Did you know that around 70 percent of children and adolescents on the autistic spectrum are diagnosed with an additional disorder — with a significant 41 percent receiving two or more "labels" on top of their autism spectrum disorder? 
Anxiety in children with autism is an especially common additional diagnosis, with a meta-analysis indicating that nearly 40 percent of autistic children suffer from an anxiety or related disorder. The most common anxiety disorders in autistic children and adolescents are specific phobias, obsessive compulsive disorder (OCD) and social anxiety disorder. Generalized anxiety disorder, separation anxiety disorder, and panic disorder are likewise fairly prevalent within this population. 
Why Are Anxiety Disorders More Common In Children With Autism?
While the reasons behind the increased prevalence of anxiety disorders in children on the autism spectrum are not entirely clear at present, researchers have hypothesized that because anxiety is most likely to occur in autistic adolescents rather than younger children, the phenomenon is linked to the process of these young people becoming more aware of their own neurodiversity. The more complex social interactions autistic people are exposed to during adolescence as opposed to earlier childhood could be another cause of anxiety. 
Specific learning disabilities are likewise more frequent in children on the autistic spectrum. In autistic individuals with high IQs but co-existing learning disabilities, this disparity between high ability on the one hand, and disability on the other, might lead to a frustration that triggers anxiety.
Additionally, over-stimulating environments which are frequently subject to change — something that perfectly describes many schools — are particularly unsettling to people on the autism spectrum. As children leave the preschool years and the controlled environment of their own families behind, the chaotic aspects of life can lead to anxiety. 
Does Your Autistic Child Suffer From Anxiety? A Few Things To Watch Out For
Note that people involved in your child's life — such as the school psychologist — often use atypical behavioral characteristics to come to the conclusion that a particular child could be suffering from anxiety. These behaviors include fidgeting, problem-solving difficulties, a lack of concentration, perfectionism, rapid speech, irritability, and a lack of participation in group tasks. 
Misdiagnosing a child with high-functioning autism as suffering from an anxiety disorder is thus a danger. On the other end of the spectrum, it is also possible that an autistic child who does suffer from anxiety misses out on a correct diagnosis because the symptoms of anxiety are misconstrued as simply being characteristic for a child with autism. 
Parents of autistic children may themselves, like mental health and educational professionals, focus on a child's external behaviors when seeking to determine whether their child is anxious — and the behaviors you are seeing may not give you an adequate picture of what is going on in your child's mind. 
How Should Anxiety In Children With Autism Be Managed?
Data on the treatments that most benefit autistic children with comorbid anxiety is, unfortunately, still limited at this point in time. How should anxiety be approached in autistic children and adolescents, then?
Research shows that cognitive behavioral therapy (CBT), one of the two pillars of anxiety management in neurotypical children alongside SSRI antidepressants, is slightly less effective in children with high-functioning autism, but nonetheless has a positive effect.  Research into the possible benefits of SSRI antidepressants for anxiety in children with autism is not very extensive right now, but your child's healthcare provider may nonetheless suggest antidepressants as a means of coping with anxiety .
Another option that has been researched is the so-called Hug Machine, a device that applies pressure to the body to soothe individuals with autism. It was invented by Temple Grandin and has been shown to lead to a significant reduction in anxiety among children on the autistic spectrum. 
In addition, parents may wish to look into herbs to treat anxiety disorders in children, as well as investigating the link between childhood anxiety and nutritional deficiencies — certain supplements, such as Omega-3 fatty acids and selenium, may help reduce the symptoms of anxiety in your autistic child.