RESEARCH QUESTION Is the effectiveness of conservative policy comparable to that of surgery in children with trigonocephaly, with regard to clinical outcomes and psychosocial functioning, stress for parents, and costs? DESIGN Observational cohort study in children, aged 0 to 8 years old, with trigonocephaly, excluding metopic ridging (physiologic early closure of metopic suture), treated conservatively or surgically.

OUTCOME MEASURES Primary: Head growth decline, indicating raised intracranial pressure Secondary: fundoscopy, cognition, behavior, refraction and vision, forehead shape, quality of life, posttraumatic stress, decisional conflict, costs.

HYPOTHESIS Conservative treatment is non-inferior to surgery with regard to clinical outcomes, causes less burden of disease, and is cost-effective.

STUDY DESIGN Observational cohort study STUDY POPULATION Children, aged 0 to 8 years old, with trigonocephaly, excluding metopic ridging (physiologic early closure of metopic suture) INTERVENTION Conservative policy USUAL CARE/COMPARISON Craniofacial surgery OUTCOME MEASURES Primary: Head growth decline (head circumference in SD), indicating raised intracranial pressure Secondary: fundoscopy, cognition, behavior, refraction and vision, forehead shape, quality of life, posttraumatic stress, decisional conflict, costs. Repeated measures at 0, 2, 4, 6, 8 years of age SAMPLE SIZE/DATA ANALYSIS Eligible patients <= 3 years of age are included since Sept 2022 Sample size 440 patients <1 year of age: 195 surgery and 245 conservative

Non-inferiority with regard to head growth from 0-4 years (annual measurement) is determined using a linear mixed model adjusted for confounders:

severity of phenotype, sex, syndrome and parental factors (e.g., education). COST-EFFECTIVINESS ANALYSIS/BIA An economic evaluation is performed with the incorporation of medical costs and costs due to loss of productivity for the parents. A detailed costs-study is done for medical specialist care, surgical costs, hospitalization and other costs directly associated with the interventions. Cost prizes of surgery will be determined by the bottom-up micro-costing method. Cost-utility will be measured with QALY (based on EQ-5D utility score) gained, with confidence ellipses and acceptability curves. The impact of conservative policy versus surgery will be investigated on assurance perspective and central level. From the viewpoint of the (health care) government, a societal perspective and perspective of the "budgettair kader zorg" will be highlighted. We will provide a valid framework with budget consequences by a range of predictions. Sensitivity analysis is done.

TIME SCHEDULE Inclusion between Sept 2022 and Sept 2030. Analysis and reporting for each outcome parameter related to age is distributed from January 2025 to September 2031. New recommendation for treatment of trigonocephaly in guideline ready in 2031.

Inclusion Criteria:

• diagnosis of trigonocephaly

Exclusion Criteria:

• metopic ridge (physiologic early closure of metopic suture)

Children with mild, moderate or severe trigonocephaly, between 0 and 8 years of age, excluding metopic ridge (i.e. physiologic early closure of metopic suture).