What are the CNS findings in bacterial meningitis?

CNS Findings in Bacterial Meningitis

Clinical Presentation

The most common CNS findings in bacterial meningitis are fever, headache, neck stiffness, and altered mental status, though the classic triad of fever, neck stiffness, and altered mental status is present in only 41-51% of adult patients. 1

Core Clinical Features in Adults

• Headache occurs in 58-87% of patients 1
• Fever (>38.0°C) is present in 77-97% of cases 1
• Neck stiffness is found in 65-83% of patients 1
• Altered mental status occurs in 30-69% of patients 1
• Focal neurologic deficits develop in 15-34% of cases 1
• Coma is present in 7-16% of patients at presentation 1

Important Clinical Caveat

Characteristic clinical signs may be completely absent in bacterial meningitis, and no single clinical finding is present in all patients. 1 The sensitivity of classic meningeal signs is poor: neck stiffness has only 31% sensitivity, Brudzinski sign 9%, and Kernig sign 11% for predicting CSF pleocytosis. 1 This means absence of these findings cannot exclude bacterial meningitis.

Special Populations

• Neonates often present with nonspecific symptoms rather than classic meningeal signs 1
• Children beyond neonatal age most commonly present with fever, headache, neck stiffness, and vomiting, though characteristic signs may be absent 1
• Elderly patients may have scarce or atypical signs 2

Cerebrospinal Fluid Findings

The typical CSF profile shows pleocytosis with 80-95% polymorphonuclear neutrophil predominance, elevated white blood cell count (typically 1,000-5,000 cells/mm³), markedly decreased glucose (<40 mg/dL in 50-60% of cases), low CSF-to-blood glucose ratio (<0.4), and elevated protein levels. 1, 3, 4

Detailed CSF Parameters

• Opening pressure: Generally 200-500 mm H₂O (may be lower in neonates and infants) 1
• WBC count: Typically 1,000-5,000 cells/mm³, with a broad range of 100-110,000 cells/mm³ 1
• Neutrophil predominance: 80-95% in most cases 1, 3
• CSF glucose: <40 mg/dL in approximately 50-60% of patients 1
• CSF-to-serum glucose ratio: <0.4 (80% sensitive, 98% specific for bacterial meningitis in children ≥12 months) 1
• CSF protein: Elevated, typically around high levels 3
• CSF lactate: Often elevated with better diagnostic accuracy than WBC count for differentiating bacterial from other meningitis types 3

Critical Diagnostic Pitfall

Approximately 10% of patients with acute bacterial meningitis present with lymphocyte predominance (≥50% lymphocytes or monocytes) in CSF, which can be misleading. 1 This underscores that CSF interpretation must occur in clinical context.

Special Population Considerations

• Neonates: CSF abnormalities may be absent in a significant percentage of cases; in culture-proven neonatal meningitis, many had fewer than expected WBC/mm³ 3
• Immunocompromised patients: Normal CSF parameters do not rule out meningitis, and high clinical suspicion must be maintained until cultures are final 3
• Prior antibiotic treatment: May modify CSF findings, though WBC count may not be greatly affected 3

Neurologic Complications During Clinical Course

Half of adults with bacterial meningitis develop focal neurologic deficits during their clinical course, and one-third develop hemodynamic or respiratory insufficiency. 1

Common Acute Complications

• Seizures: Occur in up to 30% of children before admission 1
• Focal neurologic deficits: Develop in 50% of adults during disease course 1
• Altered consciousness: Ranges from confusion to coma 1, 5
• Cranial nerve palsies: May occur (though CN VI or VII palsy alone is not an indication to delay lumbar puncture) 1

Cerebrovascular Complications

• Cerebral infarctions occur frequently 1
• Subarachnoid hemorrhage may develop 1
• Intracranial hemorrhage (associated with anticoagulant use) 1
• Venous sinus thrombosis can complicate the clinical course 1

Structural Complications

• Hydrocephalus (both obstructive and communicating types) 1
• Subdural empyema may require neurosurgical intervention 1
• Brain abscess can develop 1
• Cerebral edema with risk of herniation 1

Imaging Findings and Indications

CT imaging before lumbar puncture is indicated in patients with severely altered mental status (Glasgow Coma Scale <10), focal neurologic deficits, new-onset seizures, severe immunocompromised state, age ≥60 years, history of CNS disease, or papilledema. 1

When Imaging Shows Abnormalities

• MRI is preferred over CT for superior resolution when intracranial abnormalities are suspected, though CT availability and speed are often greater 1
• Obstructive hydrocephalus typically requires placement of external ventricular drain 1
• Space-occupying lesions (subdural empyema, brain abscess) may warrant neurosurgical intervention to prevent cerebral herniation 1

Critical Management Point

Treatment should be initiated within one hour for all patients with suspected bacterial meningitis, irrespective of whether cranial imaging is performed before lumbar puncture. 1 Blood cultures should be obtained and empiric antibiotics started immediately if lumbar puncture is delayed for any reason. 3, 4

Long-Term Sequelae

One-third of patients surviving bacterial meningitis will have persisting complaints, with hearing loss being the most common severe sequela (occurring in 5-35% of patients). 1

Most Common Sequelae

• Hearing loss: 34% of children (most common sequela); bacterial meningitis is the leading cause of acquired hearing loss in children 1
• Seizures: 13% of children 1
• Motor deficits: 12% of children 1
• Cognitive defects: 9% of children, with cognitive slowness common in adults 1
• Hydrocephalus: 7% of children 1
• Visual loss: 6% of children 1
• Multiple sequelae: One in five children have multiple sequelae 1

Follow-up Recommendations

Hearing evaluation should be performed in all survivors, as 54% of pneumococcal meningitis survivors have audiometric evidence of hearing loss even without clinical suspicion. 1 Early detection is critical in young children to prevent speech development delays, and timely cochlear implantation can prevent long-term disability. 1