Acanthamoeba Rhombencephalomyelitis: Novel White Matter Tract–Based Neuroinvasive Pattern in a Retrospective Observational Study from the Kerala Amoebic Meningoencephalitis Surge, 2025

Objective: To describe a distinct rhombencephalomyelitis pattern in patients with central nervous system infection due to free – living amoebae, differing from the classical granulomatous amoebic encephalitis (GAE) phenotype. Methods: This retrospective observational study included twelve consecutive patients with brainstem and/or spinal cord involvement associated with central nervous system infection due to Acanthamoeba species at a tertiary care centre in Kerala, India. Diagnosis was confirmed by wet mount examination and amoeba species – specific 18S rRNA PCR of cerebrospinal fluid. Comprehensive evaluation excluded bacterial, mycobacterial, fungal, and autoimmune etiologies. Results: Twelve patients (seven men; median age 50 years; IQR: 33 – 60 years) presented with subacute cranial neuropathies (58.3%) and/or gait disturbance (91.7%). Seven (58.3%) were immunocompetent; only two had traditional risk factors (ritualistic nasal ablution). MR scan demonstrated involvement of the spinal cord (66.7 %), pons (58.3 %), middle cerebellar peduncle (50%), and medulla (41.7 %). This distinctive neuroimaging pattern reflected white matter tract – based spread along cerebellar peduncles and/or corticospinal tracts. CSF analysis revealed lymphocytic pleocytosis (median: 12 cells/mm ³ , IQR: 5 – 47) with hypoglycorrhachia (CSF – to – blood glucose ratio 0.45 IQR: 0.4 – 0.5). Acanthamoeba spp. were confirmed by CSF PCR in ten (83.3 %) and presumptively by wet mount examination in two (16.7%). Nine patients (75.0%) survived, of whom eight (66.7%) achieved favourable functional outcome (mRS 1 – 2) at discharge. CONCLUSION Acanthamoeba spp. causes distinct white matter tract – based non-granulomatous rhombencephalomyelitis. Neurologists should screen for free – living amoebae CNS infection even without traditional risk factors, as this pattern may mimic demyelinating disorders (NMOSD, MOGAD, ADEM, or MS). When identified early, Acanthamoeba rhombencephalomyelitis carries a favourable prognosis .