Comprehensive Study Notes on Attention Deficit Hyperactivity Disorder (ADHD) and Autism Neurobiology

Overview of ADHD Subtypes and Prevalence

• Definition and Classification: Attention Deficit Hyperactivity Disorder (ADHD) is classified as a neurodevelopmental disorder because symptoms are fundamentally present in childhood, even if a formal diagnosis does not occur until adulthood.

• Subtypes of ADHD:     * ADHD-I (Inattention): Characterized primarily by symptoms of inattention.     * ADHD-HI (Hyperactivity/Impulsivity): Characterized primarily by symptoms of hyperactivity and impulsivity.     * ADHD-C (Combined): Characterized by a combination of both inattentive and hyperactive-impulsive symptoms.

• Diagnostic Clarification: "Attention Deficit Disorder" (ADD) is noted as not being a valid current diagnosis; the standardized term is ADHD with the specified subtype.

• Global Prevalence Statistics: According to Faraone et al. (2021), the estimated global prevalence is:     * Children and Youth: $5.9\%$     * Adults: $2.5\%$

DSM-5 Diagnosis: Primary Criteria (Criteria A)

• General Definition: Criteria A requires a persistent pattern of inattention and/or hyperactivity-impulsivity that interferes with functioning or development. These are characterized by two distinct lists of $9$ symptoms each.

• Symptom Thresholds:     * For individuals under the age of $17$, six (or more) symptoms from a specific list must have persisted for at least $6$ months.     * For individuals aged $17$ and older, five (or more) symptoms are required.     * The symptoms must be to a degree that is inconsistent with developmental level and negatively impacts social, academic, or occupational activities directly.

• Inattention Symptom Examples:     * Often fails to give close attention to details or makes careless mistakes in schoolwork, work, or other activities.     * Often does not follow through on instructions and fails to finish schoolwork, chores, or duties in the workplace (e.g., the individual loses focus or is easily side-tracked).     * Often avoids, dislikes, or is reluctant to do tasks that require mental effort over a long period of time, such as schoolwork or homework.

• Hyperactivity and Impulsivity Symptom Examples:     * Often fidgets with or taps hands or feet, or squirms in seat.     * Often has trouble waiting his/her turn (e.g., while waiting in line).     * Often interrupts or intrudes on others. This includes butting into conversations, games, or activities; using other people’s things without asking or receiving permission; and, for adolescents/adults, taking over what others are doing.

• Subtype Requirement Summary:     * ADHD-I: Meets the threshold ($6+$ symptoms) from the Inattention list.     * ADHD-HI: Meets the threshold ($6+$ symptoms) from the Hyperactivity/Impulsivity list.     * ADHD-C: Meets the threshold from both the Inattention and Hyperactivity/Impulsivity lists.

• Nature of Symptoms: All diagnostic symptoms categorized under Criteria A are behavioral in nature.

DSM-5: Additional Diagnostic Criteria (Criteria B through E)

• Criteria B (Age of Onset): Several inattentive or hyperactive-impulsive symptoms must have been present before the age of $12$ years.

• Criteria C (Pervasiveness): Several symptoms must be present in two or more settings, such as at home and school/work, with friends/relatives, or in other activities.

• Criteria D (Functional Impairment): There must be clear evidence that the symptoms interfere with, or reduce the overall quality of, social, school, or work functioning.

• Criteria E (Exclusionary Criteria): The symptoms must not occur exclusively during the course of schizophrenia or another psychotic disorder. They must also not be better explained by another mental disorder, such as a mood disorder, anxiety disorder, dissociative disorder, personality disorder, or substance intoxication/withdrawal.

Current Issues in ADHD Diagnosis

• Criticism of Criteria: The DSM-5 diagnostic criteria have been criticized by researchers (Batstra & Frances, 2012) for being too wide.

• High Diagnostic Prevalence: In the US, diagnostic prevalence involves $11.4\%$ of children (Danielson et al., 2024).

• The Overdiagnosis vs. Underdiagnosis Debate: Both issues are considered genuine problems.     * Evidence for Overdiagnosis:         * Relative age effects: The youngest children in a school year have a higher probability of being diagnosed (Layton et al., 2018).         * Harmful effects: Diagnosis can be harmful in mild cases (Kazda et al., 2021).     * Evidence for Underdiagnosis:         * Racial Bias: Documented in studies such as Morgan et al. (2013).         * Gender Bias: Documented in studies such as Mowlem et al. (2019).         * Adult expression: Recognition of how symptoms manifest in adults is often delayed (Agnew-Blais et al., 2016).

