Parkinson’s Disease Is Not Only Tremor: Non-Motor Early Signs Such as RBD, Constipation and Loss of Smell, and Why to Start Rehabilitation Early

When Parkinson’s disease is mentioned, many people at once picture a hand that keeps shaking. Yet tremor is only the tip of the iceberg and is not even the earliest warning sign. What really appears first is often a set of non-motor symptoms that seem to have nothing to do with the nervous system, such as punching and kicking in sleep, years of constipation, being unable to smell chewing gum, or low mood for no reason. These early signs can appear10 to 20 years before the typical symptoms of tremor, slowness and stiffness. This article concentrates on the non-motor early signs before any tremor appears. For the four motor symptoms after diagnosis (tremor, rigidity, slowness of movement and postural instability, known by the acronym TRAP), seeParkinson’s disease symptoms and stages, and this page does not repeat them.

Why the public so often equates Parkinson’s disease with tremor

“Parkinson’s disease equals tremor” is a very widespread misunderstanding. Tremor is undoubtedly the most striking sign. It is visible and easily noticed by others, and over time it has become the public’s picture of Parkinson’s disease. Clinically, though, tremor is only one of the four motor symptoms, and many patients never have tremor as the main sign. They first have slowness of movement, muscle stiffness or an unsteady gait. In other words,having no tremor does not mean not having Parkinson’s disease, and tremor itself is far from the earliest sign.

Non-motor symptoms that can appear 10 to 20 years early

Medical research over the past decade or so has brought an important change in thinking: Parkinson’s disease does not form overnight but goes through a long prodromal phase. During this time the degeneration in the brain has quietly begun, but not yet seriously enough to cause tremor or slowness, and in its place some non-motor symptoms appear first.

A large case-control study in the UK using primary care records (Schrag, Lancet Neurology 2015) analysed data from 8,166 patients with Parkinson’s disease and 46,755 controls, and found that prodromal features such as constipation and tremor were already significantly more common than in controls 5 to 10 years before diagnosis. The International Parkinson and Movement Disorder Society (MDS) has also formally drawn up research diagnostic criteria for prodromal Parkinson’s disease (Heinzel, Mov Disord 2019), which list loss of smell and RBD among the prodromal markers with strong clinical evidence and a high weighting. These non-motor early signs are therefore not baseless and are early signals supported by the consensus of the international medical community.

The four main non-motor early signs

The four early signs below differ in the strength of their medical evidence and how closely they are linked. When reading, keep one general principle in mind: these early signs are highly sensitive but have low specificity. They are quite common in people with Parkinson’s disease, but also very common in the general public, so having just one of them usually does not mean Parkinson’s disease, and there is no need for undue alarm.

RBD: punching and kicking, shouting or falling out of bed in sleep

Among the four early signs, REM sleep behaviour disorder (RBD) is the most specific. In ordinary dreaming, the muscles of the body automatically go into a locked state, which stops the limbs from moving with the dream. In people with RBD this physiological mechanism fails, so after falling asleep they shout, punch and kick according to the content of the dream, and in severe cases fall out of bed or injure the person beside them by mistake.

A prospective study across 24 research centres following 1,280 patients with idiopathic RBD (Postuma, Brain 2019) found that after a diagnosis of RBD, about 6.3% each year converted to Parkinson’s disease, dementia with Lewy bodies or multiple system atrophy, and the 12-year cumulative conversion rate was as high as 73.5% or more. A meta-analysis combining 8 studies (Zhang, BMC Neurology 2017) found that the proportion of people with Parkinson’s disease who have RBD symptoms is 23.6%, far higher than 3.4% in healthy controls (pooled OR 5.69).

Long-standing constipation

Long-standing constipation is one of the early signs most often taken lightly in the prodromal phase of Parkinson’s disease, and it is closely linked to what the medical profession calls the gut-brain axis. A 24-year study following 6,790 men (Abbott, Neurology 2001) showed that people who had a bowel movement less than once a day had about4.5 times the risk of developing Parkinson’s disease later.

