Country star Brad Paisley's actress wife has opened up about her mother's battle with dementia in a new magazine essay.

Kimberly Williams-Paisley reveals she feels she is slowly losing her mum to primary progressive aphasia, a rare form of the disease, in a heart-breaking piece published by Redbook magazine.

The Father of the Bride star writes, "I've watched a passionately joyful woman, a devoted mother, an engaged listener and friend deteriorate and transform into someone almost unrecognizable.

"It's been agonizing to slowly lose her."

Williams-Paisley reveals her mother's condition has robbed her of her ability to write and recall words - and she can't even pronounce her grandson Jasper's name correctly.

And the actress admits the decision to move her mum from the home she shares with her husband and family to a care facility was heart-wrenching but unavoidable.

She adds, "The move was the hardest change my tight-knit family has ever had to endure.

"Our visits were agonizing for me. I couldn't look at her without seeing a fading picture of who she used to be. I resented this mostly manic, dangerous, crazy woman who had taken over my mother's body."