Genetic and Environmental Influences on ADHD

• Heritability: ADHD is highly heritable, with genetic influences accounting for $70-80\%$ of variance in twin studies (Faraone et al., 2021).

• Familial Risk: First-degree relatives are $5$ to $10$ times more likely to have ADHD compared to the general population.

• Genetic Complexity: ADHD is polygenetic, with more than $27$ involved genes identified to date.

• Diagnostic Tools: Genetic testing is currently not a valid diagnostic tool for ADHD and will likely remain difficult to develop into one due to this polygenetic nature.

• Environmental Factors: These factors are diverse and interact with genetics. Examples include:     * Prenatal maternal stress.     * Low birthweight.     * Food dye exposure.     * Note: Each individual environmental contributor typically has a small effect.

Stability and Change Over the Lifespan

• The Remission Paradox: Binary diagnosis suggests malleability, as only $15\%$ of diagnosed children meet diagnostic thresholds in adulthood. However, symptom-based analysis suggests chronic consistency.

• Longitudinal Findings: Cross-sectional research indicates $40\%$ to $60\%$ of diagnosed children show symptoms in adulthood. Longitudinal work indicates only approximately $10\%$ of diagnosed children show sustained remission (Sibley et al., 2022).

• Symptom Evolution: Hyperactivity levels tend to decline during adolescence, but this likely reflects the internalization of symptoms rather than disappearance. Inattention tends to remain stable into adulthood.

• Chronic Impact: Even in cases of partial remission, ADHD is associated with reduced educational and occupational attainment, including lower salaries, suggesting it is a chronic, lifespan disorder.

The Neuroscience of ADHD

• Neurotransmitter Pathways: ADHD is associated with reduced activity in specific networks:     * Reward Pathways: Regulated by dopamine.     * Alertness Pathways: Regulated by norepinephrine.     * Medication: This reduced activity is the primary mechanism targeted by stimulant medications.

• Arousal Theory: ADHD is characterized as a compensation for baseline underarousal (Sergeant, 2000).

• Brain Structure: ADHD is associated with reduced brain volume and cortical thickness in key areas like the prefrontal cortex (Valera et al., 2007).     * Often, these structural differences represent a delay in maturation rather than a permanent deficit (Shaw et al., 2007).     * Changes in cortical thickness over time can predict the remission of symptoms (Shaw et al., 2013).

• Brain Connectivity: ADHD involves atypical connectivity, specifically within the Default Mode Network (DMN), which stays active ("on") when it should be suppressed during tasks (Sonuga-Barke & Castellanos, 2007).

Cognitive Pathways of ADHD

• Pathway #1: Executive Dysfunction (Barkley, 1997):     * Core Deficit: Deficits in inhibitory control—the ability to withhold a behavioral response.     * Consequence: Failures in inhibition disrupt other executive functions like attention allocation and working memory.     * Evidence: Measured via stop-signal tasks. Participants habituate a response (pressing for letters but not numbers), then an exception is added (pressing for letters except 'X'). Researchers measure commission errors (pressing when 'X' appears).     * Note: Not all ADHD patients show these deficits (Willcutt et al., 2005).

• Pathway #2: Delay Aversion (Sonuga-Barke et al., 2003):     * Core Deficit: Abnormal reward processing where delay is perceived as more aversive.     * Standard Gratification Task: Children with ADHD prefer a small reward now over a large reward later. However, reward size is confounded with wait time.     * Modified Task: If a child must wait the same amount of total time regardless of choosing the small or large reward, children with ADHD do not differ from neurotypical children.

• Dual Pathway Model: Proposed by Sonuga-Barke, suggesting ADHD can be caused by executive dysfunction, delay aversion, or both. However, many children with ADHD do not show clinically unusual levels of either (Nigg et al., 2005).

Neurological Theories of Autism Review

• Brain Anatomy: While autistic brains are healthy and often do not differ in gross anatomy from neurotypical brains, they are wired differently (Belmonte, 2004; Courchesne & Pierce, 2005).

• Wiring Hypotheses:     * Local Hyperconnectivity: High density of local neuronal connections explains sensory oversensitivity.     * Long-range Hypoconnectivity: Deficits in long-distance connections make distributed processing more difficult.

• Default Mode Network (DMN) in Autism: The DMN is involved in cognitive abilities and emotion regulation. Its development parallels cognitive and social development.

• Symptom Correlation: DMN connectivity and the severity of autistic symptoms are negatively correlated; lower connectivity is associated with higher symptom severity (Yerys et al., 2015).