Tuberculous Meningitis
• Lyme Disease (Neuroborreliosis)
• Brain Abscess

Laboratory Tests & Biomarkers

• CSF WBC Count: Elevated, typically > 1000 cells/microL with > 80% polymorphonuclear leukocytes (PMNs).
• CSF Glucose: Low, typically < 40 mg/dL or CSF/Serum glucose ratio < 0.4.
• CSF Protein: Markedly elevated, typically > 200 mg/dL.
• CSF Gram Stain: Positive in 60-90% of untreated patients; reveals organism morphology (e.g., Gram-positive diplococci for pneumococcus).
• Blood Cultures: Positive in 50-75% of cases.

Imaging Modalities & Findings

• CT Head without Contrast:
• MRI Brain with Gadolinium:

• Incubation
  Duration varies (2-10 days), asymptomatic or mild upper respiratory symptoms.
• Acute Meningitic Phase
  Rapid onset of fever, headache, meningismus, and potentially altered mental status over 24-48 hours.
• Complicated/Encephalitic Phase
  Development of seizures, focal neurologic deficits, profound coma, and signs of brain herniation.

Second-Line & Adjunctive Therapy

Targeted therapy based on culture/susceptibilities. For penicillin-allergic patients (severe): Meropenem 2 g IV q8h + Vancomycin. For Listeria in penicillin-allergic patients: Trimethoprim-sulfamethoxazole 5 mg/kg IV q6h.

Surgical & Procedural Management

Rarely required unless there is a complication such as a subdural empyema, brain abscess requiring drainage, or severe hydrocephalus necessitating an external ventricular drain (EVD).

Recommended Lifestyle Changes

• Complete the entire course of prescribed antibiotics.
• Undergo formal audiological evaluation 4-6 weeks after discharge.
• Report any persistent headaches, memory issues, or weakness.

Patient Counseling & Advice

Inform patients and families that bacterial meningitis is a life-threatening emergency. Discuss the potential for long-term neurological sequelae, including hearing loss and cognitive deficits. Close contacts may require chemoprophylaxis if the pathogen is Neisseria meningitidis or Haemophilus influenzae type b.

Follow-Up & Monitoring Schedule

Neurological examination and audiometry. Monitor for delayed complications such as hydrocephalus. Ensure appropriate vaccinations post-recovery, especially assessing for immune deficiencies if recurrent.

Preventive Strategies

Vaccination is the cornerstone of prevention. Administer Pneumococcal vaccines (PCV15/20, PPSV23), Meningococcal vaccines (MenACWY, MenB), and Haemophilus influenzae type b (Hib) vaccine according to age and risk guidelines. Close contacts of patients with N. meningitidis should receive chemoprophylaxis (e.g., Ciprofloxacin 500 mg PO single dose, or Rifampin 600 mg PO BID for 2 days).

Overall mortality in adults is ~15-20%. S. pneumoniae carries a higher mortality (20-30%) compared to N. meningitidis (10%). Up to 30% of survivors suffer significant sequelae, including deafness, paresis, or cognitive impairment.