This condition is often associated with impulsivity, short attention span, distractibility and difficulties with organization and planning. While formerly, the disorder was separated into ADHD and ADD, now the two subtypes are described as ADHD – Combined Hyperactive and Inattentive type and ADHD – Inattentive type.

Despite being the most commonly studied and diagnosed mental disorder in children and adolescents, the precise cause or causes are unknown in the majority of cases. Genetic factors are estimated to make up about 75% of the risk. Nicotine exposure during pregnancy may be an environmental risk. It does not appear to be related to the style of parenting or discipline.

 It affects about 5–7% of children when diagnosed via the DSM-IV criteria and 1–2% when diagnosed via the ICD-10 criteria. As of 2019, it was estimated to affect 84.7 million people globally. Rates are similar between countries and differences in rates depend mostly on how it is diagnosed. 

ADHD is diagnosed approximately two times more often in boys than in girls, although the disorder is often overlooked in girls because they tend to manifest the inattentive type and so their symptoms are often less disruptive. 

About 30–50% of people diagnosed in childhood continue to have symptoms into adulthood and between 2–5% of adults have the condition. In adults, inner restlessness and anxiety, rather than hyperactivity, may occur. Adults often develop coping skills which compensate for some or all of their impairments. The condition can be difficult to tell apart from other conditions, as well as from high levels of activity within the range of normal behavior.

Unfortunately, the symptoms of ADHD are not specific to or limited to ADHD. For example, inattention can be seen in children with severe anxiety, lead toxicity, emotional turmoil/trauma, or concussive brain injury. Thus, ruling out other causes of inattention is critical to the proper treatment of any child considered to have ADHD. Furthermore, hyperactivity and impulsivity are characteristics of childhood bipolar disorder. Evidence suggests that treating early bipolar disorder with stimulant medications because it has been mistaken for ADHD, can lead to a worse course of disease progression. For these and other reasons, it is important not to confuse ADHD with other conditions which present with similar symptoms.

Perfusion Brain SPECT scans in the Differential Diagnosis of ADHD

Perfusion brain SPECT scans allow us to see different levels of function in the living brain because blood flow (perfusion) in the brain is a direct correlate of brain activity. This can allow a visual distinction between persons with idiopathic (true) ADHD and persons with ADHD-like symptoms caused by other disorders (toxicity, injury, bipolar disorder). In idiopathic ADHD, there is often a decrease in brain activity in the prefrontal cortex, particularly during concentration tasks. This decrease is the opposite of what should occur with a concentration task. Using SPECT scans, the decrease in activity is visualized as a decrease in perfusion.

1. Prefrontal Cortex (PFC)Functions (consideration, decision and action center of the brain responding to integrated sensory input)
PFC, an area vitally involved in executive functions such as Attention, Focus, Concentration, Organization, Abstract Thinking, Forethought, Planning, Cognitive Flexibility, Insight, Learning From Mistakes, Integrating Working Memory, Problem Solving, Creativity, Judgement, Reasoning, Decision-Making, Follow Through, Emotional Regulation, Empathy, Social–Relational Abilities.

Figure 1: Increased activity (perfusion) can be seen in the prefrontal cortex (encircled and red arrow) during a concentration task in a neurotypical person (yellow increases to brown and red on the color scale).

2. What Happens When the PFC Doesn’t Work Correctly : Short Attention Span, Inattention, Distractibility, Lack of Insight, Not Learning from Mistakes, Poor judgment, Lack of forethought, Impulsive, Emotionally Impulsive, Irritability, Difficulty making decisions/problem-solving, Forgetful, Misplace Things, Procrastination, Lack of empathy, Insensitive. Problems with sense of self, difficulty in interpersonal relationships. Actions not congruent with values and goals.

Figure 2: In patients with ADHD, activity (perfusion) decreases in the prefrontal cortex (encircled and red arrow) during a concentration task (red and white dropping to green on the color scale), making it more difficult to attend, make decisions, regulate impulses, and learn from past mistakes.

In contrast, patients with symptoms of ADHD, such as hyperactivity, impulsivity, inattention, and irritability who actually have bipolar disorder can show markedly increased cortical activity in general and, not a decrease in PFC activity, but an increase during concentration tasks.

Figure 3: Patient with bipolar disorder showing generalized markedly increased activity in the cerebral cortices (red and white) and an increase in PFC activity (encircled) during a concentration task.

Prefrontal Cortex Functions

PFC Dorsal lateral
  • Attention
  • Planning
  • Follow through
PFC Inferior orbital
  • Impulse control
  • Inhibition
  • Judgment
  • Empathy
  • Ethics
  • Morality

PFC Problems

PFC Dorsal lateral
  • Inattention
  • Lack of forethought
  • Procrastination
PFC Inferior orbital
  • Impulsive
  • Disinhibited
  • Poor judgment                                                       
  • Lack of empathy
  • Lack of ethics

Some forms of idiopathic ADHD show decreased function and therefore deceased blood flow or other changes in the temporal lobes. Some studies have shown matching decreases in the cerebellum, reflecting the cerebellum’s role in cognitive tasks.

As discussed above, some patients will present with ADHD symptoms, but have a different cause for their symptoms. Below is an example of a 11 year old girl who presented with academic difficulties, restlessness, and disruptive behavior in class. Her scan does not show ADHD; rather she presents with decreased temporal lobe function (areas of green in the temporal lobes).

Traumatic brain injury can also lead to symptoms of inattention, concentration problems, and impulsivity. This 26 year old male suddenly developed problems with focus and concentration. He was missing meetings, losing things, and missing planes in his travel-intensive career. A SPECT scan revealed significant brain injury. On questioning, he recalled that he had fallen off a 20 foot cliff and lost consciousness while on assignment. ADHD was not the diagnosis. Traumatic brain injury was the correct diagnosis.

Toxic brain injury can also lead to ADHD-like symptoms. This 16 year old was diagnosed with ADHD and was medicated with stimulants. Nonetheless, he continued to have poor academic performance, impulsivity, poor concentration, and was late for work or school often. In fact, his symptoms seemed to be getting worse on stimulant medication. SPECT scans revealed markedly scalloping which also got worse over time. Investigation for possible toxins in the patient’s environment revealed cadmium toxicity from the family’s well-water.

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