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Nonverbal autism is known as a subset of autism where the person does not speak. While most autistic children eventually begin to speak, there is a significant minority who will remain nonverbal.

Background
It was formally estimated that 25% to 50% of children diagnosed with autism spectrum disorder (ASD) did not develop spoken language beyond a few words or utterances. More recent research reports that 25-30% of individuals with ASD will not develop functional language. citation here. statement from recent research regarding prevalence of functional language. Despite the growing field of research on ASD, there is not much information available pertaining to people with autism who never develop functional language; that, in fact, nonverbal autistic individuals are considered to be underrepresented in all of autism research. Because of the limited research on nonverbal autism, there are not many validated measurements appropriate for this population. For example, while they may be appropriate for younger children, they lack the validity for grade-school-aged children and adolescents and have continued to be a roadblock for nonverbal autism research. Often in autism research, nonverbal autistic individuals are subgrouped with LFA, categorized by learning at most one word or having minimal verbal language.

Most of the existing body of research in nonverbal autism focuses on early interventions that predict successful language outcomes. Research suggests that acquiring language before age five is a good indicator of positive child development, and the likelihood of acquiring functional language in the future past this age is minimal, that early language development is crucial to educational achievement, employment, independence during adulthood, and social relationships.

Most children with ASD can be diagnosed between age two and three because of their behaviors and lack of social skills. They can also have gastrointestinal problems, seizures, and sleep disorders.

Early predictors
The causes of nonverbal autism are unknown. However, there appears to be a relationship between joint attention and verbal communication. Joint attention occurs between two individuals when one draws the other’s attention to an object through gesturing (i.e. eye gazing, pointing). The ability to achieve joint attention at an early age plays a significant role in language development, and studies indicate severe lapses in joint attention in children with autism. In one study, researchers suggest that a displayed pattern of delays, absences, or a general impaired response to stimuli (hyporesponsiveness) and a fascination with intense or repetitive stimulation (sensory seeking) is more likely in nonverbal children with autism, suggesting that both hyporesponsiveness and sensory seeking is related to poor communication outcomes in children with ASD.

The amygdala theory
There is a growing body of tentative evidence indicating the amygdala's involvement in the development of autism. The amygdala theory of autism focuses on the importance of the amygdala in relation to social functioning and observes that autism is largely a severe impairment of social functioning. The amygdala is thought to be associated with the fight or flight response in animals and its activity is heavily correlated with fear in humans. Additionally, it has been heavily implicated in relation to social functioning in various animal studies. Evidence suggests an amygdala hyperactivity model may be more accurate than one comparing it to a lesion.

Lesion studies have shown that amygdala damage results in severe social impairment among animal models. Vervet monkey mothers with amygdala lesions were shown to be much less caring with their young neglecting and even abusing them. Rats with amygdala ablations become much more docile. Monkeys with lesions to the anterior temporal lobe develop a disorder known as Klüver–Bucy syndrome, characterized by loss of fear, hypersexuality, hyperorality, and an inability to recognize visual objects (often, but not always).

Evidence shows the amygdala accounts for the emotional, oral, and sexual abnormalities listed above. These abnormalities coincide with several characteristics of the diagnostic guidelines for autism, at least passably for an animal model.

Post-mortem analysis of humans shows an increased neuronal density in the amygdala in autism compared to controls, indicating a potential linkage and supporting the hyperactivity model.

Several studies presented subjects with ASD photographs of human eyes and had them report the emotional state of the person in the picture. A smaller amygdala was associated with increased response time but not decreased accuracy. There was also significantly less amygdaloid activation in the brains of those with ASD, than controls. Subjects compensated for this lack of amygdaloid activity with increased activation in the temporal lobe, and are associated with verbally labeling images. This activity is thought to imply less usage of emotional/social cues to identify objects and rather more objective, factually based processing. One may extrapolate from this model that patients with autism may learn that a specific facial configuration represents an emotional state and what that emotional state implies socially, but they may not come to truly understand how that person feels. This supports a theory of mind deficit, or inability to empathize with others – a characteristic symptom of ASD.

