Richard Trosch, M.D, Movement Disorder Specialist, Director, Parkinson's and Movement Disorders Center, Southfield, MI, Member, Michigan Parkinson Foundation Professional Advisory Board.
In January, the US Food and Drug Administration approved the use of ioflupane iodine-123 injection or DaTscan,developed by GE healthcare, as an aide in the differentiation of Parkinsonian syndromes from essential tremor. DaTscan is a radionucleotide agent with a high affinity for the dopamine transporter (DaT), a protein found on dopamine neurons. A specific marker for DaT, DaTscan produces images that provide visual evidence based on the density of dopamine transporters. Once injected, DaTscan will bind to or "tag" healthy dopamine neurons in the striatal region of the brain. The radioactivity from this drug may then be visualized by single photon emission computed tomography or SPECT, an imaging technology that is relatively inexpensive and widely available.
DaTscan is now approved in the U.S. for the diagnostic differentiation between Parkinsonism vs. essential tremor. In the European Union, where DaTscan has been available since 2000, it is also indicated for the diagnosis of Lewy Body dementia versus Alzheimer's disease. The projected cost for a DaTscan will be about $1500 to $2000 for the compound, with additional charges of the procedure fee and diagnostic interpretation. Insurance coverage has been established for Medicare and Blue Cross Blue Shield and it is anticipated that coverage by other medical insurers will follow.
How is it administered?
Prior to the DaTscan injection, an oral administration of 120 mg potassium iodine is recommended to minimize unnecessary thyroid uptake of radioiodine. The DaTscan agent is delivered as an injection via an intravenous catheter. After three to six hours, the patient is placed in the SPECT scanner and a detector, called a gamma camera, is used to visualize the distribution and quantity of the DaTscan compound within the brain. This procedure will take about 30 minutes to complete.
The DaTscan agent works by binding to or "tagging" the DaT protein, located on dopamine brain cells. This is used to visualize the location and density of dopamine brain cells, which "light up" on the image. If the parts of the brain where dopamine cells should be remain dark on the scan, an expert reader may diagnose early Parkinson's disease or Parkinsonism. DaTscan has been shown to have a 97.5% diagnostic sensitivity, meaning that it will be abnormal in 97.5% of patients with Parkinson's disease. Because other Parkinsonian syndromes, including Progressive Supranuclear Palsy (PSP), Multiple Systems Atrophy (MSA) and Corticobasal Degeneration (CBD) also have a dopaminergic loss, similar to Parkinson's disease, DaTscan will not be able to discriminate between these disorders.
Reported adverse effects from the DaTscan agent include headache, dizziness, nausea, vertigo, dry mouth, and injection site pain and hypersensitivity reactions. In the European experience, with over 300,000 doses dispensed, no serious adverse effects have been reported. DaTscan in contraindicated in patients with a know sensitivity to iodine, in women who are pregnant or breastfeeding, and in patients with impaired kidney or liver function. Also, certain prescription medications will need to be stopped prior to this study. Although this agent is a radiopharmaceutical, the minimal amount of radiation exposure is considered clinically insignificant.
How will DaTscan be used?
Currently, there is no definitive test for PD. Establishing a diagnosis of Parkinson's disease remains a clinical process, hinging on the patient's description of their symptoms and the physician's findings on neurological examination. The accuracy of a diagnosis of PD is dependent upon the skill and experience of the examining physician and remains a "best guess". Studies examining the accuracy of this guess have found an error rate of about 25 to 30%. Improved diagnostic accuracy can be achieved by following a patient's clinical course over time and by demonstrating an improvement with levodopa treatment. A diagnosis of "definite" Parkinson's disease, however, requires pathologic confirmation. During life, if the classic Parkinson's disease symptoms, signs and response to levodopa are present, a diagnosis of "probable PD" may be made. Clearly, objective measures to aide in the diagnosis of PD are needed and much work is being done to discover biomarkers for this illness.
DaTscan will serve as a diagnostic adjunct, meaning it can provide additional information about a patient's condition. It cannot be used to establish a diagnosis of PD, however, it may provide additional information to help establish or refute a particular diagnosis. The utility of DaTscan in the diagnosis of PD will be limited to special circumstances. If you have already received a diagnosis from an expert and are responding well to dopaminergic therapy, PET and SPECT scans would not add any new information and therefore likely to be unnecessary. Several experts have suggested that approximately 5% of Parkinsonism/PD cases may be DaTscan candidates, with the majority of Parkinson's disease patients not requiring this test. Instead, DaTscan will be reserved for cases where there is diagnostic uncertainty, for atypical cases of tremor, when the distinction between PD and essential tremor is unclear, in psychogenic Parkinsonism, and for patients who have a difficult time accepting the diagnosis of Parkinson's disease.
Within the neurologic community, skeptics have voiced concerns about the potential for inappropriate testing, overuse and/or misinterpreted testing. Whether an internist or general neurologist should ever order a DaTscan has been the subject of considerable debate. Clearly, most Parkinson's disease patients will not benefit from a DaTscan and the non-specialist may not be trained to recognize when this test is required. A less expensive alternative, a consult with a movement disorders specialist, provides greater diagnostic accuracy and will prevent unneeded testing.