Holoprosencephaly (HPE) is a complex forebrain congenital malformation with widely variable outcomes. It represents a disorder of ventral induction, which begins in the fifth gestational week. Its main feature is forebrain cleavage failure, which prevents the brain complete division into right and left hemispheres, the normal development of midline structures, and the deep brain structure. Based on the severity of prosencephalic cleavage failure, three classic forms (lobar, semilobar, and alobar) were described, and subsequently, interhemispheric variant (syntelencephaly) and septopreoptic variants were proposed. This review proposes a practical imaging approach to diagnosing HPE spectrum disorders, allowing an easier recognition and earlier diagnosis, which is essential for prenatal care and adequate parental counseling. In addition, we intend to simplify the understanding of HPE through a didactic discussion, schematic illustrations, and descriptions of each entity's current classification and critical neuroimaging features, as well as the main differential diagnosis of HPE. Holoprosencephaly (HPE) results from an incomplete midline cleavage of the forebrain (prosencephalon). It includes a wide spectrum of intracranial and craniofacial midline defects and a myriad of clinical manifestations, consisting of neurologic impairment and dysmorphism of the brain and face. It is the most common malformation of forebrain development. Evidence suggests that HPE can be present either sporadically or can have a syndromic association. The defect associated with HPE occurs at approximately 3 to 4 weeks post-conception (between day 18 and day 28 of embryonic life) and is a disorder of gastrulation. The HPE phenotype continuum has been divided into 3 categories by DeMyer and Zeman and modified by DeMyer. These categories include: Alobar: This is the most severe and most common form (two-thirds of all HPE cases), characterized by a complete failure to partition the forebrain into the left and right hemispheres; this results in a single, centrally located ventricle. Semilobar: Partial forebrain cleavage Lobar: Almost complete forebrain cleavage Middle interhemispheric variant (syntelencephaly): A rare variant with an abnormal midline union of the posterior frontal and parietal lobes. Septo-preoptic variant: This rarest variant is mild, and the midline fusion is restricted to the telencephalon's septal or preoptic region.

Holoprosencephaly (HPE) is a severe and complex congenital brain malformation caused by a defect in the midline cleavage of the prosencephalon during early embryonic development. It is the most common prosencephalic malformation in humans and is categorized into three classical forms based on the severity of this cleavage defect: alobar, semilobar, and lobar HPE. A milder interhemispheric variant, called syntelencephaly, is also considered a form of HPE. This condition may arise from genetic, environmental, or teratogenic factors. Patients with HPE often present with facial dysmorphia, with severity generally correlating with the extent of brain malformation. HPE is diagnosed prenatally through ultrasound and brain magnetic resonance imaging (MRI). Through this case report and a review of the literature, we discuss the etiopathogenic and diagnostic aspects of HPE, along with the management of this congenital malformation, illustrated by the antenatal diagnosis of a newborn with alobar HPE, confirmed by brain MRI at 32 days of age.

