Physical aggression -- hitting, biting, scratching, spitting, and otherwise lashing out -- is not uncommon in someone in the later stages of a dementia illness such as Alzheimer's disease.
There are two main reasons why people with dementia may turn violent:
Personality changes brought on by the disease include loss of inhibition and self-control. A mild-mannered individual may do things he or she never would have previously.
Emotional or physical discomfort is the top trigger for physical aggression. The person feels insecure, threatened, angry, tired, embarrassed, humiliated, or otherwise vulnerable but lacks the ability to communicate these emotions in a socially acceptable way.
How to prevent aggression
Try to keep the person calm, secure, and comfortable. Make sure the person is dry (if he or she wears adult sanitary products) and is neither hungry nor thirsty. People with Alzheimer's forget to eat and can't always tell you what they need.
Keep to basic household routines. Ideally, sleep and meals happen in a predictable way every day. Ideally, the person with Alzheimer's gets fresh air (weather permitting) every day and/or gets a little exercise, even if it's just walking through the house.
Keep a written log of what was happening just before violent outbursts. Try using the ABC method to understand Alzheimer's behavior. You may soon see a pattern. If bathing tends to spark violence, for example, can you tell what seems most upsetting about it? If it's being cold, maybe you can turn up the heat, shut the bathroom door, and run towels and a robe in the dryer before you begin.
Prepare the person for triggers as best you can. Obviously you can't preempt every upset -- if a substitute care helper shows up, you still need the help of that person even if the new face is upsetting to the person with Alzheimer's. But while it's not usually productive to rationalize with someone who has dementia, telling them about an upcoming change is considerate and may offer a little preparation. Keep your tone calm and upbeat -- letting your own frustration show through words or body language will only make your loved one tense and more on edge.
How to Respond in the Heat of the Moment
Try to stay calm. Don't fight back or raise your voice. Even cues that you're nervous might get picked up by someone with Alzheimer's, and that can increase the aggression. Leave the room if you need to pull yourself together.
Stay safe. Obviously you don't want your charge to fall or hurt herself, but your own safety needs to be paramount. Step back if the person is out of control, rather than stepping in to restrain or overpower.
Don't argue. Make it your goal to avoid escalating the behavior, not to get your way or prove yourself right.
Resist the temptation to punish. The notion of cause and effect is beyond the cognition of someone with serious dementia. Issuing consequences (no snack, a lecture) will only add to the person's upset, and to the violence.
Distract. Try breaking the mood by stopping and starting again in 15 minutes. Change to a new activity, or even just move to a new room. If bathing has gotten off on the wrong foot, for example, switch to something you know your loved one enjoys -- listening to music, having a snack. Then get back to the bath later, taking care to eliminate or soften the trigger if you can. (Maybe you play the favorite music in the bathroom this time.)
Self-soothe in healthy ways. After a troubling incident, take care of yourself, too. Call a friend or reach out to an online Alzheimer's forum. Do not isolate yourself physically from others (a common practice, since caregivers grow afraid to have others see their loved one "this way").
As a Final Resort
Ask the doctor about medication. Nobody likes to think about worst-case scenarios, but sometimes, in serious situations, prescription medications (ranging from antidepressants to antipsychotics) are used to curb physical aggression.
Consider -- or at least stay open-minded about -- a new living situation. Signs that home care may no longer be viable include violent episodes that become routine (weekly or more often), medication that doesn't help, and yourself or a family member being injured or at risk of injury.