What are the differences in MRI brain findings between Tuberculosis (TB) meningitis and viral meningitis?

MRI Differentiation Between TB Meningitis and Viral Meningitis

The key MRI difference between TB meningitis and viral meningitis is that TB meningitis typically shows basal meningeal enhancement, tuberculomas, and vascular complications, while viral meningitis (particularly HSV) characteristically affects the temporal lobes and limbic structures with T2/FLAIR hyperintensity.

Characteristic MRI Findings in TB Meningitis

• Thick basal meningeal enhancement in the cisterns is the hallmark feature, visible after gadolinium administration on T1-weighted images 1, 2
• Tuberculomas (single, multiple, or miliary) appear as ring or nodular enhancing lesions that are characteristically hypointense on T2-weighted images 1, 2
• Vascular complications are common, with MR angiography showing focal arterial narrowing, particularly affecting the terminal segment of internal carotid artery and proximal segments of middle and anterior cerebral arteries 1
• Infarcts (both hemorrhagic and bland) are frequently detected, especially in the basal ganglia and internal capsules 1
• Hydrocephalus and cranial nerve enhancement may be present due to the predominant basal meningitis 1, 3

Characteristic MRI Findings in Viral Meningitis/Encephalitis

• Early MRI changes in HSV encephalitis 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 4, 5
• Diffusion-weighted MRI (DWI) is especially sensitive for detecting early changes in HSV encephalitis 4, 5
• MRI obtained within 48 hours of hospital admission is abnormal in approximately 90% of patients with HSV encephalitis 4, 5
• The limbic encephalitis pattern is highly specific (87.5%) for PCR-confirmed HSV encephalitis 4, 5
• Later in the disease course, hemorrhagic changes may develop in the affected areas 4, 6

Diagnostic Approach

• MRI should be performed as soon as possible on all patients with suspected meningitis/encephalitis, ideally within 24 hours of hospital admission, but certainly within 48 hours 4, 5
• If MRI is not immediately available, CT scanning may exclude structural causes of raised intracranial pressure but has limited sensitivity (approximately 25%) for viral encephalitis 4
• CSF analysis remains essential for diagnosis of both conditions:
  • TB meningitis typically shows lymphocytic pleocytosis, markedly elevated protein, and low glucose 7, 3
  • Viral meningitis also shows lymphocytic pleocytosis and elevated protein, but glucose is usually normal or only mildly reduced 4, 5
• PCR testing of CSF is crucial for definitive diagnosis in both conditions 7, 8

Clinical Implications of Correct Diagnosis

• Early diagnosis and treatment are critical for both conditions to reduce mortality and neurological sequelae 4, 8
• TB meningitis requires prolonged multi-drug antituberculous therapy (typically 12 months) plus corticosteroids 7, 3
• HSV encephalitis requires immediate treatment with aciclovir (10 mg/kg three times daily) for 14-21 days 4, 5
• Follow-up MRI may be useful to assess treatment response and disease progression in both conditions 5, 8

Important Pitfalls to Avoid

• Relying solely on CT scan for diagnosis, as it has low sensitivity for both conditions, especially early in the disease course 4
• Delaying treatment while waiting for definitive diagnostic confirmation - empiric therapy should be initiated based on clinical suspicion and initial CSF findings 4, 7
• Failing to consider TB meningitis in patients from high-risk populations or with immunocompromise 4, 3
• Overlooking the possibility of co-infection, particularly in immunocompromised patients 4, 8