There is no single blood or imaging test that can definitively diagnose Parkinson’s disease. In most cases, the diagnosis is made on the basis of classic symptoms, a typical medical history and characteristic findings on physical exam. However, in some individuals, the diagnosis is more challenging. Patients who present only with a tremor may have Parkinson’s disease or may have essential tremor, a completely different disorder. Some individuals may have signs and symptoms of parkinsonism that are actually secondary to other conditions, including multiple small brain infarcts (strokes) or exposure to certain drugs. In situations like these, where the physical exam or symptoms may be atypical for Parkinson’s disease, an imaging test called a DaTscan may help to either confirm the diagnosis of Parkinson’s or make it unlikely. This information can play a critical role getting individuals with movement disorders the appropriate treatment, particularly early in the course of their disease.
What is a DaTscan?
DaTscan is the trade name for Ioflupane I-123, which is a radiotracer used in a nuclear medicine imaging test called brain SPECT. DaTscan binds to the presynaptic dopamine active transporter (DAT) on neurons that communicate with areas controlling movement, including the striatum. The hallmark of Parkinson’s disease is the loss of these neurons and a decrease in dopamine; when these neurons die, there are fewer dopamine transporters, leading to decreased activity on the brain scan.
For the classic motor symptoms of Parkinson’s to be present, typically 50% or more of these neurons must be lost. DaTscan is able to detect this decreased activity early in the course of Parkinson’s, when the diagnosis may still be uncertain.
When is DaTscan helpful?
There are certain clinical scenarios in which DaTscan SPECT is most helpful.
- Differentiating between Parkinson’s and essential tremor – in an individual whose sole symptom is tremor, it can be difficult to make a definitive diagnosis. DAT scans are abnormal in patients with Parkinson’s, but normal in patients with essential tremor.
- Differentiating between Parkinson’s disease and drug-induced parkinsonism – anti-psychotic medications used to treat psychiatric illnesses (including schizophrenia) work by blocking dopamine receptors. Due to this, these patients may develop motor problems mimicking those found in Parkinson’s disease. However, DAT scans are normal in drug-induced parkinsonism.
- Differentiating between Parkinson’s disease and psychogenic parkinsonism – occasionally individuals have motor symptoms that arise from psychological rather than neurodegenerative causes. DAT scans are normal in individuals with psychogenic parkinsonism.
It is also important to be aware of the limitations of DAT scans. Any disease process that causes loss of the presynaptic dopamine neurons will result in an abnormal scan. DaTscan is not able to differentiate between Parkinson’s disease, progressive supranuclear palsy, multiple systems atrophy and other neurodegenerative diseases affecting the dopamine neurons in the brain.
DaTscan can play a very useful role in the diagnosis of Parkinson’s disease, but typically only early in the course of the disease, in certain clinical settings. A neurologist specializing in movement disorders is best qualified to determine if this is the right exam for you.