The five-year-old ran into my exam room with his mother trailing behind. He wore a Transformers T-shirt and jeans that each bore signs of a recent encounter with a chocolate bar. Immediately he took a toy train apart and scattered the pieces all over the floor. “The kindergarten teacher said she doesn’t think Jason belongs in the class,” the mother said to me. “But we’re not sure.”
Jason’s pediatrician had referred him to me because of his hyperactive behavior. “New patient to me,” her note said. “No old records available. Very hyperactive, difficult to examine, possible
developmental delay: refer to developmental pediatrician.”
Having been a general pediatrician for many years before specializing in developmental pediatrics, I sympathized with her. The 20 minutes allotted for a standard exam wasn’t nearly enough to try to figure out what was going on with this child.
Jason was now busy with a ball, but then quickly moved to a book and began turning the pages and pointing to every picture, labeling each one: “House! Duck! Train!” Then he was off to crash two trucks together.
His mother looked at him uncomfortably, clearly unsure whether she should try to guide him or let him alone. “It’s OK, nothing here is breakable,” I reassured her. “Tell me what he’s like at home.”
“He’s into everything, just like he is here,” she said. “He can’t sit still for a minute. That’s probably why the kindergarten teacher doesn’t think he belongs there. But...” She paused, as if trying to decide whether or not to say something.
“But there are other things that concern you, besides his hyperactivity?” I ventured. She looked relieved. “Yes,” she said. “I don’t know, it just seems like it takes him longer to learn things than other children. We don’t have any other kids, so we can’t compare him that way, but when he’s with other kids his age...it seems like he just doesn’t get it.”
When I observed the way Jason played with each toy, he seemed immature. His play didn’t have the imagination or complexity I would expect at his age. He was more interested in the toys for younger children than in those that should interest a 5-year-old. His vocalizations tended to be single words or sometimes phrases of two or three words.
When I had him play with the blocks, puzzles, crayons, and other materials I use to assess a child’s developmental status, his difficulties became evident. He could copy a circle but not a cross or a square. He could count two blocks but not more than that. Overall his developmental skills seemed more like those of a 3-year-old than a 5-year-old. The question was, why?
As I delved further, nothing stood out in his medical or social history to account for his delays. Then I asked about the family history. “Is there anybody on either side of the family who was slow to talk or had learning problems?” His mother paused for a moment and then said, “Yes, I have two cousins who are mentally retarded.”
“Are they boys or girls?”
“Boys,” she replied.
“Are they on your mother’s side or your father’s?”
“They’re my mother’s sister’s kids.”
“Does she have other children?”
“Yes, she has two daughters, too, but they’re fine. Oh, wait. I just remembered that one of them has a little boy. I don’t see her very much, since they moved to another state, but I think my mom said that they were worried because he was almost two and wasn’t talking yet.”
There was a pattern. Three males in the family had developmental delays or mental retardation, but their mothers and sisters were OK. That gave me the clue I needed. I suspected that Jason had Fragile X syndrome.
Fragile X is the most common inherited cause of mental retardation—now also referred to as intellectual disability—affecting about 1 in 4,000 children. The X in the name refers to the X chromosome, one of the paired sex chromosomes: Women have two X chromosomes, while men have one X and the Y that contains all the coding for maleness. A girl gets one X from her father and one from her mother; a boy’s X chromosome has to come from his mother, since he gets the Y from his father.