She had always been painfully shy. As a child she was never without her satin comfort blanket, and in her first year of school she answered the teachers in just the smallest wisp of a voice. Then when she was six years old her voice began to disappear altogether, like the little white dot when you turn off an old-fashioned TV set—tiny, tinier, gone. Unless she was with her family, the words just would not come out.

Fifteen years ago, when she was 17, Anna Clark sat on her bed with her diary. "I feel like I'm in the middle of an airport tarmac," she wrote, "with everyone else outside in the green fields. I feel like someone trapped in a box."

"The limits of my language means the limits of my world," wrote the philosopher Ludwig Wittgenstein. Anna's world was shrinking as her peers' was opening up. Her mom still did everything for her, even after she graduated from high school. "I think I may be a helpless sort of person," she wrote in her diary.

The boredom, the scrutiny, the sitting outside eating lunch on her own; it was too much, and Anna's frazzled 15-year-old body revolted. Panic attacks and major depression saw her taken out of school for "quite a length of time", and when she returned she had a teacher's aid by her side.

Being an outcast is painful, but it's also painfully dull. Without the usual trappings of teenage girlhood—the closeness of a best friendship, endless phone calls about nothing—school life was a desolate blur. "Day in and day out," she wrote in her diary. "So monotonous."

Anna became known as "the girl who doesn't talk." If she needed something she had to scribble a note to the teacher, and at lunchtime she ate outside on her own.

No one knew it then but the freeze would plough on, ambivalent as any force of nature as it swallowed up what remained of her childhood. "In junior high I managed to speak a little to a couple of friends, but I regressed in high school," she says. "I couldn't speak to any of the students or the teachers. Some of the other students would speak to me but I couldn't reply, so they mostly avoided me."

"I can see myself sitting with the teacher in my classroom," she explains by email. "She wanted me to read out loud to her, but I couldn't. I was frozen."

To avoid episodes like that, she avoids the things that together make up a life: a job, eating out, holidays, public transport, shopping. Years ago, it dawned on her that even if she could suddenly speak, she wouldn't know how to hold a conversation. Even walking down the street makes her nauseous. So does entering a store—"I always fear the shop assistant coming to ask me if I need help with anything."

When Anna tries to talk, rational thought is eclipsed by deep shame. No one likes me, she thinks. Or, Everyone thinks I'm strange. Then her whole body goes into shutdown mode. "My face contorts. My throat feels closed, my stomach churns, my knees shake..."

She can say goodbye to extended family when they visit—if she stands behind a door or a wall.

"We can all withhold speech out of choice, but it's generally short-lived and feels nothing like being unable to speak," says Alison Witgens, a speech therapist at St. George's hospital in London who has authored several books about SM. Sufferers, she explains, "inwardly panic, become physically frozen and cannot utter a sound."

SM is psychogenic, which means it manifests physically but its root cause is psychological. People with the condition aren't mute because there is something wrong with their vocal cords, or to hide a speech defect. They simply have a debilitating fear of sounding strange or stupid. The key thing to know is that sufferers do not choose not to talk—their silence feels beyond their conscious control, even when it forecasts the very scrutiny they dread.

Sufferers can speak normally in "select" situations, but otherwise remain mute. The most common pattern is the ability to talk at home with family, but not in public or social situations, including school, university, with colleagues, to strangers—anywhere and anyone outside their safety zone.

Selective mutism, or "SM" for short, is a largely unknown anxiety disorder that can also be described as a phobia of talking.

But if she can talk, asked her brother and her father from the sidelines, then why doesn't she?

Thoughts of the future began to nag: One day her mother would be old and it would rest on her, Anna, to do things for the both of them. Things like going to the shops. It was unthinkable—the wall separating her from the speaking world was too high and too thick.

Anna, who writes with the slightly formal eloquence of someone educated but untouched by conversational shortcuts, lives in rural New Zealand with her mom and receives a sickness benefit. The only time she leaves home is to see her psychiatrist. She types out messages on her phone for him to read. "I need to feel happier and less anxious," she says, "before I can even try to attempt to speak."

There are some small exceptions: she can say goodbye to extended family when they visit if she stands behind a door or a wall. And she can whisper "thank you" in shops sometimes if she's away from her hometown, where everyone knows her.

Driving is off-limits, too. What if she got into an accident and had to get out to speak with the other driver? "Or if I ever had to stop for a police officer for something, like a random breath test. I fear I would freeze and be unable to say anything, and they would wonder what was wrong with me." She got her license two years ago but only because her brother came along to the lessons and spoke for her.

Other anxiety disorders can develop later in life, but SM almost always appears in sufferers between three and six years old, says Dr. Woodcock. "At some point in their lives, children get treatment for it. Parents keep pursuing and pursuing until they find something that works."

