Klinefelter syndrome (KS) is one of the most common sex chromosome abnormalities and several cases are documented where children with KS present with epilepsy. Also, cognitive function deficits are common, despite high-functioning cases might occur. However, most reported cases are children or young adults.

We report a case of a 56-year-old patient with KS who presented in our clinic with potentially epileptic events. We found left-lateralized temporal intermittent rhythmic delta in routine electroencephalogram (EEG), high-frequency oscillations (HFOs, > 80 Hz) in high-density EEG (HD-EEG) that where pronounced over the left hemisphere, as well as left-temporal and paraventricular structural changes. Despite the patient was cognitively high functioning with excellent verbal and episodic memory performance, executive function tests involving inhibitory skills yielded significant differences against an age and sex-matched control group (N = 18).

This case suggests that high-functioning patients with KS may present with epilepsy-typical activity, that could be indicative for late-onset epilepsy. Diagnostic means of routine EEG can be complemented by HFO analysis of HD-EEG. Normal to high cognitive functioning may co-occur with deficits in single domains such as inhibition.