Radioulnar synostosis-developmental delay-hypotonia syndrome, also known as Der Kaloustian-McIntosh-Silver syndrome, is an extremely rare syndrome with synostosis described in about 4 patients to date with clinical manifestations including congenital unilateral radioulnar synostosis, generalized hypotonia, developmental delay, and dysmorphic facial features (long face, prominent nose and ears).

Occipitofrontal (head) circumference greater than 97th centile compared to appropriate, age matched, sex-matched normal standards. Alternatively, a apparently increased size of the cranium.