Beverly Frey of New Tripoli, Pa., suspected something was wrong with her son, Craig, at a very early age.
"From the time he was about 1, he was a head-banger and had very bad vision," Frey said.
When he was 8, his head-banging was severe enough to cause significant damage. "He had a retinal detachment caused by head-banging, and he ultimately lost his left eye," she said.
Her son, who is now 24, was also intellectually disabled, developmentally delayed and had a lot of difficulty sleeping.
"I remember sending him to pre-school and telling the teacher he was up until 3 a.m.," Frey said.
Mutations on a gene that make a brain protein cause the other 10 percent of cases. SMS occurs in about 1 in 25,000 births. Most people with SMS have the same characteristics, although they vary in severity.
"The hallmarks of SMS are a developmental delay; intellectual disability; extremely poor sleep and sleep disturbances; and negative behaviors, such as aggression, self-injurious behaviors and tantrums," said Dr. Lorraine Potocki, associate professor of molecular and human genetics at Baylor College of Medicine and Texas Children's Hospital in Houston.
Sarah Elsea, associate professor of pediatrics and human and molecular genetics at Virginia Commonwealth University in Richmond, said, "The biggest issues for most parents are the sleep disturbances and behavioral problems. Tantrums can occur from toddler age through adulthood. If there's any change in a routine, that often sets off tantrums."
Sleep problems tend to show up very early, even during infancy.
Said Potocki: "As infants, they sleep too well. By the time they get to be a toddler, they're up several times a night."
Adults and children with SMS also tend to be obese, although experts disagree on how prevalent weight problems are among this population.
"By 14 years old, 90 percent are overweight or obese," Elsea said. "We think it's associated with altered gene expression in the hypothalamus, which causes them to keep eating."
Despite their weight problem, people with SMS are not prone to obesity-related medical problems, such as Type 2 diabetes, although experts don't know why. They also don't seem to be at higher risk for any other medical problems.
"These children are often considered normal very early on," said Dr. Ellen Magenis, a co-discoverer of the syndrome and a professor emeritus at Oregon Health and Sciences University in Portland. "Then, you may start to see signs before the first year. They may start banging their heads against their cribs."
SMS also causes unusual facial features, such as a recession of the nose and lower jaw.
"But they're not so unusual in their appearance that you instantly begin to think of some malformation disorder," Magenis said.
The developmental delay and sleep issues may also not be obvious until children are a little older. Even then, doctors may not recognize the signs of SMS because they don't know much about it, experts say.