PBA can affect those with neurological conditions or injuries such as stroke, multiple sclerosis, or Alzheimer's disease.
Andy Tyerman, consultant clinical neuropsychologist of brain injury charity Headway, says: "The term refers to uncontrolled expression of emotion that is disproportionate or inappropriate to the social context and may be inconsistent with what the person is actually feeling.
"A person might also appear very distressed about something that would previously have been only slightly upsetting."