"Only 62 percent [of pre-retirees] have talked to an advisor about their retirement plan," John Carter, president of Nationwide Financial, told AdvisorOne, "but of those, just one in five have discussed health care costs." He noted that 43 percent said they were going to talk to "a" financial advisor, not necessarily the advisor they've been working with.

"This is a wake-up call for advisors to be proactive about education," Carter said.

Furthermore, 12 percent of pre-retirees say they plan to switch advisors, but of those, 54 percent say they wouldn't if their current advisor could help them plan for health care coverage in retirement.

Affluent pre-retirees also need help understanding Medicare, the survey found. Only 20 percent say they are confident in their knowledge of the program, and more than half say it is very or extremely important to their retirement planning to educate themselves on Medicare coverage.

Pre-retirees estimated Medicare would cover 68 percent of their health care costs in retirement, but the Employee Benefit Research Institute estimates it will only cover about 51 percent. When asked how retirees calculated how much Medicare would cover, most said they just guessed. Fifteen percent came to that conclusion after their own research and 7 percent asked friends who had already retired.

Almost half of respondents understand health care will be their biggest expense in retirement, but they still dramatically underestimate how much it will cost. Pre-retirees estimated they would spend an average $5,621 on health care, but a 2010 survey by Fidelity put the average out-of-pocket cost of health care for a 65-year-old couple between $250,000 and $430,000.

Inflation risk is one reason pre-retirees have difficulty appreciating how much health care will cost once they retire. "It's difficult for individuals to know what prescriptions and doctors visits will cost," Carter says. "They see the cost going up through inflation and they don't see it slowing."

Longevity risk is another reason for pre-retirees' severe underestimation of health care costs, Carter says. "Individuals aren't recognizing their need for nursing home care," he notes.

"Years ago, when 401(k)s were first being established, advisors committed themselves to learning about IRA rollovers, what you can and can't do," Carter says. "It took effort, education and commitment. Advisors need to dedicate themselves to education and understanding so they can advise their clients on health care costs' impact on their retirement plans."

Harris Interactive conducted the survey for Nationwide in January 2012 among 625 pre-retirees over 55 with at least $250,000 in household assets who plan to retire by 2020. Harris surveyed another 625 retired adults over 65 with at least $250,000 in household assets.