Alobar

Alobar is the most severe form of holoprosencephaly, a spectrum of congenital forebrain malformations arising from incomplete division of the embryonic prosencephalon during early gestation. In alobar holoprosencephaly, the cerebral hemispheres fail to separate, producing a single, unpaired cerebral ventricle and a lack of an interhemispheric fissure. The term contrasts with semilobar and lobar holoprosencephaly, which show progressively greater hemispheric cleavage.

The condition results from disrupted signaling during early brain development, typically in the first trimester. Defects in sonic hedgehog (SHH) pathway genes and other genetic factors can contribute, and associations with chromosomal abnormalities such as trisomy 13 (Patau syndrome) are reported. Environmental risk factors, including maternal diabetes, have also been implicated. Alobar holoprosencephaly is often accompanied by midline facial defects, such as cyclopia or closely spaced eyes (hypotelorism), nasal anomalies, and a variable degree of facial dysmorphism.

Clinical features vary but are typically apparent prenatally or at birth. Infants may present with severe neurodevelopmental impairment, seizures, feeding difficulties, and hypotonia. The prognosis is poor, with many affected pregnancies resulting in stillbirth or neonatal death; a subset may survive with profound disability. Prenatal imaging, especially ultrasound and fetal magnetic resonance imaging, can diagnose alobar holoprosencephaly before birth. Postnatal imaging with MRI or CT confirms the diagnosis by showing a monoventricle, fused thalami, and absence of the expected hemispheric division, often with associated facial anomalies.

Etymology traces to al- (without) and lobar (lobes), reflecting the absence of cerebral lobation in this form.