29 adult MAS patients (86.2% right-handed) enrolled in a natural history study completed neuropsychological evaluation and neuroimaging (side of cranial fibrous dysplasia: n_left=6, n_right=4, n_bilateral=18, n_none=1; side of ventricular enlargement: n_left=5, n_right=1, n_bilateral=4, n_none=18).

Patients had a mean IQ of 97.9 (SD=14.8). More patients had abnormalities on the left side of the brain than the right. A one-sample Wilcoxon signed-rank test showed a significantly higher discrepancy between verbal and nonverbal subtests in either direction on the WAIS-IV intelligence scale relative to the normative sample (V=318.5, p=.009). Individuals affected by bilateral cranial fibrous dysplasia and/or enlarged ventricles generally performed worse on measures of IQ than those who were unilaterally-affected regardless of the extent of the lesions.

Most patients did not exhibit marked cognitive impairment, although 17.2% of patients had IQs more than 1 SD below the mean (<85). Patients with bilateral brain abnormalities exhibited lower IQ than those with unilateral involvement. The rate of WAIS-IV verbal-nonverbal discrepancy in adult MAS patients was significantly higher than in the general population, suggesting possible lateralization of brain involvement. Future research should investigate the correlation between affected side and specific cognitive deficits in MAS as well as the possible role of plasticity during brain development.