The third time my husband complained about not being able to refill his subway card, I began to worry that the vending machine wasn’t the problem. When I accompanied him the next morning, I knew it wasn’t.

My jaw dropped as I watched him squint and poke blindly at the screen, misreading prompts and misunderstanding the payment option buttons. Once he finally selected a dollar amount and tried to pay, he needed my help to insert his transit debit card into the correct slot. When he was prompted to enter his PIN, he typed in our ZIP code and got an error pop-up.

“See?” he said. “It doesn’t work!”

I nodded, finished the transaction and watched him pass through the turnstiles to work. Back on the street, I replayed the weird disconnects he had been having over the past year: the visual problems he blamed on dyslexia, the forgetfulness I attributed to undiagnosed A.D.H.D., all the odd lapses at stores, banks and in our own home that he claimed were the fault of other people, gadgets and glitches. They had all seemed like random one-offs until now.

Seven neurology exams, two M.R.I.s and one heartbreaking neuropsychology test later, my husband, at 59, learned that he had Alzheimer’s, a rare early-onset subtype called posterior cortical atrophy. This means the plaque buildup in his brain started its dirty work with his visual processing before taking aim at his short-term and working memory.

By the last appointment in a yearlong series of humiliations for him and gut punches for me, we discovered that he could no longer read properly, sign his name on a line, remember the date or often why we were seeing a neurologist in the first place. The doctors were amazed that he was still working at a city agency and commuting. I shared their view about working (and it turned out that most of his office mates had been covering for him), but not about commuting.

Like many native Brooklynites, he had been riding the subway his entire life — the system is baked into his long-term memory. He not only knew the lines and their quirky permutations but also where to stand on a platform so the doors of an incoming train would open right in front of us. I constantly ribbed him for being a train nerd. It was no accident that our son’s first toys were model subway cars.