Is Parkinson’s disease an unique clinical entity? Rigid or tremor dominant PD: Two faces of the same coin.
Parkinson’s Disease (PD) is a disorder known to be a result of dopamine deficiency, a chemical in the brain influential in different networks in the brain. Research has identified different clinical subtypes of PD, involving different brain networks and having different disease courses. Some studies have identified four patterns of clinical presentation in PD patients:
- Benign form (pure motor form): predominant slowness of movement
- Mixed motor-non-motor form: slower movement, gait disturbance, behavioral changes (i.e., apathy, emotional disturbance, cognitive changes), and a slow progression
- Non-motor dominant form: lowest motor burden and highest non-motor burden
- Non-motor symptoms can include: sleep disturbance/fatigue, cognitive changes, emotional disturbance, apathy, gastrointestinal changes, urinary changes; sensory changes, low blood pressure, among others.
- Motor dominant form: predominant motor symptoms of PD with faster disease progression
Overall, most research studies have focused on the two most common clinical pattern of PD presentation:
- Akinetic-rigid type: slowness of movement accompanied by muscle stiffness and resistance to passive movement
- Tremor-predominant type: prominent tremor of one or more limbs with a relative lack of significant rigidity and slowness of movement
This study examined the cognitive and behavioral changes in the two common PD presentations by comparing 432 PD patients to 457 control cases. Each patient underwent a neurological examination (including extensive neuroimaging techniques to confirm PD diagnosis) and neuropsychological assessment (including cognitive and mood assessment) at the beginning of the study and again every 6 months for 24 months.
- At baseline: Compared to control cases, PD patients showed worse performance on both cognitive (including language and executive functioning measures) and mood assessments (including depression, anxiety and apathy), but both groups performed similarly on a brief cognitive screen (Montreal Cognitive Assessment).
- At baseline: The Akinetic-rigid subtype had higher levels of depression, apathy and lower performance on measures of executive functioning compared to the Tremor-predominant group, who had higher levels of anxiety.
- At 24 months follow up: There was a similar symptom pattern in group difference, with the Akinetic-rigid subtype showing a more severe disease progression compared to the Tremor-predominant type.
- Presents with severe gait and motor impairment with worse clinical progression
- Shows more impairment in executive functioning including divided attention, inhibition (the voluntary or involuntary restraint of a behavior or act), and word generation
- Shows more symptoms of depression and apathy
- Show some degree of depression but more anxiety
- Show lower executive functioning (word production) compared to control but better compared to akinetic-rigid type
In conclusion, research studies have hypothesized different explanations for the different clinical presentations in PD, including the involvement of different brain networks and dopamine level changes in different parts of the brain.