My husband was diagnosed with Parkinson’s about three years ago. He is doing well physically but declining cognitively.

Most cognitive problems are medication related.I was on 4 times the meds i really needed,resulting in many problems,forgetfulness,sudden sleep attacks,memory failure.Now just 3 25/10 and 1- 200 mg comtan better than before pd.
Old saying ifyoudont use it,you can lose it,lol

He has had a cognitive assessment......3 hours worth.

The multi diagnosed Parkinson's patient is not unheard of, however, it's best to get a second opinion to make sure he is not having a change in his mental status.

As cwillie said I'd have another assessment done my gma was originally diagnosed Parkinsons but as her symptoms progressed she was actually classic Lewey Bodies, we had to bring this to the Docs attention as he wasnt very familiar with Lewey Bodies...

Yes - Parkinson's dementia possibility due to PD in itself. The physical symptoms of PD gets easier compared to the cognitive/ dementia. Everything gets more challenging with it. Work closely with the doctors for any changes, they can suggest and work more with it, my dad still is a work in progress, Stay strong PD family.

Parkinson's dementia is a possibility, or it might be that (like a relative of mine) the parkinson's like symptoms are a result of Lewy body dementia and not Parkinson's at all, or he may have mixed dementia - I think you need to ask his doctors for a cognitive assessment and some further investigation.