In most cases, polio may not have the severe long-term effects on survivors that experts have previously feared, a new study finds.

A team from the Mayo Clinic found that survivors did much better as they aged than had been expected, with most only suffering a modest increase in muscle decline over time. Very few of the study participants had to adapt their homes or move out due to crippling symptoms, the researchers said.

"Our polio studies have demonstrated that these aging survivors of poliomyelitis do slowly weaken over time, however, this change is very modest in the majority," said study author Dr. Eric J. Sorenson, a Mayo Clinic neurologist.

His team published its findings in the August issue of the Journal of the Peripheral Nervous System.

For many childhood polio survivors, their battle with a potentially crippling and fatal virus that ravaged the world over half a century ago is more than a terrible memory.

Decades later, symptoms can resurface or worsen, usually marked by pain and weakness that affects survivors' quality of life. The fear of further physical deterioration and disability often plagues polio survivors, Sorenson added.

But the new study "allows us to be reassuring even to those polio survivors with significant deficits," Sorenson said. That means a better long-term prognosis, which is the main concern of patients.

In the study, the Mayo group tracked changes in strength and motor function for 50 polio survivors (average age 53) in Olmsted County, Rochester, Minn., for a period of 15 years. The participants were an average of 40 years past their childhood experience with polio when the study began. Participants also answered a related questionnaire.

According to the researchers, participants did lose some strength over the 15-year study period, but this decrease was relatively small. Motor neurons -- which control motor function -- dropped off with advancing age at a rate that was comparable to non-polio survivors. The majority of study participants did complain of progressive weakness over the 15-year study period, however.

Survivors who were most affected by polio earlier in life, or with the greatest residual weakness, were most likely to be aware of their slow deterioration, the researchers said, perhaps because they had learned to expect a decline. However, measurements suggest that these individuals didn't get worse any faster than others in the study.

"This study sheds some objective light on polio, showing that at least some of the deterioration is neurological and that the decline in function is real, not imagined. So, it's reassuring to polio survivors that their decline has some neurologic basis," said Dr. Nancy Green, Medical Director at the March of Dimes.

Post Polio Syndrome (PPS), which occurs in two-thirds of survivors, is the re-emergence or worsening of residual deficits decades after the childhood infection, Green said. Problems can include worsening weakness and/or pain, as well as problems eating, swallowing, or breathing.

According to another expert, a loss of motor neurons -- the nerve cells in the spinal cord that control muscles -- occurs in normal aging, as well. But since polio has already depleted survivors of many of these neurons earlier in life, they have less of a reserve when they start aging.

"There's a threshold of strength required to do anything, like lifting half a gallon of milk. For people without polio, there's a big reserve of strength, and even if they lose a little, they can compensate, whereas polio survivors have less strength to start, so every bit seems to count for more," said Dr. Julie Silver, director of the International Rehabilitation Center for Polio at Spaulding Rehabilitation Hospital, in Framingham, Mass.

Polio survivors are also at a higher risk of other musculoskeletal problems, such as osteoarthritis and overuse injuries, which are often treatable. The tricky part about labeling all physical problems as PPS is that it may overlook other conditions, Sorenson said. That means survivors may not get the appropriate attention or treatment that they should.

"I think as they age, almost all polio survivors have special needs, but while they may experience increased disability, they do much better with proper intervention and treatment," said Silver, who is also an assistant professor in the department of physical medicine and rehabilitation at Harvard Medical School. For example, many polio survivors who rely on crutches or a cane can develop carpal tunnel syndrome, which can be surgically cured, she said.