Epilepsy in Down syndrome rises steeply once dementia becomes symptomatic

Design

Multicentre observational longitudinal study across seven international cohorts, 2012 to 2025

Population

4,804 adults with Down syndrome, mean age 43 years; 20.3% with Alzheimer's disease dementia

Primary outcome

Active epilepsy prevalence and cumulative incidence relative to symptomatic Alzheimer's disease, plus survival and cognitive trajectory

Effect

Cumulative incidence 9.5% (95% CI 7.8-11.3) at dementia diagnosis rising to 56.9% (51.7-61.7) at nine years; adjusted odds ratio 1.33 per year (1.22-1.45); mortality hazard ratio 2.10 (1.57-2.82) with late-onset myoclonic epilepsy

Seven international cohorts contributed 4,804 adults with Down syndrome, mean age 43, followed longitudinally between 2012 and 2025. A fifth had Alzheimer's disease dementia and a further 6.7% were at the prodromal stage. The question was when epilepsy appears relative to dementia, and what it means once it does.

The answer is a steep, near-linear climb after symptomatic Alzheimer's disease begins: cumulative epilepsy incidence of 9.5% (95% CI 7.8-11.3) at the point of diagnosis, reaching 56.9% (51.7-61.7) nine years later. Risk rose with each year since diagnosis (adjusted odds ratio 1.33 per year), with severe-to-profound intellectual disability, and with APOE e4 carriage, independently of age. Seizures were mostly myoclonic or tonic-clonic. Late-onset myoclonic epilepsy carried roughly double the mortality (hazard ratio 2.10, 95% CI 1.57-2.82) and faster cognitive decline.

One practical finding cuts against habit: interictal EEG abnormalities had little diagnostic utility here. The diagnosis is clinical, and the myoclonic jerks that precede it are easily attributed to the dementia itself or dismissed as startle by carers who have stopped expecting anything new. Asking about morning jerks at each review of an adult with Down syndrome and symptomatic dementia costs nothing and is where this finding lands.

• Ask carers directly about morning myoclonic jerks at every review after a dementia diagnosis
• Expect rising risk each year after symptomatic Alzheimer's disease, not a fixed baseline
• Give more weight to the history where intellectual disability is severe or APOE e4 is known
• Do not rely on a normal interictal EEG to exclude it
• Record epilepsy onset — it marks a change in prognosis, not just a new medication

Why it matters

It reframes myoclonic jerks in this group from a curiosity of advanced dementia into a marker that changes prognosis.

Don't overread it

This was observational — it shows when epilepsy appears and what it predicts, not that treating it changes survival.

The statistics, in plain English

The cumulative incidence figures describe the proportion affected by a given time, so 56.9% at nine years is not an annual rate but the accumulated total. The odds ratios are expressed per year since diagnosis, which is why 1.33 compounds into a large difference over several years. The mortality hazard ratio of 2.10 is an association: epilepsy may be marking more advanced disease rather than causing the excess deaths, and this design cannot separate the two.

Read the rest in the app

You have read your two free briefings this month. The app carries all 27 specialties, every morning, free — and this finding is waiting in it.

Get Daily Doctor — free

Scan to keep reading on your phone. No account needed to start.