Studies attempting to see what clusters of symptom patterns emerge from the evidence, however, have found more latent class subtypes of ADHD based on ADHD symptom type, commorbidity with oppoistional defiant disorder (or its close cousin conduct disorder), and commorbidity with anxiety disorders. As the study explains:
Characteristics of the Sample
The total sample consisted of 1,010 individuals, 55% of whom were male; 10.6% were ages 4 to 11 years at intake, 26.6% were 12 to 17 years, and 62.8% were 18 years and older. Based on our clinical assessment, 49.6% of subjects were affected with ADHD, 46.6% were unaffected, and 3.8% had an indeterminate diagnosis. . . .
LCA of ADHD
LCA [Latent Cluster Analysis] using the 18 VAS-P [diagnostic test] items in 107 children revealed a best fit for a five-cluster model and LCA in 269 adolescents produced a six-cluster model as the best fit; LCA using the 18 VAS-P items in 634 adult subjects found a seven-cluster model fit the data best. . . .
Common to all of the age groups were clusters demonstrating severe combined ADHD symptoms, moderate combined symptoms, mild inattentive symptoms, and few ADHD symptoms.
A talkative-hyperactive cluster was found in 4- to 11-year-olds; a similar group was found in the adults but with lower symptom severity. This group was not found in the adolescents.
Two symptom clusters were found in the adult and adolescent age groups but not in 4- to 11-year-olds: a severe inattentive ADHD cluster and a mild combined ADHD cluster.
With the exception of the three symptom clusters mentioned above (talkative-impulsive, severe inattentive, and mild combined), similar clustering trends were found in the three age groups. However, the older age groups showed a marked decrease in symptom severity scores for hyperactivity questions . . . .
[W]e compared ADHD status as defined by the DSM-IV best estimate and posterior cluster membership. . . . The proportion of ADHD cases affected in particular clusters was similar between age groups with the exception of the mild combined ADHD symptom cluster.
All of the individuals who were assigned to the severe combined and the severe inattentive groups had DSM-IV ADHD. Most of the individuals assigned to the moderate combined group had DSM-IV ADHD. The proportion of affected individuals in the mild combined cluster differed between adults and adolescents. Adolescents assigned to this cluster were largely affected with DSM-IV ADHD; adults assigned to this cluster were a mixture of affected and unaffected individuals.
LCA of VAS-P ADHD and Comorbid Symptoms
In LCA of ADHD and comorbid symptoms in children ages 4 to 11, a six-cluster model showed the best fit; seven-cluster models provided the best fits for the adolescents and adults. . . . Overall, the pattern of ADHD symptom endorsement among the clusters resembled the LCAs limited to only ADHD symptoms. Inclusion of comorbid symptoms appeared to separate certain ADHD subgroups.
In 4- to 11-year-olds, the severe combined cluster split into two clusters, one with high anxiety symptom endorsements and one with low anxiety. Those with higher anxiety also had higher ODD compared to the group with lower anxiety.
In 12- to 17-year-olds, the symptom endorsements for ADHD items appear similar after addition of comorbid symptoms, the exception being the disappearance of a cluster corresponding to mild inattentive symptoms. There the two groups displaying severe combined ADHD symptoms appeared to differ most dramatically on externalizing symptoms, although there were differences in internalizing symptoms to a lesser extent. The two groups displaying predominantly inattentive symptoms differed in the extent of internalizing symptoms.
In adults, like adolescents, a cluster demonstrating mild inattentive symptoms was no longer present when comorbid symptoms were added to the analysis. The cluster size of the talkative-impulsive group also decreased after comorbid symptoms were added. In addition, in the adult group, both internalizing and externalizing symptoms appeared to differentiate clusters displaying similar ADHD symptoms. The two moderate combined ADHD groups in adults appeared to differ most notably in internalizing symptoms.
DSM-IV and the Results Compared
The results are something of a muddle.
The Purely ADHD Symptom Based Clusters
Looking at ADHD symptoms alone, there were six clusters of people exhibiting some symptoms (in addition to a category for asymptomatic people). All six were found in adults. But, one category "hyperactive-talkative" wasn't found in teens, and two categories (mild combined and severe inattentive) weren't found in pre-teens.
Three of the six clusters categories (hyperactive-talkative, mild combined and mild inattentive) were predominantly below the clinical threshold to diagnose ADHD under DSM-IV, except in the case of most of the mildly inattentive pre-teens.
Two of the clusters (severe combined type and severe inattentive type) seems to be a close match to combined type and predominantly inattentive DSM-IV diagnoses.
A "moderate combined type" cluster was largely above the clinical threshold of the current DSM-IV standard, but fit a mix of predominantly hyperactive, predominantly inattentive and combined type cases in the DSM-IV system.
The Clusters Produced With Co-Morbidity Considered
Considering comorbid oppositional defiant disorder and anxiety disorders as well as ADHD symptoms produces twelve clusters of people exhibiting some symptoms which don't neatly align with the ADHD symptom alone clusers or show stability across age ranges. Nine of the twelve clusters were found in only one of the three age groups (pre-teen, teen and adult). Only two clusters (moderate in everything and severe in everything) were found in all age groups.
