Meet Alfie, one of the mascots of the Amp Research Lab.
Alfie is almost 16 years old, which makes him about 85 in human years. He's doing remarkably well heal...th-wise, but he's starting to have some sensory decline (notably hearing and vision loss) and signs of cognitive decline, or dementia. Similar to older humans, these are all common afflictions that increase with age. Also similar to humans, it can be very difficult to tease apart the effects of sensory changes and signs of cognitive changes. For example, Alfie often wakes up during the night ready to start his day. This is common behaviour with dementia. However, it may be that he simply does not have the sensory input that would tell him that it's nighttime, if he cannot see that it's still dark and cannot hear activity (or lack thereof) from other members of the household.
With humans, we can test for hearing, vision, and cognitive decline separately, but there's a lot of evidence that our cognitive tests can be affected by sensory changes (e.g., DuPuis et al., 2014; Jorgensen et al., 2016). Some cognitive tests ask the patient to listen to words and then recall the words later. If the words aren't heard properly in the first place, it's unlikely that they'll be remembered well!
Another strategy we might try with humans is a trial with hearing aids to see whether increased auditory input will help with some of the signs of confusion to find out how much of the apparent cognitive decline was just due to not hearing and being connected with the surrounding auditory world (e.g., Palmer et al, 1999). As we do not yet have hearing aids for dogs, we muddle along as best we can, practicing patience and celebrating Alfie's abilities, including his ongoing desire for LOOOONG walks every day and his still excellent sense of smell.