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Incidence is estimated at 1/15,000 births. Males are more frequently affected than females (sex ratio of 2:1) and the frequency of trigonocephalic twins is unexpectedly high.

The premature closure of the metopic suture results in deformation of the anterior portion of the calvarium and a triangular-shaped forehead. In mild forms, only prominent ridging of the metopic suture is visible; while in more severe forms marked narrowing of the frontal and temporal regions affects the supraorbital rims leading to hypotelorism. The psychomotor development of patients is usually normal and the majority of cases are mild.

The underlying genetic cause of isolated trigonocephaly remains to be delineated. However, the concordance rate of isolated trigonocephaly in monozygotic twins is 43%, suggesting that both genetic and environmental factors are involved in the etiology of this disorder.

Diagnosis is based on clinical and ultrasound examination, radiological evaluation by X-rays, and 3D CT scans and/or MRI of the skull. Since fusion of the metopic suture normally occurs during early childhood, CT scans for metopic synostosis should be performed before 6 months of age to avoid misdiagnosis. Premature fusion of the metopic suture may occur both in syndromic and in nonsyndromic forms of synostosis.

The differential diagnosis should include trisomy 13 and other chromosomal disorders (distal monosomy 9p and 11q), as well as C syndrome (see these terms), and is based on the presence of multiple anomalies (including facial dysmorphism, organ anomalies and intellectual deficit) in the syndromic forms. Fetal exposure to valproic acid during pregnancy usually leads to trigonocephaly, which in this case is often associated with intellectual deficit and dysmorphic features, and therefore fetal valproate syndrome (see this term) should also be included in the differential diagnosis.

Most cases are sporadic but familial forms with apparently autosomal dominant transmission have been reported (representing 7-8% of all cases).

Surgery is the only treatment for correction of the skull deformation. Primary orbitocranioplasty is performed according to different techniques including reshaping with metallic fixation or use of primary bone grafting and resorbable fixation to expand the frontal region. Anterolateral expansion of the supraorbital bar and stabilization of the construct via bone grafting and resorbable fixation give the best esthetic outcome with a low incidence of reoperation.

The clinical outcome for patients having undergone surgery is usually satisfactory with a low requirement for additional interventions, especially in the mild forms. Recurrence of a prominent metopic ridge may occur in some rare cases.