In many ways, those who have an SCT profile have the opposite symptoms of those with classic ADHD: Instead of being hyperactive, extroverted, obtrusive, and risk takers, those with SCT are passive, daydreamy, shy, and "HYPO"-active in both a mental and physical way. They also don't have the same risk factors and outcomes. Their demeanor is sluggish, as if "in a fog" and logically they also process information more slowly. A key behavioural characteristic of those with SCT symptoms is that they are more likely to appear to be lacking motivation. They lack energy to deal with mundane tasks and will consequently seek things that are mentally stimulating because of their underaroused state. Those with SCT symptoms show a qualitatively different kind of attention deficit that is more typical of a true information input-output problem, such as memory retrieval and active working memory. Conversely, those with the other two subtypes of ADHD are characteristically excessively energetic and have no difficulty processing information.
It is thought that SCT, ADHD-PI, and ADHD are due to variations in the availability of dopamine and norepinephrine, and/or the efficiency of the large chemical structures of the specific receptors and re-uptake receptors. This would explain the efficacy of stimulants such as amphetamines on the treatment of ADHD and SCT.
Those with ADHD-PI often respond well to amphetamines, such as the prescription medication Adderall. While methylphenidate and amphetamines have many similar effects on patients (both inhibit reuptake of the neurotransmitters dopamine and norepinephrine, for example), amphetamines also promote release of those neurotransmitters. This positive effect appears to support the hypothesis that SCT is related to neurotransmitter deficiencies.
Selective attention difficulties of those with SCT manifests itself academically, in that they are prone to making more mistakes while working. Those with classic ADHD do not have this difficulty. Those with SCT have difficulty with verbal retrieval from long term memory, but may have greater visual spatial capabilities. They have deficits in working memory which has been described as the ability to keep multiple things in mind for manipulation, while simultaneously keeping this information free from internal distraction. Consequently, mental skills such as calculation, reading, and abstract reasoning are often more challenging for those with SCT. They also have a more disorganized thought process, a greater degree of sloppiness, and lose things more easily. They tend to have a greater degree of comorbid learning disabilities. Instead of having greater difficulty selecting and filtering sensory input, as is in the case of SCT, people with other types of ADHD have problems with inhibition.
Studies indicate that comorbid psychiatric problems are more often of the internalizing variety with SCT, such as anxiety, depression, and social withdrawal. Their typical shy nature and slow response time has often been misinterpreted as aloofness or disinterest by others. In social group interactions, those with SCT may be ignored. Those with the other types of ADHD are more likely to be rejected in social situations, because of more intrusive or aggressive behavior. Those with classic ADHD also show externalizing problems such as substance abuse, oppositional-defiant disorder, and, to a lesser degree, conduct disorder.
Sluggish Cognitive Tempo symptoms were removed from the Inattention symptom list in 1988 because of poor negative predictive power for the inattentive subgroup, and because DSM contributors and editors wanted the inattentive symptoms to be identical for all ADHD subgroups. The presence of the SCT symptoms tended to predict inattention, but the absence of these symptoms did not predict the absence of inattention. This analysis did not take into account the possibility that the SCT symptoms could help predict a distinct grouping within the ADHD/PI subgroup and that the ADHD/PI subgrouping could be heterogeneous in nature.
In the DSM-IV, with its new classification of symptoms for predominately inattentive ADHD, 50 to 70% of those with a ADHD-PI diagnosis have subclinical levels of hyperactivity-impulsiveness symptoms. People with ADHD combined type (ADHD-C) and predominantly hyperactive–impulsive type (ADHD-PHI) may outgrow some, or most of their hyperactive symptoms during or after childhood, while inattentive symptoms typically remain into adulthood. In contrast, those with SCT have had only inattentive features from a young age with little to no history of hyperactivity-impulsiveness. Dr. Russell Barkley has proposed that the DSM-IV designation of ADHD-PI be used only for those displaying purely inattentive symptoms and that those who have had a history of any hyperactivity be designated as ADHD combined subtype. Currently, one can have a few hyperactive symptoms and still receive a diagnosis ADHD-PI. Others believe that SCT should be classified as a new separate disorder when the DSM is next updated.
Adele Diamond has recently postulated that the core cognitive deficit of those with ADHD-PI (ADD), is working memory, or, as she coined in her recent paper on the subject, "childhood-onset dysexecutive syndrome". She states: