Neurodevelopmental disorder

Neurodevelopmental disorders (NDDs) are impairments caused by abnormal development of the brain or central nervous system. The impairments can also be caused by damage in early childhood. Among the most well-known NDDs are autism spectrum disorders (ASD), attention deficit hyperactivity disorder (ADHD), and learning disorders such as dyslexia.

Diagnosis of these disorders at an early stage can resolve some effects and alleviate many symptoms. Research into likely genetic connections remained ongoing and supported by some studies, but the causes were still largely unknown. Some factors may include lead and other environmental contaminants as well as fetal exposure to medications, alcohol, smoking, and other substances. NDDs are more often diagnosed in males than females, although the ratio varies by disorder. For example, the male-to-female ratio for the diagnosis of ASD is 4:1 and 2:1 for ADHD in children.

Background

Although the term neurodevelopmental disorders is a modern invention, documentation of symptoms dates to at least the eighteenth century. Melchior Adam Weikard, a German physician, described the symptoms of attention deficit disorder in a book published in 1775. Terms likely considered offensive in modern times were used as early as the sixteenth century to describe symptoms of intellectual disabilities. In 1800, a feral boy of about twelve was found in France. He was dubbed Victor, the Wild Boy of Aveyron. Descriptions suggest the child was on the autism spectrum.

Over the centuries, researchers developed new theories of human development. They also began to learn how the human brain functions and develops. The brain comprises multiple regions, although it can be classified into three major parts. The brain stem links the upper brain and the spinal cord and controls involuntary functions, such as breathing, and reflexes. The cerebellum, which is located behind the brain stem, is involved in balance and coordination. The largest part of the brain, the cerebrum, is involved in memory and learning. It triples in size during the first year of life. The brain contains a network of nerve cells called neurons that have branch-like structures. The neurons communicate with one another using chemical and electrical signals.

The brain forms networks of neurons while it is developing and uses them to process information by passing signals at connections called synapses. These synapses, which begin to form during the second trimester of prenatal development, are gaps between the neurons. At birth, the brain has about the full number it will ever have, although the brain continues to grow in size, doubling in size the first year. The human body develops up to twice as many synapses in the first three years of life as it will have in adulthood. Throughout childhood and adolescence, the body eliminates many of these synapses.

The process of removing synapses, sometimes called blooming and pruning, is affected by the child’s experiences in early life. Genes oversee formation and placement of neurons, but the brain itself completes its own design based on sensory input, an ability called plasticity. The overabundance of synapses allows the infant and toddler brain to receive stimulation more efficiently. Research has shown, for example, that children who hear more speech growing up retain more synapses between neurons in the language area of the brain. Synapses are strengthened with use, while those rarely engaged stay weak. Weak synapses are more likely to be lost to pruning. Pruning is a way for the brain to retain the most efficient way to process information and remove the rest.

While positive experiences have an important impact on brain development in the early years, so do negative experiences. The effects of neglect, poverty, violence, and other factors on early childhood development has been the subject of study for decades.

Overview

The Diagnostic and Statistical Manual of Mental Disorders (DSM), published by the American Psychiatric Association, places many common neurodevelopmental disorders in children and adolescents into categories. Attention deficit disorders include ADHD combined type and ADHD inattentive type (ADD). Cerebral palsy, developmental dyspraxia, and developmental oral apraxia are classified as developmental movement disorders. Disorders called global developmental disorders that were previously known as "mental retardation" comprise the intellectual disability category. Language and learning disorders include expressive or receptive language disorders, articulation disorders, stuttering, dyslexia (reading difficulty), dyscalculia (mathematics), dysgraphia (writing), and nonverbal learning disorder. Autism spectrum disorder is its own category. Many neurodevelopmental disorders become evident in early childhood. Others begin in older teens and young adults.

Research into the causes of many disorders remained ongoing, but scientists have found evidence that multiple factors may be involved. Intellectual disabilities are often linked to genetic disorders. For example, researchers have found many genes that are risk factors of autism spectrum disorder. Many genes are related to schizophrenia, although no single genetic alteration seems to predominate in cases. Other studies have looked outside for possible causes, including substances in the environment. For example, some researchers believe endocrine disruptors, which include pesticides, medicines, and industrial pollutants, may be related to autism spectrum disorder, ADHD, intellectual disability, and other disorders. Lead poisoning can cause developmental delay and learning difficulties.

People with attention deficit hyperactivity disorder often seem restless or impulsive. The disorder is often diagnosed soon after children begin attending school, when education officials note they seem to have trouble concentrating and following directions. Most patients are diagnosed between the ages of six and twelve years. The three main types are predominantly hyperactive-impulsive, predominantly inattentive, and combined ADHD. Predominantly hyperactive-impulsive typically involves fidgeting, talking, moving frequently, and not staying with a task for long. Individuals are more likely to take risks and have more accidents. Individuals with predominantly inattentive ADHD may be easily distracted. They may have trouble focusing on activities such as conversation. Those with combined ADHD will have symptoms of both other varieties. Risk factors of ADHD include premature birth, low birth weight, brain damage or injury, a family history of ADHD, and prenatal exposure to alcohol, cigarette smoke, drugs, and other toxins. Treatment for ADHD may include behavioral therapy, medications, and psychotherapy, either alone or in combination. Medications are either nonstimulants or stimulants. Stimulants stimulate people without ADHD but have the opposite effect on those with the disorder.

Autism spectrum disorder covers a wide range of conditions, from mild to severe. The symptoms can be evident in infants, but are often noticed when a child is about eighteen months old. Signs of ASD include trouble communicating and difficulties in social situations. Individuals may be sensitive to smells, sounds, or textures. They may try to avoid touch, such as hugs, and avoid eye contact. They may not speak or may begin speaking later than others. Actions may include repetitive behavior, such as flapping hands or repeating the same phrase, and self-harm, such as banging their head. Other signs may include having trouble expressing emotions and not being able to understand abstract ideas or pretend play. Genetic factors appear to be related to ASD. Prenatal factors and environment may also be risk factors. Studies have found no link to childhood vaccinations. Treatment depends on the severity of the disorder. Many people with mild autism complete their education and are successful in their chosen careers. Others may require lifelong supervision and care. Generally, treatment may include behavior therapy, family therapy, speech and communication therapy, and medications. Early diagnosis and intervention are regarded as crucial for those with ASD.

A diagnosis of cerebral palsy can cover a range of disorders caused by damage to the immature brain, often before birth. CP affects movement, muscle tone, and posture. For example, individuals may be floppy, have rigid limbs, move involuntarily, and have abnormal reflexes. The eye muscles, muscles used in swallowing, and joints may also be affected. Some patients can walk, while others are unsteady. Some patients may have intellectual disabilities, while others do not. Blindness, deafness, and epilepsy may also occur with CP. The condition may affect a limb, one side of the body, or the whole body. A number of factors have been linked to CP, including lack of oxygen to the brain during labor and delivery, fetal stroke, gene mutations, and infections during infancy that cause inflammation in the brain or head. Some infections and exposures the pregnant person experiences, including rubella, zika virus, syphilis, and a parasitic infection called toxoplasmosis, can increase a baby’s chances of having CP. Other risk factors include breech birth position, low birth weight, and multiple babies.

Global developmental disorder includes a range of intellectual disabilities. Signs and symptoms include hitting developmental milestones such as rolling over, crawling, and walking much later than usual; talking late or having trouble speaking; having trouble solving problems; having trouble remembering things; needing help with tasks such as getting dressed or going to the bathroom longer than developmentally appropriate. These disorders may be caused by prenatal exposure to toxic substances such as alcohol, infections, malnourishment, brain trauma, and genetic conditions. Treatment may include physical, behavioral, occupational, or speech and language therapies, as well as early childhood special education.

Bibliography

“Attention Deficit Hyperactivity Disorder (ADHD).” National Health Service, 24 Dec. 2021, www.nhs.uk/conditions/attention-deficit-hyperactivity-disorder-adhd/. Accessed 7 Oct. 2024.

Cheroni, Cristina, et al. “Autism Spectrum Disorder at the Crossroad Between Genes and Environment: Contributions, Convergences, and Interactions in ASD Developmental Pathophysiology.” Molecular Autism, vol. 11, no. 1, p. 69, 10 Sept. 2020. doi:10.1186/s13229-020-00370-1. Accessed 7 Oct. 2024.

“Fetal Alcohol Spectrum Disorder: About, Causes and Co-occuring Conditions." Government of Canada, 29 Nov. 2023, www.canada.ca/en/public-health/services/diseases/fetal-alcohol-spectrum-disorder.html. Accessed 7 Oct. 2024.

Iyengar, Sandhyaa, et al. “Adverse Childhood Experiences in Children with Neurodevelopmental Concerns.” Pediatrics, vol. 144, no. 2 Meeting Abstract, 1 Aug. 2019, pediatrics.aappublications.org/content/144/2‗MeetingAbstract/46. Accessed 4 Feb. 2021.

Morris-Rosendahl, Deborah, and Marc-Antoine Crocq. “Neurodevelopmental Disorders—The History and Future of a Diagnostic Concept.” Dialogues in Clinical Neuroscience, vol. 22, no. 1, Mar. 2020, pp. 65–72. doi:10.31887/DCNS.2020.22.1/macrocq. Accessed 7 Oct. 2024.

“Neurodevelopmental Disorders.” NeuronUP, neuronup.us/neurorehabilitation/neurodevelopmental-disorders/. Accessed 7 Oct. 2024.

Niebuhr, Virginia. “Neurodevelopmental Disorders; Mood and Anxiety Disorders.” The University of Texas Medical Branch, 20 Oct. 2017, www.utmb.edu/pedi‗ed/CoreV2/Development/Development8.html. Accessed 4 Feb. 2021.

"What Are Neurodevelopmental Disorders?" MRC Centre for Neurodevelopmental Disorders, devneuro.org/cndd/#faqs-home. Accessed 7 Oct. 2024.