What Is Parkinson's Disease? Causes, Symptoms, and Treatment

What Is Parkinson's Disease?

Parkinson's disease (PD) is a progressive neurodegenerative disorder that primarily affects movement, caused by the loss of dopamine-producing neurons in a region of the brain called the substantia nigra. As these neurons die, the brain produces less dopamine — a neurotransmitter essential for coordinating smooth, purposeful movement — leading to the characteristic motor symptoms of Parkinson's.

Parkinson's affects approximately 1 million Americans and 10 million people worldwide, making it the second most common neurodegenerative disease after Alzheimer's. Incidence increases dramatically with age — most diagnoses occur after 60 — but approximately 10–15% of cases are "young-onset" (before age 50).

The Cardinal Motor Symptoms

Parkinson's is diagnosed clinically based on motor symptoms, at least two of the following four must be present:

• Tremor at rest: The most recognizable symptom — a rhythmic, "pill-rolling" tremor typically beginning in one hand, present at rest, and reduced with intentional movement. Absent in about 30% of patients.
• Bradykinesia (slowness of movement): Required for diagnosis. Everything moves slower — getting up from a chair, walking, turning over in bed, buttoning a shirt. The face may show reduced expression (hypomimia or "masked face").
• Rigidity: Stiffness of the muscles, causing resistance to passive movement. "Cogwheel rigidity" — the characteristic ratchet-like resistance felt when moving a PD patient's limb — is distinctive.
• Postural instability: Impaired balance and righting reflexes, appearing later in the disease. Increases fall risk significantly.

Non-Motor Symptoms

Parkinson's is far more than a movement disorder. Non-motor symptoms can precede motor symptoms by years and significantly impact quality of life:

• REM sleep behavior disorder (RBD): Acting out dreams during sleep — often the earliest symptom, appearing 10–20 years before motor symptoms. Strong predictor of future Parkinson's or Lewy body dementia.
• Constipation and GI dysfunction: The enteric nervous system (gut) is heavily affected; constipation often precedes motor symptoms by years.
• Anosmia (loss of smell): Frequently an early symptom
• Cognitive impairment and dementia: Mild cognitive impairment is common; Parkinson's dementia develops in up to 80% of patients over time
• Depression and anxiety: Extremely common; partly neurobiological (dopamine and serotonin dysfunction), partly reactive to diagnosis
• Autonomic dysfunction: Orthostatic hypotension (blood pressure drop when standing), urinary problems, excessive sweating

Causes: Alpha-Synuclein and Lewy Bodies

The pathological hallmark of Parkinson's is the accumulation of abnormally folded alpha-synuclein protein into aggregates called Lewy bodies within neurons. These aggregates spread through the nervous system in a predictable pattern (Braak staging), possibly explaining why non-motor symptoms precede motor symptoms — the pathology begins in the olfactory bulb and lower brainstem before reaching the substantia nigra.

Why alpha-synuclein misfolds and aggregates is incompletely understood. Genetics play a role: mutations in LRRK2, PARK7, SNCA, PINK1, and PRKN increase risk. Environmental factors — pesticide exposure (particularly paraquat and rotenone), head trauma, and gut microbiome alterations — are also implicated.

Treatment

Levodopa: The Gold Standard

Levodopa (combined with carbidopa to prevent peripheral conversion) is the most effective treatment — the brain converts levodopa to dopamine, partially replacing what's lost. It provides substantial motor symptom relief but doesn't slow disease progression. Over time, its benefits fluctuate ("wearing off" between doses) and can cause dyskinesias (involuntary movements).

Dopamine Agonists

Pramipexole, ropinirole, and rotigotine mimic dopamine's effects. Used alone or with levodopa; less effective but with different side effect profiles. Risk of impulse control disorders (gambling, hypersexuality) is notable.

Deep Brain Stimulation (DBS)

A surgical procedure implanting electrodes in the subthalamic nucleus or globus pallidus, connected to a pacemaker-like device. DBS dramatically reduces motor fluctuations and dyskinesias for carefully selected patients and is considered when medication management becomes inadequate.

Exercise

Vigorous aerobic exercise (cycling, boxing, treadmill with harness) has the strongest evidence base for slowing functional decline in Parkinson's. It's not a cure, but regular exercise is among the most important non-pharmacological interventions.