There is/are form(s) of Motor Neurone Disease (ALS) that involve cognitive impairment, though it/they tend to have a more rapid progression than the more classic presentation.
I worked with one MND patient who had an unusually slow progression, not unlike Prof Stephen Hawking. Then a member of the family got MND, and they thought yes we know what to expect, but within a few months the relative developed dementia and died.