Home SEE WELL The Link Between Alzheimer’s Disease and the Eyes

The Link Between Alzheimer’s Disease and the Eyes

by Courtney Dryer
Alzheimer's and the eye

Alzheimer’s disease (AD) is the most common cause of dementia, an acquired progressive brain disorder. Currently, it is estimated that 44 million worldwide live with dementia, but the number is expected to triple by 2050 with an aging population. Memory loss and cognitive impairment from Alzheimer’s can interfere with one’s daily living.

Alzheimer’s disease is thought to be caused by an abnormal buildup of proteins in the brain. The initial damage occurs in the entorhinal cortex and hippocampus parts of the brain which are involved in memory.

Later into the disease process, the cerebral cortex becomes affected, which is responsible for language, reasoning, and social behavior. Eventually, other parts of the brain become damaged. Patients may have trouble finding the right words, have vision/spatial issues, and impaired reasoning or judgment in the early stages of the disease. As the disease progresses, those with AD may also have changes in their behavior or personality.

AD patients develop abnormal protein clumps (amyloid plaques) and tangled bundles of fibers (Tau) in the brain. Another finding in Alzheimer’s disease is the loss of connections between nerve cells (neurons) in the brain. Neurons transmit messages between parts of the brain, and from the brain to other parts of the body.

Who is at risk of developing Alzheimer’s disease?

The risk factors for developing Alzheimer’s are not completely known but is most likely a combination of the environment and your genes. Age is the most significant factor, and your risk doubles every five years after the age of 65. If you have a family history of AD, you have a slightly greater chance of developing the disease.

Education and physical exercise may lower your risk of developing AD, whereas mid-life high blood pressure, high cholesterol and diabetes may increase your risk. A history of head injury or trauma, smoking, and Down’s Syndrome are also risk factors.

Though the exact reason is unknown, research shows the Latino population is about one-and-a-half times as likely as older whites to have Alzheimer’s and other types of dementia, while older African Americans are about twice as likely to have the disease as older whites. The increase in risk may be due to a higher incidence of vascular disease (blood vessel disease) in those populations.

Is there a treatment for Alzheimer’s disease?

Alzheimer’s disease is currently ranked as the seventh leading cause of death in the United States. There is no cure for Alzheimer’s, but there are several medicines approved by the U.S. Food and Drug Administration (FDA) to help manage the disease’s symptoms.

Most of the current drugs work best for those with early or middle-stage Alzheimer’s. Doctors agree that medicine used to treat behavior problems should only be used after other non-drug strategies have failed.

Cholinesterase inhibitors to treat Alzheimer’s disease

Galantamine, rivastigmine, and donepezil are cholinesterase inhibitors that may help reduce or control symptoms. Cholinesterase inhibitors work by preventing the breakdown of acetylcholine, which is a brain chemical important for memory and thinking. Common side effects of this drug class are nausea, vomiting, diarrhea, insomnia, muscle cramps and weakness, fatigue, headaches, and weight loss. 

Immunotherapies to treat Alzheimer’s dsease

Lecanemab and aducanumab are immunotherapies with the FDA’s accelerated approval to treat those with early Alzheimer’s. Accelerated approval allows for the quicker use and approval of medications for serious conditions that fill an unmet medical need and have a therapeutic effect.

To gain full FDA approval, the drug companies must conduct additional studies on the benefits of the therapies. The immunotherapy drugs target the protein beta-amyloid to reduce amyloid plaques. Possible side effects of the immunotherapy drugs include ARIA, headache, dizziness, falls, and diarrhea.

Treatments for late-stage Alzheimer’s disease

Memantine is a N-methyl-D-aspartate (NMDA) antagonist used to treat the symptoms and improve the patient’s daily functions. It is believed to work by regulating glutamate, a brain chemical. The side effects of memantine include dizziness, headache, diarrhea, constipation, and confusion. 

The FDA has also approved donepezil, the rivastigmine patch, and a combination medication of memantine and donepezil for the treatment of late-stage Alzheimer’s disease.

What is the link between Alzheimer’s and the eye?

Research now shows that diseases of the brain can also affect the eyes because the optic nerve and retina are composed of brain tissue that extends outside the brain. Both Alzheimer’s disease and dementia appear to have effects on the retina and parts of the eye.

What eye testing will show Alzheimer’s disease?

Currently, Alzheimer’s disease is diagnosed after many assessments and exams to rule out other conditions. Doctors may use cognitive testing, discussions with family members, physical exams, and brain scans, but none of these tests diagnose Alzheimer’s. The disease can only be diagnosed after death by examining the brain tissue.

Eye exams of the retina can be used to diagnosis may other systemic conditions like diabetes, hypertension, stroke, and some cancers. It is possible that a dilated eye exam combined with a scan of the retina could be used to find early-stage Alzheimer’s disease. Many small studies have shown AD may cause changes in the individual retinal layers or the blood flow within the eye.

Optical coherence tomography (OCT) is a test found in eye care offices. It is currently used to evaluate many other retinal conditions by looking at the layers of the retina and may be key to the evaluation and study of patients with AD.

Fluorescence lifetime imaging ophthalmoscopy (FLIO) is a newer device that looks at specific patterns in conditions that affect the retina. FLIO has been used to measure a protein called beta-amyloid in the retina which is one of the proteins that builds up in the brains of patients with AD.

In addition, the front of the eye, called the anterior segment, contains the aqueous humor and tear film layer which may be comparable to the cerebrospinal fluid found in the brain and spinal cord.  Both fluids are enriched with molecules that have potential neurodegenerative biomarkers. Deposits of amyloid-beta (Aβ) and tau protein are present in the eyes of AD patients, and other structural and functional changes have been observed in the structure of the eyes.  

While we don’t yet know the exact eye tests needed to make an Alzheimer’s diagnosis, research demonstrates enormous potential. It is quite possible we will be diagnosing and monitoring Alzheimer’s disease by an eye exam in the future.

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