USMLE Guide: Central Nervous System Infections

Introduction

Central Nervous System (CNS) infections are a group of disorders characterized by the invasion and inflammation of the brain, spinal cord, or meninges. These infections can be caused by various pathogens, including bacteria, viruses, fungi, and parasites. Understanding the clinical presentations, diagnostic approaches, and management strategies for CNS infections is crucial for medical professionals. This guide aims to provide a concise overview of essential information for the United States Medical Licensing Examination (USMLE) related to CNS infections.

I. Pathogens Associated with CNS Infections

A. Bacterial Infections

1. Neisseria meningitidis - A gram-negative diplococcus commonly causing acute bacterial meningitis, especially in young adults.
2. Streptococcus pneumoniae - A gram-positive cocci associated with meningitis, especially in children and older adults.
3. Haemophilus influenzae - A gram-negative bacterium primarily affecting children, particularly those who are non-vaccinated.
4. Listeria monocytogenes - A gram-positive rod causing meningitis, especially in neonates, elderly, and immunocompromised individuals.
5. Mycobacterium tuberculosis - An acid-fast bacterium responsible for tuberculous meningitis, often seen in immunocompromised patients.

B. Viral Infections

1. Herpes Simplex Virus (HSV) - HSV-1 and HSV-2 can cause encephalitis, with HSV-1 being the most common etiology.
2. Varicella-Zoster Virus (VZV) - VZV can lead to encephalitis or meningitis, especially in immunocompromised individuals.
3. Enteroviruses - Coxsackievirus and echovirus are common causes of aseptic meningitis, particularly in children.
4. Arboviruses - West Nile virus, Eastern equine encephalitis virus, and St. Louis encephalitis virus are examples of arboviruses causing meningitis or encephalitis.

C. Fungal Infections

1. Cryptococcus neoformans - A encapsulated yeast causing meningitis, predominantly affecting immunocompromised individuals.
2. Histoplasma capsulatum - An intracellular fungus causing meningitis, often seen in immunocompromised patients with exposure to bird or bat droppings.
3. Coccidioides immitis - A dimorphic fungus causing meningitis, prevalent in arid regions like the southwestern United States.

D. Parasitic Infections

1. Toxoplasma gondii - Toxoplasmosis can cause encephalitis, especially in immunocompromised individuals.
2. Naegleria fowleri - An amoeba found in warm freshwater, responsible for primary amebic meningoencephalitis (PAM), a rapidly fatal infection.
3. Taenia solium - Cysticercosis occurs when larvae of this tapeworm invade the CNS, leading to neurocysticercosis.

II. Clinical Presentations

A. Meningitis

• Headache, neck stiffness (nuchal rigidity), and fever.
• Positive Kernig's and Brudzinski's signs.
• Photophobia and altered mental status.

B. Encephalitis

• Fever, headache, altered mental status, and focal neurologic deficits.
• Seizures, behavioral changes, and personality disturbances.
• Specific viral encephalitis may present with characteristic signs (e.g., HSV - temporal lobe involvement).

C. Brain Abscess

• Headache, fever, focal neurologic deficits, and signs of increased intracranial pressure.
• Possible history of trauma, chronic sinusitis, dental infection, or endocarditis.

D. Tuberculous Meningitis

• Gradual onset of headache, fever, altered mental status, and meningeal signs.
• Cranial nerve palsies, hydrocephalus, and basal exudates on fundoscopy may be present.

III. Diagnostic Approaches

A. Lumbar Puncture

• Essential for diagnosing CNS infections.
• Cerebrospinal fluid (CSF) analysis includes cell count, protein, glucose, and Gram stain/culture.

B. Imaging Studies

• Non-contrast head CT scan to evaluate for increased intracranial pressure, mass effect, or hydrocephalus.
• Magnetic Resonance Imaging (MRI) for better visualization of brain parenchyma, especially in suspected encephalitis.

C. Serologic Tests

• Detect specific antibodies (IgM/I