The ADHD Subtypes
Did you know there are different forms of ADHD? In fact, there are three distinct types. When it comes to children, ADHD Hyperactive Type (ADHD-H) is the most recognized. In adults, ADHD Inattentive Type (ADHD-I) is more common. But what sets these types apart?
As a neurodivergent clinician specializing in working with neurodivergent adults, and someone diagnosed late in life with both ADHD and Autism, I understand the abundance of misinformation about various neurotypes. In this article, I will explore the different types of ADHD and how to distinguish between them. I’ve also included information on the diagnostic criteria for ADHD to help demystify the process of diagnosis.
Contents:
Overview of the ADHD Subtypes
Before getting into the similarities and differences between the different ADHD types, let’s define them. There are three types: ADHD Inattentive type (ADHD-I), ADHD Hyperactive and Impulsive type (ADHD-H), and ADHD Combined type (ADHD-C).
ADHD-I is characterized by difficulties regulating attention
ADHD-H is characterized by impulsive and hyperactive behavior
ADHD-C is characterized by both inattention and hyperactivity/impulsivity
Let’s get into it.
ADHD Inattentive Type (ADHD-I)
ADHD-inattentive type (formally known as ADD) is marked by:
Difficulty regulating attention
A tendency to make careless mistakes
Overlooking details, distractibility
Difficulty organizing and finishing tasks
Forgetfulness
Difficulty listening
Difficulty sequencing events or following detailed instructions
Difficulty with routine chores.
Working memory and processing speed are often impacted in the context of ADHD-inattentive type (processing speed may not become impacted until too many tasks are placed on the person). This type is the most common type diagnosed among adults and females.
Inattentive ADHD does not look like the stereotypical presentation of ADHD. Most people think of a young child who is struggling to sit in their seat or running around tirelessly. However, ADHD-I can look like:
daydreaming quietly in class
feeling anxious or sad
difficulty listening (which may be attributed to being “spacey.”
shyness
people-pleasing
trouble maintaining friendships
picking at cuticles or skin
being a perfectionist
People with ADHD-I are more likely to have Sluggish Cognitive Tempo (SCT). SCT was described by Russel Barklay, and is characterized by dreaminess, mental fogginess, slow working memory, hypoactivity (less activity), staring frequently, inconsistent alertness, and excessive daydreaming. It is estimated that 30-63% of people with inattentive type ADHD also have SCT (Fassbender et al., 2015)
Inattentive ADHD may be the most difficult to diagnose as it is often dismissed as the person being “spacey”, or “apathetic”, or experiences are attributed to anxiety or a mood disorder. Even when diagnosed, they are less likely to receive medication for their ADHD. Willcutt, 2012 found that while ADHD-I was the most common subtype, people with ADHD-C were more likely to be referred for clinical services. This finding suggests that those with ADHD-I may need to take extra initiative and engage in self-advocacy to ensure they obtain the appropriate support and services.
ADHD Hyperactive and Impulsive Type (ADHD-H)
ADHD-Hyperactivity is the most common form of ADHD diagnosed in preschoolers. It is associated with behavioral difficulties in early childhood and co-occurs with Oppositional Defiance Disorder (ODD) and Conduct Disorder (CD) at high rates (Bendiksen et al., 2014). ADHD-H is characterized by the need for constant movement. Such folks often fidget, squirm, and get up from their seat to walk around or stand. Children with ADHD-H are often described as if they are “driven by a motor” (running around excessively). Hyperactivity may also show up as excessive talking, difficulty waiting their turn, and difficulty with self-control may lead to impulsive behaviors (blurting out answers, engaging in risky activities, and so forth). This type of ADHD is the most recognizable and thus diagnosed more readily than ADHD-I. It is more commonly diagnosed among children and men.
ADHD Combined Type (ADHD-C)
ADHD-C has a high prevalence rate and is the most common presentation among children.
Combined type ADHD is diagnosed when a person presents with six out of the nine symptoms of both inattention and hyperactivity. ADHD-C also co-occurs with externalizing disorders such as ODD and CD at high rates (Bendiksen et al., 2014). People with ADHD-C often have co-occurring internalizing disorders (anxiety and depression).
ADHD Screener: While we’re on the topic of the DSM and diagnosis, the ASRS is the most widely used screener for ADHD, and this instrument aligns closely with the DSM criteria.
DSM-5 Criteria for the ADHD Subtypes
ADHD-I
Here is how ADHD-I is diagnosed based on the DSM-5 criteria. Six of the following nine symptoms of inattention must be present for the last six months:
Often fails to give close attention to details or makes careless mistakes in schoolwork, at work, or during other activities (e.g., overlooks or misses details, work is inaccurate).
Often has difficulty sustaining attention in tasks or play activities (e.g., has difficulty remaining focused during lectures, conversations, or lengthy reading).
Often does not seem to listen when spoken to directly (e.g., the mind seems elsewhere, even in the absence of any obvious distraction).
Often does not follow through on instructions and fails to finish schoolwork, chores, or duties in the workplace (e.g., starts tasks but quickly loses focus and is easily sidetracked).