"The older you go, the rarer selective mutism is," says Dr. Elizabeth Woodcock, a clinical psychologist and director of the Selective Mutism Clinic in Sydney. British estimates indicate that seven in 1,000 children suffer from SM, compared with one in 1,000 adolescents and one in 2,400 young adults.

She does have a degree, "things I could do mostly without speaking." It meant living with her brother while she studied and writing emails to communicate with her tutors, but she did it. She hasn't ever had a job though, or a relationship.

Anna's family has battled both anxiety and depression over the years. When it came to inheriting severe anxiety, she was high risk. The fact it then manifested as SM was a parallel stroke of bad luck, likely triggered by starting school.

Anxiety is thought to be a blend of genetics (nature) and modelling (nurture). Most people with SM come from anxious families, says Dr. Kotrba.

The amygdala, which Dr. Kotrba describes as "our brain's watchdog", is an almond-shape set of neurons in the brain's medial temporal lobe that houses our fear response. Its job is to look out for danger, in other words. The problem is that people with SM see danger in all sorts of places. This explains their physical symptoms: "freezing up", a racing heart, muscle tension.

"There is evidence to suggest the amygdala of individuals with selective mutism is over-reactive," says Dr. Aimee Kotrba, the lead clinical psychologist at Thriving Minds Behavioral Health, a pediatric clinic in Michigan dedicated to anxiety disorders.

Anna's family did what they could—there were counsellors and antidepressants, and her school organised art therapy. But they lived rurally and had limited means.

Adult cases are therefore infinitely more complex. For the condition to have clung on into adulthood, it was either so severe that treatment failed, the right treatment wasn't in reach, or the issue wasn't addressed at all.

But for all intents and purposes it is still creaking, says Toni Pakula, co-founder of New Zealand SM charity, Voice . "The majority of people have either not heard of SM, or they misinterpret its meaning. We've had parents tell us their doctors had to google the condition."

Finally, in 1994—thanks to lobbying by The Selective Mutism Foundation —the disorder's second name, "Elective Mutism," was thrown out in recognition that sufferers do not "elect" to be mute. Only then did the door to enlightenment begin to slowly creak open.

The majority of people have either not heard of SM, or they misinterpret its meaning.

Therein followed a 100-year stretch of more or less nothing; right up until the 1990s, SM was largely ignored, considered too rare to be of public interest. The scant literature that did exist dated back to the 1960s and painted a brazenly negative picture of manipulative children who clammed up in stubborn defiance.

To understand why, it's necessary to go back—but not very far. Selective Mutism was first reported by a German physician, Kussmaul, in 1877. He came across children who "refused" to speak, and named the disorder Aphasia Voluntaria.

The few medical experts in SM, and they are exceedingly rare, use words like "hidden" and "invisible" when they talk about the condition.

Who would Anna be if she could speak? "I would definitely have a job and not be dependent on other people's help for everything," she says. "Maybe I would travel, maybe I would be married and have children. I could try to make friends. I could find what my role in life is."

Carl Sutton, who runs iSpeak —the only organisation in the world that caters to adult sufferers—has written to multiple SM experts about this omission. "But they are pretty closed-minded about it," says the 46-year-old I.T. professional from the UK. "I get blanked when I say, 'Excuse me, can you please change the info on your website and literature? Because it is incorrect.'

Most perplexing of all is the fact medical bodies, almost without exception, define SM as an exclusively childhood disorder. Despite the inclusion of adult sufferers in the official DSM description . Which means Anna remains invisible not just from public life, but also from the medical field her disability occupies.

Why the glaring omission? One explanation is that adult cases are rarer. Another is that adults with SM often hide away from the world. Even if they do leave the house, the condition isn't detectable, unlike other speech-related issues such as a stammer or Tourette's Syndrome.

"Not many people even realise there are adult sufferers," says Pakula. "It scares me to think how many lost voices may be out there—people locked away in their homes, on the streets. Some become agoraphobic, others turn to substance abuse or alcohol, and many, many more are depressed."

In short, if you have SM as an adult, good luck.

The reality for adult sufferers could not be more different. There is no literature, no ongoing research, no conferences, and certainly no specialist clinics.

And yet, in 2017—thanks largely to the internet—young children with SM are well-catered for. There is a wealth of literature available, plus support groups, ongoing research, and the odd specialist pediatric practice.

"I often feel like I'm virtually the only person standing up for the reality of this condition, never mind the existence of adults."

Only one quarter of Carl's subjects had ever received a formal diagnosis. "So," he says, "for every person who was diagnosed with SM, there could well be many more with the disorder who have not received a formal diagnosis but have been affected by SM just as much.

In 2003, he authored a study on the experiences of adult sufferers for his Masters in Psychology. He interviewed 83 people from 11 countries and noted their extreme vulnerability to depression: approximately four times higher than that of the general population.

"I exist, as do all the people I have been in contact with."

