Part 2 · Course and diagnosis
The differential
Several conditions look like Parkinson's at first, and some are far more common than people expect. In one US county, Parkinson's disease made up only 42% of new cases of parkinsonism and drugs caused 20%. 40 Follow the questions a specialist asks, watch a rest tremor and an action tremor side by side, then compare any look-alike with Parkinson's feature by feature.
A simplified teaching sequence from published criteria and guidance. Real assessments ask these questions together, not one at a time, and loop back as the illness unfolds.
Rest tremor or action tremor?
A key early question is whether a shaking hand is a parkinsonian rest tremor or the action tremor of essential tremor. Play a short examination: arm resting, lifting, held out, finger to nose, and back to rest.
Schematic traces: the amplitudes and the frequencies drawn (5 and 7 Hz) are illustrations within the published ranges. Ranges: MDS rest tremor 4–6 Hz 291; tremor consensus, parkinsonian rest tremor 4–7 Hz with higher frequencies described 31; essential tremor 4–12 Hz 198. The sources do not agree on one PD range.
Side by side with Parkinson's
Pick a condition. Each row sets one feature beside Parkinson's disease, coloured by whether it looks similar or points away. Grey means the sources behind this module do not cover that feature for that condition; it does not mean "normal".
similar to PDpoints away from PDnot covered by these sources
A summary of the table below, where every statement carries its source.
The differential, condition by condition
Click a condition to open what else the sources say about it.
| Condition | What points to it rather than Parkinson's | Key tests and findings |
|---|
Why the differential is hard
Parkinsonism has many causes
Among 364 new cases of parkinsonism in Olmsted County, Minnesota: 42% Parkinson's disease, 20% drug-induced, 17% unspecified, 14% parkinsonism in dementia and 7% other causes. Men had higher incidence for every type except drug-induced. 40
Even experts are sometimes wrong
Of 100 people diagnosed with PD by neurologists, 24 had another condition at autopsy, including PSP, MSA, Alzheimer's disease and vascular disease. Pooled accuracy of the clinical diagnosis is 80.6%. 157,309
More than one disease at once
At the UK Brain Bank, 37.2% of people with PD also had co-pathology such as DLB, MSA or Alzheimer pathology, which complicates any single-diagnosis accuracy figure. Functional symptoms can also coexist with PD. 14,112
Atypical disorders hide at first
Under the 2008 criteria, sensitivity for MSA at the first visit was only 41% for possible and 18% for probable MSA. For PSP under the older NINDS-SPSP criteria, the median was 24%. In early unclassifiable parkinsonism, the MRI hummingbird and morning-glory signs caught 35.3%. 147,234,373
What NICE recommends
Suspect PD with "tremor, stiffness, slowness, balance problems and/or gait disorders", refer quickly and untreated to a specialist, and review the diagnosis every 6 to 12 months. NICE advises against PET outside trials, acute levodopa or apomorphine challenge tests and routine smell testing for the differential. 241
Missed in the community
In a London population study, at least 15% of people with a PD diagnosis did not meet strict criteria, and about 19% of people found to have PD had not been diagnosed with it. 320