Identifying the infraslow activity associated with faciobrachial dystonic seizures in EEG

The tonic-dystonic motor spasms or faciobrachial dystonic seizures (FBDS) pathognomonic of anti-LGI1 encephalitis have been shown to be associated with a preceding cortical change visible in EEG as a phasic DC (direct current) shift, or infraslow activity (ISA).^{1, 2} At usual clinical filter settings, the ISA may be difficult to appreciate and is often unrecognized. Best seen in common average or linked ears reference montages, ISA precedes the spasms by ~1 s over the contralateral frontal lobe,² in some patients maximal over the mid frontal area and in others the frontocentral region. When reviewed using a longer time constant (e.g., the 0.07 Hz analog filter of the clinical amplifier, with no added digital high-pass filter, as shown in Video 1, Figure 1), the ISA is more readily apparent. In some patients, FBDS may be very subtle (see later examples in Video 1) and thus identifying the characteristic EEG features can have diagnostic importance.