Understanding Parkinson's Disease: A Comprehensive Patient Guide

Table of Contents

• Epidemiology: Who Gets Parkinson's Disease?
• What Exactly Is Parkinson's Disease?
• Causes and Risk Factors
• Disease Progression and Symptoms
• Treatment Approaches
• Medications for Motor Symptoms
• Managing Non-Motor Symptoms
• Surgical Options: Deep Brain Stimulation
• Key Takeaways for Patients
• Source Information

Epidemiology: Who Gets Parkinson's Disease?

Parkinson's disease incidence and prevalence increase significantly with age. Research shows that men are approximately twice as likely to develop Parkinson's as women. The incidence rates range from 47 to 77 cases per 100,000 people aged 45 or older, and dramatically increase to 108-212 cases per 100,000 people aged 65 or older.

Studies have found that White persons generally have higher incidence rates than Black or Asian persons. However, autopsy studies show similar frequencies of Lewy bodies (the hallmark protein clumps found in Parkinson's) among Black and White persons, suggesting diagnostic disparities rather than biological differences. The overall prevalence is approximately 572 cases per 100,000 people aged 45 or older.

Parkinson's carries significant health consequences, with age and sex-adjusted mortality estimated at approximately 60% higher than the general population. The economic burden in the United States alone is projected to increase from $52 billion in 2017 to $79 billion in 2037, reflecting the growing impact of this condition on healthcare systems and families.

What Exactly Is Parkinson's Disease?

For two centuries, Parkinson's disease has been diagnosed clinically based on the characteristic motor syndrome featuring bradykinesia (slowness of movement) accompanied by resting tremor, rigidity, and postural reflex impairment. These symptoms largely result from dopaminergic dysfunction in the nigrostriatal system, which controls movement.

However, Parkinson's is now recognized as a multisystem neurologic disorder that extends far beyond movement problems. Non-motor symptoms include:

• Sleep disorders, particularly REM sleep behavior disorder where patients physically act out dreams
• Cognitive impairment and eventually dementia in many patients
• Mood changes including depression and anxiety
• Autonomic dysfunction causing constipation, urinary problems, and orthostatic hypotension (blood pressure drops when standing)
• Sensory symptoms including hyposmia (reduced sense of smell) and pain

These non-motor symptoms often precede the onset of motor symptoms by many years, suggesting they may represent the prodromal (early pre-motor) phase of the disease. The International Parkinson and Movement Disorder Society has established clinical diagnostic criteria and research criteria for identifying this prodromal phase.

At autopsy, up to 90% of clinically diagnosed Parkinson's cases show intraneuronal accumulation of misfolded α-synuclein protein (called Lewy bodies and Lewy neurites). This pathology selectively affects specific brain regions including brain-stem nuclei, the peripheral autonomic nervous system, and limbic and cortical regions. The loss of dopamine-producing substantia nigra neurons is another characteristic feature.

Causes and Risk Factors

Parkinson's disease results from multiple causes involving both genetic and non-genetic factors. Genetic variants with large effects have been identified in approximately 20% of Parkinson's cases. These include:

Autosomal dominant Parkinson's with incomplete penetrance includes variants in: LRRK2 (present in 1-2% of all cases and up to 40% of familial cases) GBA1 (encoding glucocerebrosidase, present in 5-15% of cases, most common in Ashkenazi Jewish or North African ancestry populations) VPS35 and SNCA (present in less than 1% of cases)

Recessively inherited Parkinson's variants include PRKN, PINK1, and DJ1, which account for most cases that develop at a young age. These recessive variants tend to have fewer non-motor features and more prominent dystonia than typical Parkinson's.

For people without strong genetic risk factors, heritability is estimated at 20-30%, suggesting significant contributions from environmental factors. Key environmental risk factors include:

• Residential or occupational exposure to pesticides (paraquat, rotenone, 2,4-dichlorophenoxyacetic acid, organochlorines, organophosphates) or chlorinated solvents (trichloroethylene, perchloroethylene) - associated with 40% or greater increased risk
• High consumption of dairy products associated with increased risk, possibly due to bioconcentration of pesticides like heptachlor in milk
• Mild-to-moderate head injury associated with 31% to over 400% increased risk decades later
• Less consistent associations with metal exposure, type 2 diabetes, certain inflammatory disorders, and infections

Protective factors that appear to decrease Parkinson's risk include cigarette smoking, caffeine consumption, and increased physical activity. Most researchers believe that combinations of genetic susceptibility and environmental exposures rather than single factors determine individual risk.

Disease Progression and Symptoms

Parkinson's disease typically begins with motor symptoms that are often asymmetrical - affecting one side of the body more than the other. Patients initially experience slowness of movement and tremor that eventually progress to bilateral bradykinesia, rigidity, tremor, and gait and balance disorders.

The time course of progression varies significantly between individuals. Eventually, these symptoms lead to functional impairment and loss of independence, often from the combined effects of motor and cognitive decline, falls, and fractures.

