Cerebellar and vermal volumes were measured by quantitative MRI studies in 46 right-handed boys with attention deficit hyperactivity disorder (ADHD) and 47 matched healthy controls, and results were analyzed at the Child Psychiatry Branch, National Institute of Mental Health, Bethesda, MD. Both area and volumetric analyses showed a smaller cerebellar vermis in boys with ADHD, and the posterior inferior lobules VIII to X were particularly involved. Total cerebral volume was 6% smaller in the ADHD group, also. WISC-R vocabulary and block design scores were significantly lower in ADHD compared to control children. The findings were not related to prenatal alcohol exposure nor to methylphenidate treatment. A cerebello-thalamo-prefrontal circuit dysfunction is postulated in ADHD. 
COMMENT. The cerebellar vermis is smaller in right-handed boys with ADHD, and the reduction in volume involves particularly the posterior inferior lobules VIII to X. The smaller cerebral volumes might explain the significantly lower IQ scores in ADHD children compared to controls. Previous studies by Dr Castellanos at the NIH showed that ADHD was associated with loss of normal asymmetry in the caudate nucleus, smaller right anterior frontal area, and smaller cerebellum, and IQ scores correlated with total cerebral volume. Researchers at the Masssachusetts General Hospital, Boston, have reported a smaller splenium of the corpus callosum in ADHD children, and the Johns Hopkins group of investigators found a smaller globus pallidus volume (see Progress in Pediatric Neurology III, 1997;p 212). These studies add to the growing volume of data supporting a structural brain pathology as the basis for ADHD. 
A failure to inhibit or delay response, a central executive function, is a major problem in ADHD. These MRI studies suggest that the cerebellum is involved in the frontal-striatal dysfunction postulated in ADHD. Cognitive dysfunction, visuospatial deficits, and mutism are reported in association with cerebellar lesions.