Objective:

Describe an unusual presentation of antibody-negative rhombencephalitis with
granulomatous pathology associated with mediastinal seminoma.

Background:

Neurologic syndromes associated with seminoma are typically antibody-mediated, most
commonly linked to KLHL11, and usually present as limbic or brainstem encephalitis.
Granulomatous CNS inflammation has not been well described in this setting.

Design/Methods:

CASE REPORT

Results:

A 33-year-old male presented with subacute diplopia, vertigo, and gait ataxia following
several months of constitutional symptoms, including weight loss. MRI brain showed a
T2/FLAIR hyperintense lesion involving the right dorsal medulla, cerebellopontine angle,
and cerebellum, with associated nodular enhancement. CSF demonstrated lymphocytic
pleocytosis with elevated protein and a persistent unique oligoclonal band. Extensive
infectious, rheumatologic, and paraneoplastic antibody testing, including KLHL11 and
Ma2 in serum and CSF, was negative, with no evidence of neuronal staining on
composite tissue. Despite initial clinical improvement with corticosteroids, imaging
demonstrated radiologic progression. Brain biopsy revealed necrotizing granulomatous
inflammation, with broad infectious and neoplastic evaluation, including PCR and
metagenomic next-generation sequencing, unrevealing. Whole-body PET-CT imaging
revealed two hypermetabolic anterior mediastinal nodules, and surgical resection
confirmed a primary mediastinal seminoma. Testicular ultrasound was negative. The
patient demonstrated marked clinical and radiographic improvement with
corticosteroids, cyclophosphamide, and chemotherapy.

Conclusions:

This case highlights a steroid-responsive rhombencephalitis with biopsy-proven granulomatous inflammation occurring in association with primary mediastinal seminoma. In the absence of an identifiable infectious or systemic granulomatous process, findings support a paraneoplastic immune-mediated mechanism. This expands the spectrum of seminoma-associated neuroimmunologic disease and underscores the value of comprehensive malignancy screening and early tissue diagnosis (low threshold to biopsy) in atypical CNS inflammatory presentations.