Richieri Costa-da Silva syndrome is a rare, genetic, myotonic syndrome characterized by childhood onset of progressive and severe myotonia (with generalized muscular hypertophy and progressive impairment of gait), short stature, skeletal abnormalities (including pectus carinatum, short, wedge-shaped thoracolumbar vertebrae, kyphoscoliosis, genu valgum, irregular femoral epiphyses), and mild to moderate intellectual deficiency. No facial dysmorphism nor joint limitation is associated. There have been no further descriptions in the literature since 1984.

Distance between the commissures of the mouth more than 2 SD below the mean. Alternatively, an apparently decreased width of the oral aperture (subjective).