A: There are 3 overarching causes of an inability to speak: psychiatric changes, neurologic changes, and changes to the mouth and throat.

Mutism is an inability or refusal to speak. It can be congenital – meaning that the reason your character can’t speak is something they were born with – or it can be acquired.

1. Psychiatric Changes

There are several different psychiatric conditions that can cause mutism.

  • Catatonia is a psychiatric syndrome that is often associated with mutism. Catatonia can be caused by a variety of conditions ranging from severe depression to schizophrenia.
  • Developmental delays, such as in children with autism, Down syndrome, and other congenital syndromes, may also cause delayed language development and subsequent mutism. Some people with language delays may never learn to speak, but may communicate using sign language, hand signals, communication boards, or other assistive devices.
  • Selective mutism is a type of anxiety disorder, in which your character cannot speak in certain situations (school, work, etc.) despite being able to speak in other situations. Selective mutism usually affects children and often symptoms appear before age 5. While people with selective mutism often “grow out” of their inability to speak, they will usually still have signs of social anxiety. Interestingly, selective mutism is generally not associated with trauma or traumatic events.

2. Neurologic Changes

There are a lot of neurologic conditions that can cause an inability to speak. There are three main neurologic conditions that cause inability to speak for three very different reasons. Most of the time, these conditions are caused by stroke or traumatic brain bleeds in particular areas of the brain.

  • Aphasia is the inability to understand or express speech. The most common type of aphasia is Broca’s aphasia. Caused by damage to the brain’s language centers, Broca’s aphasia causes an inability to express language. Your character will be able to understand what is said to them, but won’t be able to find words to reply. They might be able to read but unable to write. It’s an incredibly frustrating condition. Broca’s aphasia is most often caused by stroke, but it can also be caused by traumatic brain injury, ruptured brain aneurysms (hemorrhagic stroke), and brain tumors.
  • Apraxia of Speech (AOS) is the inability of the brain to communicate with the physical structures of the mouth and throat in order to make words. Your character will know what words they want to say (and thus should be able to write), but their brain has trouble getting the message through. For this reason, your character will be able to make sounds, but they may not be understandable. AOS is most often caused by stroke, but it can also be caused by traumatic brain injury, ruptured brain aneurysms (hemorrhagic stroke), and brain tumors.
  • Akinetic mutism is a rare condition that is characterized by a complete lack of motivation to do – well, anything. A person with akinetic mutism will be totally conscious but will not move, despite hunger, thirst, or pain. People with Creutzfeld-Jakob disease – also called “Mad Cow Disease” – often develop akinetic mutism, though that is not the only cause.
  • Dementias can also cause an inability to speak. Severe Alzheimer’s disease can cause aphasia, often first spotted by their inability to name everyday objects. Other types of dementia, such as frontotemporal dementia (FTD) or “Pick’s disease” can also cause mutism.
  • Cerebellar mutism is a rare condition of mutism that occurs after brain surgery on a specific type of tumor called a medulloblastoma. This condition is most often seen in children, since they are more likely to develop this type of tumor.

3. Changes to the Mouth & Throat

Physical damage to the structures required for speech – the mouth, tongue, and larynx in particular – can difficutly speaking. These organic causes of muteness can be acquired or they can be congenital. Often, injury to these structures will result in a soft or a hoarse voice, rather than a complete inability to speak.

  • Dysarthria is difficulty speaking due to trouble with the muscles used for speech. Rather than causing an inability to speak, dysarthria causes slurred, slowed, monotone, or irregular speech. Dysarthria tends to be caused by neurologic diseases that affect the muscles, such as ALS (“Lou Gehrig’s disease), cerebral palsy, multiple sclerosis, myasthenia gravis, and Parkinson’s disease. Dysarthria can also result if the nerves innervating the muscles of the neck and face are damaged.
  • Laryngeal trauma – trauma to the voice box – will cause a hoarse voice, trouble speaking, neck swelling, and a specific type of noisy breathing, called stridor. Untreated, your character may need to be intubated to prevent their airway from swelling shut.
  • Vocal cord paralysis occurs when the muscles of the vocal cords aren’t getting the signal from the nerve that controls them. Symptoms include a breathy or hoarse voice, trouble breathing, trouble coughing, and a flat-sounding voice. It can be caused by trauma to the neck, certain infections (Lyme disease, mononucleosis, even possibly COVID-19), and tumors of the neck and throat.
  • Glossectomy (tongue removal) is a time-honored way of silencing characters. After all, what better way to shut your character up than to brutally cut out their tongue? But that’s not the only way for your character to lose a tongue – cancers of the tongue, mouth, and throat can also necessitate its removal. Depending on how much of the tongue is left (glossectomies can be partial, half, or full), your character may be unable to talk or swallow.

Speech Language Pathologists (SLP)

Speech Language Pathologists (SLPs) are healthcare professionals specializing in speech and swallowing. Whether your character is suffering from aphasia, dysarthria, or even language delay, SLPs have techniques that will help to improve their communication. If your character is going to suffer from a speech difficulty (and your story is set in contemporary USA), consider utilizing an SLP in your story who can help them recover their ability to communicate.

A final note

Note that I didn’t mention deafness on here. Most people who are deaf aren’t mute – they are able to speak quite fluently using sign language. Furthermore, many deaf people can speak verbally as well, particularly if they were hearing during language development.