Often has difficulty organizing tasks and activities (e.g., difficulty managing sequential tasks; difficulty keeping materials and belongings in order; messy, disorganized work; has poor time management; fails to meet deadlines).
Often avoids, dislikes, or is reluctant to engage in tasks that require sustained mental effort (e.g., schoolwork or homework; for older adolescents and adults, preparing reports, completing forms, reviewing lengthy papers).
Often loses things necessary for tasks or activities (e.g., school materials, pencils, books, tools, wallets, keys, paperwork, eyeglasses, mobile telephones).
Is often easily distracted by extraneous stimuli (for older adolescents and adults, may include unrelated thoughts).
Is often forgetful in daily activities (e.g., doing chores, running errands; for older adolescents and adults, returning calls, paying bills, keeping appointments)
ADHD-H
To meet the criteria of ADHD-H, six of the following nine symptoms of hyperactivity must be present for the last six months:
1. Often fidgets with or taps hands or feet or squirms in seat.
2. Often leaves seat in situations when remaining seated is expected (e.g., leaves his or her place in the classroom, in the office or other workplace, or in other situations that require remaining in place).
3. Often runs about or climbs in situations where it is inappropriate. (Note: In adolescents or adults, may be limited to feeling restless.)
4. Often unable to play or engage in leisure activities quietly.
5. Is often “on the go,” acting as if “driven by a motor” (e.g., is unable to be or uncomfortable being still for an extended time, as in restaurants, meetings; may be experienced by others as being restless or difficult to keep up with).
6. Often talks excessively.
7. Often blurts out an answer before a question has been completed (e.g., completes people’s sentences; cannot wait for turn in conversation).
8. Often has difficulty waiting their turn (e.g., while waiting in line).
9. Often interrupts or intrudes on others (e.g., butts into conversations, games, or activities; may start using other people’s things without asking or receiving permission; for adolescents and adults, may intrude into or take over what others are doing).
ADHD-C
In order to be diagnosed with ADHD-C based on the DSM-5, one must meet at least six of the ADHD-I criteria of the ADHD-H criteria for the last six months.
Prevalence of the ADHD Subtypes
Given how the prevalence rates ebb and flow throughout the lifespan, it is helpful to think about these subtypes with a bit of fluidity. The diagnosis provided at one specific time may differ from that provided at another specific time in a person’s life. For example, a young child may be diagnosed ADHD-H, and in mid-childhood meet the full criteria for ADHD-C, and in adulthood, only meet the criteria for ADHD-I. Whether a person has ADHD or not, how the symptoms present at a given point in time or a given context has a great deal of flux, and thus it’s helpful to hold the framework of subsets with flexibility.
I found the breakdown of the prevalence throughout the lifespan quite interesting. In a comprehensive metanalysis of ADHD studies Willcutt, 2012 observed the following prevalence rates for ADHD subtypes:
ADHD-I
Preschool: 23% of children with ADHD
Elementary School: 45% of children with ADHD
Adolescence: 72% of teens with ADHD
Adult: 47% of people with ADHD--the most common subtype among adults
ADHD-H
Preschool: 52% of children with ADHD
Elementary: 26% of children with ADHD
Adolescence: 14% of children with ADHD
ADHD- C
Preschool: 25% of children with ADHD
Elementary: 29% of children with ADHD
Declined in samples of adolescents and adults
Similarities and Differences Across ADHD Subtypes
While their presentation differs in several ways--other characteristics are consistent across the board. Some have suggested ADHD-I be considered a separate condition; however, it appears that there is significant enough neuropsychological overlap that ADHD-I should not be treated as a separate disorder. It’s helpful to note that many of the topics below (executive functioning, processing speed) have a lot of variability within the research studies, which speaks to the complexity and variance of ADHD.
Similarities Across ADHD Subtypes
Executive Functioning
The studies on executive functioning (EF) and ADHD subtypes are inconsistent. In the late 1990s, Barkley (1997) hypothesized that ADHD-C and ADHD-H were associated with executive functioning deficits while ADHD-I was not (Geurts et al., 2005). However, this theory has largely fallen out of favor as there is a robust body of research that points to the likelihood that executive functioning challenges are similar across both groups.
(Geurts et al., 2005) studied a group of ADHD-C and ADHD-I children to see if they demonstrated differences in executive functioning and, thus, if they represented distinct disorders. Their findings suggest a similar impact on executive functioning, particularly impairment in inhibition, was noted in both groups.
In a study done by Bahçivan et al., 2015 found similar executive functioning between ADHD-I and ADHD-C on measures of inhibition, set-shifting, verbal fluency, cognitive flexibility, and planning; however, ADHD-I demonstrated significantly higher performances in verbal working memory and verbal category-shifting than children in the ADHD-C group. ADHD-I struggled more with indecision than ADHD-C.
In a study done by Nigg et al, 2002, they compared executive functioning among ADHD-I and ADHD-C and found that the groups did not differ on most domains (ADHD-C struggled with planning more, and motor inhibition was elevated among boys with ADHD-C).
