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142 Chapter 8 DISCUSSION In this study focusing on ventriculomegaly and TH in children with Crouzon syndrome, we have identified three main findings. First, we aimed to determine how ventriculomegaly and TH develop and progress over time. We found that ventriculomegaly is present in 29% at onset, the prevalence increases to 49% shortly after skull expansion, mostly in the first 1.5 years, then declines and normalizes over time and following treatment, remaining relatively stable from 5 years of age onward. TH is present in 11% at onset, with time and despite treatment (ie, vault expansion, ETV, or VP-shunting), prevalence increases to 46%, with the biggest increase happening in the first 2.5 years. Second, we aimed to determine how ventriculomegaly and TH relate to one another. We found that FOHR and tonsil position were associated, and that a 10% increase in FOHR was associated with a +1.6mm increase in tonsil position. Third, we aimed to determine which clinical traits (if any) were associated with TH or ventriculomegaly. We found that FOHR is associated with head-circumference but not with closed lambdoid sutures before 1 year of age, and TH is associated with closed lambdoid sutures before 1 year of age, but not with head-circumference. The prevalence of 49% of ventriculomegaly is in line with the reported prevalence of 30%–70% in children with Crouzon syndrome.18-20 The prevalence of %46 of patients with TH ≥ 5+mm is similarly in line with reported prevalence of %70–%38 in Crouzon patients.21, 22 In the majority of our patients, ventriculomegaly preceded the development of TH. However, our cohort shows different orders of occurrence of ventriculomegaly and TH, which illustrates the unpredictable nature of developing ventriculomegaly and/or TH. In line with studies that have shown that premature closure of the lambdoid sutures is associated with development of TH ≥ +5mm,23, 24 we found a strong association between closed lambdoid sutures within the first year of life and a +6.990mm increase in tonsil position (95% CI [3.614–10.276]). We found no evidence for an association between tonsil position and head-circumference. A study by Coll et al. showed a statistically significant association between the presence of hydrocephalus and TH in Crouzon patients, as determined by a chi-square test.20 This study expands on that finding by demonstrating that a 10% increase in FOHR was associated with a +1.6mm increase in tonsil position. Many theories have been postulated to explain hydrocephalus in syndromic craniosynostosis.18, 25 However, to date, no unifying theory has been able to explain all variations of manifestations of hydrocephalus and TH.11, 26 In Crouzon patients, there have been big differences in the prevalence of hydrocephalus and TH ≥ 5+mm on their

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