Introduction
Attention deficit hyperactivity disorder (ADHD) remains a controversial diagnosis despite being one of the most, if not the most, researched conditions in child psychiatry. Why might this be? Some of the reasons relate to a lack of agreement among professionals and the public in the degree of activity and distractibility that should be regarded as normal in childhood and at what age overactivity should be regarded as abnormal. In other words, ADHD is a developmental condition and developmental norms are difficult to establish, particularly when there are no gold standard objective measures. In addition, there may be cultural and geographical differences in diagnostic conventions.
Rutter and Sroufe [1] outlined three key issues for developmental psychiatry: (1) The understanding of causal processes, (2) the concept of development, and (3) continuities between normality and pathology. Rutter and Sroufe also described the rising influence of behaviour genetics on our understanding of causality as well as issues in the progressive reorganisation of behaviour as the organism transacts with the environment during development. Finally, the delineation of what is involved in the continuities and discontinuities between normality and pathology is a central concern of developmental psychopathology. They point out that that regardless of whether the underlying liability to psychopathology is dimensional with a continuum spanning normality and pathology, categorical decisions will often be required for practical decision-making purposes.
Historically, a lecture by George Still [2] described a group of children who manifested a deficit in “volitional inhibition” or moral control. Still described an overrepresentation of males, increased alcoholism, and criminality, as well as a familial disposition to the disorder. He also observed the possibility of acquired injury giving rise to the disorder. The latter idea was reenforced by the pandemic of encephalitis lethargica that swept Europe in 1917–18, which was followed by a syndrome of overactivity and distractibility. Subsequent inability to demonstrate “hard” neurological signs gave rise to theories of minimal brain dysfunction (MBD) [3, 4]. A further important historical milestone was the finding by [5] that Benzedrine, a central nervous system stimulant, had a controlling effect on the overactive behaviour of children treated for spina bifida. This serendipitous discovery brought the syndrome into the arena of psychopharmacology, with subsequent controversies and public concerns that are still current despite numerous effectiveness studies [6, 7].
Genetic influences
Some of the underlying issues that contribute to the ADHD dilemmas relate to genetics, gender differences, comorbidity, and classification systems. There are at least two disparate approaches to analysis of behaviour, one focused on discrete behavioural categories, and the other on continua throughout the population [8]. For example, Levy and colleagues [9] showed that the heritability of DSM-III-R defined ADHD was not statistically different whether ADHD was defined as a continuum or a category. That is, the DSM-III-R (8/14) and DSM-IV (6/9) cut-offs are arbitrary points along dimensions of inattention and hyperactivity-impulsivity that we all share, and studies may reflect the initial classification methods.
Despite the above findings, the issue of ADHD subtype definition remains clinically important. For example, Rasmussen and colleagues [10] investigated familial clustering of latent class and DSM-IV defined attention-deficit/hyperactivity (ADHD) subtypes. The investigators utilised logistic regression to assess the clustering of same and different subtype combinations among twin and twin-sibling pairs and whether genetic influences contributed significantly to the observed patterns of subtype combinations the same and different subtype combinations among siblings. They concluded that with the exception of the DSM-IV hyperactive-impulsive subtype and the severe hyperactive-impulsive latent class, all other sibling DSM-IV and latent class ADHD subtypes consistently exhibited same-subtype clustering with MZ probands, DZ probands, and their siblings in both samples. The overall pattern of findings in both samples was thought to indicate significant genetic influences contributing to patterns of subtype concordance, and in general provided empirical validity for DSM-IV subtypes.
Classification issues and ADHD: DSM-IV versus DSM5
The fifth edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM5) was released in May 2013 [11]. A number of issues have relevance for ADHD. These include the removal of the multi-axial assessment while not directly involving ADHD does affect the context in which the diagnosis is made. The apparent reasoning is that there was no evidence of its use by U.S. clinicians, despite some concerns that a broad assessment is useful for treatment planning and teaching. According to Frances [9], this change “would result in the loss of much valuable clinical information. Multi-axial diagnosis provides a disciplined approach to distinguishing between state and trait (Axis I versus Axis II) and to determining the contributions of medical conditions (Axis III) and of stressors (Axis IV) to the diagnosis and treatment of psychiatric disorders” [12].
