| Summary: | Antibodies directed against the voltage-gated potassium channel complex (anti-VGKCs) are implicated in several autoimmune conditions including limbic encephalitis and epilepsy. However, emerging evidence suggests that only specific subtypes of anti-VGKCs are pathogenic. We present the case of a 55-year-old man who initially presented with focal unaware seizures and behavioural changes mimicking anti-VGKC-seropositive encephalitis that further progressed to parkinsonism with evidence of frontotemporal dementia and pre-synaptic dopaminergic deficit. Aggressive treatment with immunotherapy was ineffective, and antibody subtyping later revealed the anti-VGKC antibodies to be negative for leucine-rich glioma-associated 1 (LGI1) and contactin-associated protein-like 2 (CASPR2) – the two known pathogenic subtypes. The clinical relevance of so-called “double-negative” anti-VGKCs (i.e., those not directed towards LGI1 or CASPR2) has been called into question in recent years, with evidence to suggest they may be clinically insignificant. Our case emphasises the importance of antibody subtyping in cases of anti-VGKC seropositivity; negative results, particularly when combined with a poor response to immunotherapy, should prompt a rapid reconsideration of the working diagnosis.
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