The 67-year-old actor initially suffered from aphasia, a language disorder caused by brain damage.
But since last year, "Bruce's disease has progressed and we now have a more specific diagnosis: frontotemporal dementia," his relatives said in a statement.
"Unfortunately, the communication difficulties are just one symptom of the illness Bruce is dealing with," the family shared, saying they were "relieved to finally have a clear diagnosis."
Frontotemporal degeneration (FTD) is a neurodegenerative disease, related to Alzheimer's disease.
People with FTD may experience memory problems, behavioral changes, or difficulty speaking or moving.
“Today, there is no cure for this disease, a reality which we hope can change in the years to come,” the family stressed.
"As Bruce's condition progresses, we hope the media will focus on bringing this disease to light, which needs much more awareness and research," his loved ones added.
© 2023 AFP