According to the NHS, selective mutism "prevents children speaking in certain social situations, such as school lessons or in public", but it does acknowledge that if left untreated it can continue into adulthood.
For 35-year-old Sabrina Branwood from Rochdale, the disorder makes it impossible for her to talk in certain social situations. She used an app on her tablet to share her story with BBC Two's Victoria Derbyshire programme, revealing that "when people ask me questions, my anxiety makes it hard to think".
Sadly, Sabrina's phobia also makes it difficult to talk to certain people, including close family members. After her grandmother had a stroke, Sabrina became so anxious she was unable to talk to her, and was unable to tell her directly that she loved her before she died.
"When people ask me questions, my anxiety makes it hard to think," said Sabrina via her app. "Trapped. I'm not silent because I don't want to talk. I would rather talk freely but it's very difficult and complicated. Having selective mutism can feel like you're living your life in a box. The box is see-through so you can see out and hear people, but you can't leave no matter how hard you try."
"You can shout inside the box as loud as you like but nobody can hear you," she continued. "They can't hear you cry when you're hurt or scared."
Selective mutism typically develops during early childhood, when children are put into a different social setting, such as nursery. More common in girls, it manifests itself as nervousness, difficult behaviour when returning back into the family environment, and literally being unable to speak (rather than choosing not to).
Alison Wintgens, national adviser for selective mutism at the Royal College of Speech and Language Therapists, told the BBC that care for adults with selective mutism is "extremely under-researched, with a real deficiency of services because it so often falls under the radar".
For Sabrina, her fear of speaking has led to her feeling like she has missed out on so much, with her condition growing increasingly difficult to manage as she gets older.
"When you're a child your parents can do lots more to help, and speak for you, but as an adult you're meant to do all that stuff yourself and people expect you to be able to, but I can't," she said.
However, she remains optimistic that she will recover one day, and is currently studying for a degree in psychology from Open University so that she can help other sufferers.
For more information about selective mutism, visit iSpeak.
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