Non-motor symptoms frequently precede motor problems by decades. These include:

• Hyposmia (reduced sense of smell)
• Autonomic dysfunction including constipation, urinary issues, and orthostatic hypotension
• REM sleep behavior disorder where patients act out dreams
• Cognitive changes such as visuospatial or executive dysfunction

Cognitive decline associated with mild cognitive impairment or Parkinson's disease dementia develops in approximately 10% of patients annually. About 38% of clinically diagnosed Parkinson's cases and 89% of dementia with Lewy bodies cases also have Alzheimer's-associated pathological features, indicating frequent overlap between these neurodegenerative conditions.

Treatment Approaches

Regular exercise, a healthy diet, high-quality sleep, and avoidance of adverse exposures provide a foundation for managing Parkinson's at any stage and have been associated with reduced mortality. Unfortunately, no available pharmacologic therapy has definitively been shown to slow the progression of Parkinson's disease despite nearly four decades of clinical trials.

Researchers believe that interventions before motor symptoms occur, or when only biomarker evidence of disease is present, may increase the potential for neuroprotection. Current research is exploring treatments targeting genetically defined subpopulations with variants in GBA1 or LRRK2, as well as investigating glucagon-like peptide receptor agonists.

Symptom management must be individualized since Parkinson's manifests and evolves differently in each person. A multidisciplinary approach is ideal, involving a neurologist, mental health professional, neurosurgeon, and physical, occupational, and speech therapists. The needs of patients, family members, and caregivers should be reassessed regularly, including advanced care planning and, in severe cases, referral to hospice care.

Medications for Motor Symptoms

Oral formulations of levodopa remain the main treatment for motor symptoms, although tremor may be less responsive than bradykinesia and rigidity in some patients. If patients don't respond to levodopa, the diagnosis may need to be reconsidered.

The duration of effect after a levodopa dose (called "on" time) typically lasts several hours but begins to shorten after an average of 4 years. This leads to motor fluctuations where "on" time is interspersed with periods of reduced symptomatic benefit ("off" time). Strategies to address these fluctuations include:

• Increasing the overall dose or frequency of administration
• Switching to extended-release formulations
• Adding catechol-O-methyltransferase (COMT) inhibitors or monoamine oxidase B (MAO-B) inhibitors
• Using non-dopaminergic medications like amantadine and istradefylline

Common side effects of levodopa include dyskinesia (involuntary movements), hallucinations, behavioral problems, orthostatic hypotension, and nausea. Dopamine agonists are used less commonly now due to their unfavorable side-effect profile including nausea, somnolence, sleep attacks, impulse-control disorders, and peripheral edema.

For severe or frequent "off" episodes, on-demand strategies include subcutaneous injection or sublingual apomorphine and inhaled levodopa. Advanced cases may use continuous enteral delivery of levodopa through an intrajejunal pump or subcutaneous delivery systems.

Managing Non-Motor Symptoms

Non-motor symptoms contribute greatly to the disease burden but have limited evidence-based treatment guidelines. Parkinson's-related dementia may respond modestly to acetylcholinesterase inhibitors or memantine, with only rivastigmine classified as clinically useful based on evidence-based reviews.

Depression and anxiety can be treated with selective serotonin-reuptake inhibitors, serotonin-norepinephrine reuptake inhibitors, or less commonly, dopamine agonists, with careful attention to drug interactions. Autonomic symptom management remains a key unmet need, with various approaches used for specific symptoms:

• Drooling managed with sublingual atropine drops or salivary-gland botulinum toxin injections
• Constipation managed with increased dietary fiber, stool softeners, or laxatives
• Sleep disorders potentially helped with cognitive behavioral therapy, melatonin, or low-dose clonazepam

Surgical Options: Deep Brain Stimulation

Deep-brain stimulation (DBS) therapy involves intracranial placement of thin leads into either the subthalamic nucleus or globus pallidus regions of the brain. These leads connect to a neurostimulator placed under the skin in the subclavicular region, delivering electrical stimulation that can be adjusted to target individual symptoms over time.

DBS improves quality of life and alleviates motor fluctuations, providing an average increase of 3-4 hours of "on" time per day. Candidates for DBS typically have motor fluctuations that are poorly controlled by medication. The procedure, patient selection, and ongoing device management are typically performed at specialized centers with expertise in movement disorders.

Key Takeaways for Patients

Parkinson's disease is a complex neurological disorder that affects much more than movement. Understanding that non-motor symptoms often appear years before movement problems can lead to earlier diagnosis and intervention. While no treatment currently slows disease progression, many strategies can effectively manage symptoms and improve quality of life.

Patients should work with a multidisciplinary team to develop individualized treatment plans that address both motor and non-motor symptoms. Staying informed about new research developments, particularly those targeting specific genetic subtypes, may provide future opportunities for more personalized treatment approaches.

Most importantly, maintaining regular exercise, a healthy diet, good sleep habits, and avoiding known risk factors can help manage symptoms and potentially improve outcomes throughout the disease course.

Source Information

Original Article: "Parkinson's Disease" by Caroline M. Tanner, M.D., Ph.D., and Jill L. Ostrem, M.D.

Publication: The New England Journal of Medicine, 2024;391:442-52

DOI: 10.1056/NEJMra2401857

This patient-friendly article is based on peer-reviewed research from leading Parkinson's disease experts at the University of California, San Francisco. The information has been comprehensively translated from the original scientific review while preserving all factual content, statistical data, and clinical recommendations.