USMLE Guide: Bacterial Meningitis

Introduction

Bacterial meningitis is a serious infection of the meninges, the protective membranes surrounding the brain and spinal cord. It is a medical emergency that requires prompt diagnosis and treatment. This USMLE guide provides a comprehensive overview of bacterial meningitis, including its etiology, clinical features, diagnostic approach, treatment, and potential complications.

Etiology

Bacterial meningitis can be caused by various pathogens, but the most common bacteria responsible for this infection are:

1. Neisseria meningitidis: A gram-negative diplococcus commonly associated with meningococcal meningitis.
2. Streptococcus pneumoniae: A gram-positive cocci responsible for pneumococcal meningitis, often seen in children and adults.
3. Haemophilus influenzae: A gram-negative coccobacillus that historically caused severe meningitis in children but is now less prevalent due to vaccination.
4. Listeria monocytogenes: A gram-positive bacteria commonly affecting pregnant women, newborns, and immunocompromised individuals.

Clinical Features

Patients with bacterial meningitis typically present with the following clinical features:

1. Fever and chills: Most patients exhibit high-grade fever and may experience rigors.
2. Headache: Severe headache is a common symptom due to meningeal inflammation.
3. Nuchal rigidity: Neck stiffness is a hallmark sign, limiting flexion of the neck.
4. Altered mental status: Patients may appear confused, lethargic, or even comatose.
5. Photophobia: Sensitivity to light is often reported by patients.
6. Kernig's and Brudzinski's signs: These physical examination findings can help confirm the diagnosis of meningitis.

Diagnostic Approach

The diagnostic workup for suspected bacterial meningitis should include the following investigations:

1. Lumbar puncture: Essential for cerebrospinal fluid (CSF) analysis, including cell counts, glucose, protein, and Gram stain.
2. Blood cultures: Obtaining blood cultures prior to antibiotic administration is crucial for identifying the causative organism.
3. Imaging: Computed tomography (CT) scan of the head may be performed in specific cases to rule out contraindications for lumbar puncture.

Treatment

Prompt initiation of antibiotic therapy is vital in bacterial meningitis management. Empiric antibiotic therapy should cover the most likely pathogens, including:

1. Ceftriaxone or cefotaxime: Effective against Neisseria meningitidis, Streptococcus pneumoniae, and Haemophilus influenzae.
2. Ampicillin: Added to the regimen to cover listeria monocytogenes in patients at risk (e.g., neonates, pregnant women, elderly, immunocompromised).

Complications

Bacterial meningitis can lead to various complications, such as:

1. Cerebral edema: Swelling of the brain due to increased intracranial pressure.
2. Hydrocephalus: Abnormal accumulation of cerebrospinal fluid in the brain.
3. Seizures: Resulting from inflammation and irritation of the brain.
4. Septic shock: A life-threatening condition characterized by systemic infection and hypotension.
5. Hearing loss: Sensorineural hearing loss can occur, particularly in children.

Conclusion

Bacterial meningitis is a serious infection that requires early recognition and prompt treatment to improve patient outcomes. Understanding the etiology, clinical features, diagnostic approach, treatment, and potential complications is crucial for medical professionals preparing for the USMLE examination and for providing effective patient care.