Clinical Overview of Central Nervous System (CNS) Infections

Meningitis Presentation

In bacterial meningitis, over 95% of patients present with at least two of the following "classic" symptoms: fever, headache, nuchal rigidity (neck stiffness), and an altered mental state. Other common findings include nausea, vomiting, photophobia, and confusion.

In infants, the presentation may be more subtle or non-specific, including fever, diarrhea, lethargy, feeding difficulties, and respiratory distress. A bulging fontanelle or seizures may appear in later stages.

Physical Examination Signs

Clinicians rely on specific physical signs to identify meningeal irritation:

1. Nuchal Rigidity: This is the stiffness of the neck muscles, making it painful or difficult for the patient to flex the neck forward.
2. Brudzinski’s Sign: When the clinician gently flexes the patient's neck toward the chest, the patient involuntarily flexes their hips and knees to lessen the stretch on the inflamed meninges.
3. Kernig’s Sign: With the patient in a supine position and the hip flexed at 90°, the clinician attempts to extend the knee. A positive sign is indicated by resistance or pain that prevents the knee from extending beyond 135°.

Exam Tip: If a patient presents with a high fever, headache, and a positive Brudzinski's sign, acute bacterial meningitis should be a top differential diagnosis. If the patient also exhibits a hemorrhagic rash, suspect meningococcal meningitis.

Classification and Etiology

CNS infections are categorized by their duration and the causative agent involved.

1. Acute Bacterial (Pyogenic) Meningitis

This is a medical emergency. The causative agents vary significantly by age group:

• Newborns: Group B Streptococcus, S. pneumoniae, Listeria monocytogenes, and E. coli.
• Babies and Children: S. pneumoniae, Neisseria meningitidis, and H. influenzae.
• Teens and Young Adults: N. meningitidis and S. pneumoniae.
• Older Adults: S. pneumoniae, N. meningitidis, Group B Streptococcus, H. influenzae, and Listeria monocytogenes.

2. Viral Meningitis (Aseptic)

Viral meningitis typically presents with an acute onset of low-grade fever, headache, photophobia, and nuchal rigidity. Unlike bacterial meningitis, patients usually remain alert unless the infection progresses to encephalitis. Common causes include:

• Enteroviruses (e.g., echoviruses and coxsackie) — the most common cause.
• Herpes simplex virus (HSV) and Varicella.
• Other causes: Mumps, CMV, EBV, and HIV.

3. Chronic (Subacute) Meningitis

This category includes infections that follow a more indolent or slow course, such as Tuberculous (TB) meningitis. TB meningitis may initially present with non-specific symptoms like malaise and anorexia before progressing to classic meningeal signs. Other causes include fungal, neoplastic, parasitic, and rickettsial infections.

4. Encephalitis and Cerebral Abscess

While meningitis involves the membranes, encephalitis involves inflammation of the brain parenchyma itself. Cerebral abscess refers to a localized collection of infection within the brain tissue.

Diagnosis and Management

Diagnostic Approach

Early identification is paramount. Essential investigations include:

• Lumbar Puncture (LP) & CSF Analysis: To check for cell count, protein, glucose, and culture.
• Blood Cultures and CBC: To identify systemic infection.
• Neuroimaging (CT & MRI): To assess for complications like edema or abscess.
• Other tests: Serum glucose (to compare with CSF glucose), procalcitonin, and PCR assays.

Management Strategies

Management is divided into supportive care and specific antimicrobial therapy.

Empirical Antibiotic Selection (Based on Age):

For bacterial meningitis, Dexamethasone (0.6 mg/kg/day) is often administered to reduce inflammation. Viral meningitis is largely managed with supportive care, though specific antivirals like Acyclovir are used for HSV.

Complications and Prevention

If left untreated, meningitis can progress rapidly to seizures, focal neurological signs, and death. Even with treatment, survivors may face significant long-term complications:

• Neurological: Multiple seizures, cortical blindness, mental-motor retardation, paralysis, ataxia, and memory problems.
• Other: Hearing loss, cranial nerve dysfunction, renal problems, and bone/joint arthritis.

Prevention is heavily reliant on vaccination programs, including vaccines for Haemophilus influenzae type b (Hib), Neisseria meningitidis (serogroups A, C, Y, W135, and B), and Streptococcus pneumoniae.