In some people, the signs of attention deficit hyperactivity disorder (ADHD) seem obvious -- fidgeting constantly, having difficulty paying attention in school or at work, and leaving every task unfinished. Based on these and other symptoms, doctors have designated different types of ADHD, which are used to diagnose ADHD and prescribe treatment (often stimulant medications, such as Ritalin).

Yet the symptoms of ADHD may mimic those of other conditions, and sometimes the signs are subtler and harder to distinguish. One psychiatrist, Daniel Amen, MD, believes that to get a truly accurate diagnosis of ADHD it's necessary to look inside the brain to see how well various areas are functioning. He has developed his own set of ADHD subtypes based on brain scans of children with ADHD, which he says can better target treatment and determine whether it's working.

How Is ADHD Usually Diagnosed?

Most psychologists, psychiatrists, and pediatricians diagnose ADHD based on a series of inattention and hyperactivity symptoms outlined in the Diagnostic & Statistical Manual for Mental Disorders (DSM-IV). For someone to be diagnosed with ADHD, the behaviors must have lasted for at least six months, and be severe enough to disrupt school and other aspects of the individual's life.

Inattention symptoms include:

Not paying attention to detail
Making careless mistakes
Failing to pay attention and keep on task
Not listening
Being unable to follow or understand instructions
Avoiding tasks that involve effort
Being distracted or forgetful
Losing things that are needed to complete tasks

Hyperactivity-impulsivity symptoms include:

Fidgeting
Squirming
Getting up often when seated
Running or climbing at inappropriate times
Having trouble playing quietly
Talking excessively or out of turn
Interrupting

Based on these criteria, the DSM-IV identifies three subtypes of ADHD:

1. ADHD, Combined Type: Both inattention and hyperactivity-impulsivity symptoms.

2.ADHD, Predominantly Inattentive Type: Inattention but not hyperactivity-impulsivity symptoms.

3. ADHD, Predominantly Hyperactive-Impulsive Type: Hyperactivity-impulsivity but not inattention symptoms.

What Are Amen's Types of ADHD?

Amen, who is a child, adolescent, and adult psychiatrist, and medical director of the Amen Clinics in California, Washington, and Virginia, has used a combination of symptoms and brain scans to come up with his own types of ADHD.

Amen considers these to be the hallmark symptoms of ADHD:

Short attention span
Distractibility
Disorganization
Procrastination
Poor internal supervision

Based on these symptoms, and the use of SPECT brain scans to highlight activity in the parts of the brain related to attention, short-term memory, and forethought, Amen described these six types of ADHD:

Type 1 -- Classic ADHD. All of the hallmark symptoms of ADHD, plus hyperactivity and impulsivity. Responds well to stimulant medications.
Type 2 -- Inattentive ADHD. All of the hallmark features of ADHD, but instead of hyperactivity, there is low energy. Also responds well to stimulant medications.
Type 3 -- Overfocused ADHD. All of the hallmark features of ADHD, in addition to negative thoughts and behaviors, such as opposition and arguing. Tends to respond better to an antidepressant (SSRI, such as Prozac) combined with a stimulant.
Type 4 -- Temporal Lobe ADHD. The hallmark features of ADHD, plus irritability, aggressiveness, and memory and learning problems. Responds better to antiseizure medications (Neurontin) than to stimulants.
Type 5 -- Limbic ADHD. Combines ADHD with depression and low energy and decreased motivation. Responds better to stimulating antidepressants (such as Wellbutrin) than to stimulants.
Type 6 -- The Ring of Fire. Cross between ADHD and bipolar disorder. Characterized by moodiness, aggressiveness, and anger. Anticonvulsants or newer antipsychotic medications tend to work better than stimulants.