Congenital external hydrocephalus: A rare presentation of lobar holoprosencephaly in a neonate.

External hydrocephalus is a rare condition characterized by abnormal cerebrospinal fluid (CSF) accumulation in the subarachnoid space, often associated with developmental anomalies. Lobar holoprosencephaly, a milder form of holoprosencephaly, can manifest with hydrocephalus due to its impact on brain development and CSF dynamics. This case report describes a neonate with congenital external hydrocephalus secondary to lobar holoprosencephaly, highlighting the diagnostic imaging findings and management approach. A neonate presented with progressive macrocephaly, irritability, altered sensorium, and poor feeding. Antenatal ultrasound at 32 weeks of gestation revealed macrocephaly and hydrocephalus, leading to a cesarean delivery at 38 weeks. Physical examination showed an occipitofrontal circumference of 45 cm, exceeding the 97th percentile for age. Magnetic resonance imaging (MRI) revealed fused frontal horns of the lateral ventricles, hypoplasia of the posterior corpus callosum, and extensive extra-axial CSF accumulation compressing the brain parenchyma. The CSF collection showed complete suppression on FLAIR imaging, confirming its nature, and a cortical vein sign indicated an enlarged subarachnoid space rather than a subdural hygroma. A diagnosis of lobar holoprosencephaly with congenital external hydrocephalus was made. The patient underwent peritoneal shunting to alleviate intracranial pressure, significantly reducing head circumference to 38 cm. Postoperative recovery was uneventful, and the parents were counseled on genetic testing and long-term follow-up. This case underscores the importance of detailed neuroimaging in differentiating external hydrocephalus from other pathologies and highlights the role of surgical intervention in improving outcomes. Early diagnosis and a multidisciplinary approach are vital for managing complex congenital anomalies such as lobar holoprosencephaly.