There is 30-something Declan from Ireland, a quantity surveyor. He can't speak to anyone from his past who knew him as a selectively mute kid. "It felt like there was a second heart in my gut that shot out streams of cold sickly sweet liquid," he says, describing his silent childhood. "When the moment came for action [to talk], this liquid froze and expanded and constricted my throat and body. The more I tried to fight it, the more intense it got."

Sooner or later, when you have SM, your silence will be misunderstood. People assume you are just outrageously rude, or intellectually impaired. That's why the members of Space Cafe are so willing to help outsiders understand.

You're screaming the words in your head but they just won't move down to your mouth.

Most have never seen a doctor about their SM, let alone one who actually knows about it, let alone one who actually knows about SM and has experience treating it. They are just relieved to have found a place where they don't stick out: "I always thought it was just me." "Me too." "I always thought I was alone." "I think we all did."

Some group-members report progress, like making a phone call while no one was looking. Others lament steps taken backwards. "Is it possible to live with SM and be happy?" they ask. "What techniques or therapy are working for you?"

At SM Space Cafe, a closed Facebook support group, 553 people from around the world are busy comparing notes and posting motivational quotes ("It's a slow process, but quitting won't speed it up"). A young woman has photographed her homemade art: four butterflies with the words "One day, we will all find our wings and be free."

Two things worry her: "The lack of healthcare and self care [among sufferers], because they are afraid of having to speak to a doctor, dentist or health specialist." And her clients' inability to form romantic relationships. Thanks to the absence of formative interactions in adolescence, the risk of a lonely life is perilously high.

Marian Moldan, a clinical social worker, runs a group in New York for young adults with SM. She says it is absolutely "imperative" that adult sufferers are acknowledged by the wider medical community, in order for healthcare services to catch up.

She can't work, and her partner has to make phone calls for her. If it were up to Becky, "I'd be a chatty outgoing person—which I am, when I'm at home. But there's no one there to see it; no one sees the real me."

Becky, 29, from Yorkshire, can talk to most people, but only one or two words. "After that, my mind goes blank, so someone has to step in and save me." It's like there's something physical blocking your voice, she explains. "You're screaming the words in your head but they just won't move down to your mouth."

Gabrielle, a 28-year-old from Pennsylvania, woke up one day when she was nine with a cold, and found her voice was gone. "When my voice came back," she says, "I just didn't use it."

When he was six, Declan's parents took him to see a speech therapist. "I think they thought I didn't speak because I didn't know how to pronounce words properly," he recalls, "but I did. I never knew why I was there."

In most countries, accessing public or subsidized mental health services require a referral from a general practitioner. This is yet another hurdle, says Toni Pakula of Voice. "In most cases, seeing a doctor would be very difficult to do for a sufferer of SM on their own."

But physical proximity to an expert like Kotrba is the exception, not the rule. And therapy is expensive. Especially if, like many with SM, paid employment is not an option.

"It's also important to find out if the psychologist has experience working with anxiety—especially social anxiety or adult SM—and if they use cognitive behavioural therapy."

That might sound hopeless, says Dr. Kotbra, but successful treatment is possible: "The key is a motivation or desire to change, and a willingness to challenge themselves to engage in social activities, even when that's difficult.

"Finally, mutism becomes a habit or part of their behavioural repertoire, as well as part of who they are, which makes the behaviour even more difficult to change."

"Second, many grownups with SM have loved ones and colleagues who 'accommodate' their lack of speech, so they're less likely to feel motivated to speak.

Dr. Aimee Kotrba, one of the only anxiety specialists contacted with some experience with adult cases of SM, says such cases tend to be "quite hard". First, she explains, they rarely self-refer to treatment as it's "anxiety provoking."

Even if they did manage to secure access to cut-price treatment, "I'm not sure there would be anyone with awareness of SM in adults working for those services. So I think seeking help—and then being able to get the right help from people who understand—would be a very challenging task."

The late writer Maya Angelou had selective mutism between eight and 13 years old. "I thought of my whole body as an ear," she wrote, "so I can go into a crowd and sit still and absorb all sound. That talent or ability has lasted and served me until today."

"If one can't speak, one truly listens," emails Carl Sutton, of iSpeak. "One lives within the thoughts and feelings of other people and can have a profound level of empathy. It's not the case for everybody, and on the whole SM is an ordeal, but struggling with communication teaches you what it is to be human."

For Anna, reflecting on what good or essential human truths might be extracted from the torture of SM is a hard ask. Her depression is bad at the moment and she has little energy.

"I have heard other people say that having SM has made them better listeners and observers, but I actually have not noticed that with myself," she emails.

"I have actually found my anxiety makes me less observant and I miss hearing important things. I think the thing that having SM does is give you an ability to empathise with other people who feel isolated or different.

"Thank you,

"Anna."

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*Some names have been changed to protect anonymity.