This story has been updated.

Comedian Robin Williams was grappling with severe depression when he committed suicide Monday, and on Thursday we learned that he also was in the early stages of Parkinson's disease. Sadly, the two conditions are often found together.

In a 2012 study conducted by the National Parkinson Foundation, 61 percent of 5,557 Parkinson's patients surveyed reported that they also suffered from depression, with symptoms that ranged from mild to severe.

Both conditions are associated with a shortage of dopamine, a neurotransmitter that helps regulate movement and control the brain's pleasure center.

"Dopamine is a feel-good chemical. If you are low in dopamine, you are not going to feel so good," said Joyce Oberdorf, president and CEO of the National Parkinson Foundation. "There are [also] other neurotransmitters that can be low."

A separate study published Friday found that newly-diagnosed Parkinson's patients have higher rates of depression, anxiety, fatigue, and apathy than a control group of people without Parkinson's. Researchers from the Raymond and Ruth Perelman School of Medicine at the University of Pennsylvania found that 13.9 percent of patients had symptoms of depression when they were diagnosed with Parkinson's, a proportion that rose to 18.7 percent after 24 months. Just 6.6 percent of people without the disease had depression, and that dropped to just 2.4 percent after 24 months.

Despite their depressive symptoms, most of the Parkinson's patients who also had that condition were not treated with anti-depressants at any point in the two-year study. The findings were published in the journal Neurology.

Parkinson's is an incurable, progressive, neurological disease that afflicts about 1 million people in the United States. It affects the motor system, leaving victims with tremors, a stiff or awkward gait and repetitive, involuntary muscle movements known as dyskinesias.

In its early stages, Parkinson's is manageable through a variety of medications that increase the supply of dopamine, keep it circulating in the brain longer and make it work better, Oberdorf said. Quality of life is often high for the first five to seven years of the disease, although it varies among individual sufferers, she said.

Later, as the disease progresses, the ability to make dopamine and neurological receptors for the chemical declines, and symptoms often worsen. People can live for decades with Parkinson's, and the progress of the disease varies, Oberdorf said.

No one is entirely sure what causes the disease, but the current belief is that it is a combination of genetic susceptibility and environmental causes such as head trauma and exposure to pesticides. Deaths from Parkinson's usually are caused by falls or pneumonia, as the ability to swallow diminishes, Oberdof said.

Williams has been open about his battle with drugs and alcohol but wasn't ready to share his Parkinson's diagnosis publicly, his wife, Susan Schneider, said in a statement she released Thursday. Actor Michael J. Fox suffers from the disease and has started a foundation to raise money for research.