The estimated prevalence of depression in PD varies according to the type of mood assessment applied (interview versus self-report questionnaire), diagnostic criteria or definition of depression (the number and type of symptoms required), and research setting (community-based versus neurology clinic).^1,2 However, overall the data suggest that at any given time 20–40% of individuals with PD are experiencing depression of some type. This is a higher rate than that found in the general population.^3–5 Depression can be difficult to diagnose in PD because of the overlap between symptoms of depression and PD. For example, the biological symptoms typical of depression—such as low energy, insomnia or excessive sleep, weight loss, diminished sexual function, and an emotionless face—can be directly related to the neuroanatomical disruption characteristic of PD. These symptoms are not necessarily evidence of depression and, conversely, the psychomotor slowing of depression might be accidentally overlooked in a patient with PD.^2,6

Depression in PD could be considered an understandable reaction to a disabling chronic illness. However, some researchers believe that depression may be a part of the disease and caused by neurological changes.^2,7 This notion is supported by the fact that depression sometimes precedes the diagnosis of PD. Of course, the cause of depression in patients with PD could be a combination of a subjective reaction to the illness and the brain changes brought about by the disease.⁵ The consensus is that depressive symptoms should be addressed and treated by physicians regardless of whether these symptoms are part of PD itself or due to a separate cause.^1,8,9

The psychiatric complications of PD require attention because they can exacerbate the already considerable physical challenges brought on by the disease. In fact, an international survey of patients with PD discovered that depressive symptoms were the most important factor in patient quality-of-life ratings. ^10,11 Another reason to treat depression in PD is that reports of care-giver burden correlate significantly with patients’ depression and quality of life.¹²

It has been recommended that optimal anti-Parkinson symptom treatment should be the first step in the treatment of PD. It is notable that some PD medications may have an antidepressant effect of their own. The antidepressant effect of dopamine agonists such as pramipexole is probably due to stimulation of D3 dopamine receptors, whereas the drug’s effect on PD symptoms is related to the D2 dopamine receptor.³ Dopamine agonists may be helpful, in particular for patients who experience on–off motor fluctuations, with their depressive symptoms being related to the ‘off’ periods. When the use of an antidepressant is considered in a patient with PD, its potential adverse effects and interactions with PD drugs must be weighed against the effects of the depression itself.³ Although it should be noted that the majority of the studies did not include a placebo control, there is evidence that antidepressant medications can have a significantly positive effect on depression in PD.^1,2,6 There are different classes of antidepressant medication. The selective serotonin re-uptake inhibitors (SSRIs) (e.g. Zoloft^®, Prozac^®, Paxil^®, and Celexa^®) are prescribed most often in PD patients with depression. In general, the SSRIs are safer and better tolerated by patients than the tricyclic antidepressants (TCAs) (e.g. Elavil^®, Tofranil^®, and Pamelor^®). For example, the SSRIs have fewer cardiac and cognitive adverse effects and, in addition, they can effectively treat anxiety and pain, which are also common in patients with PD.^2,3,8