Alzheimer’s disease causes worsening memory loss and brain degeneration over time. Not every case looks the same. The timing of diagnosis and the symptoms someone has can vary depending on the type of Alzheimer’s.

Here’s what you need to know about four types of Alzheimer’s disease.

1. Late-Onset Alzheimer’s Disease

Late-onset dementia is most commonly caused by Alzheimer’s disease. It’s what people usually think of when they think about Alzheimer’s. “Late-onset” means symptoms begin when a person is 65 years old or older.

The main symptom of late-onset Alzheimer’s is memory loss. It can start in the early stages with forgetting words or struggling to find the right one. Over time, people will forget names, details about themselves, and their life stories. As their cognitive function worsens, people may also experience personality changes and struggle with daily activities.

Alzheimer’s also affects:

• How, when, and for how long people can focus
• How people act and their mood
• How well people can plan
• How people perform basic tasks

Late-onset Alzheimer’s disease often begins slowly. Over time, people will have more and more symptoms that their family members and friends will notice.

Older age is the main risk factor for late-onset Alzheimer’s disease. Having a family history of late-onset Alzheimer’s is another risk factor, though it’s not a guarantee of developing the condition.

Scientists have identified certain genes that are linked with Alzheimer’s. The apolipoprotein E (APOE) e4 genetic variant is the main genetic risk factor associated with late-onset Alzheimer’s disease. Having two copies of this genetic variant increases the risk, but it’s important to understand that this gene is a risk factor and not a direct cause of Alzheimer’s. Having the gene does not mean a person will develop Alzheimer’s.

2. Early-Onset Alzheimer’s Disease

Early-onset Alzheimer’s disease, or younger-onset Alzheimer’s disease, affects people who are younger than 65. Many people affected by early-onset Alzheimer’s are in their 40s or 50s. Alzheimer’s symptoms disrupt their family and working lives. Early-onset Alzheimer’s is less common than the late-onset version, but it's not clear exactly how many people are affected by it.

While the APOE e4 variant can play a role, early-onset Alzheimer's is more commonly linked to other, more rare genetic variations (such as in the APP, PSEN1, and PSEN2 genes). Unlike the APOE e4 variant, these other genetic variations lead to familial Alzheimer's disease (FAD), which is a specific kind of early-onset Alzheimer’s.

Early-onset Alzheimer’s has the same set of symptoms as late-onset Alzheimer’s. People may struggle to remember, especially when it comes to new information they have recently learned. They may also have issues solving problems that used to be easy for them.

Getting diagnosed with early-onset Alzheimer’s disease can be a challenging process because healthcare providers aren’t looking for dementia in middle-aged people. If you’re concerned about early-onset Alzheimer’s, get a comprehensive health checkup from a specialist who understands the condition.

They’ll evaluate you for Alzheimer’s, as well as for other conditions that may explain your symptoms. If you can bring a list of symptoms that you or a loved one has experienced, this may help them make a diagnosis.

3. Familial Alzheimer’s Disease

Familial Alzheimer’s disease is caused by specific genetic changes separate from those associated with other types of Alzheimer’s. These changes cause less than 1 percent of the total number of Alzheimer’s cases. People with FAD start experiencing cognitive decline in their 30s or 40s.

FAD is considered a form of early-onset Alzheimer’s because it starts at a younger age, but it is unique because of its strong genetic cause and the pattern of inheritance from one’s parents. This is why genetic counseling and testing are so important for families with a history of FAD.

While only a few hundred families around the world seem to be affected by FAD, the inheritance patterns in these families are very strong. That’s because these genetic changes are dominant. If a parent has a mutation in one of these genes, their children have a 50 percent chance of inheriting that mutation. Having a single copy of one of these genetic variants is enough to develop familial Alzheimer’s disease.

If you’re concerned about FAD, talk to your doctor about genetic counseling and genetic testing.

4. Atypical Alzheimer’s Disease

For about 5 percent of people with Alzheimer’s, memory problems don’t appear until later, as the disease progresses. They will experience other behavioral changes first. When this happens, it’s called atypical Alzheimer’s disease.

There are different types of atypical Alzheimer’s. The two most common types are discussed below.

Frontal Variant Alzheimer’s Disease

In 1 out of 50 people diagnosed with Alzheimer’s, the disease will affect the frontal lobes first. This area of the brain isn’t usually affected until much later in the stages of Alzheimer’s disease. People diagnosed with frontal variant Alzheimer’s disease (FvAD) will experience changes to their behavior or problems with executive functioning before they experience memory issues.

Behavior changes may include suddenly changing their eating habits, losing all inhibitions, reacting inappropriately to other people’s feelings, losing motivation to do activities they love, or developing repetitive actions.

Executive functioning problems may include not being able to follow directions with a lot of steps, fixating on things or people, struggling with decision-making, getting distracted easily, or copying what other people do.

The symptoms of FvAD can often be mistaken for another condition called frontotemporal dementia, as both can cause early changes in personality and behavior. A skilled neurologist should perform a full workup to determine the underlying cause.

Posterior Cortical Atrophy

Posterior cortical atrophy (PCA) happens when the back, or posterior, part of the brain is damaged. Alzheimer’s is the most common cause of this damage, but other things can cause it, too. Most people with PCA start having symptoms when they’re between 55 and 65.

The first symptoms of PCA are usually visual. A person may have perfect eyesight, but they may no longer be able to judge distance. They might struggle to recognize objects and people they know or have problems knowing where their body is located in space. Over time, they’ll experience more of the common symptoms of Alzheimer’s disease, as outlined above.

It can take a long time to get an accurate diagnosis of PCA because people may assume they have a problem with their eyes rather than a change in their brain function. If you think you or your loved one might be living with PCA, ask your doctor about it as soon as you can.

Talk to Your Doctor

You should talk to your doctor if you’re concerned about your risk of dementia or if you notice early symptoms in yourself or a loved one. An accurate diagnosis is key to understanding what’s going on, getting access to different treatment options, and planning for the future.

Talk With Others Who Understand

On myALZteam, people share their experiences with Alzheimer’s, get advice, and find support from others who understand.

Have you or a loved one been diagnosed with a type of Alzheimer’s? Let others know in the comments below.