If you or a loved one is living with Parkinson’s disease (PD), you’ve probably spent a lot of time researching the condition and trying to understand its causes, symptoms, and progression. But even the most detail-oriented search may not uncover every fact about Parkinson’s, because the disease affects people in so many different ways.

These six essential facts about Parkinson’s will help deepen your understanding of the condition. Ultimately, more knowledge can empower you to advocate for yourself or a loved one, helping ensure appropriate care and treatment.

1. Not Everyone With Parkinson’s Disease Experiences Tremor

Many people think of tremor, or involuntary shaking, as the most common symptom of Parkinson’s disease. Although it’s true that tremor is what often prompts people to seek a Parkinson’s diagnosis, not everyone with the condition experiences tremor.

Around 70 percent to 90 percent of people living with Parkinson’s disease will have tremor symptoms during the course of the disease, according to the Parkinson’s Foundation. In some cases, tremor occurs internally, which may feel a shaking sensation inside the body. This often occurs in the limbs, chest, or abdomen, and it isn’t visible from the outside.

2. Loss of Smell Can Be an Early Symptom

An early sign of Parkinson’s disease that’s often overlooked is a reduced sense of smell. Hyposmia (loss of smell) doesn’t affect everyone with Parkinson’s, but it can affect people long before movement symptoms develop — sometimes up to a decade beforehand.

People who lose their sense of smell without another identifiable cause have a 50 percent or greater chance of developing Parkinson’s within five to 10 years.

A changing sense of smell is a common topic on MyParkinsonsTeam. “Lost any decent sense of smell a while ago,” one member said. Another noted, “Sometimes I can’t smell anything — or everything smells the same.”

People who lose their sense of smell without another identifiable cause appear to have a 50 percent or greater chance of developing Parkinson’s within five to 10 years.

3. Parkinson’s Can Affect Younger People

Aging is considered the main risk factor for Parkinson’s — the average age at diagnosis is 60. However, in rare cases, the disease affects people who are younger than 40. Anyone diagnosed below 50 is classified as having young-onset Parkinson’s disease, which makes up about 10 percent to 20 percent of all Parkinson’s cases.

In people with young-onset Parkison’s, the disease usually progresses more slowly than in typical Parkinson’s, and some symptoms can be different. Dystonia (muscle stiffness or cramping) and dyskinesia (involuntary body movements) are common early symptoms in younger people. However, cognitive impairment, such as memory loss and confusion, and trouble with balance tend to be less common in younger people with Parkinson’s.

4. Parkinson’s Is More Common in Men

The Parkinson’s Foundation states that men are 1.5 times more likely than women to develop Parkinson’s disease. The disease also tends to affect males and females differently, according to a styudy in the Journal of Parkinson’s Disease. In females, motor symptoms tend to occur later, and one of the earliest symptoms is tremor, according to the study. The authors report that females have a higher risk of postural instability (problems with balance and staying upright) and frequent falls. In males, posture problems and worsening cognitive abilities are more common, per the study.

Men are 1.5 times more likely than women to develop Parkinson’s disease, according to the Parkinson’s Foundation.

Studies continue to explore potential reasons for these differences. Researchers believe that hormones, genetics (inherited traits), or environmental factors may play a role.

5. Inheriting Parkinson’s Is Unlikely

Although some people with Parkinson’s disease have a family history of the condition, most do not. Parkinson’s is attributed to inherited genes in about 10 percent to 15 percent of cases. Scientists believe Parkinson’s results from a combination of genetic and environmental factors, but they haven’t yet pinpointed an exact cause of Parkinson’s.

MyParkinsonsTeam members frequently discuss concerns about family members who have a medical history of Parkinson’s disease. “I have a sister, two cousins, and a trio of aunts who had PD,” one member wrote. Another said, “My husband and his brother both developed it at the same time. He hasn’t had genetic testing to see if that was the cause or not, but it seems like a possibility.”

You can have genetic testing done to see if you have a mutation (change) in a gene associated with Parkinson’s. However, even if your test results show the mutation, your chances of developing Parkinson’s are still low based on a number of factors. For instance, you may have other genes that protect you from the disease. Researchers are still studying the specific role genes play in the development of Parkinson’s.

6. Parkinson’s Can Cause Psychosis

Delusions and hallucinations — the two main types of Parkinson’s psychosis — occur in 20 percent to 40 percent of people with the disease. Psychosis causes nonmotor symptoms that affect the mind and can increase in the disease’s later stages.

Hallucinations are perceptions of things that aren’t real and can involve vision, hearing, smell, taste, or physical sensations. Delusions are irrational beliefs or thoughts that can cause strong feelings such as jealousy, paranoia, agitation, or anxiety. Psychosis can cause behavioral changes and severely affect quality of life and daily activities for people living with Parkinson’s and their caregivers.

Delusions and hallucinations — the two main types of Parkinson’s psychosis — affect 20 percent to 40 percent of people with the disease.

Psychosis in Parkinson’s can occur because of side effects from medications, changes in brain cells as the disease progresses, or delirium (sudden confusion or unawareness of surroundings) caused by an infection or other health problems. Treatment may involve adjusting medications that boost dopamine to control movement disorders and, if needed, prescribing antipsychotic drugs to manage more severe symptoms.

“My husband has PD dementia. He suffers from delusions and some paranoia. The past week or so has been difficult. He believes the delusion so strongly that it keeps him stressed. At times he won’t eat, and he is having trouble sleeping,” one MyParkinsonsTeam member shared.

“I had hallucinations years ago with amantadine, but the doctor reduced the dosage and they stopped,” another member wrote.

Always work carefully with your doctor before making medication changes. Regular communication with your healthcare provider ensures that your treatment plan is tailored to your needs.

Talk With Others Who Understand

MyParkinsonsTeam is the social network for people with Parkinson’s disease and their loved ones. On MyParkinsonsTeam, more than 106,000 members come together to ask questions, give advice, and share their stories with others who understand life with Parkinson’s.

What Parkinson’s facts do you find the most interesting? Share your experience in the comments below, or start a conversation by posting on your Activities page.