The Great Imitator in the Dura: A Radiologic Transformation of West Nile Encephalitis
Amanpreet Sidhu1, Kishan Patel2, Geethika Ameneni2, Sagi Elhassan1
1Department of Neurology, University of Arizona, 2University of Arizona School of Medicine
Objective:
To report an unusual case of West Nile meningoencephalitis presenting with isolated pachymeningeal enhancement that evolved into diffuse leptomeningeal involvement, expanding the recognized imaging spectrum of West Nile neuroinvasion.
Background:
Neuroinvasive West Nile virus (WNV) infection classically manifests with leptomeningeal or parenchymal inflammation, most often involving the basal ganglia, thalami, or brainstem. Isolated pachymeningeal enhancement and subdural effusions are distinctly uncommon and rarely described. Recognizing this divergent imaging pattern is critical to prevent diagnostic delay and unnecessary investigation for mimicking entities such as autoimmune pachymeningitis or intracranial hypotension.
Results:
A 71-year-old man with no significant medical history was found unresponsive in his hotel room; two days earlier he had been at neurologic baseline. On admission, he was obtunded and non-verbal, without focal deficits or trauma. Initial MRI with gadolinium revealed asymmetric pachymeningeal enhancement (left > right) without leptomeningeal involvement or parenchymal lesions. CSF analysis showed lymphocytic pleocytosis (52 WBC/µL).
Repeat MRI two days later demonstrated diffuse pachymeningeal enhancement with new bilateral subdural effusions and mild downward brainstem crowding. Despite these changes, the patient’s mental status remained unchanged. A third MRI several days later revealed resolution of the subdural effusions but new diffuse leptomeningeal enhancement bilaterally, marking a striking compartmental shift. Differential diagnoses at that stage included IgG4-related disease, autoimmune encephalitis, and viral meningoencephalitis. Repeat CSF showed 22 WBC/µL. Serum WNV IgM returned positive, confirmed by CSF IgM, establishing the diagnosis of West Nile meningoencephalitis.
Conclusions:
This case broadens the radiologic spectrum of West Nile neuroinvasive disease, demonstrating an evolving trajectory from pachymeningeal to leptomeningeal enhancement with transient subdural effusions. The dural-predominant phase may represent an underrecognized inflammatory interface in viral meningoencephalitis. Clinicians should maintain suspicion for WNV even when imaging is atypical and prioritize serologic testing despite negative molecular assays.
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