Environmental risk factors

Initial conceptualizations posited that deprived early environments caused these disorders. The growing amount of genetic data has meant that formative assumptions such as Kanner’s `refrigerator’ mothers are now recognized as false (Kanner, 1943).

No environmental factors are necessary or sufficient causes of the neurodevelopmental disorders, even though several perinatal factors have been associated with some of the disorders (e.g. Autism: Larsson et al. 2005). Other environmental risks have been identified in exceptional cases. For instance, toxic exposures to mercury and lead have been associated with isolated cases of autism and intellectual deficits respectively (e.g. Lanphear et al. 2005; DeSoto & Hitlan, 2007).

The utility of this criterion to define a cluster of neurodevelopmental disorders is restricted by the potential importance of perinatal risks that are difficult to define. The limited number of studies that examine environmental risks across disorders or symptom domains also restricts the importance of this criterion when defining a Neurodevelopmental cluster.

Shared neural substrates and biomarkers

Neurodevelopmental disorders are thought to evolve through aberrant processes not typically observed in any phase of normal brain development that alter trajectories in normal brain development (e.g. in Autism: Courchesne et al. 2001). It is likely that abnormal brain development is a product of gene and perinatal environment. Brief Report On Neurodevelopmental Disorders Essay. This may explain the persistence of these disorders.

The precise pathophysiology of the neurodevelopmental disorders is unknown although deviations in cortical structures have been identified. The cortical profiles of the learning, communication and motor disorders are the least studied and therefore this review uses the PDDs as an example. In the PDDs, abnormalities in head size and cerebral volume occur. Macrocephaly (head circumference above the 97th percentile) is one of the most consistent physical findings in children with Autism (e.g. Lainhart et al.1997; Stevenson et al. 1997) but at birth these children tend to have smaller heads than normally developing children, with some reports that a marked growth occurs between 6 and 12 months of age (Courchesne et al. 2003; Hazlett et al. 2005). Magnetic resonance imaging studies confirm this macrocephaly, showing that Autism is associated with abnormally large brain volumes, although the details differ between studies. A recent meta-analysis of 46 imaging studies found volume increases in total brain, cerebral hemispheres, cerebellum and caudate nucleus but a reduction in the corpus callosum (Stanfield et al. 2008). There is less evidence that macrocephaly is a defining feature of Asperger’s disorder and there has been little investigation of this characteristic in CDD (Cederlund & Gillberg, 2004). In contrast to Autism, children with Rett’s disorder are defined as having a normal head size prior to the onset of regressive symptoms (6–12 months of age; APA, 1994, p. 72, Criteria A and B). The cause(s) of these abnormalities are unclear.

Other neural and biomarker anomalies are evident. Electroencephalography abnormalities occur in some PDDs; for example in autism (Kagan-Kushnir et al. 2005; Coben et al. 2008) and in Rett’s disorder (Kaufmann & Moser, 2000). Abnormalities of serotonin concentration and changes in cholinergic and GABAergic indices have also been associated with some of the neurodevelopmental disorders (Chugani et al. 1999; Bethea & Sikich, 2007) but these findings are far from specific, and none can be considered as either necessary or sufficient to cause the disorder or to provide a `neurodevelopmental’ physiological marker.

Although it is presumed that the abnormal developmental trajectory of these disorders are caused by some form of abnormal brain pathology, the identified abnormalities are seldom precisely defined, and to the extent that they have been examined across diagnoses, they seem to be non-specific.

Shared temperamental antecedents

Determining temperamental antecedents in young children affected by developmental disabilities is difficult. In cases where the individual is born with the disorder (MR, PDD) it is not possible to determine the pre-existing temperament. Unlike the Emotional and Externalizing clusters, there is no evidence of a common temperament associated with the disorders considered for inclusion in the Neurodevelopmental cluster. Brief Report On Neurodevelopmental Disorders Essay.

Symptom similarity and shared cognitive and emotional processing abnormalities

The neurodevelopmental disorders are similar in that they `manifest… delay/deviance in maturationally influenced psychological features (i.e. the skills cannot develop unless the necessary neural structure is available)’ (Rutter et al. 2006, p. 278). The disorders in this cluster therefore share the feature of `impairment in development’. Moreover, compared to other conditions, such as ADHD, that also involve neurodevelopmental perturbations, the conditions included in the current cluster involve a more stable perturbation in development that persists in the majority of cases. The impairment(s), however, may be in one or several areas, and the pattern of impairment serves to specify the disorder. The developmental areas include deficits in cognition, social interaction, communication and normative behaviour. In the majority of cases, deviance rather than delay in reaching developmental milestones seems a more appropriate characterization, again differentiating the conditions in the current cluster from other so-called `childhood’ disorders. Given the integral nature of these domains to everyday life, some degree of functional impairment is also evident. Longitudinal studies show the persistence of the impairment into later life (Maughan & Hagell, 1996; Stothard et al. 1998; Howlin et al.2000; Mawhood et al. 2000; Clegg et al. 2005).

