CK syndrome is a rare, genetic, X-linked syndromic intellectual disability disorder characterized by mild to severe intellectual disability, infancy-onset seizures, post-natal microcephaly, cerebral cortical malformations, dysmorphic facial features (including long, narrow face, almond-shaped palpebral fissures, epicanthic folds, high nasal bridge, malar flattening, posteriorly rotated ears, high arched palate, crowded teeth, micrognathia) and thin body habitus. Long and slim fingers/toes, strabismus, hypotonia, spasticity, optic disc atrophy, and behavioral problems (aggression, attention deficit hyperactivity disorder and irritability) are additional features.

The middle finger is more than 2 SD above the mean for newborns 27 to 41 weeks EGA or above the 97th centile for children from birth to 16 years of age AND the five digits retain their normal length proportions relative to each other (i.e., it is not the case that the middle finger is the only lengthened digit), or, Fingers that appear disproportionately long compared to the palm of the hand.