A 64-year-old man presents with multiple daily brief (1–3 second) dystonic posturing of the left face and ipsilateral arm, occurring up to 50 times per day. He also reports subacute memory loss, confusion, and insomnia over 4 weeks. Serum sodium is 126 mEq/L. MRI brain shows bilateral medial temporal T2/FLAIR hyperintensity with preserved diffusion. CSF shows mildly elevated protein without pleocytosis or oligoclonal bands. An EEG is performed. The pattern marked **B** in the diagram—brief subclinical or ictal discharges over fronto-central or temporal regions correlating with the dystonic episodes—is characteristic of which antibody-mediated encephalitis?
A. Anti-CASPR2 encephalitis with neuromyotonia
B. Anti-LGI1 encephalitis with faciobrachial dystonic seizures
C. Anti-NMDA receptor encephalitis with orofacial dyskinesias
D. Anti-GABA-B receptor encephalitis with occipital seizures
Explanation
Why Anti-LGI1 encephalitis with faciobrachial dystonic seizures is right
The EEG pattern marked B—brief subclinical or ictal discharges over fronto-central or temporal regions during the dystonic episodes—is pathognomonic for anti-LGI1 encephalitis presenting with faciobrachial dystonic seizures (FBDS). The clinical triad of brief dystonic posturing (face and arm), subacute memory loss with SIADH-pattern hyponatremia, bilateral medial temporal hyperintensity on MRI, and characteristic EEG findings of subtle ictal discharges during the brief episodes (often subclinical on scalp recording) uniquely define anti-LGI1 disease. LGI1 is a secreted neuronal protein at the synapse associated with the VGKC complex. FBDS are often the earliest and most distinctive feature, preceding overt limbic encephalitis by weeks to months, and are pathognomonic when combined with the imaging and serological findings (Irani et al., Lancet Neurol 2010; Harrison's 21e).
Why each distractor is wrong
Anti-NMDA receptor encephalitis with orofacial dyskinesias: Affects predominantly young women, often with ovarian teratoma; presents with prominent psychiatric symptoms, seizures, movement disorders, and autonomic instability. The EEG pattern and clinical phenotype differ markedly; FBDS are not characteristic, and the demographic (older male) is atypical.
Anti-CASPR2 encephalitis with neuromyotonia: Associated with Morvan syndrome, characterized by neuromyotonia (continuous muscle fiber activity), insomnia, and autonomic symptoms. EEG findings and the specific pattern of brief dystonic seizures are not typical; the clinical presentation centers on muscle stiffness and myokymia rather than FBDS.
Anti-GABA-B receptor encephalitis with occipital seizures: Often associated with small-cell lung cancer; presents with seizures (typically generalized or occipital), cognitive decline, and ataxia. The characteristic brief fronto-central/temporal ictal discharges during dystonic episodes and the FBDS phenotype are not features of anti-GABA-B disease.
High-YieldNEET PG
Anti-LGI1 encephalitis is defined by faciobrachial dystonic seizures (brief, repetitive, dystonic posturing of face and arm) with characteristic fronto-central/temporal ictal EEG discharges, bilateral medial temporal hyperintensity, and SIADH-pattern hyponatremia—often in older men, rarely paraneoplastic, and dramatically responsive to immunotherapy.
Irani SR, et al. Lancet Neurol. 2010;9(10):979–990. FBDS and anti-LGI1 encephalitis. Harrison's Principles of Internal Medicine, 21e. Autoimmune Encephalitis section.
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