Depression and anxiety are twice as common in people who have recently been diagnosed with Parkinson's disease compared with the general population, according to a new US study.

While Parkinson's patients are often prone to depression, it is often left untreated in these patients.

Researchers from the University of Pennsylvania studied 423 newly-diagnosed Parkinson's patients and monitored the state of their mental health over a period of two years.

At the start of the study, it was found that 14% of patients with Parkinson's had depression, compared with 6.6% of a group of volunteers in the study who did not have the disease.

When the study participants were followed up, there was a small increase in the frequency and seriousness of depression in the patients with Parkinson's, while in the non-Parkinson's group a decrease was recorded.

At the start of the study, 16% of patients with Parkinson's were taking an antidepressant drug, and this increased  to one-in four of the Parkinson's patients two years later.

But significantly, it was found that around 70% of Parkinson's patients with depression were still not being treated with medication, highlighting the under-recognition and under-treatment of depression in people with Parkinson's disease.

Experts also believe that depression can in some cases be a signal of Parkinson's developing at a later stage.

The research is published in the journal Neurology.