Loss or reduction of the sense of smell

Reduced sense of smell (hyposmia) or loss of smell (anosmia) is very common in people with Parkinson’s disease, and also the easiest to miss. Because not being able to smell is not as conspicuous as tremor, many patients never notice that their sense of smell is fading. A meta-analysis combining 12 studies (Bagherieh, Curr J Neurol 2023) found that the pooled prevalence of olfactory dysfunction in people with Parkinson’s disease is about64%, and in some studies it is over 90%. The prodromal criteria of the International Parkinson and Movement Disorder Society (MDS) also list loss of smell as a prodromal marker with a markedly raised positive likelihood ratio (Heinzel, 2019).

Depression and anxiety

Emotional problems can likewise appear many years before the motor symptoms, yet they are often put down simply to character or environmental stress. A meta-analysis (Broen, Mov Disord 2016) showed that the pooled prevalence of anxiety disorders in people with Parkinson’s disease is about31%. These ups and downs of mood come in part from degeneration of the brain’s chemical messengers, and the patient is not just overthinking or dwelling on things. The people around the patient should never dismiss it lightly with a remark to look on the bright side.

Local survey: over 80% of patients underestimated their risk before diagnosis

The public’s lack of awareness of early signs is especially marked in Hong Kong. A survey carried out under the HKU Jockey Club 「柏動愛」 Parkinson’s disease care programme, organised by the School of Nursing of the University of Hong Kong, the 行為健康教研中心 and the Hong Kong Society for Rehabilitation, found thatover 80% of people with Parkinson’s disease had thought before diagnosis that their chance of the disease was very small, and that among people without the disease, over 60% of respondents also underestimated the risk. The programme specifically pointed out four neglected early signs: punching and kicking in sleep (RBD), constipation, loss of smell and low mood, which agree with the focus of this article.

When to be alert and seek medical help

After describing several early signs, the most important thing is to avoid causing needless alarm. It is stressed again here: these non-motor signs are highly sensitive but have low specificity. Constipation, loss of smell, limb movements in sleep and low mood are all very common in the general public, and one symptom alone never means Parkinson’s disease will follow.

The difference between Parkinson’s disease and dementia

Many people hear the words brain degeneration and worry about becoming demented, so the difference between the two must be made clear. Parkinson’s disease mainly affects the patient’s ability to move, causing tremor, stiffness, slowness and balance difficulty, while most patients keep normal thinking and cognition for a long time. This differs in essence from dementia, which mainly causes decline in memory, judgement and logical thinking.

Pooled research data show that the rate of new dementia in people with Parkinson’s disease is about 4.5% a year (Gibson, Mov Disord 2024), and mostly falls on a minority of patients in the late stage of the disease and at older ages, so a diagnosis never means dementia. The public should not equate Parkinson’s disease with dementia.

Other related conditions: young-onset patients, postural hypotension and night-time urination

Besides the four main early signs above, Parkinson’s disease has some less familiar features, which are described only briefly below and not examined in depth:

What to do once early signs are noticed, and rehabilitation training

The aim of knowing these early signs is not to cause alarm but to prompt action: notice early and start rehabilitation early. International clinical guidelines advise that, early in the course, referral to a therapist with clinical experience in Parkinson’s disease can be considered, to help the patient build an exercise habit early and maintain the ability to be active, and home rehabilitation support is one practical arrangement. It must be explained, though, that the effects of exercise and rehabilitation training are mostly small to moderate, and some are only short term. Their core role is to delay disability and maintain function and not to cure or reverse the disease, so the key is consistency and enough training.

1. 1
   Notice the early signs

   Watch whether you or a family member has several non-motor early signs at the same time, especially RBD together with other signs.
2. 2
   Neurological assessment

   See a neurologist for a formal diagnosis. This step must be handled by the doctor and cannot be replaced by rehabilitation training.
3. 3
   Refer for rehabilitation early

   Whether or not the diagnosis has been made, the person can first have a functional assessment and listen to advice on exercise, to build an exercise habit early.
4. 4
   Continuing training in sufficient amount

   According to individual need, use PT, ST and OT services together, and slow the decline in physical function through continuing training.

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