It can be caused by motor impairment (for example, weakness or paralysis of the facial muscles), as in Parkinson's disease, or by other causes, such as psychological or psychiatric factors (for example, if a patient does not feel emotions and thus does not show any expression).
The condition can result from motor impairments, like those seen in Parkinson's disease, or from other causes, including psychological or mental health factors. For instance, it might occur when someone cannot feel emotions and thus doesn't show any facial expressions.