Holoprosencephaly is a rare brain malformation consisting of impaired midline cleavage of the embryonic forebrain presenting with variable features of craniofacial dysmorphism. It affects 1 in 10,000 live births occurring more in females than males. We present a case of alobar HPE and aim to raise awareness on the importance of early prenatal detection and counselling. We present a case of 3200-gram female baby, born by spontaneous vaginal delivery with APGAR scores of 5 and 6 in the first and fifth minute of life respectively. On admission, the baby was lethargic, had central and peripheral cyanosis, hypothermic with temperature of 32.1 °C, respiratory rate of 65 breaths/min, heart rate of 135 beats/min and oxygen saturation of 94% with an oropharyngeal airway and on oxygen support via a face mask. She had microcephaly, hypotelorism, and a small nose with a single imperforate nostril. She was diagnosed to have alobar holoprosencephaly with cebocephaly. A computed tomography scan of the brain revealed a cephalohematoma in the vertex and an intranasal soft tissue density lesion blocking the entrance measuring approximately 10 × 8.5 mm. Absence of the corpus callosum and septum pellucidum with a resulting monoventricle formed from the lateral ventricles, the fusion of the thalami and a sizeable arachnoid cyst involving the left cerebellar hemisphere were evident. She was started on IV antibiotics and IV fluids. Non-invasive airway management was opted for by the ENT team based on the condition of the baby. She succumbed to death 6 days post admission due to severe respiratory failure. The types of HPE are alobar, semi lobar, lobar and interhemispheric variants. Alobar HPE is the most severe form and is incompatible with life. Clinical presentation entails facial dysmorphism with features of hypotelorism, microcephaly and a blind ended nostril. Alobar and semilobar HPE can reliably be diagnosed with ultrasound during the first and second trimesters of pregnancy. Absence of choroid plexus and fused cortex are pathognomonic characteristic on ultrasound and CT scan respectively. Alobar holoprosencephaly is a rare brain malformation which is incompatible with life. Prenatal ultrasound screening of the foetus brain is essential and reliable in making a diagnosis. Absence of the "butterfly" sign in the foetal brain ultrasonography should raise a high index of suspicion for brain malformation with unfavourable outcome. Legal medical termination of pregnancy may serve as an early intervention.

Adult form and function are dependent upon the activity of specialized signaling centers that act early in development at the embryonic midline. These centers instruct the surrounding cells to adopt a positional fate and to form the patterned structures of the phylotypic embryo. Abnormalities in these processes have devastating consequences for the individual, as exemplified by holoprosencephaly in which anterior midline development fails, leading to structural defects of the brain and/or face. In the 25 years since the first association between human holoprosencephaly and the sonic hedgehog gene, a combination of human and animal genetic studies have enhanced our understanding of the genetic and embryonic causation of this congenital defect. Comparative biology has extended the holoprosencephaly network via the inclusion of gene mutations from multiple signaling pathways known to be required for anterior midline formation. It has also clarified aspects of holoprosencephaly causation, showing that it arises when a deleterious variant is present within a permissive genome, and that environmental factors, as well as embryonic stochasticity, influence the phenotypic outcome of the variant. More than two decades of research can now be distilled into a framework of embryonic and genetic causation. This framework means we are poised to move beyond our current understanding of variants in signaling pathway molecules. The challenges now at the forefront of holoprosencephaly research include deciphering how the mutation of genes involved in basic cell processes can also cause holoprosencephaly, determining the important constituents of the holoprosencephaly permissive genome, and identifying environmental compounds that promote holoprosencephaly. This article is categorized under: Congenital Diseases > Stem Cells and Development Congenital Diseases > Genetics/Genomics/Epigenetics Congenital Diseases > Molecular and Cellular Physiology Congenital Diseases > Environmental Factors.

Middle interhemispheric (MIH) variant of holoprosencephaly (HPE) or syntelencephaly is a rare prosencephalic cleavage disorder. In literature, few cases of accurate prenatal diagnosis have been reported. We report on four additional prenatally diagnosed cases. Between 2012 and 2017, four cases of MIH HPE were retrieved. Data on prenatal imaging, genetic analysis, and pathological investigation are collected. A "PubMed" and "Trip database" search were conducted revealing six papers reporting on 11 prenatally diagnosed cases. Four additional cases of MIH HPE were diagnosed at an earlier gestational age (between 17 and 25 weeks of gestation) compared with 11 cases from the literature review (15-39 weeks). First trimester transvaginal ultrasound facilitates correct differentiation between the severe HPE variants. Frequent association with ZIC2 mutation was found in nearly 50% of the cases (5/11) compared with one case in our series. MIH variant of HPE is detectable from the early second trimester and should be considered in the differential diagnosis when the cavum septi pellucidi (CSP) is absent. Genetic analysis and autopsy should be conducted to investigate this more recent and rare variant.