Dr. Margaret G. McKernan, MD Alzheimer’s disease is the most common form of dementia, a term describing the loss of memory and other cognitive functions to the point of interfering with daily life. Alzheimer’s is predominantly a disease of late middle and old age, affecting up to 13% of individuals over the age of 65. The disease is progressive, meaning that the symptoms of memory loss, personality change and confusion continue to worsen over time, to the point where the individual is unable to interact with the world around them.
What is PET-CT?
PET-CT stands for Positron Emission Tomography (PET) – Computer Tomography (CT), a medical imaging study that combines a standard CT scan with a map of metabolic activity in the brain. The scan uses radiolabeled glucose to measure the level of activity is different areas of the brain; the more active cells are, the more glucose they need for energy. A standard PET-CT scan demonstrates a pattern of hypometabolism, or decreased activity, in the temporal and parietal lobes in the brains of patients with Alzheimer’s disease.
The search for a more definitive diagnosis for Alzheimer’s disease
While this pattern of decreased activity in specific parts of the brain is suggestive of Alzheimer’s disease, it is not a definitive diagnosis. Physicians perform extensive evaluations on patients with memory loss and cognitive decline to try to determine the exact cause and type of dementia. A specific diagnosis of Alzheimer’s allows for the prompt initiation of appropriate therapy and counseling.
One of the fairly specific signs of Alzheimer’s disease is the presence of clumps of a specific protein (beta-amyloid protein) intermixed with brain cells into a complex called a beta-amyloid plaque. Individuals with Alzheimer’s disease have a moderate to large number of these plaques in their brains. Unfortunately, up until now, the only way to assess for the presence and number of these plaques in the brain has been to perform an invasive brain biopsy (sampling of the tissue of the brain).
A new PET-CT technique for earlier, more specific diagnosis
Earlier this year, the US Food and Drug Administration approved a new compound called Amyvid (Florbetapir F 18). This drug binds directly to the abnormal beta-amyloid plaques in the brain, and produces a radioactive signal that can be measured by the PET scanner. This allows imagers to produce a three-dimensional map of the number and location of the abnormal amyloid plaques in the brain. A positive study, demonstrating a moderate or large amount of plaque, is highly suggestive of Alzheimer’s disease. A negative study, demonstrating little or no plaque, makes Alzheimer’s disease unlikely.
Earlier diagnosis of Alzheimer’s disease can improve quality of life
In 2011, Alzheimer’s Disease International (ADI) released a report detailing the social and economic cost of a delayed diagnosis of Alzheimer’s disease. It estimated that at least three-quarters of people worldwide living with dementia had not yet been properly diagnosed. This delay in diagnosis prevents access to medical therapy and counseling that can significantly improve the quality of life of both patients and caregivers. Furthermore, an early diagnosis can yield net savings of over 10,000 USD per patient in health care costs.
Treatment for mild to moderate Alzheimer’s usually consists of medications called cholinesterase inhibitors, including donepezil (Aricept). This drug likely works by preventing the breakdown of acetylcholine, a neurotransmitter (brain chemical) important for memory and thinking. Aricept has been shown to increase memory and brain function in patients with early Alzheimer’s, improving quality of life. It may also slow the pace of cognitive decline, giving individuals with Alzheimer’s more months or years of independence. The earlier in the course of Alzheimer’s disease the drug is administered, the more effective it is, highlighting the need for early diagnosis.
Hope for the future
Scientists now have a detailed understanding the mechanism of beta-amyloid plaque formation in the brain. There are multiple drugs in the development stage that attempt to either block the formation of plaques or remove the plaques from the brain tissue. One possible mechanism is to prevent beta-amyloid fragments from clumping together to form the damaging plaques. Researchers have been able to achieve this in the laboratory, and clinical trials are underway.
This new PET-CT technique may prove a useful tool in identifying patients who may be candidates for these targeted drug therapies, initially in clinical studies and then eventually in clinical practice. Overall, this technique represents an important new weapon in the fight against Alzheimer’s disease.