To describe a novel “Todd’s pupil” sign in a rare case of persistent Episodic Unilateral Mydriasis (EUM), indicating coexisting Epilepsy in Psychogenic Non-Epileptic Seizures (PNES).

Although rare and typically linked with migraine, EUM has also been associated with epilepsy. Precise mechanisms remain unclear; nonetheless, it is hypothesized that frontal cortex projections to the ipsilateral pretectum and mesencephalic tegmentum could modulate pupillary pathways. Therefore, since bilateral mydriasis is common during seizure activity, a frontal epileptic focus might inhibit ipsilateral dilation, causing contralateral mydriasis only. Postictal unilateral mydriasis may thus represent a lateralizing sign analogous to Todd’s paralysis, reflecting transient postictal inhibition or exhaustion of these cortical pathways affecting autonomic pupillary control. While “Todd’s paralysis” traditionally refers to transient postictal weakness contralateral to the seizure focus, we propose that a similar mechanism could explain transient contralateral mydriasis, hereby “Todd’s pupil” sign.

A 24-year-old woman with PNES was admitted for evaluation of a new type of seizure-like episodes associated with persistent postictal EUM. Examination revealed anisocoria due to an unreactive, right-sided mydriasis. She reported that her pupil typically returned to normal within five days, displaying a “Todd’s pupil” behavior.

Neuroimaging was unrevealing. Video-electroencephalography captured five stereotyped events, some during sleep, with semiology of abrupt tonic asymmetric arm extension with left elbow slightly flexed, followed by rightward body and arms turning, suggesting supplementary motor area involvement. A subset of events showed an electroencephalographic signature of low-amplitude fast activity over the left frontocentral region. She was diagnosed with left frontal epilepsy associated with EUM and started on lacosamide.

This case illustrates a “Todd’s pupil” sign, where persistent EUM acts as a postictal lateralizing feature of focal epilepsy, supporting a functional connection between frontal cortical networks and pupillary control pathways. Recognizing this sign could aid in diagnosing coexisting epilepsy in PNES, prompting further evaluation and treatment.