What are the differences in MRI brain findings between Tuberculosis (TB) meningitis and Herpes Simplex Virus (HSV) meningitis?

MRI Differentiation Between TB Meningitis and HSV Meningitis

The key MRI differences between TB meningitis and HSV meningitis are that TB meningitis typically shows basal meningeal enhancement, hydrocephalus, and tuberculomas, while HSV meningitis/encephalitis characteristically shows temporal and limbic involvement with early gyral edema and high signal intensity on T2-weighted and FLAIR images.

Characteristic MRI Findings

HSV Meningitis/Encephalitis

• Early MRI changes occur in the cingulate gyrus and medial temporal lobe, showing gyral edema on T1-weighted images and high signal intensity on T2-weighted and FLAIR images 1
• Diffusion-weighted MRI (DWI) is especially sensitive for detecting early changes in HSV encephalitis 1
• MRI obtained within 48 hours of hospital admission is abnormal in approximately 90% of patients with HSV encephalitis 1
• Later hemorrhagic changes develop in affected areas 1
• The limbic encephalitis pattern is highly characteristic with a reported specificity of 87.5% for PCR-confirmed HSV encephalitis 1

TB Meningitis

• Predominantly basal meningitis with enhancement of the meninges 2, 3
• Hydrocephalus is a common finding due to inflammatory exudates blocking CSF flow 4
• Tuberculomas (small intracranial lesions) may be visible 4
• Ischemic brain injury secondary to tuberculous vasculitis 4
• Signs of increased intracranial pressure 5

Clinical Context for Interpretation

• HSV encephalitis is a medical emergency requiring immediate treatment with aciclovir (10 mg/kg three times daily) if suspected, without waiting for confirmation 6
• TB meningitis is also a medical emergency, and empirical anti-tuberculosis therapy should be started promptly in all patients in whom the diagnosis is suspected 2
• MRI should be performed as soon as possible on all patients with suspected encephalitis/meningitis, ideally within 24 hours of hospital admission 1
• If MRI is not immediately available, CT scanning may exclude structural causes of raised intracranial pressure but has limited sensitivity (approximately 25%) for HSV encephalitis 1

Diagnostic Approach

For HSV Meningitis/Encephalitis

• CSF PCR for HSV DNA remains the gold standard for diagnosis 1
• CSF typically shows lymphocytic pleocytosis, elevated protein, and normal glucose 6
• HSV-2 meningitis more commonly affects women 6
• HSV meningitis should be distinguished from HSV encephalitis, which is a much more severe infection with high neurologic morbidity and mortality 6

For TB Meningitis

• CSF examination typically shows clear and colorless fluid with low, predominantly lymphocytic leukocytosis, raised protein and low glucose levels 4
• Decision to treat is mostly clinical because of difficulty in demonstrating TB bacilli on routine smear and time needed for culture 4
• Supporting evidence includes positive TB contact, Mantoux skin test, chest radiograph, PCR for tuberculosis on CSF, and demonstration of TB bacilli from extraneural sources 4

Treatment Implications of Correct Diagnosis

HSV Meningitis/Encephalitis

• HSV encephalitis requires 14-21 days of IV aciclovir 6
• For first episode HSV-2 meningitis, aciclovir 10 mg/kg IV every 8 hours until resolution of fever and headache, followed by valaciclovir 1 g TID to complete a 14-day course 6
• Suppressive therapy with valaciclovir 500 mg BID is not recommended for prevention of recurrent HSV-2 meningitis 6

TB Meningitis

• Treatment should consist of 4 drugs (isoniazid, rifampicin, pyrazinamide, ethambutol) for 2 months followed by 2 drugs (isoniazid, rifampicin) for at least 10 months 2
• Adjunctive corticosteroids (either dexamethasone or prednisolone) should be given to all patients with TB meningitis, regardless of disease severity 2
• Recent research suggests potential benefits from increased doses of rifampicin, addition of linezolid or fluoroquinolones, or adjunctive aspirin combined with corticosteroids 7

Important Considerations

• Both conditions can be fatal if not diagnosed and treated promptly 6, 2
• Follow-up MRI may be useful to assess treatment response and disease progression 1
• Imaging findings should always be interpreted in conjunction with clinical presentation, cerebrospinal fluid analysis, and other laboratory tests 1
• HIV status should be determined in all patients with suspected TB meningitis, as it complicates management 2