In this study, distinct empathic profiles in children with ASD and CD-CU+ were found. Furthermore, the work demonstrates improvement of empathic skills throughout childhood and adolescence, which is comparable for individuals with psychiatric disorders and control children. These results yield implications for further research as well as for therapeutic interventions.

According to DSM-IV TR and ICD-10, a diagnosis of autism or Asperger Syndrome precludes a diagnosis of attention-deficit/hyperactivity disorder (ADHD). However, despite the different conceptualization, population-based twin studies reported symptom overlap, and a recent epidemiologically based study reported a high rate of ADHD in autism and autism spectrum disorders (ASD). In the planned revision of the DSM-IV TR, dsm5 (www.dsm5.org), the diagnoses of autistic disorder and ADHD will not be mutually exclusive any longer. This provides the basis of more differentiated studies on overlap and distinction between both disorders. This review presents data on comorbidity rates and symptom overlap and discusses common and disorder-specific risk factors, including recent proteomic studies. Neuropsychological findings in the areas of attention, reward processing, and social cognition are then compared between both disorders, as these cognitive abilities show overlapping as well as specific impairment for one of both disorders. In addition, selective brain imaging findings are reported. Therapeutic options are summarized, and new approaches are discussed. The review concludes with a prospectus on open questions for research and clinical practice.

With the present review, we intend to highlight the importance of considering the age- and development-dependent occurrence of comorbidity in ADHD and to outline distinct trajectories of symptom progression with possible impact on course and outcome of ADHD. The review will focus on introducing the concepts of "developmental epidemiology" and "developmental comorbidity". Psychiatric and non-psychiatric age-dependent comorbidity can be seen in the majority of children, adolescents and adults with ADHD, resulting in a severe impairment of everyday life with considerable functional and psychosocial problems. Concerning the temporal order of occurrence, psychiatric conditions may be present before the appearance of first definite ADHD symptoms ("pre-comorbidity", such as temperament factors, sleep disturbance, autism spectrum disorders and atopic eczema). They may coincide with the time when ADHD symptoms reach a clinically significant level ("simultaneous comorbidity": enuresis, encopresis, developmental dyslexia). The majority of comorbidity, however, appears after the onset of ADHD in the course of disease ("post-comorbidity": tic disorder, depression and suicidality, anxiety disorders, obsessive compulsive disorder, bipolar disorder, conduct and substance use disorders, obesity and personality disorders). The aetio-pathophysiology of ADHD and its comorbid disorders and also the nature of comorbidity itself being highly heterogeneous, we additionally discuss possible models of comorbidity. In the future, longitudinal data on distinct patterns of symptom and comorbidity progression would help to refine disease classification systems, strengthen the power of future genetic studies and finally allow for more specific treatment strategies.

Most likely, no single ADHD biomarker can be identified. However, the use of a combination of markers may help to reduce heterogeneity and to identify homogeneous subtypes of ADHD.

Measuring biomarkers to identify and assess illness is a strategy growing in popularity and relevance. Although already in clinical use for treating and predicting cancer, no biological measurement is used clinically for any psychiatric disorder. Biomarkers could predict the course of a medical problem, and aid in determining how and when to treat. Several studies have indicated that of candidate psychiatric biomarkers detected using proteomic techniques, cholesterol and associated proteins, specifically apolipoproteins (Apos), may be of interest. Cholesterol is necessary for brain development and its synthesis continues at a lower rate in the adult brain. Apos are the protein component of lipoproteins responsible for lipid transport. There is extensive evidence that the levels of cholesterol and Apos may be disturbed in psychiatric disorders, including autistic spectrum disorders (ASD). Here, we describe putative serum biomarkers for psychiatric disorders, and the role of cholesterol and Apos in central nervous system (CNS) disorders.