Differentials
Encephalitis
SIGNS / SYMPTOMS
Abnormal cerebral function, such as altered behavior and speech or motor disorders, particularly when associated with fever, suggests a diagnosis of encephalitis.
INVESTIGATIONS
Cranial imaging done by CT or MRI scans.
Viral meningitis
SIGNS / SYMPTOMS
Relevant exposure history.
No differentiating symptoms and signs.
INVESTIGATIONS
Cerebrospinal fluid (CSF) pressure is usually normal.
CSF WBC count may be normal or 10-500/microliter and has a lymphocytic differentiation.
CSF glucose is normal and CSF protein slightly elevated.
Bacterial culture of CSF is negative.
Polymerase chain reaction for enteroviruses and herpes viruses.
Procalcitonin is usually normal.
Drug-induced meningitis
SIGNS / SYMPTOMS
No differentiating symptoms and signs.
History of culprit drug use (e.g., nonsteroidal anti-inflammatory drugs, trimethoprim/sulfamethoxazole, amoxicillin, ranitidine).
INVESTIGATIONS
This is a diagnosis of exclusion. Cerebrospinal fluid typically shows a neutrophilic pleocytosis. Symptoms resolve once the offending drug is stopped.
Tuberculous meningitis
SIGNS / SYMPTOMS
History of contact or resident in endemic area.
Symptoms and signs of pulmonary and extraneural disease.
INVESTIGATIONS
Cerebrospinal fluid (CSF) smear and culture: sensitivity of smear >50% if repeated drops of CSF sediment dried on a slide and then stained and examined at length. Culture requires large volume for maximum sensitivity.
Skin testing or interferon-gamma-based blood tests for exposure to Mycobacterium tuberculosis supportive, but negative results do not exclude diagnosis of tuberculosis.
Fungal meningitis
SIGNS / SYMPTOMS
Presentation is often insidious with onset of headache and fever over weeks or months. A rash resembling molluscum contagiosum may be present in disseminated cryptococcal disease.
INVESTIGATIONS
Testing cerebrospinal fluid (CSF) for cryptococcal antigen has a sensitivity of almost 100% for cryptococcal meningitis.
In HIV-positive patients the fungal burden is high, leading to high CSF pressures. CSF leukocyte count may be low. India ink stain or cryptococcal antigen is usually positive.
HIV-negative patients have higher CSF leukocyte counts, and India ink stain is positive in only half of cases.
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