Dementia is defined as the "loss of multiple acquired cognitive and emotional abilities sufficient to interfere with daily activities". More than 50 diseases produce dementia (Alzheimer's account for 70%, prevalence in PD is 20 - 30%).
Dementia varies in its specific presentation depending on which particular abilities are compromised. Disabilities can occur in memory, language, spatial perception, cognition, attention, high-level motor control, emotion, and motivation. The key is how it adversely affects daily functioning.
In PD it's often difficult to tell because of the possibility of co-morbid Alzheimer's (both increase with age) and the cognitive effect of anti-parkinsonian treatment.
Difficult condition...