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Neurodevelopmental Disorders

Neurodevelopmental disorders include the syndromes previously known as developmental disorders. The prefix neuro- was added to the label in DSM-5, reflecting the fact that the disorders always involve the nervous system.

Neurodevelopmental disorders typically show themselves early, often before school age. They come in several major groups:

Many neurodevelopmental disorders (in addition to those with known genetic causes) tend to run in families. They are presumed to have genetic origins.

Other suspected causes include environmental toxins of all types. Research into neurodevelopmental disorders has investigated effects ranging from maternal drug use such as alcohol and tobacco use, mercury, PCBs, lead, pesticides, flame retardants, components of commercial plastic products, byproducts of gasoline and diesel fuel combustion, and naturally occurring poisons such as arsenic.

Many studies have found weak associations between toxins and developmental problems. However, such research is necessarily correlational (no group of children can be randomly assigned to toxin exposure).

Confounded variables are always present in such correlational research, because the same group might be exposed to multiple risk factors, such as poverty, tobacco use, lead contamination, and pollution. All of this makes interpretation of research difficult.

In the United States, the National Toxicology Program (NTP) concluded that childhood lead exposure was associated with reduced cognitive function and lower academic achievement. The Environmental Protection Agency similarly determined that mercury exposure could have neurotoxic effects on humans, as verified by aftereffects of mercury poisoning incidents in Japan and elsewhere.

Neurodevelopmental disorders include most of the well-known syndromes that include intellectual disability, such as Down syndrome. Other developmental disorders affect only one isolated part of the intellect.

An example is dyslexia (classified as a communication disorder) which specifically involves reading difficulties. Other developmental disorders are complex but distinctive syndromes with widely varying effects, such as autism.

What all developmental disorders have in common is that they represent unusual limits or blockades on development. Alternatively, a development problem can produce excesses in behavior, such as uncontrolled motor movements or activity levels that impede learning, as with ADHD (attention deficit syndrome with hyper­activity). Sometimes more than one disorder co-exists in the same child.

Down Syndrome

Down Syndrome is the most common form of intellectual disability. It is caused by a common genetic problem that can be diagnosed while a baby is still in the womb. Outcomes vary widely; some individuals with Down Syndrome attend college, while others are profoundly disabled.

What is Down Syndrome?

Physical symptoms of Down Syndrome children may include the following:

–short, wide, or broad body parts: e.g. a short thick neck; a large tongue which may protrude involuntarily; small, broad, flat hands and feet; lack of overall body height

–a distinctive facial appearance, including small teeth; underdevel­oped bones of the nose; small, round, bulging or slanted-looking eyes with a fold of skin near the bridge of the nose

–a variety of medical problems, not inevitable, but more common among Down Syndrome children: heart disorders, eye problems, a rare type of leukemia, missing or misaligned teeth, and metabolic irregularities

Why is it not a category in DSM-5?

Down Syndrome is not explicitly address­ed in DSM-5 because DSM is a diag­nostic manual, and Down Syndrome can be diagnosed with certainty using genetic tests. If DSM-5 were to include every genetic disorder, it would probably triple in size. If Down Syndrome is accompanied by intellectual disability, that has a DSM-5 code.

The ICD (International Catalog of Diseases) includes codes for Down Syndrome. It also has separate codes for each typical symptom of Down Syndrome, such as an abnormally high-arched palate.

What does it mean to say "trisomy of chromosome 21"?

Down Syndrome is caused by failure of chromosome pair 21 to separate during meiosis in the developing human ovum or egg. The Down Syndrome child ends up with 47 chromosomes instead of 46, because there are three chromosomes where there should be two. That is the meaning of the word trisomy–literally three-ism–when people describe the genetic basis of Down Syndrome as trisomy of chromosome 21.

What is Lesch-Nyhan syndrome and what point does it illustrate here?

Down Syndrome dominates public discussions of developmental disabil­ities because it is the most common, affecting about 1 in 800 children. But there are hundreds, perhaps thousands, of more rare genetic syndromes, each with its own unique characteristics.

For example, Lesch-Nyhan Syndrome, afflicts 1 in 50,000 boys. For those affected, it is an all-consuming problem. Victims of Lesch-Nyhan Syndrome mutilate themselves if given a chance.

If restrained, they appear happy; if released, they cry, bite themselves, and strike out at objects and people. Most die before adulthood. Many other rare genetic disorders produce equally unique and distinctive symptoms.

Children with Down Syndrome are characteristically good-natured and friendly. Each is an individual, however, and cases vary dramatically.

Relatively new to the awareness of clinicians is dual-diagnosis or "co-morbidity." In these cases, Down Syndrome is combined with a known psychiatric syndrome like autism spectrum disorder (ASD) or obsessive-compulsive disorder (OCD). Dr. George T Capone wrote:

Sometimes parents bring their child with Down syndrome to the clinic–not always for the first time–and they are deeply distraught about a change in their child's behavior or development. Sometimes they describe situations and isolated concerns that worry them such as their child has stopped learning new signs or using speech.

...When they call to him, he doesn't look at them. Maybe he isn't hearing well? He will only eat 3 or 4 foods. The suggestion of a new food, or even an old favorite, brings about a tantrum like no other.

...Getting him to stop staring at the lights is sometimes difficult and may result in a scene. He requires a certain order to things. Moving a chair to another spot in the room upsets him until it is returned to its usual spot. (Capone, 2012)

Dr. Capone is describing typical early symptoms of autism. "Until recently, it was commonly believed that the two conditions could not exist together," he notes. Clinicians are awakening to the fact that they can co-exist.


Capone, G.T. (2012) Down syndrome and autistic spectrum disorder: A look at what we know. [National Down Syndrome Society] Retrieved from:

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