Is faciobrachial dystonic seizure a misnomer? A case report of atypical leg-predominant tonic-dystonic attacks in LGI1-associated encephalitis

BACKGROUND: Antibody‑mediated encephalitis due to leucine‑rich glioma‑inactivated 1 (LGI1) is a frequent cause of immune‑responsive paroxysmal motor phenomena in older adults. The disorder is commonly associated with brief, stereotyped involuntary movements traditionally labeled faciobrachial dystonic seizures (FBDS). However, the "faciobrachial" descriptor may underrepresent the true phenotypic spectrum. We report a case of LGI1‑associated autoimmune encephalitis presenting with leg‑predominant tonic-dystonic attacks, expanding the recognized clinical presentation and highlighting variability in semiology. CASE PRESENTATION: A 62‑year‑old man presented following multiple falls caused by sudden, repetitive involuntary movements primarily affecting the left lower extremity. Episodes were short‑lived, painless, and occurred dozens of times daily, often provoked by initiation of voluntary movement and preceded by a brief sensory warning. Consciousness was consistently retained, although some attacks spread to involve the ipsilateral arm and face. Neurological examination, cognitive assessment, brain MRI, and EEG were unremarkable. Cerebrospinal fluid analysis demonstrated mildly elevated protein without pleocytosis. Antiseizure therapy provided no benefit. High‑dose intravenous corticosteroids resulted in minimal improvement, whereas plasma exchange led to complete symptom resolution. Serum autoimmune testing was positive for LGI1-IgG antibody and a low-titer P/Q-type calcium channel antibody, suggesting an autoimmune encephalitis- most likely LGI1 encephalitis. FDG-PET/CT showed no malignancies or hypermetabolic abnormalities. CONCLUSIONS: This case highlights that LGI1‑associated paroxysmal events may present as leg‑predominant tonic-dystonic attacks, rather than the more typical faciobrachial distribution. Clinicians should maintain a high index of suspicion for autoimmune causes in late‑onset paroxysmal movement disorders, as prompt immunotherapy can prevent falls, reduce morbidity, and significantly alter disease course. Recognition of serum‑positive, imaging‑negative LGI1 autoimmunity is particularly important, as such presentations may lack common cognitive or limbic features and risk diagnostic delay.