Cognitive impairment is a key feature of the neurodevelopmental disorders. For most of the conditions included in the cluster, standardized tests exist for documenting the severity of the impairment. For example, cognitive impairment characterizes MR, learning, motor, and communication disorders, where standard measures exist for characterizing the impairment. The PDDs also involve cognitive impairment, although less standardized tests exist for identifying associated cognitive deficits in the PDDs than for other disorders included in this cluster. The nature of cognitive impairment varies among the disorders, ranging from deficits confined to specific core processes to more generalized problems. The notion of the presence of relatively `preserved’ capacities in the face of specific impairment is well established and even in disorders such as MR, where more pervasive deficit is expected, patterns of relative strengths and weaknesses are still evident (Sattler, 1992). With regard to MR, pervasive and significant deficit (>2 s.d.) in general intellectual functioning and significant limitations in adaptive functioning are expected and form part of the definition of the syndrome. With regard to the disorders that fall under the umbrella of PDD, at least half have intellectual impairment (Chakrabarti & Fombonne, 2001; Yeargin-Allsopp & Boyle, 2002). Autism is associated with more severe MR, whereas in Asperger’s disorder, by definition, intellectual impairment is deemed to be not `clinically significant’, but deficits in domains other than intellectual functioning, particularly social communication, support the inclusion of Asperger’s disorder here. For each of the PDDs, perturbations in social processing are detectable on standardized measures, such as the Autism Direct Observation Scale (ADOS, Lord et al. 1989). By contrast, the learning, motor and communication disorders are defined by more isolated deficits in the relevant domain(s), discordant with the individual’s chronological age and measured intelligence. Brief Report On Neurodevelopmental Disorders Essay.

Although the presence of aberrant development sets children with the neurodevelopmental disorders apart from the general population, this factor is not a point of absolute distinction from adults and children with disorders represented in other clusters. In fact, developmental delay and overt cognitive impairment are associated with multiple syndromes. Indeed, the disorders represented in the Neurocognitive cluster also have cognitive deficits as a key feature, as noted by Sachdev et al. (2009). A minor lowering (0.5 s.d.) of intellectual functioning is also associated with some externalizing disorders (Sattler, 1992; Sattler & Hoge, 2006; Krueger & South, 2009). In the neurodevelopmental disorders the cognitive symptoms arise early and persist as the defining feature of the syndromes. The developmental impairments seem to have neural underpinnings. Cognitive deficits in the neurodevelopmental disorders are persistent, and this is supported by psychometric tests used to reliably track these deficits. In the future, the availability of such tests may allow characterization of the persistence of the core cognitive deficits in other clusters.

High rates of co-morbidity

Grouping the neurodevelopmental disorders presumes that within-cluster co-morbidity is higher than between-cluster co-morbidity. Few studies, however, have examined the differences between these disorders, and unlike analyses of the Emotional and Externalizing clusters there have been no analyses of within- versus between-cluster co-morbidity rates. Thus, for current purposes it is best to examine the co-morbidity of these disorders at the symptom level, and as noted in the last section, there is substantial overlap for all the neurodevelopmental disorders in terms of cognitive, social and communicative deficits, and particularly for intellectual impairment. Stuttering, for example, is associated with delays in the acquisition of language and with poor articulation independent of the stutter itself (Andrews & Harris, 1964), and MR occurs in most Autism cases (for reviews see Fombonne, 1999, 2003).

Co-morbidity is not exclusively limited to the disorders within the Neurodevelopmental cluster. MR (or intellectual impairment) is a key feature in the dementias of the Neurocognitive cluster (Sachdev et al. 2009), and emotional and externalizing symptoms may also co-occur with some of the neurodevelopmental disorders (e.g. Kim et al. 2000). Although the within-cluster co-morbidity between the neurodevelopmental disorders, particularly at the symptom level, supports grouping these disorders together, future analyses comparing co-morbidity rates across disorders and clusters may provide more robust support for this cluster.

Course of illness

The neurodevelopmental disorders have an early age of onset. Given the significant role of genetics in the development of all the neurodevelopmental disorders, it is presumed that these disorders are present from birth. For the PDDs, developmental problems may be recognized in the first year of life (Adrien et al. 1993; Osterling & Dawson, 1994; Baranek, 1999; Werner et al. 2000) although symptom severity and parental concern regarding their child’s development can influence when the child receives a diagnosis (Stone et al. 1994; De Giacomo & Fombonne, 1998; Twyman et al. 2009). Autism diagnoses typically do not occur until 6 years of age (Howlin & Moore, 1997; Howlin & Asgharian, 1999) but Autism does tend to be diagnosed earlier than Asperger’s disorder (mean age of Asperger’s diagnosis is 11 years; Howlin & Asgharian, 1999). The onset of the early regressive symptoms in Rett’s and CDD may mean that these conditions are diagnosed earlier than both Autism and Asperger’s disorder.  Brief Report On Neurodevelopmental Disorders Essay.Some learning disorders and milder forms of MR may only be detected at a later age when academic testing occurs, typically in elementary school.

The course of the neurodevelopmental disorders is usually continuous, but some speech and motor disorders may be exceptions to this trend (Communication disorders: Stothard et al. 1998; Johnson et al. 1999; Kloth et al. 1999; Clegg et al. 2005; Bloodstein & Ratner, 2008. Learning disorders: Maughan & Hagell, 1996; Shaywitz et al. 1999; Mattison et al. 2002. Motor Disorder: Cantell et al. 1994, 2003; Sugden & Chambers, 2007. PDDs: Howlin et al. 2000, 2004; Mawhood et al. 2000; Cederlund et al. 2008). CDD and Rett’s disorders by definition have a deteriorating course. Although the clinical profile in each neurodevelopmental disorder does change as the child matures, evidence of the underlying deficit typically persists. A persistent course, more than any other single characteristic, distinguishes the child-onset conditions included in this cluster from those included in other clusters. In some cases where treatment is focused on function, a disorder may seem to remit. However, even here, treatments do not alter course per se; rather, they provide the individual with different skill sets, such as facilitating interpersonal and occupational skills for individuals with MR to compensate for a stable underlying deficit.