A 62-year-old man presents with a 3-month history of rapidly progressive cognitive decline, myoclonus triggered by startle, and cerebellar ataxia. Brain MRI shows cortical ribboning and hyperintensity in the caudate and putamen on DWI sequences. His EEG is shown in the diagram. The pattern marked **A** — generalized periodic sharp wave complexes at 1 Hz — is characteristic of which of the following conditions?

Explanation

## Why Sporadic Creutzfeldt-Jakob disease (sCJD) is right The clinical presentation (rapidly progressive dementia, myoclonus, cerebellar ataxia, cortical ribboning on MRI) combined with the EEG finding of **generalized periodic sharp wave complexes at 1 Hz** is pathognomonic for sporadic CJD. This EEG pattern appears in 60–70% of sCJD cases as the disease progresses and reflects the underlying spongiform degeneration, neuronal loss, and astrogliosis caused by accumulation of misfolded prion protein (PrP^Sc). sCJD accounts for ~85% of all CJD cases, with median onset at 60–65 years and median survival of 5–6 months. The combination of clinical features, MRI findings (caudate/putamen hyperintensity, cortical ribboning on DWI), and this characteristic 1 Hz periodic sharp wave pattern is the diagnostic hallmark of sCJD per updated CDC/MRC criteria. ## Why each distractor is wrong - **Familial Alzheimer disease**: While it causes progressive dementia, it does NOT produce the characteristic 1 Hz periodic sharp waves on EEG. FAD typically shows background slowing and theta activity, not periodic sharp complexes. The myoclonus and rapid progression over months (not years) are atypical for FAD. - **Variant Creutzfeldt-Jakob disease (vCJD)**: Although vCJD is a prion disease linked to bovine spongiform encephalopathy, it typically affects younger patients, presents with psychiatric symptoms first, and shows the **pulvinar sign** on MRI. Critically, vCJD does NOT show the 1 Hz periodic sharp wave complexes on EEG—this pattern is absent in vCJD and is specific to sporadic CJD. - **Rapidly progressive frontotemporal dementia**: FTD with TDP-43 pathology causes cognitive decline and behavioral changes but does NOT produce the characteristic 1 Hz periodic sharp waves. FTD EEGs typically show nonspecific slowing without the periodic sharp complex pattern seen in prion disease. **High-Yield:** 1 Hz periodic sharp wave complexes on EEG = sporadic CJD (not variant CJD); combine with rapid dementia + myoclonus + MRI cortical ribboning/caudate-putamen hyperintensity for diagnosis. [cite: Harrison 21e Ch 432; Adams Neurology 11e Ch 33]