Prodromal Alzheimer’s Disease

Prodromal Alzheimer’s disease represents a critical window in the progression from normal cognition to dementia, when mild cognitive impairment begins to signal deeper brain changes that may lie ahead.

Table of contents

• What is Prodromal Alzheimer’s Disease?
• Symptoms and Cognitive Changes
• Biomarkers and Diagnosis
• Disease Progression
• Current Research and Treatment Approaches

What is Prodromal Alzheimer’s Disease?

Prodromal Alzheimer’s disease is the period of time when a person experiences mild cognitive impairment (MCI) that may lead to dementia. This is also referred to as MCI due to Alzheimer’s disease, and it is the stage where there are clear symptoms of brain dysfunction, though the person has not yet developed full dementia^[1][2].

This stage comes after the preclinical phase of Alzheimer’s disease, when brain changes are occurring but symptoms are not yet obvious. During the prodromal stage, symptoms become noticeable to the individual and their loved ones. The person may struggle with tasks that were once easy, but they can still maintain some level of independence^[1].

Alzheimer’s disease is a progressive neurodegenerative disorder, which means it damages and destroys brain cells over time. This process is characterized by the buildup of abnormal proteins in the brain, including amyloid plaques and neurofibrillary tangles made of tau protein. These changes begin 15 to 20 years before obvious cognitive symptoms appear^[2].

People with Alzheimer’s disease progress from their normal baseline cognitive abilities through subtle changes in the preclinical stages to obvious symptoms of brain dysfunction in the prodromal stage. Eventually, they reach Alzheimer’s dementia, which significantly impairs their ability to perform daily activities that they were previously able to do independently^[2].

Symptoms and Cognitive Changes

The prodromal stage of Alzheimer’s disease brings changes that are more noticeable than normal aging but do not yet severely impact daily living. Memory loss is typically the most prominent symptom, affecting the ability to recall recent events. However, other cognitive functions also begin to decline^[1].

During the prodromal phase, dementia due to Alzheimer’s disease is preceded by about five to six years of accelerated decline in multiple cognitive functions. Studies show that five to six years before diagnosis, the rate of global cognitive decline sharply accelerates by more than 15-fold. The acceleration in decline occurs slightly earlier for semantic memory (memory for facts and concepts) at 76 months before diagnosis, and working memory (the ability to hold and manipulate information) at 75 months, compared to other cognitive functions^[4].

In the prodromal and mild phases of Alzheimer’s disease, researchers have observed an early increase in certain biomarkers of brain damage. This is followed by behavioral disturbances and deficits in executive functions, which are mental skills that help with planning, organization, and decision-making. Negative symptoms tend to predominate in the prodromal phase, including apathy (lack of interest or motivation), inflexibility, and loss of insight^[5].

People in the prodromal stage may experience difficulty with tasks that require complex thinking or problem-solving. They might struggle to follow multi-step instructions, have trouble managing finances, or find it challenging to plan and organize activities. These difficulties stem from the brain changes already underway, even though the person can still function relatively independently^[2].

The prodromal stage can be distinguished from normal aging by considering factors such as education level, cultural background, and primary language. Health professionals look for subtle cognitive changes that go beyond what would be expected from typical aging^[2].

Biomarkers and Diagnosis

Researchers and clinicians use biomarkers, which are measurable indicators of disease, to help identify prodromal Alzheimer’s disease. These biomarkers can provide evidence of the brain changes characteristic of Alzheimer’s disease before dementia develops^[1][2].

The National Institute on Aging and Alzheimer’s Association have established definitions for different stages of preclinical Alzheimer’s disease based on biomarker findings. These include evidence of amyloidosis (amyloid protein buildup) detected through PET imaging (a type of brain scan) or cerebrospinal fluid analysis (testing of the fluid surrounding the brain and spinal cord). More advanced stages show evidence of both amyloidosis and neurodegeneration, which is the progressive loss of nerve cells^[2].

Prodromal Alzheimer’s disease can be defined as having mild cognitive or behavioral impairment with relatively preserved functional independence, measured by specific clinical scales. Some experts suggest that a score equal to 0.5 on the global CDR Dementia Staging Instrument plus National Alzheimer’s Coordinating Centre behaviour and language domains may be useful to define prodromal stages^[5].

