71 Microstructures | Trigonocephaly INTRODUCTION Trigonocephaly, caused by prenatal closure of the metopic suture, is the second most common form of single-suture craniosynostosis.1 The presentation of metopic synostosis is highly variable and ranges from a mild phenotype to a severe phenotype with the classic wedge-shaped skull, depending on when the suture closes during gestation. Children with metopic synostosis are at risk for neurocognitive disorders, such as behavioral problems and delays in speech and language development.2, 3 Patients with moderate and severe phenotypes undergo surgical correction of the frontal bones and supraorbital rims, with the aim to prevent potential restriction of brain development, to reduce the risk of raised intracranial pressure, and to improve esthetic outcomes. However, the functional indication for surgical intervention has been under debate. Recent studies have shown that the percentage of patients with trigonocephaly with preoperative papilledema as a sign of intracranial hypertension is negligible (<2%).4 In addition, preoperatively patients with trigonocephaly show a normal intracranial volume similar to that of healthy age-matched controls.5 Furthermore, cerebral blood flow of the frontal lobes in unoperated patients with trigonocephaly up to the age of 18 months is not significantly different from control patients as shown with arterial spin labelling-magnetic resonance imaging (MRI).6 Finally, the triangular shape of the forehead tends to improve over time, although it is not known to what extend this self-correction occurs. To date, the exact underlying pathophysiology of metopic synostosis and its relation with neurocognitive disorders is unclear. There are two predominant theories on the relation between metopic synostosis and potential altered neurodevelopment. One theory states that metopic synostosis is part of a bone malformation of the frontal bones which in turn leads to mechanical restriction of brain development.7 The second theory proposes an intrinsic brain anomaly in which the exerted pressure by the frontal lobes as driving force of suture patency is failing.5, 7-9 In line with the latter theory, previous studies have demonstrated that the frontal intracranial volume is smaller in patients with trigonocephaly than in controls and that neurodevelopmental disorders occur in both unoperated patients with a mild trigonocephaly phenotype as well as in operated patients with a severe phenotype.2, 3, 5 In addition, recent studies have shown an overlap in genetic mutations between patients with trigonocephaly and patients with neurodevelopmental disorders.10-12 These findings suggest that aberrant neural development, especially of the frontal lobe, is caused by an inborn brain problem, rather than mechanical restriction. Insight into the microarchitecture of the white matter of the unoperated brain could improve our understanding of the pathophysiology of metopic synostosis and its relation to altered neural development. MRI with diffusion tension imaging (DTI) can 5
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