While studies are mixed, it appears there is consensus that similar neuropsychological processes are at play within all subtypes of ADHD, and all experience executive functioning difficulties. However, these will vary from person to person and will likely be influenced by other factors (gender, temperament, presence of co-occurring conditions).
Auditory Processing
Auditory processing disorder is common among ADHDers. In a study done involving ADHD children and adolescence, no difference within auditory processing was found among subtypes (Ghanizadeh, 2009). ADHD children with co-occurring ODD had higher rates of auditorily processing problems and were more likely to under-register sound (hyp0sensitive) (Ghanizadeh, 2009).
Sensory Processing
While sensory processing problems are not technically part of the DSM criteria, there is increasing awareness around the high co-occurrence of sensory processing problems and ADHD. A high percentage of ADHD children also have sensory processing problems. In a meta-analysis, Ghanizadeh, 2011, found a significant relationship between sensory processing problems and ADHD (both subtypes).
Children with ADHD have more difficulties in tactile processing (Hern, 1992)
Sensory over-responsivity in ADHD is associated with anxiety. ADHD with sensory sensitivity (sensory over-responsivity) had higher anxiety levels than ADHD children without (Ghanizadeh, 2011).
The level of touch sensitivity is higher in ADHD females than ADHD males (Ghanizadeh, 2011). One study found that sensitivity to touch was higher in girls, but ADHD boys were not different from control boys Bröring, 2008Balance control and vestibular systems: More than a third of ADHD children experience poor balance and coordination, which is related to sensory inputs and integration and difficulty inhibiting movement (Ghanizadeh, 2011).
Sensory processing problems can be useful for differentiating ADHD children from non-ADHD children. However, it doesn’t distinguish ADHD from other conditions (such as autism). (Ghanizadeh, 2011).
ADHD children with sensory sensitives typically have another co-occurring condition, such as anxiety. (Ghanizadeh, 2011).
ADHD children with co-occurring ODD have higher rates of sensory processing differences (particularly auditory processing). Such children tend to be under-responsive to sounds and sensory input.
Differences Across ADHD Subtypes
Processing Speed
Processing speed is associated with how quickly our brains can process and work with information and stimuli. ADHDers don’t necessarily have impaired speed overall, but processing speed slows when the tasks become more complex or more demands are clumped together (Kibby et al., 2018). While working memory difficulties occurs in both subtypes, it appears that processing speed is more consistently impacted in ADHD-I. As reported in Adalio et al, 2019, children with ADHD-I have been shown to have more processing speed deficits when compared to:
typically developing children (e.g., Arnett, Pennington, Willcutt, DeFries, & Olson, 2015; Rossi et al., 2015)
children ADHD-H (e.g., Hellwig-Brida, Daseking, Petermann, & Goldbeck, 2010)
children with ADHD-C (e.g., Mayes, Calhoun, Chase, Mink, & Stagg, 2009)
Similarly, adult ADHD with hyperactivity features is associated with faster processing speed (Nigg et al., 2005), while ADHD-inattentive type is associated with slower processing speed (Hunt, Bienstock, & Qiang, 2012; (Nigg et al., 2005)
Age of Onset/Age of Diagnosis
Symptoms of ADHD-I may show up later in life and are certainly more likely to be diagnosed later in life than ADHD-H and ADHD-C. Willcutt, 2012 found that approximately 10-25% of ADHD children had an age of onset after seven years of age—and that this occurred more frequently among children with ADHD-I. Complicating matters, ADHD-I may be attributed to anxiety or other mood disorders and thus may be diagnosed later in life.
Impairment Differences
First of all, I don’t love the use of the word “impairment,” and yet at the same time, assessing the impact of the symptoms/traits on a person’s functioning is an integral part of providing medical assessments. But it’s an important thing to understand here, and here is why:
In order to get a diagnosis of ADHD, a person must show “impairment” in more than one environment. However, emerging research suggests that this criterion may contribute to people with ADHD-I and ADHD-H being missed more often. Emerging research suggests that impact on functioning (i.e., impairment) has some differences among the three subtypes. While ADHD-C often shows “impairment” in both school and home (or work and home), a subset of people with ADHD-I and ADHD-H only shows significant impairment in a single setting at a single point in time (Willicutt, 2012).
ADHD-I is most likely to show impairment in school because of the attention demands. At the same time, ADHD-H is more likely to show impairment at home due to behavioral difficulties, and people with ADHD-C are the most likely to show impairment in multiple contexts. This is a significant consideration because according to the DSM-5 a person must meet “impairment” in more than one domain. This makes us vulnerable to missing people with ADHD-I and ADHD-H. As Willcutt, 2012 observes, these findings “suggest that the validity of the cross-setting impairment criterion should be systematically evaluated in future studies to clarify the costs and benefits of its inclusion as a diagnostic criterion in the DSM-5 or other future diagnostic systems.”
Summary: The ADHD Subtypes
To summarize, ADHD is complex, it can show up in a multitude of ways, the presentation may shift and turn throughout the lifespan, and if you have the inattentive type you may need to advocate a bit more to ensure your ADHD symptoms are recognized and treated. Thanks for being here and learning with me. I’ll see you next Monday for more #ADHD misdiagnosis.
Citations
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