The prevalence of autistic spectrum disorders (ASD) is higher in Deaf people than in hearing people. However, conditions that mimic ASD associated with language deprivation are even higher and many people who have ASD appear to have an uncomfortable relationship with sound.
The causes of ASD are still not really understood. Whilst a genetic link has been confirmed, there are also causal associations between neurological vulnerability and ASD.
Sometimes the cause of deafness threatens the functional integrity of the brain (e.g. CMV, meningitis, rubella, prematurity and some syndromic causes of deafness) so it is not surprising to find that rates of ASD rise. As survival rates for infants with these problems rise, so will the proportion of Deaf people who have ASD.
Diagnosis of deafness and autism
There is an international shortage of professionals who have experience of working with Deaf people who have ASD and even fewer who are deaf themselves or who can sign. Hence, misdiagnosis (both over and under-diagnosis) is more common than in an equivalent hearing population. However, even for clinicians who are experienced in working with Deaf people, accurate diagnosis of ASD is made difficult by the features of many Deaf people’s development that result in conditions that can create a presentation that mimics ASD.
Just as increasing rates of ASDs have been reported in the general population, increasing rates of ASD have been identified among children who are Deaf. Furthermore, research has indicated that when compared with estimates among the general population a greater than expected number of children who are Deaf are also reported to have an ASD. In looking at rates of hearing loss among children with autism, Rosenhall and colleagues (1999) reported a prevalence of profound hearing loss to be 10 times greater than the 0.1 to 0.2% commonly reported within the general population.
For example, similar to children with autism, children who are Deaf may demonstrate language delays, and theory of mind delays.
Currently, distinguishing symptoms of autism as distinct from features commonly associated with hearing loss depends largely on the clinical knowledge of professionals due to limited research describing this complex comorbidity. With few providers trained in both autism and deafness and several potential overlapping symptoms, it is not surprising that research suggests limited diagnostic agreement among professionals when diagnosing ASD among Deaf children.
Language deprivation in childhood
Up until approximately the time children go to primary school (5-7 years) the human brain is primed to develop sophisticated, abstract and social language. Without a rich language environment in these early years, language deprivation (permanently stunted language) is possible. This is alarmingly common in Deaf people and particularly shocking in that much of it is preventable. Recent research has found that 20% of Deaf children were unable, despite normal non-verbal IQs, to be interviewed either in speech or sign because of poor language skills.
Without language it is difficult to achieve the degree of social interaction to learn about the mental states of other people and how their behaviour may relate to these. This can result in poor Theory of Mind (ToM) and it is clear that it is language deprivation and not deafness that is linked to poor ToM. Other research has found that, whilst Deaf children of deaf parents develop ToM at the same time as hearing children of hearing parents (around 3-5 years), Deaf children of hearing parents often have significant delays in ToM.
It is possible to develop ToM late, and it is certainly better than none at all. However, without it the child misses significant opportunities to develop socially and cognitively by aiding in new learning and social situations, for example dealing with peer groups, authority relationships with teachers, and romantic relationships.
Crucial in the assessment of a Deaf child is to analyse the degree to which they are motivated to communicate, indicating autistic features, rather than if they are able to communicate.
Language Deprivation in Adults
Some researchers have suggested that the effects of language deprivation in adults who have been deaf since birth, or at least their formative years, is a separate and sometimes distinct diagnostic category. Historically the problems with dysfluency and emotional, behavioural and social difficulties experienced by some Deaf adults have been described. More recently, the term of Language Deprivation Syndrome (LDS) has been proposed.
At All Saints Hospital we are interested in exploring LDS as the link between deafness and autism in the aspects of assessment and treatment. The overlap and distinct features of the three conditions are summarised in the figure below.
Professor Brendan Monteiro, All Saints Hospital