Studies conducted specifically on nonverbal autistics provide similar evidence. Brain studies have shown several amygdaloid impairments among those with ASD. The amygdala in those with nonverbal autism have less volume compared to controls, contain a higher density of neurons suggesting hyperconnection, and show a negative correlation between amygdala size and impairment severity among subjects.

Infantile autism is actually associated with an oversized amygdala, there are developmental theories as to how this may occur. Research on major depressive disorder has shown that excessive activation such as stress or fear leads to allostasis, or degeneration of the neurons involved in creating the phenomenon. Initial hypertrophy results in atrophy and reduction of brain size in the given region. Over time, this occurs in patients with severe depression and they develop a decreased amygdala size. Some scientists theorize that this is happening early during infancy the autistic brain, accounting for the initial overgrowth and later observed size reduction.

When eye tracking software is employed to record where subjects focus their visual attention on images of human faces, small amygdala volume is associated with decreased eye fixation. Eyes are considered to be especially important for establishing human connection and conveying emotion, thus fixation on them is considered to be a crucial part of identifying people and emotions in a social setting.

In addition to a negative correlation to eye fixation studies showed a smaller amygdala was associated with impairment in nonverbal communication skills as well. This suggests that the amygdala is critical in developing all types of communicative abilities, not just verbal. This suggests the amygdala may play a crucial role in relating to other humans in a way that allows for behavioral mimicry.

Among patients with nonverbal ASD researchers could predict symptom severity based on amygdala activity. Those with the least amygdala activity had the most impaired nonverbal communication abilities, those with the most activity had the strongest communication abilities.

The development of language, similar to the development of most physical skills, relies heavily on mimicry of other humans. ASDs are known to impair one's ability to focus on and relate with people possibly as a result of a damaged amygdala. Nonverbal autistics will often be able to learn more basic communicative skills such as pointing to objects or selecting a picture from a list. These skills are far more simple and do not require the degree of personal connection needed for language development.

It is important to note that these studies must be considered with great caution. Cross-sectional studies can only suggest so much about the pathology of a disorder. Further study, particular longitudinal studies, are needed to gain a more complete understanding. It is also important to recognize that most disorders arise from a complex interworking of the entire brain and restricting a theory to one subsystem would be a mistake, this theory merely suggests how the amygdala may be involved with develop of ASD and provides evidence to support an association.

Language outcomes
For nonverbal grade school children and adolescents with autism, communication systems and interventions have been implemented to enhance language and communication outcomes. Speech-generating devices such as tablet computers use visual displays for children who lack verbal language, giving them the task of selecting icons indicating a request or need. For adolescents with nonverbal autism, interventions can condition them to learn more advanced operations on speech-generating devices that require more steps (i.e. turning on device, scrolling through pages), which would allow them to enhance their communicative abilities independently.

The picture exchange system (PECS) is a form of spontaneous communication for children with autism in which an individual selects a picture indicating a request. PECS can be utilized in educational settings and at the child’s home. Longitudinal studies suggest PECS can have long-term positive outcomes for school-aged children with nonverbal autism, specifically their social-communicative skills, such as higher frequencies of joint attention and initiation, and duration of cooperative play, which are all important roles in improving language outcomes.

It has also been suggested that a significant stage in acquiring verbal language is learning how to identify and reproduce syllables of words. One study found that nonverbal and minimally verbal children with autism are capable of enhancing their oral production and vocalizing written words by isolating each syllable of a word one at a time. The process of breaking down a syllable at a time and having it visually displayed and audibly available to the child can prompt him or her to imitate and create nonrandom and meaningful utterances.

Most of these studies contain small sample sizes and were pilot studies, making additional research significant to assess whether these findings can be generalized to all age groups of the same population. Furthermore, most studies on nonverbal autism and speech-generating device communication were based on more basic skills, such as naming pictures and making requests for stimuli, while studies in advanced communication is limited.

