Introduction

Functional seizures (FS) are common, and distinguishing FS from epileptic seizures (ES), even with gold standard investigations can sometimes be difficult (e.g. with frontal lobe seizures). The heartbeat-evoked potential (HEP) has been shown to correlate with the degree of a patient’s bodily attention (Coll et al. 2021). Previous work demonstrated diagnostically useful differences in the change in interictal to pre-ictal HEP between patients with FS and those patients with epilepsy (Elkommos et al. 2022).

Aims

We sought to replicate our previous findings in a larger sample, and secondly extend them to the study of semiological subtypes of FS and ES. We therefore compared the interictal, preictal and postictal HEP in patients with FS, epilepsy and both, and between semiological groups, using retrospective EEG data.

Methods

We identified 150 FS and 194 ES from video telemetry (VT) recordings of patients with ES, FS or both. Semiologies of FS were grouped according to Asadi-Pooya et al’s 4-way semiological classification (Asadi-Pooya 2019). ES were classified according to the ILAE semiological classification (Fisher et al. 2017). FS of the generalized motor and focal motor type were grouped into the “motile” group, and FS in the akinetic features or subjective symptoms were grouped as “non-motile”. ES with generalized or focal motor, automotor or hypermotor features were grouped as “motile”, and dialeptic and akinetic ES and auras were grouped as “non-motile”. HEP was calculated by averaging EEG segments time-locked to R-waves, correcting for pre-R baseline, and measuring the average voltage between 0.455 and 0.595 seconds after the R wave. We compared ES and FS of equivalent semiology.

Results

Analysis was performed with frequentist ANCOVA and Bayesian ANCOVA including heartrate in the null model. We found that non-motile FS were associated with an increase in preictal HEP from interictal whereas motile FS were associated with a drop in preictal HEP (At C4: Bayes factor (BF) 3.81, p value 0.01, At F8: BF 378.5, p value <0.001, using the sum of C4 and F8: BF 1366, p value <0.001). The HEP appears to normalise postictally in functional seizures. We also found that changes in HEP distinguished FS and ES with similar semiology (p=0.02).

These findings suggest that non-motile FS are a response to increased bodily attention, whereas motile FS are a response to decreased bodily attention (i.e. dissociation). Furthermore, the HEP exhibits potential as a biomarker for discriminating FS from ES with similar semiology.