(Victo Ngai/For The Washington Post)

Parents instinctively know the sound of their child’s voice, the voice that they alone can pick out of a cacophonous playground. When that voice suddenly goes away, you become desperate to hear it, even for just a moment, and every moment without it is a torment. When our son stopped talking, our sense of loss was painful and acute.

In the winter of 2010, my son, who was then 3 years old, had developed a curious habit of not answering when we spoke to him. It was that unforgettable Snowmageddon winter, when we were enduring a record-breaking series of snowstorms. One night, we’d eaten dinner watching through the window as the snow softly fell, piling up like fresh sheets out of the dryer. My son had refused to eat most of his dinner and was trying to get down from the table. As was customary in our house, I reminded him to ask, “May I be excused?” He shook his head and kept his mouth shut.

I was annoyed, but I thought little of the episode. It didn’t occur to me until much later that he didn’t speak a single word to me or my husband, Eric, for more than two hours.

Later that week, when I picked him up at preschool and asked him about his day, he didn’t answer again, just shaking his head. I pulled his teacher aside. “I’m just wondering,” I began, “does he ever talk in class?”

The teacher cocked her head. “No, not really, now that I think about it.”

“How long has this gone on?”

“A month, maybe?” A month of complete silence at school? I was stunned.

Over the next few weeks, his periods of speaking at home shrank, while the length of his silences expanded. Stranger still, he sometimes replaced his speech with a series of “mmm” sounds, his lips tightly sealed, as if the words were trapped behind a door. In one stretch, he didn’t say a single word to us, or to anyone, for two weeks straight. When he finally spoke, it was like a feather on a breeze, the words gone as quickly as they arrived.

At preschool, a classmate gave my son a nickname — “the talk-no-talk boy” — after my son hadn’t talked there at all for three months, and he retreated even further.

At first I thought it was an odd but passing phase for a 3-year-old. But soon, with myconcern rising, I took to the Internet and found that this condition had a name: selective mutism, or SM. It is a rare anxiety disorder related to social phobia that affects 7 out of every 1,000 children, according to one SM research organization. I also consulted with our pediatrician, who ruled out autism, but I learned that selectively mute children can have similar difficulties communicating. Our pediatrician referred us to a child psychologist who confirmed that our son had selective mut­ism.

Typically, SM kids speak in certain places where they’re most comfortable, such as home, but not in other locations, such as school or day care. This was the case with our son. For selectively mute kids, I read, even whispering with one other person can feel like speaking to a crowd of thousands.

Reading about SM, I felt a shock of recognition. Growing up, I, too, had suffered from a public-speaking phobia, and I rarely participated in class discussions. In high school, I would lose sleep for nights on end before giving an oral report.

Although I’ve mostly conquered those fears, painful, embarrassing memories rushed to the surface as I perused Web sites about the condition. And along with them came a deep sense of guilt. I learned that three-fourths of children with SM have a parent with social anxiety. If anxiety was in my son’s makeup, I was convinced that I had put it there.

As I searched, I also saw frightening references to Seung-Hui Cho, the troubled young man who killed 32 people and wounded 25 others at Virginia Tech in April 2007, before turning the gun on himself. In addition to schizophrenia and depression, Cho suffered from selective mutism. He spent his childhood unable to communicate with peers — and was routinely bullied in return. I didn’t want that for my son.

Several family members and friends tried to calm us, saying that our son was simply shy and that he would grow out of it. At our pediatrician’s recommendation, I called a child psychologist who had experience treating kids with SM. We were soon in therapy — all three of us — where our son found a kind and understanding soul who knew that his inability to speak was far more than just shyness. In therapy we realized what we had long suspected: that selective mutism is merely the outward sign of an inward struggle that can make children feel utterly alone.

I have since learned that our experience mirrored that of other families. Naomi, a District mother of two who asked that her last name not be published to preserve her family’s privacy, says that her SM daughter once sat through her pre-school lunch without eating because she couldn’t figure out how to open her lunchbox or ask for help. Another time, her daughter walked back from the playground to school with one shoe because she couldn’t tell her teacher that she’d lost the other one. And yet the preschool director and teachers told Naomi that she shouldn’t be concerned because her daughter was so young.

“But even then it didn’t feel right,” Naomi says. “I knew from early on that she was experiencing real fear around speaking. When other children approached, even children she knew, I could feel her whole body tense up. It was really this kind of paralysis and an almost physical inability to speak.”

Marian Moldan, a licensed clinical social worker and principal of Childhood Anxiety Solutions in New York, specializes in the treatment of SM in part because she suffered from it herself as a child.

“People just thought I was really shy,” says Moldan, who never spoke throughout elementary school. When she began middle school, she took advantage of the fresh start offered by a new school to start answering teachers’ questions out loud. But without treatment or guidance, her social skills lagged. “I didn’t know how to negotiate any conflicts,” she says. “I didn’t know how to enter a social group. I was pretty much isolated.”

She brings that personal experience to bear when she counsels children and teens and through “Charli’s Choices,” a book that helps adults and kids who are interacting with SM children. Among the suggestions is to frame questions as a choice — “Do you want the red marker or the blue marker?” — which is far more likely to draw a verbal response than an open-ended query. Like other SM specialists, Moldan recommends a process of “baby steps,” which might begin with a socially anxious child making eye contact or waving goodbye to someone. These tasks sound simple but can be huge steps for a child having trouble communicating. The idea is to help SM children become comfortable interacting with others until they eventually begin talking again.

Over a period of more than two years, and with guidance from our therapist, this is the approach we took with our son. We removed all pressure on him about speaking while encouraging play dates with trusted friends and other interactions — such as hanging out after school in his classroom — that would expand his comfort zone. It took a long time, but his words eventually came back — in waves. First, he began talking to me and my husband, then to other family members, then to close friends and, finally, to his teachers and classmates.

Today, he is a happy, outgoing — and talkative — 8-year-old, although still more introverted than many of his peers. Not too long ago, I marveled when he happily got up in front of his class and read a story he had written. In those early days, I cradled every word he said like a bird in my hand, and I still do now, when his words rush forth on every topic imaginable. But in those times when he is silent and thoughtful, when words aren’t required, I’ve come to treasure those, too.

O’Connell is a writer based in Arlington. Her Web site is www.kimaoconnell.com.