White Matter Microstructure in ASD and ADHD: A DTI Study

Background:  

Autism spectrum disorder (ASD) and attention deficit hyperactivity disorder (ADHD) are neurodevelopmental disorders. Previous researches have shown high rate of ADHD comorbidity (30-50%) in individuals with ASD. According to the Diagnostic and Statistical Manual of Mental Disorders—5th edition (DSM-5), ASD and ADHD can be diagnosed together.

 Structural and functional magnetic resonance imaging (MRI) has shown brain abnormalities in ASD and ADHD. Recent neuroimaging technique makes it possible to investigate white matter fiber structure using Diffusion Tenor Imaging (DTI). Not only the localized brain abnormality, but impaired structural brain connectivity are considered to be related the core feature of ASD and ADHD. However, there has been no neuroimaging research that investigated the white matter microstructure in ASD with ADHD.

Objectives:  

The aim of the present study is to reveal the commonality and difference of white matter microstructure between ASD with and without ADHD.

Methods:  

 A total of 89 participated in this study; 41 ASD without ADHD (ASD-), 20 ASD with ADHD (ASD+), 28 Normal Controls (NC). Two medical specialists diagnosed ASD and ADHD according to DSM-5 criteria. All participants do not have mental retardation. All participants are adults (age > 20 years old). The three groups did not differ statistically in their chronological age and estimated IQ.

MRI scans were conducted using a 3.0 Tesla Siemens scanner. DTI data were processed using programs in the FMRIB Software Library (FSL) version 5.0. The FA map of each subject was calculated using the DTIFIT program implemented in FSL. TBSS (Tract-Based Spatial Statistics) was used for voxelwise statistical analysis. All FA data were normalized into a common space and averaged to create a mean FA image. FSL Randomise performed group comparison of FA maps. The statistical threshold was defined at p < 0.0000001 (uncorrected for multiple comparisons).

Results:   The two disorder groups showed reduction of FA extensively in anterior corpus callosum (CC) compared to NC group. Several previous studies have found the lower FA of CC in both ASD and ADHD. This finding suggested reduced FA of CC in ASD regardless of ADHD comorbidity. Lower FA of bilateral superior longitudinal fasciculus (SLF) in ASD+ compared to ASD-. The SLF is a part of attentional network. Several previous studies have found lower FA of SLF in both ASD and ADHD. The finding suggested that the lower FA of SLF in ASD is contributed by ASD+, not by ASD-. The FA of right sagittal stratum including inferior longitudinal fasciculus (ILF) and inferior fronto-occipital fasciculus is lower in ASD- compared to ASD+. The ILF is considered to play important role in recognition of facial expressions.  The finding suggested that there is some difference in the neural substrate of social disability between ASD- and ASD+. 

Conclusions:   Compared to NC, ASD+ and ASD- had lower FA in anterior CC. Between ASD+ and ASD-, FA value is different in some regions such as SLF and ILF. The present study suggested that the neural substrate of ASD- and ASD+ has some commonality and difference in the white matter microstructure.