Blood-based biomarkers are emerging as important tools for diagnosis. Neurofilament light (NfL), a marker of nerve cell damage, has shown promise in identifying and tracking disease progression. Studies have found that NfL levels are significantly increased in the prodromal phase compared with healthy individuals, but lower than in patients with mild dementia. High NfL levels at baseline have been identified as strong predictors of disease progression^[5].

In research settings, scientists are studying substances like hemoprotein neuroglobin (Ngb), which is present mainly in the brains of mammals. Studies in mice have found that Ngb levels rise with age in relationship to high cerebrospinal fluid amyloid beta levels, suggesting it may play a protective role in the progression of Alzheimer’s disease^[1].

Disease Progression

The prodromal stage of Alzheimer’s disease represents a critical period when intervention might be most effective. Understanding how quickly the disease progresses during this stage is important for both patients and healthcare providers^[6].

Research has shown that prodromal Alzheimer’s disease is not a stable condition. At one-year follow-up, more than half of patients in the prodromal phase (51.2%) had converted to dementia. This high conversion rate demonstrates that the prodromal stage is a dynamic and rapidly changing period in the disease course^[5].

By contrast, people who do not develop Alzheimer’s disease show little cognitive decline over time. The sharp acceleration in cognitive decline that marks the transition from prodromal Alzheimer’s to dementia stands in clear contrast to normal aging^[4].

The progression of Alzheimer’s disease follows a continuous model of disease expression. There is a gradual decline in brain and behavioral function that appears to parallel closely the trajectory of brain changes occurring over time. In the very early stages, when the damage is limited, symptoms may not be apparent at all. As the damage accumulates over time, early symptoms emerge in the prodromal stage, followed later by fully developed clinical disease in the dementia stage^[3].

The rate of cognitive decline during the prodromal phase can vary between individuals. Factors such as vascular disease and vascular risk conditions may influence how Alzheimer’s symptoms develop. Clinical conditions related to Alzheimer’s disease and vascular problems often occur together and frequently share common risk factors^[3].

The duration from the onset of prodromal symptoms to dementia diagnosis has been estimated at approximately five to six years on average, though this can vary considerably between individuals. Some estimates range from about one year to more than ten years^[4].

Current Research and Treatment Approaches

Researchers are intensely focused on the prodromal stage of Alzheimer’s disease because early intervention during this phase may offer the best opportunity to slow or prevent progression to dementia. Understanding the changes that occur during the prodromal stage is critical for developing effective prevention and treatment strategies^[1][6].

Early identification of prodromal Alzheimer’s disease could be particularly important because recent clinical trials of disease-modifying agents suggest that these treatments work best when started at early stages of the disease. Both the International Working Group and the American Alzheimer’s Association have worked to define the boundaries of the preclinical and prodromal stages to facilitate earlier diagnosis and treatment^[2].

One major area of research focuses on brain energy metabolism and mitochondrial function, which refers to how cells produce energy. Problems with energy production in brain cells are among the earliest changes in Alzheimer’s disease. These energy deficits appear early in the prodromal phase and in those at risk for Alzheimer’s disease. Researchers are exploring treatments that target the energy production system in cells rather than single components, with approaches personalized to each person’s specific energy production problems^[6].

Clinical trials are investigating whether combining lifestyle interventions with medical treatments can benefit people with prodromal Alzheimer’s disease. One multinational study tested a multimodal lifestyle intervention that included nutritional guidance, exercise, cognitive training, management of vascular and metabolic risk factors, and social stimulation. Some participants also received medical food supplements. The study found good feasibility and adherence to these interventions in prodromal Alzheimer’s disease, with participants showing improvements in healthy diet measures^[9].

Various pharmaceutical approaches have been tested in prodromal Alzheimer’s disease. For example, six-month treatment studies have been conducted to evaluate the effects of experimental medications on cognitive function and disease progression in patients with suspected prodromal Alzheimer’s disease^[11].

Advanced brain imaging techniques are being developed to better identify which individuals with prodromal Alzheimer’s disease will progress to dementia. Machine learning models using brain metabolism imaging data have shown promise in distinguishing between people with prodromal Alzheimer’s who will remain stable and those who will progress to dementia. Such tools could help identify optimal candidates for treatment^[12].

Researchers continue to study the natural history of prodromal Alzheimer’s disease, the rate of conversion to fully symptomatic disease, and factors that predict progression. This knowledge is essential for designing effective clinical trials and developing interventions that can delay or prevent the onset of dementia^[5][10].