Autism prodrome 20 of 89
above average IQ (including those who would meet criteria for Asperger syndrome),
diagnosis does not occur until school age or even older; it is now common in many
countries for autism to be diagnosed by the age of two or three (Charman & Baird,
2002; Charwaska, Klin, Paul, Macari, & Volkmar, 2009; Mandell, Novak, &
Zubritsky, 2005). This means that, in combination with the high recurrence rate in
siblings, younger siblings of children with a diagnosis can be studied from a young
age, including from birth and potentially even during pregnancy.
This has led to considerable interest and research activity internationally to
study cohorts of ‘at risk’ younger siblings of children with a diagnosis from the first
years of life. This contrasts to many other disorders in this special issue in which high
risk genetic designs involve following offspring (sometimes as young as infancy) of
parents diagnosed in adulthood (e.g. Bipolar disorder: Chang, et al., 2003; Psychosis:
Schubert & McNeil, 2004; Schizophrenia: Bota, Sagduyu, Filin, Bota, & Munro,
2008). However, there are exceptions including studying younger siblings of children
with ADHD (Faraone, Biederman, Mennin, & Russell, 1998) and dyslexia (Carroll &
Snowling, 2004). In this section we review studies of younger siblings of children
with autism and their contribution to our knowledge regarding the early markers and
prodrome of ASD.
Many investigators are currently conducting prospective longitudinal studies
of younger siblings of children with ASDs who as a group are at higher risk to
develop an ASD compared to the general population (for a summary; see Table 3).
The majority of literature regarding the developmental trajectories of these younger
siblings reports on the early development of younger siblings as a group without
reference to later diagnoses. Therefore these data do not necessarily pertain to our
discussion of the prodrome because, whilst these groups of siblings include siblings