Two of the most important early signs of Parkinson’s disease now recognized by researchers are loss of smell and a sleep disorder called REM sleep behavior disorder (RBD). Both can appear years, sometimes over a decade, before the tremor and stiffness most people associate with the disease. The prodromal phase of Parkinson’s, the period before movement symptoms show up, can last up to 20 years. During that window, the brain is already changing, and these two signs are among the earliest red flags.
Loss of Smell
A reduced or absent sense of smell, known as hyposmia, is one of the strongest early indicators of Parkinson’s. In more than 90% of people eventually diagnosed with Parkinson’s, smell problems appear before any motor symptoms do. This isn’t the temporary loss you’d get from a cold or sinus infection. It’s a gradual, persistent fading of the ability to identify or detect odors, and many people don’t even notice it happening.
The reason smell goes first has to do with where the disease starts in the brain. Parkinson’s involves the buildup of a misfolded protein that damages and kills nerve cells. According to a widely accepted model of how the disease progresses, the earliest damage occurs in the olfactory bulb, the brain’s smell-processing center. As the abnormal protein accumulates there, it triggers a cycle of inflammation and further protein buildup that destroys neurons. This happens long before the protein reaches the deeper brain areas responsible for movement.
Smell loss can precede a Parkinson’s diagnosis by as long as 10 years. Standardized scratch-and-sniff tests exist that can measure your ability to identify odors. One commonly used version picks up smell deficits in about 82% of people with Parkinson’s, though it also flags some people who don’t have the disease (specificity around 66%). That means a smell test alone isn’t enough for a diagnosis, but a persistently poor score, especially combined with other risk factors, is worth paying attention to.
REM Sleep Behavior Disorder
REM sleep behavior disorder is a condition where you physically act out your dreams. During normal REM sleep, your body is essentially paralyzed so you don’t move while dreaming. In RBD, that paralysis fails. People kick, punch, shout, or thrash during vivid dreams, sometimes injuring themselves or their bed partners. It’s different from occasional restless sleep or sleepwalking. RBD episodes are dramatic and often violent, and the person typically remembers the dream content.
RBD is now considered one of the most powerful predictors of Parkinson’s and related diseases. More than 80% of people diagnosed with RBD eventually develop a neurodegenerative condition, most commonly Parkinson’s or a closely related form of dementia. In one long-term study, the average duration of RBD symptoms before a baseline assessment was about 9 years, and many participants went on to develop Parkinson’s during follow-up. RBD, like smell loss, can appear a full decade before motor symptoms emerge.
The biological explanation follows the same pattern as smell loss, just one step deeper in the brain. After damaging the olfactory bulb, the misfolded protein progresses into the lower brainstem, where it disrupts the circuits that control muscle paralysis during sleep. This is why RBD often appears after smell loss in the sequence of early symptoms, though both can show up well before tremor or slowness of movement.
Other Early Warning Signs
Smell loss and RBD get the most attention, but they aren’t the only non-motor symptoms that appear early. Constipation is another significant one. A meta-analysis combining data from over 740,000 participants found that people with constipation had 2.27 times the odds of later developing Parkinson’s compared to people without it. That association held even when constipation was assessed more than 10 years before diagnosis. The cause is similar: the misfolded protein damages nerve cells in the gut wall and the vagus nerve, slowing intestinal movement.
Other commonly reported early symptoms include urinary urgency, excessive saliva, anxiety, and forgetfulness. People with early Parkinson’s report a higher total number of these non-motor symptoms compared to healthy individuals of the same age. No single symptom confirms the disease, but a cluster of them, especially smell loss plus RBD plus constipation, paints a much clearer picture.
Why These Signs Matter Now
For most of Parkinson’s history, diagnosis only happened after movement symptoms appeared. By that point, a substantial portion of the brain’s dopamine-producing cells are already lost. Recognizing that the disease has a long pre-motor phase has shifted the focus toward catching it earlier, when future treatments might be able to slow or prevent progression.
One promising development is a skin biopsy test. Researchers have shown that small 3-millimeter skin samples taken from the neck, knee, and ankle can detect the same abnormal protein that builds up in the brains of Parkinson’s patients. In a study of more than 400 participants, the test identified 93% of people with clinically confirmed Parkinson’s, while only 3% of healthy controls tested positive. The levels of the protein in skin samples also correlated with how severe the disease was. This kind of test could eventually allow doctors to confirm what early symptoms like smell loss and RBD are suggesting.
Retinal thinning detected through eye scans is another area of active investigation. People with Parkinson’s show thinning in specific layers of the retina, and the same misfolded protein has been found in inner retinal tissue. However, these changes overlap with normal aging and other eye conditions, so retinal scans aren’t reliable enough on their own to serve as a diagnostic tool yet.
What to Do if You Notice These Signs
If you’ve noticed a persistent change in your sense of smell that isn’t explained by allergies, a cold, or COVID, or if a bed partner has told you that you act out your dreams violently during sleep, these are worth bringing up with a neurologist. Neither symptom guarantees you’ll develop Parkinson’s. Smell loss has many causes, and RBD can occasionally occur in isolation. But in combination with each other or with symptoms like chronic constipation, they form a recognizable pattern.
A neurologist can assess your symptoms using standardized smell tests, sleep studies to confirm RBD, and brain imaging such as a dopamine transporter scan to check whether the dopamine system is already affected. For people under 50, or those with multiple close relatives who have Parkinson’s, genetic testing for specific gene variants is also an option. Early identification won’t change the disease’s course with current treatments, but it opens the door to clinical trials, lifestyle planning, and being first in line as new therapies become available.