MRI Findings in Suspected Encephalitis with Cerebellar Involvement
In a patient presenting with prolonged fever, ataxia, nystagmus, bradycardia, and altered mental status, obtain an urgent MRI brain without and with IV contrast to evaluate for infectious encephalitis, particularly tick-borne or enteric fever-related CNS infection, looking specifically for signal abnormalities in the basal ganglia, thalamus, cerebellum, and brainstem. 1
Recommended MRI Protocol
MRI head without and with IV contrast is the imaging test of choice when intracranial infection is suspected. 1 This clinical presentation—combining fever, cerebellar signs (ataxia, nystagmus), bradycardia, and altered mental status—strongly suggests infectious encephalitis requiring contrast-enhanced imaging to detect inflammatory changes and enhancement patterns. 2
Key Sequences to Include:
- Diffusion-weighted imaging (DWI) to identify acute ischemic changes, cytotoxic edema, and areas of restricted diffusion seen in encephalitis 2
- T2-weighted and FLAIR sequences to detect parenchymal signal abnormalities and inflammatory changes 2
- Susceptibility-weighted imaging (SWI) to identify microhemorrhages that may occur with severe encephalitis 2
- Post-contrast T1-weighted sequences to evaluate for meningeal or parenchymal enhancement 1
Expected MRI Findings Based on Clinical Presentation
Infectious Encephalitis Patterns:
Signal abnormalities in deep gray matter structures are characteristic of severe tick-borne encephalitis, specifically involving the basal ganglia and thalamus, typically without contrast enhancement in early stages. 3 These findings correlate with severe nerve cell degeneration, inflammatory cell infiltrates, and neuronophagia documented in neuropathological studies. 3
Cerebellar and brainstem involvement should be specifically evaluated given the ataxia and nystagmus, as rhombencephalitis (hindbrain inflammation) can present with increased FLAIR signal in the pons and cerebellum. 4
"Starry sky" pattern of diffuse, innumerable punctate foci of diffusion restriction with susceptibility-weighted signal attenuation throughout cerebral hemispheres may be seen in severe rickettsial infections like Rocky Mountain Spotted Fever. 5 This pattern represents widespread microinfarcts from vasculitis.
Specific Anatomical Regions to Evaluate:
- Basal ganglia and thalamus: Look for T2/FLAIR hyperintensity, which is characteristic of tick-borne encephalitis 3
- Cerebellum: Evaluate for signal abnormalities and swelling, as acute cerebellar ataxia can occur with enteric fever 6
- Brainstem (particularly pons): Assess for FLAIR hyperintensity suggesting rhombencephalitis 4
- Cerebral cortex: Check for multifocal areas of restricted diffusion or enhancement 5
- Meninges: Evaluate for leptomeningeal enhancement suggesting meningitis 1
Clinical Context and Differential Diagnosis
The combination of bradycardia with fever (relative bradycardia) is a critical clue pointing toward enteric fever (typhoid), which can cause CNS complications including acute cerebellar ataxia. 6 This finding, combined with ataxia and altered mental status, makes enteric fever with CNS involvement a strong diagnostic consideration. 7, 6
Tick-borne encephalitis must be considered if there is any history of tick exposure, as it characteristically causes fever and altered mental status with specific MRI findings in deep gray matter. 3
Rickettsial infections should remain on the differential, particularly if a petechial rash develops, as these can cause severe encephalitis with distinctive MRI patterns. 5
Timing and Urgency
MRI should be performed within 24-48 hours of presentation for suspected encephalitis, ideally within 24 hours. 2 The American College of Radiology emphasizes that MRI has superior sensitivity compared to CT for detecting encephalitis, particularly when CT is normal or shows only subtle abnormalities. 2
Do not delay empiric antimicrobial therapy while awaiting MRI, as mortality predictors in acute febrile encephalopathy include refractory seizures, GCS <8, bradycardia, and shock. 7 The presence of bradycardia in your patient is particularly concerning as an independent predictor of mortality. 7
Common Pitfalls to Avoid
Do not rely on CT alone in this clinical scenario. MRI is far more sensitive for detecting encephalitis, with CT often appearing normal in early or mild cases. 1, 2 The American College of Radiology specifically notes that 70% of patients with missed ischemic stroke diagnoses presented with altered mental status, highlighting the inadequacy of CT in detecting subtle parenchymal abnormalities. 1, 2
Do not order MRI with contrast only. There is no relevant literature supporting MRI with IV contrast alone; always obtain sequences both without and with contrast when infection is suspected. 1
Do not assume normal MRI excludes encephalitis. MRI is usually normal in mild forms of tick-borne encephalitis, and severe cases may take time to develop characteristic findings. 3 Clinical correlation and CSF analysis remain essential. 1
Impact on Clinical Management
MRI findings lead to changes in clinical management in 76% of patients with acute disorders of consciousness, including revised diagnoses (20%), revised levels of care (21%), improved diagnostic confidence (43%), and improved prognostication (33%). 1, 2 This high impact rate justifies urgent MRI in your patient with this concerning constellation of symptoms.