On 2015 Jun 10, David Keller commented:

Depression caused by early PD should respond to dopaminergic medication, not an SSRI or psychotherapy

Gustafsson and colleagues demonstrate an association between depression and a later diagnosis of Parkinson disease (PD). If a patient's depression is due to reduced brain dopamine levels caused by alpha-synucleinopathy involving the substantia nigra, then that patient's depression is probably a pre-motor symptom of their PD, rather than a psychological illness. Furthermore, any form of SSRI or psychotherapy should be futile to alleviate the depression caused by early PD; however, a therapeutic trial of levodopa or a dopamine agonist might be remarkably effective at relieving depression caused by low brain levels of dopamine. Dopaminergic therapy might also relieve a depressed patient's "psycho-motor retardation" if it is actually early Parkinsonian bradykinesia.

It might be difficult to reliably determine by clinical examination alone which depressed patients are actually exhibiting pre-motor PD, and a therapeutic trial of SSRI therapy should be avoided for these patients (since SSRI's can lower brain dopamine levels). Similarly, a therapeutic trial of dopaminergic medication in depressed patients who do not have early PD would be a waste of time if their psychiatric depression would respond better to SSRI therapy and / or psychotherapy.

What this all suggests is that scintigraphic dopamine transporter imaging of the brain (the "DaT scan") may be helpful when a depressed patient fails to respond to SSRI therapy and does not exhibit psychological depression. A DaT scan which diagnoses early pre-motor PD in such a patient could be useful if it discloses that the proper therapy for that patient's depression would be dopaminergic medication, rather than an SSRI and / or psychotherapy. If SPECT scanning lacks the spatial resolution to be helpful in identifying depression due to early PD, then perhaps PET imaging could be employed.

On 2015 Jun 10, David Keller commented:

Depression caused by early PD should respond to dopaminergic medication, not an SSRI or psychotherapy

Gustafsson and colleagues demonstrate an association between depression and a later diagnosis of Parkinson disease (PD). If a patient's depression is due to reduced brain dopamine levels caused by alpha-synucleinopathy involving the substantia nigra, then that patient's depression is probably a pre-motor symptom of their PD, rather than a psychological illness. Furthermore, any form of SSRI or psychotherapy should be futile to alleviate the depression caused by early PD; however, a therapeutic trial of levodopa or a dopamine agonist might be remarkably effective at relieving depression caused by low brain levels of dopamine. Dopaminergic therapy might also relieve a depressed patient's "psycho-motor retardation" if it is actually early Parkinsonian bradykinesia.

It might be difficult to reliably determine by clinical examination alone which depressed patients are actually exhibiting pre-motor PD, and a therapeutic trial of SSRI therapy should be avoided for these patients (since SSRI's can lower brain dopamine levels). Similarly, a therapeutic trial of dopaminergic medication in depressed patients who do not have early PD would be a waste of time if their psychiatric depression would respond better to SSRI therapy and / or psychotherapy.

What this all suggests is that scintigraphic dopamine transporter imaging of the brain (the "DaT scan") may be helpful when a depressed patient fails to respond to SSRI therapy and does not exhibit psychological depression. A DaT scan which diagnoses early pre-motor PD in such a patient could be useful if it discloses that the proper therapy for that patient's depression would be dopaminergic medication, rather than an SSRI and / or psychotherapy. If SPECT scanning lacks the spatial resolution to be helpful in identifying depression due to early PD, then perhaps PET imaging could be employed.