The DSM-IV-R classification Pervasive Developmental Disorder – Not otherwise Specified (PDD-NOS) is based on the symptoms for autism and includes a wide variety of phenotypes that do not meet full criteria for autism. As such, PDD-NOS is a broad and poorly defined residual category of the autism spectrum disorders. In order to address the heterogeneity in this residual category it may be helpful to define clinical and neurobiological subtypes. Multiple complex developmental disorder (MCDD) may constitute such a subtype. In order to study the neurobiological specificity of MCDD in comparison with other autism spectrum disorders, we investigated brain morphology in children (age 7–15 years) with MCDD compared to children with autism and typically developing controls.

Structural MRI measures were compared between 22 high-functioning subjects with MCDD and 21 high-functioning subjects with autism, and 21 matched controls.

Subjects with MCDD showed an enlarged cerebellum and a trend towards larger grey-matter volume compared to control subjects. Compared to subjects with autism, subjects with MCDD had smaller intracranial volume.

We report a pattern of volumetric changes in the brains of subjects with MCDD, similar to that seen in autism. However, no enlargement in head size was found. This suggests that although some of the neurobiological changes associated with MCDD overlap with those in autism, others do not. These neurobiological changes may reflect differences in the developmental trajectories associated with these two subtypes of autism spectrum disorders.