Help! How do I deal with my dad's delusion of thinking my mom is having an affair?

This fixation on one topic while seeming otherwise normal is called an obsession. Check medical issues first - maybe a UTI is going on again.
Next, get him seen by a neurologist or psychiatrist who has some interest in geriatric patients. This can be treated with medication...and if left untreated, he could try to injure your mother.
Let us know how things are going. Take care.

I hope his geriatric assessment will uncover what the cause is. It is important to make sure that his physician knows about this delusion. The doctor hopefully will want to conduct further tests or send him to a neurologist.

As far as your dad being delusional in the hospital, there is a condition called 'Hospital Delirium' or 'Hospital Dementia' that elderly people can suffer from. My mom had it when she was in the hospitals and SNFs - in her mind, there were wild parties going on in the staff lounge and everybody was smoking pot and/or drunk. Trying to convince her otherwise was useless.

Since then, I've done a little reading on the subject and some of the articles I've read state that the delusions can continue even after release. It is usually a sign of cognitive decline.

It's worth mentioning the delusion timeline to the doctor when he goes in for his assessment.

Edited to add: My mom said I was smoking 'funny cigarettes' and she could smell it after she was discharged and came home. I wasn't. Every now and then she will say she smells pot smoke in the house, even though nobody here smokes pot.

This isn’t much of an answer, but perhaps you and the rest of the household can use professional earplugs. When he starts, put the plugs in. He can argue as much as he wants, you won’t hear it. It might stop him when he realises it’s a waste of time.

It sounds like your father suffers from dementia which is causing him to have this delusion of your mother having an affair. He can 'promise' to stop the accusations until the cows come home, and you can threaten him all you'd like, but he cannot HELP it if dementia is the cause for these delusions! Screaming matches aren't helping anyone here either. What can you do to HELP this situation, is the question? What happened when you called an ambulance for him last time? Was an antibiotic prescribed to get rid of a UTI or was there no infection to cure? You don't say. In any event, your father is suffering here which in turn is causing the rest of you to suffer. The key is to find out what is happening to dad to cause these delusions, not to be screaming at him to stop. I guarantee you he's not happy to be thinking his wife is having an affair; it's upsetting him to have that thought! THIS is how you have to look at the situation; put his shoe on your foot and imagine how you would feel if you thought your wife was cheating on you? In his mind, she IS, and he's reeling over it! Lashing out. Something is causing this; either dementia or an infection that's causing dementia-like behavior to crop up. It's not that he 'won't stop the delusion'...........he CAN'T stop it! Once you understand that fact, then you can seek help for him accordingly w/o threats and screaming matches.

At the very least, his PCP should be able to prescribe a mild sedative for dad while a diagnosis is sought. The body language YOU show HIM needs to be calm and relaxed at all times, even if you are not feeling that way. Showing him you're so upset will only take the situation from 0-100 in 60 seconds flat! Use soothing words and actions with dad; which is more likely to calm him down than anything else.

When dealing with dementia or dementia like behaviors, keep these tips in mind:
The “Dont's”
· Do not reason and argue
· Do not demand that they reason or problem-solve
· Do not demand that they remember
· Do not demand that they get their facts straight
· Do not correct their ideas or scold them
· Do not reorient them
· Do not think that they are being uncooperative on purpose
· Do not think that they really do remember, but are pretending not to
· Do not use a “bossy” dictatorial attitude in care
· Do not act with impatience
The Do's
· Enter into their frame of reality, or their 'world'
· Be aware of their mood or state of mind
· Use few words and simple phrases
· OR use no words, just friendly gestures and simple motions
· Do everything slowly
· Approach from the front
· Wait for a slow response
· Constantly reassure them that everything is 'OK'
· Keep people with dementia comfortable 'in the moment' - every moment
· Maximize use of remaining abilities
· Limit TV or radio programs which they may feel are frighteningly real
· Maintain privacy
· Provide a safe physical environment
Language Needs
· Use short words
· Use clear and simple sentences
· Speak slowly and calmly
· Questions should ask for a “yes” or “no” answer
· Talk about one thing at a time
· Talk about concrete things; not abstract ideas
· Use common phrases
· Always say what you are doing
· If they repeat their question, repeat your answer as you did the first time · Give them a longer time to process information
· Wait patiently for a response
· Be accepting of inappropriate answers and nonsense words
· Speak softly, soothingly and gently

Care Needs
· Be aware of your body language and use it to communicate relaxation and reassurance
· Be sincere
· Use a soft, soothing touch
· Be aware of the individual’s unique triggers
· Be aware that a person with dementia may not accurately judge whether a situation is threatening to them
· They may respond to fear, pain or anxiety by defending themselves with what we call “aggression”
You need to change your behaviour to adapt to the dementia because the person with the disease cannot.

Good luck!

Carevdad
Has your dad been treated for a UTI with a cultured test? If he is being prescribed with an antibiotic without a proper test, the antibiotic may not knock it out. Take him to an urgent care and have him tested. Ask for a culture to be developed. This usually takes a day or two. You can find out if he has an infection right away but it takes time to figure out which antibiotic will be most effective.
If he doesn’t have a UTI then he needs to be treated for his delusions. Your getting upset won’t help you, mom or dad.
Has dad been to a neurologist?
A UTI can bring about dementia like symptoms. People with dementia can’t control their thoughts. Their brains are broken.
Get dad some help for all your sakes.

Carevdad, has your Dad seen a neurologist to see if possibly he may have dementia?

From what you have written, that seems to be a normal pattern of delusion. If that is the case, you will not be able to change his mind. Best to just agree with him [even though you know it is not true], and go on to another subject. The neurologist can prescribe some meds that will help with this issue.