Parkinson's disease is a progressive neurogenerative disease that causes nerve cells (or neurons) in the area of the brain that controls movement to weaken and/or die. While healthy neurons produce a chemical called dopamine, which the brain needs a certain amount of in order to regulate movement, weakened neurons produce lower levels of dopamine. What causes these neurons to weaken is currently unknown.

Some patients with Parkinson's disease also suffer from a decline in norepinephrine, a chemical that transmits signals across nerve endings and controls various functions, such as blood pressure and heart rate.

More than 10 million people worldwide are currently living with Parkinson's disease and nearly one million will be living with the disease in the United States this year, according to the Parkinson's Foundation.

Risk factors

One's risk of developing PD can depend on the following factors:

• Gender. Men are 1.5 times more likely to have PD than women.
• Age. The risk of PD increases with age, although some people are diagnosed with early-onset PD before the age of 50.
• Genetics. Most cases occur in patients with no familial link to PD, but some have an inheritance pattern involving certain altered genes that could increase the risk of developing PD.
• Environmental causes. Studies have shown a link between exposure to chemicals used in pesticides and herbicides—as well as metals and organic pollutants—and Parkinson's disease.
• Head injury. Repeated blows to the head can increase one's risk of developing PD, however, it doesn't guarantee a diagnosis.

Symptoms of Parkinson's disease

These common symptoms of Parkinson's disease often begin gradually and progress over time:

• Shaking or tremor
• Rigid muscles
• Difficulty walking
• Unsteady balance
• Poor posture
• Slowing of body movements (bradykinesia)

As the disease continues to progress, additional symptoms can occur such as slurred or soft speech, trouble chewing and/or swallowing, memory loss, constipation, trouble sleeping, loss of bladder control, anxiety, depression, inability to regulate body temperature, sexual dysfunction, decreased ability to smell, restless legs and muscle cramps.

Diagnosis

There are currently no blood or lab tests that can be used to diagnose Parkinson's disease. Neurologists make a diagnosis based on their patient's medical history and an examination.

According to the Parkinson's Foundation, at least two of the four following symptoms must be present over a period of time for a neurologist to consider Parkinson's disease as a diagnosis:

• Shaking or tremor
• Bradykinesia
• Stiffness in arms, legs and/or trunk
• Balance issues

There are a number of conditions that closely mimic Parkinson's disease, such as:

• Essential tremor. Essential tremor is an action tremor, meaning that the involuntary shaking increases when you move and try to use your hands. In Parkinson's disease, tremors occur mainly at rest, and activity reduces the symptoms.
• Normal pressure hydrocephalus. This condition, seen primarily in older people, happens when there is an excess of the fluid surrounding the brain and spinal cord. The classic symptoms are an unsteady gait, problems with memory and thinking, and urinary incontinence.
• Dementia with Lewy bodies. Symptoms seen early on in PD can also develop during the late stages of dementia with Lewy bodies, a disease that causes problems with mood, thinking and movement and is characterized by abnormal protein deposits called Lewy bodies in the brain.
• Multiple system atrophy. A progressive neurogenerative disorder, multiple system atrophy involves the death of different types of nerve cells in the brain and spinal cord. Symptoms include slowness of movement, tremor, stiffness, and impaired speech.
• Corticobasal syndrome. CBS is a condition that affects movement, language or both. The cause is currently unknown, but some people with CBS have a build-up of tau in their brains similar to that of Alzheimer's disease patients.
• Progressive Supranuclear Palsy. This rare condition, which damages nerve cells in the brain, can cause loss of balance, eye problems, unexplained falls, and stiffness.

Parkinson's disease treatment

Although there is no cure for Parkinson's disease, neurologists can recommend various medications, surgeries and/or therapies to relieve symptoms.

Medication. The most common medications used to treat Parkinson's disease are formulated to help maintain, replenish or mimic dopamine and other chemicals in the brain:

• Levodopa replenishes the brain's decreasing supply of dopamine,
• Carbidopa is used in combination with levodopa (e.g. Sinemet) to help reduce the side effects associated with levodopa, such as nausea, low blood pressure, and restlessness.
• Dopamine Agonists mimic the effects of dopamine. They can be used to delay starting levodopa or can be used in addition to it.
• Amantadine can boost levels of dopamine that are already present in the brain.
• Anticholinergic medications are prescribed to help reduce tremor.
• Catechol-O-methyltransferase (COMT) inhibitors block enzymes that break down dopamine and thereby prolong dopamine's action in the brain.
• Monoamine Oxidase (MAO) B Inhibitors also blocks an enzyme that breaks down dopamine and allows it to function for a longer period.
• Adenosine Receptor Antagonist may keep muscles working more normally in what are known as "off" periods when patients are taking levodopa/carbidopa.

Finding the right combination of PD medications can take time in order to find what works best with the least amount of side effects, which can include nausea, low blood pressure, dizziness, constipation, insomnia, hallucinations, and dyskinesia (uncontrolled body movements). Your doctor may prescribe additional medications to relieve some of these side effects or to relieve the non-motor symptoms commonly associated with Parkinson's disease.

Deep Brain Stimulation. Typically reserved for those who don't respond well enough to medication, deep brain stimulation is a surgical procedure where thin metal wires are placed in the brain and programmed to send electrical pulses that aid in controlling motor symptoms. In general, it is considered for patients who have had PD for four years or more and it is not recommended for patients with dementia.

Focused Ultrasound. A non-invasive option that is FDA-approved to treat tremor-predominant Parkinson's disease as well as essential tremor. Focused ultrasound for tremor destroys brain cells that cause motor problems. Unlike deep brain stimulation, focused ultrasound therapy is permanent and irreversible. It is also being tested for other PD symptoms.

Other Therapies. Sports and exercise regimens, such as walking, non-contact boxing, biking, yoga and tai chi, have been shown to improve balance, motor control, and strength in PD patients. Physical and occupational therapy can also aid in improving gait, flexibility, speech and the ability to perform everyday activities, such as eating and dressing.

Image: MarianVejcik/Getty Images

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