https://www.dailymail.co.uk/news/article-10669119/Bruce-Willis-announces-diagnosed-brain-condition-step-away-acting.html

Bruce Willis has reportedly been struggling with cognitive
issues on the sets of his films for years - and even needed
an earpiece to feed him lines - long before his family
announced on Wednesday that the famed actor had been diagnosed
with a brain condition.

In a statement on social media, Willis' family told his fans
that he had been diagnosed with aphasia, a brain condition that
affects his ability to understand language, and will therefore
be stepping away from acting.

But an unnamed source told Page Six his declining cognitive
ability had been an open secret in Hollywood as the actor
repeatedly had trouble acting in his films.

'Everybody knew, the cast and crew,' the unnamed source said,
adding that Willis was 'using earpieces, hearing things, for
them to feed him the lines,' and it 'was increasingly
difficult to have him on screen.'

.. . .

The generic term is "aphasia" - a brain condition/damage
issue that impairs the ability to understand language.
It is usually the result of a stroke, though other
degenerative nerve conditions can produce it. As Willis
has been getting progressively worse it's probably a
degenerative condition rather than a stroke.

This differs from "ataxia" which means an impaired ability
to PRODUCE speech.

Hmmm ... how MUCH quality dope did Willis consume during
his career ? :-)

Michael J. Fox developed a Parkinsons-like affliction rather
early in life. A known cause is consuming kitchen-sink MDMA
("xtacy") which was extremely popular during his youth.
Some crap batches produced instant Parkinsonism, search for
"frozen addict".