C syndrome, also known as Opitz trigonocephaly syndrome, is characterized by trigonocephaly, severeintellectual disability, hypotonia, variable cardiac defects, redundant (extra folds of) skin, joint and limb abnormalities, and unusual facial features such as upslanted palpebral fissures (upward pointing outside corners of the eyes), epicanthal folds, depressed nasal bridge, and low-set, posteriorly rotated ears.


  • Anteverted nares
  • Clinodactyly of the 5th finger
  • Cognitive impairment
  • Cryptorchidism
  • Depressed nasal bridge
  • Epicanthus
  • Female pseudohermaphroditism
  • Gingival overgrowth
  • Hypoplasia of the ear cartilage
  • Long philtrum
  • Low-set, posteriorly rotated ears
  • Microcephaly
  • Narrow forehead
  • Short neck
  • Short nose
  • Trigonocephaly
  • Upslanted palpebral fissure
  • Triangular-shaped head
  • Premature joining of skull bones
  • Narrow forehead
  • Pointed forehead
  • Flat nasal bridge
  • Broad nasal bridge
  • Short nose
  • Vertical folds over inner corners of nose
  • Deeply furrowed palate
  • Ear abnormalities
  • Crossed eyes
  • Bent joints
  • Fixed joint positions
  • Loose skin
  • Mental retardation


This condition is genetically heterogeneous, meaning that there is evidence of more than one type of inheritance. While many cases are sporadic, autosomal recessive, autosomal dominant, and germline mosaicism have all been suggested. At least some cases of C syndrome have been caused by dysfunction of the CD96 gene.


Surgery can greatly reduce the visual effects of trigonocephaly. It can also help address comorbid elevated intracranial pressure.


  • NIH