When dementia may not be dementia: autoimmune encephalitis and rapid cognitive decline

By Aliya Yu, PharmD Candidate

Rapidly progressive dementia (RPD) is one of the most alarming presentations in neurology. Unlike more familiar forms of dementia that develop gradually over years, RPD involves a rapid deterioration in memory, thinking, behaviour, and daily functioning over weeks or months. For patients, families, and clinicians, the speed of decline creates an urgent question: what is causing it, and can it be treated?

Traditionally, rapidly progressive dementia has been associated with devastating neurodegenerative diseases such as Creutzfeldt–Jakob disease (CJD), a rare neurological disease known for causing severe and rapidly worsening cognitive decline. However, emerging research highlights another important possibility. Autoimmune encephalitis (AE), a group of inflammatory brain disorders in which the immune system mistakenly attacks the brain. Although considered relatively uncommon, AE can affect people across a wide range of ages and often presents with symptoms such as memory loss, confusion, psychiatric changes, seizures, movement disorders, and cognitive decline. Causes vary by subtype and may involve neuronal autoantibodies, underlying tumours, infections, or other immune system triggers.

A recent multicentre study by van Steenhoven et al. published in Neurology investigated the causes of rapidly progressive dementia in 147 adults across centres in the Netherlands. The researchers, which included Encephalitis International’s Scientific Advisory Panel member Prof. Maarten Titulaer, found that autoimmune encephalitis was the most common treatable cause of rapidly progressive dementia in the cohort.

This finding matters because, unlike many neurodegenerative conditions, autoimmune encephalitis may respond to treatment. Depending on the subtype and clinical presentation, treatment can include immunotherapies such as corticosteroids, intravenous immunoglobulin (IVIG), plasma exchange, or other medications that suppress abnormal immune activity. Earlier recognition and treatment have been associated with improved outcomes, making timely diagnosis essential.

One of the study’s key messages is that recognising autoimmune encephalitis in patients with rapid cognitive decline requires attention to clinical clues that may not always be obvious. At the same time, these features can contribute to misdiagnosis or delayed diagnosis, particularly when symptoms overlap with neurodegenerative disease or present in subtle, unexpected ways.

Seizures emerged as one of the strongest diagnostic indicators. Patients with autoimmune encephalitis were significantly more likely to experience seizures than patients with other diagnoses. Yet these seizures were not always dramatic convulsions. Many patients experienced subtle focal seizures: brief, repetitive episodes that may involve changes in sensation, awareness, behaviour, or autonomic symptoms (them easier to overlook during early evaluation).

The study also highlighted important differences between autoimmune encephalitis subtypes. Anti-LGI1 encephalitis was particularly common, especially among patients presenting with seizures. In contrast, autoimmune GFAP astrocytopathy more frequently appeared in patients without seizures and often presented with psychiatric symptoms, movement disorders, or rapid cognitive decline.

Despite these clinical clues, diagnosing autoimmune encephalitis in the setting of rapidly progressive dementia is not always straightforward. Some patients may lack classic inflammatory findings, while others present with symptoms that closely resemble neurodegenerative disease. Certain forms of autoimmune encephalitis may also require specialised antibody testing that is not included in every standard diagnostic panel. The study’s authors therefore emphasise the importance of comprehensive antibody testing when evaluating patients with rapidly progressive dementia.

For patients and families facing rapid cognitive decline, these findings carry an important message. While some causes of rapidly progressive dementia remain difficult or impossible to treat, not all rapid decline represents irreversible neurodegeneration.

Autoimmune encephalitis can present with symptoms that overlap considerably with dementia, including memory loss, behavioural changes, psychiatric symptoms, and declining daily functioning, contributing to diagnostic uncertainty. Yet unlike many dementias, it may be treatable.

As research continues to improve our understanding of autoimmune brain disease, this study reinforces a simple but powerful principle: when cognitive decline moves quickly, searching for treatable causes matters.

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To view the full paper, click here.

For more information on autoimmune encephalitis, click here.

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