X‐Linked Hereditary Hydrocephalus Diagnosed in the Fetal Period With Adducted Thumb and a Novel L1CAM Variant: A Case Report

X-linked hereditary hydrocephalus (XLH) is a congenital form of hydrocephalus caused by variants in the L1CAM gene on the X chromosome. Diagnosis is often made prenatally via ultrasound or magnetic resonance imaging (MRI), but specific features such as adducted thumbs are subtle and easily missed. We report a case in which prenatal MRI at 34 weeks gestation revealed fetal hydrocephalus and an adducted thumb, suggestive of XLH. Postnatal genetic testing confirmed a previously unreported frameshift variant in the L1CAM gene, c.2248dup (p.Tyr750LeufsTer36). The male infant required neurosurgical intervention and was also diagnosed with Hirschsprung's disease. Genetic testing confirmed that the mother was a heterozygous carrier. In a subsequent pregnancy, non-invasive prenatal testing (NIPT) predicted a female fetus with no hydrocephalus. This case highlights the importance of thorough imaging and genetic evaluation in suspected XLH, especially given the increasing discovery of novel pathogenic variants.