Three of the twelve clusters were ADHD at only subclinical levels by current DSM-IV standards (hyperactive-talkative with moderate ODD, mild combined with mild ODD and anxiety disorder, and mild inattentive with mild ODD and anxiety disorder). A fourth cluster of the twelve, the moderate ADHD with mild ODD and mild anxiety disorder cluster, found only in adults, was predominantly subclinical by current DSM-IV standards: about one in five were predominantly inattentive ADHD and one or two were predominantly hyperactive/impulsive by current ADHD standards.
Three of the twelve clusters mapped fairly closely to combined type ADHD by current DSM-IV standards (severe combined type with severe ODD and severe anxiety disorder, severe combined type with moderate ODD and moderate anxiety disorder, severe combined type with severe ODD but not anxiety disorder), although in older age ranges there is some shift from a combined type to a predominantly inattentive type under DSM-IV. Severe combined type ADHD is apparently almost always accompanied by moderate to severe ODD, but sometimes comes with anxiety disorders and sometimes doesn't.
As in the ADHD symptom only analysis, the latent cluster analysis conducted with comorbidity considerations produces a moderate ADHD combined type cluster that includes a large share of all predominantly hyperactive diagnoses under the DSM-IV system, in addition to a significant number of predominantly inattentive diagnoses, and a significant number of combined type diagnoses. The cluster also is accompanied by moderate ODD and moderate anxiety disorder, and is one of just two that is found in all age groups.
The twelve cluster analysis also produces four clusters that align roughly with a predominantly inattentive diagnosis under DSM-IV, but none of them are found in more than one age group. In pre-teens, a significant number of mild attention cases that are overwhelmingly subclinical in teens and adults rate as predominantly inattentive ADHD under DSM-IV. One cluster of inattentive teens has no real comorbid conditions, while the other has moderate ODD and severe anxiety disorder. In adults, the only predominantly inattentive cluster exhibits moderate ODD and mild anxiety disorders.
One important finding of the study is that there are a significant number of people out there with subclinical levels of ADHD symptoms who clearly cluster separately from people with few symptoms at all. For pre-teens, there is about one subclinical case for every two diagnosable cases. For adults, there are about two subclinical cases for every one diagnosable case. For teens, the ratio is in between (but closer to the pre-teen ratio).
The inconstant cluster patterns for predominantly inattentive type ADHD may, in part, have something to do with this condition being less consistently diagnosed than combined type ADHD whose hyperactivity symptoms are far less subtle.
The moderate to severe combined type diagnosis declines greatly with age, while the inattentive type becomes more common. This confirms prior research showing that hyperactive symptoms of ADHD are more prone to abate with age than inattentive symptoms, and suggests that a diagnostic tool for adult ADHD might benefit from treating what would be a subclinical level of hyperactivity symptoms in a child or a teen as more significant when seen in an adult.
The finding of the study that moderate ADHD cases cluster together, rather in the subtypes of the current DSM-IV diagnosis, is notable. This may be a product of the diagnostic instrument which is tuned to discriminating subtypes in severe cases rather than mild ones. Subtyping by relative symptom severity, rather than on an absolute scale, might produce less puzzling results.
The other important finding of the study is that combined type ADHD is highly comorbid with oppositional defiant disorder, to the point where it isn't at all obvious that they are separate conditions at all. Put another way, the symptoms of ODD in DSM-IV may be what I have called "non-diagnostic symptoms" of ADHD combined type. The DSM-IV doesn't used these ODD symptoms to diagnose ADHD combined type, but these symptoms are correlated strongly enough with the definition of combined type ADHD that they could be used to diagnosis it.
Since this correlation isn't nearly so strong in the inattentive type, one wonders if it wouldn't be appropriate to add the symptoms of oppositional defiant disorder to the list of hyperactivity symptoms and redefine ODD to exclude cases comorbid with combined type ADHD (favoring ADHD over ODD on the theory that ODD is a more "loaded" diagnosis and sounds more like a recidivist delinquent determination by legal authorities than a true psychiatric diagnosis in many cases). ODD, Conduct Disorder and Anti-Social Personality disorder cover a myriad of sins. A lot of ODD, Conduct Disorder and some cases of Anti-Social Personality Disorder (or whatever it is that they call it these days) seem to basically involve poor management of the hyperactivity-impulsivity dimension of ADHD rather than something distinct. The rest of anti-social personality disorder cases, in contrast, involve something entirely different in root cause, mechanism and more carefully defined symptoms: psychopathy. Someone who is hyper and impulsive and defiant and doesn't know how to manage that but has empathy for others is probably ADHD combined type. Sometone who is cold as ice and lacks empathy but can consicously gaslight and glibly lie is probably a psychopath.
The comorbidity of ADHD of both DSM-IV types with anxiety disorders is high, but the fit is not nearly so perfect. There are plenty of people who are in clusters with ADHD combined type or ADHD inattentive type who lack anxiety disorders, while almost everyone with ADHD combined type has corresponding levels of ODD. This intuitively makes sense. Lots of people with ADHD are the opposite of the classic anxiety disorder diagnosis, laid back to a fault rather than borrowing worry that they don't need. So, maintaining separate diagnosis regimes for ADHD and anxiety disorders, as the DSM-IV status quo does, makes sense.