Organic causes
Organic causes of mutism may stem from several different sources. One cause of muteness may be problems with the physiognomy involved in speech, for example, the mouth or tongue. Mutism may be due to apraxia, that is, problems with coordination of muscles involved in speech. Another cause may be a medical condition impacting the physical structures involved in speech, for example, loss of voice due to the injury, paralysis, or illness of the larynx. Neurological damage due to stroke may cause loss or impairment of speech. Neurological damage or problems with development of the area of the brain involved in speech production, Broca's area, may cause muteness. Trauma or injury to Broca's area, located in the left inferior frontal cortex of the brain, can cause muteness. Muteness may follow brain surgery. For example, there is a spectrum of possible neurobehavioural deficits in the Posterior Fossa Syndrome in children following cerebellar tumor surgery.

Psychological causes
When children do not speak, psychological problems or emotional stress, such as anxiety, may be involved. Children may not speak due to suffering from Selective mutism. Selective mutism is a condition in which the child speaks only in certain situations or with certain people, such as close family members. Assessment is needed to rule out possible illness or other conditions and to determine treatment. Prevalence is low, but not as rare as once thought. Selective mutism should not be confused with a child who does not speak and cannot speak due to physical disabilities. It is common for symptoms to occur before the age of five. Not all children express the same symptoms.

Selective Mutism may occur in conjunction with Autism Spectrum Disorder, or other diagnoses. Differential diagnosis between Selective Mutism and language delay associated with Autism Spectrum Disorder or other disorders is needed to determine appropriate treatment.

Adults who previously had speech and subsequently ceased talking may not speak for psychological or emotional reasons, though this is rare as a cause for adults. Absence or paucity of speech in adults may also be associated with specific psychiatric disorders.

Developmental/Neurological causes
Absence of speech in children may involve communication disorders or language delays. Communication disorders or developmental language delays may occur for several different reasons.

Language delays may be associated with other developmental delays. For example, children with Down syndrome often have impaired language and speech.

Children with Autism, categorized as a neurodevelopmental disorder in the DSM-V, often demonstrate language delays. Recent studies have found that children with Autism and language delays are often more able to benefit from treatment services to help build language than was previously believed.

Treatment
For language delays or communication disorders in children, early assessment is strongly recommended. Language delays may impact expressive language, receptive language, or both. Communication disorders may impact articulation, fluency (stuttering) and other specified and unspecified communication disorders. Treatment focuses on the diagnosed condition. For example, Speech and Language Services may focus on the production of speech sounds for children with phonological challenges. Overall, early intervention for young children with language or other developmental delays is strongly recommended.

For toddlers with language delay who may also have Autism, early intervention services focusing on speech production is strongly recommended. When absence of speech is observed in children who may also be diagnosed with Autism, assessment is also strongly recommended. Intervention services and treatment programs have been specifically developed for children with Autism and language delays. For example, Pivotal Response Treatment is a well-established and researched intervention that includes family participation. Mark Sundberg's Verbal Behavior framework is another well-established assessment and treatment modality that is incorporated into many Applied Behavior Analysis (ABA) early intervention treatment programs for young children with autism and communication challenges.

Treatment for absence of speech due to Apraxia, involves assessment, and, based on the assessment, Occupational Therapy, Physical Therapy, and/or Speech Therapy.

Treatment for Selective Mutism involves assessment, counseling, and positive supports.

Treatment for absence of speech in adults who previously had speech involves assessment to determine cause, including medical and surgery related causes, followed by appropriate treatment or management. Treatment may involve counseling, or rehabilitation services, depending upon cause of loss of speech.

Management
Management involves use of appropriate assistive devices sometimes called alternative or augmentative communications. Suitability and appropriateness of modality will depend on users physical abilities and cognitive functioning.

Augmentative or assistive communication technology ranges from elaborated software for iPads to enable complex communication with an auditory component to less technologically involved strategies. For example, a common method involves use of pictures that can be attached to velcro strips to create an accessible communication modality that does not require the cognitive or fine motor skills needed to manipulate an iPad.

Speech-generating devices can help individuals with speech deficiencies associated with medical conditions that affect speech, communication disorders that impair speech, or surgeries that have impacted speech. Speech-generating device s continue to improve in ease of use.