A second question relates to preschool ADHD and whether ADHD is a valid diagnosis, and at the other end of the spectrum whether fewer symptoms should be required for a diagnosis of adult ADHD. A related question is the age at which first symptoms need to be observed and whether this should be increased. The broad question of whether a separate childhood classification of disorders should be retained rather than a combination with appropriate adult diagnoses is combined with the suggestion that ADHD should be classified as a neuro-developmental disorder. ADHD is classified as a neuro-developmental disorder and there is no separation of childhood and adult sections in the DSM5, with childhood qualifications in the appropriate sections. DSM5 has changed the requirement for onset to below 12 years rather than 7 years, and above age 17 requires a threshold of only 4 symptoms of inattention and hyperactivity. Also for the impulsivity section, additional items “acting without thinking,” “uncomfortable doing things slowly,” and “difficulty resisting temptation” were considered but ultimately not included. In general, these changes reflect the greater emphasis of DSM5 on spectra rather than categorical diagnoses and also a greater emphasis on developmental disorders. While these changes do not appear to be major, their implications for diagnostic epidemiology and treatment approaches to ADHD are for future determination. It remains to be seen whether the more “dimensional” approach will continue to give rise to questions about the boundary for treatment versus non-treatment. It is also unclear whether future revision of the ICD-10 [13] will follow the somewhat more dimensional DSM5 changes or maintain a more categorical approach.
According to Frances [9], “The greatest general impact would come from the suggestion to eliminate the ‘clinical significance’ criterion required in DSM-IV for each disorder that has a fuzzy boundary with normality (about two-thirds of them).” “These were included to ensure the presence of clinically significant distress or impairment when the symptoms of the disorder in mild form might be compatible with normality” [12]. Frances maintains, “Removing this requirement would reduce the role of clinical judgment as a gatekeeper in determining the presence or absence of mental disorders and thus would increase the already swollen rates of psychiatric diagnosis.” He also suggests, “It has been widely accepted for several decades that adding dimensions would help to solve the categorical system’s problem with fuzzy boundaries—thus improving the accuracy and precision of psychiatric diagnosis. Unfortunately, however, the field has never achieved consensus on which dimensions to choose and how best to measure them. Moreover, and most crucial, clinicians find dimensional ratings far too unfamiliar and cumbersome for use in everyday practice and all efforts to include even a few simple dimensional ratings into previous DSM’s have been met by clinician resistance and neglect” [12].
Gender differences in ADHD
Gaub and Carlson [14] reported a meta-analysis of gender differences in ADHD, which found that non-referred girls with ADHD displayed lower levels of inattention, internalising behavior, and peer aggression than boys with ADHD, while clinic-referred samples displayed similar levels of impairment and comorbidity. Gershon reported a meta-analysis comparing results with the Gaub and Carlson study and to examine potential moderators of effect size estimates [15, 16]. The results indicated that females manifested significantly less externalizing problems, but significantly more internalising problems than ADHD males (the latter in contrast with Gaub and Carlson). ADHD females performed worse on Full Scale and Verbal IQ. Teachers rated ADHD females as less inattentive and having fewer externalising problems than ADHD males. Clinically referred samples tended to manifest more severe symptoms than community samples. They also found a possible gender bias in rating scales, but despite this there were large gender differences in the manifestation of ADHD symptoms and correlates.
Levy and colleagues investigated patterns of comorbidity in the three DSM-IV [15] subtypes of ADHD—predominantly inattentive (I), predominantly hyperactive impulsive (HI), and combined (C)—with oppositional defiant disorder (ODD), conduct disorder (CD), separation disorder (SA), and speech and reading problems in a large sample of twins and siblings (2173 males, mean age 10.69 years, and 2197 females, mean age 10.75 years) [17, 18]. The findings showed significant between-group differences in males and females for inattention and hyperactivity-impulsivity symptom count with higher ODD and CD in males, and higher SA in females. Separation anxiety was higher in females in the inattention subtype.
