As a top adviser in a multinational financial services company, Susan Grant 59, was used to effortlessly juggling numbers, client details and other complex information in her head. So when she found her thoughts so muddled she couldn't remember if she needed to take the interstate north or south to drive home, she knew something was wrong. “I'd find myself talking to clients and unable to think of a word,” she recalled. “So I'd stop in the middle of a sentence — and then I couldn't remember the sentence itself.” A series of medical tests resulted in six diagnoses —...