Dear Carol: My sister’s husband has mixed dementia including Lewy body and Alzheimer’s. He’s been telling my sister that she’s not really his wife, but an “impostor.” He sometimes shouts at her to get away from him and has even tried to hit her.

They had a great marriage for 48 years before his dementia and she’s been doing everything she can to learn how to help him. We both know that this aggressive behavior is due to the disease but no matter the cause, she’s afraid of him. What's maybe even more upsetting is that she's the only one who triggers his anger. He's OK with other people. He has an appointment with his neurologist in a few weeks, but meanwhile, how does she cope? — WL.

Dear WL: I’m sure your support means the world to your sister. Being thought an impostor by her husband is devastating, of course, but his anger and threats of violence are also major concerns.

First, suggest that she consider new medications as a potential cause, especially if this is a sudden change. Medications that can help one person can cause others to have worse symptoms, and people with Lewy body dementia sometimes react differently.

Her husband should also be checked for infections, including a urinary tract infection. These can have extreme effects on older people, particularly those with dementia.

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It seems as though your sister's husband may be suffering from Capgras syndrome. Much simplified, Capgras syndrome is a type of delusional misidentification syndrome (DMS). It happens because of a disturbance in the brain’s facial recognition system and isn’t uncommon with dementia of several types, including Lewy body, frontotemporal, and Alzheimer’s disease. It can also happen at times after strokes, as well as with some mental illnesses.

Hard as this is for your sister, if she can remember that these events are also anxiety-producing for her husband, it will be helpful. Calmness is key because if she gets upset, he’ll get more upset, which could make him more inclined to strike out physically. She is not responsible for what's happening, but a calm response can help her manage the situation. Try to reassure her that the most common subjects of this misidentification are people closest to them, often a spouse.

She should contact his neurologist right away because the doctor may be able to prescribe something that could help her husband at least temporarily until the appointment. Her husband's primary care doctor should also be informed so that health checks can be done if necessary. Many caregivers in similar situations are forced to place their spouse or parent in a care facility for their own safety. Your sister may need to consider this if no solution is found.

You can help by reminding your sister that if her husband had control, he’d be devastated knowing that he might be endangering her. Reassure her that she is the last person he’d want to harm, but she needs to protect herself for both of their sakes.

Carol Bradley Bursack is a veteran caregiver and an established columnist. She is also a blogger, and the author of “Minding Our Elders: Caregivers Share Their Personal Stories.” Bradley Bursack hosts a website supporting caregivers and elders at www.mindingourelders.com. She can be reached through the contact form on her website.