Though Parkinson's disease can't be definitively diagnosed until certain motor symptoms are present, some early warning signs are now well-understood to be connected to Parkinson's disease. These signs may crop up years before the classic motor issues, like a tremor.

We asked neurologists at the Johns Hopkins Parkinson's Disease and Movement Disorders Center to explain what these early warning signs are and their likelihood of leading to a diagnosis of Parkinson's disease.

How early can Parkinson's disease be diagnosed?

A: A true determination of Parkinson's disease is a clinical diagnosis, which means certain motor symptoms have to be present, but we now know more about some early signs of Parkinson's disease that, while they don't always lead to the condition, are connected.

In terms of how early we can detect, we can detect a mutation that is associated with an increased risk of Parkinson's as early as birth. In the minority of patients who may have a known Parkinson's-related genetic mutation (about 10 percent), that gene could be tested for at any time in life. At the same time, that's not diagnosing Parkinson's; it's just identifying the risk.

Early warning signs are what we call prodromal, or preclinical, symptoms. Prodromal symptoms are an early warning sign that someone might get Parkinson's disease. Though some of these symptoms have a very high probability of signaling future Parkinson's, having one or more of them is still not a 100 percent probability. Some prodromal symptoms are loss of sense of smell, REM behavior disorder, anxiety or depression, and constipation.

Do people actually lose their sense of smell with Parkinson's?

A: Yes. It's a condition called anosmia, and if you have it with no other disease (no other cause of a loss of sense of smell), you have at least a 50 percent chance of developing Parkinson's disease in the next five to 10 years. What happens is that alpha-synuclein, the protein that clumps in the part of the brain that regulates dopamine and leads to Parkinson's disease, also aggregates in the olfactory bulb, the part of the brain responsible for your sense of smell. This happens well before the protein accumulations cause motor symptoms.

What is REM behavior disorder, and how is it connected to Parkinson's?

A: REM behavior disorder is different than other sleep problems, like insomnia. People who have it may jerk or kick — it's as though they are acting out their dreams. In a similar pattern to anosmia, people with idiopathic (meaning there's no known cause for it) REM sleep behavior disorder have at least a 50 percent chance of eventually developing Parkinson's disease.

How is constipation an early warning sign of Parkinson's? It's such a common problem.

A: It's not as specific as other prodromal symptoms, like anosmia. The rate at which people with chronic and unexplained problems with constipation develop Parkinson's disease is not as easy to pin down. But if someone has unexplained, persistent constipation, it should at least be noted, as it could be considered prodromal.

Depression and anxiety are also early warning signs of Parkinson's. How so?

A: Like the other symptoms discussed here, late-onset depression and anxiety (without a history of these symptoms earlier in life) are nonmotor prodromal manifestations of the condition. It's not that everyone who is depressed will get Parkinson's, and the numbers are lower than they are for symptoms like anosmia and REM behavior disorder. But the link is important to explore, and we are doing more research on it all the time.