Cerebellar Ataxia as the Initial Manifestation of Anti-NMDAR Encephalitis Without Prodrome: A Case report

Background: Anti-N-methyl-D-aspartate receptor (anti-NMDAR) encephalitis is a potentially reversible autoimmune encephalitis, most commonly affecting young women and children. It typically follows a prodromal illness and progresses to prominent neuropsychiatric symptoms, movement disorders, seizures, and autonomic instability. While cerebellar ataxia may occur during the disease course, it is rarely the initial or isolated presenting feature, particularly in adults, and may result in diagnostic delay. Case Presentation: We report a case of a woman in her 20s with no prior medical history who presented with progressive cerebellar ataxia, including unsteady gait and impaired fine motor coordination, without preceding systemic or prodromal symptoms. Three days after admission, she developed a generalised tonic-clonic seizure followed by rapid-onset behavioural changes, hallucinations, and cognitive decline. Neurological examination revealed cerebellar signs including dysmetria, horizontal nystagmus, and a positive Romberg test. Brain MRI was unremarkable. Electroencephalography showed diffuse delta slowing. Cerebrospinal fluid analysis revealed lymphocytic pleocytosis and tested positive for anti-NMDAR antibodies. There was no evidence of malignancy on pelvic ultrasound or CT imaging. The patient was treated with first-line immunotherapy, including intravenous immunoglobulin and corticosteroids. Her neuropsychiatric symptoms improved significantly over the following two weeks. At one-month follow-up, she had near-complete recovery in mobility and cognition. Conclusion: This case highlights an unusual adult presentation of anti-NMDAR encephalitis in which cerebellar ataxia appeared before other symptoms. Clinicians should consider autoimmune encephalitis in patients with isolated cerebellar signs followed by rapid neurological decline, even without prodromal features or imaging abnormalities. Early diagnosis and prompt immunotherapy are essential for favourable outcomes.