At this time, there is no confirmatory test for PD. The best testing we have is functional brain imaging to assess dopamine concentration in the "basal ganglia" or "striatum." This area of the brain demonstrates a decrease in dopamine concentrations with PD, and may be imaged using a Positron Emission Tomography (PET) scan or a SIngle Photon Emission Computed Tomography (SPECT) scan. PET scanners with dopamine-type imaging are not widely available, and dopamine scanning with SPECT is not available in the United States at this time. However, a SPECT imaging molecule may be approved in 2010, and there are others in development for use in the US.
Short of functional brain imaging, Neurologists look for signs and symptoms associated with PD to lead to diagnosis. Early symptoms include: resting tremor, difficulty with handwriting, dressing, cutting food, and other fine motor tasks. It is also helpful to see if one side of the body is more affected than the other.
Early signs are termed "cardinal characteristics," and include resting tremor, bradykinesia (slowness of movement), and rigidity (stiffness of the limbs, when moved through a range of motion by the examiner). If two of three of these signs are present, there is a 70% chance that the condition is PD.
The next step is to treat with medication, and if a significant benefit is seen, the chance that it is PD increases to 90%. Before initiating therapy, it is important to define these expectations, and remember, if you barely show signs of PD, you may not show a dramatic or significant improvement. In addition, the medications suggested in early PD, differ significantly from the medications used in more symptomatic PD. I prefer to start drugs that may slow disease progression in many patients, particularly in very mild cases, and use more potent therapies later.
If there is no symptomatic benefit, you should discuss other possibilities with your doctor. I would consider brain imaging in some instances. Symptoms that lead to other diagnostic concerns include: frequent falling as a first sign or early in the disease, double vision (particularly when reading), frequent crying or laughing at minor things, a hand that seems to have a "mind of its own" or that will not move the way a patient wants it to move, urinary incontinence, or significant memory disturbances. In addition, a highly symmetric presentation (both sides affected equally), or lack of tremor will push me to consider other diagnoses.