Teaching Case

Term infant with hydrocephalus, hepatomegaly, micropenis and hydronephrosis.

Examination demonstrates an advanced pattern of hydranencephaly with ballooning of the supratentorial and infratentorial compartments resulting in a meningocele through the lambdoid suture at lambda secondary to motion, elevation of the tentorium in near vertical fashion and ballooning of the anterior and middle cranial fossa. Detailed evaluation demonstrates a residual brainstem of similar calibre as the small cervical spinal cord terminating superiorly in a diencephalon remnant with hypothalamic tissue outlining inferior recesses of the dilated third ventricle. There is no significant cortical mantle of the cerebral hemispheres beyond the floor of the dilated anterior and middle cranial fossa. No recognisable cortical mantle is identified in the cerebellum, replaced by posterior fossa ballooning.

Hydranencephaly (HE) is a rare entity that occurs in utero, in which the cerebral hemispheres are completely or almost completely missing which is transformed into a membranous sac filled with cerebrospinal fluid, glial tissue, and ependyma [1]. The cerebellum, midbrain, basal ganglia, thalami and choroid plexus are generally not involved.
Hemihydranencephaly is less common disorder characterised by a unilateral absence of one cerebral hemisphere.
Several aetiologies have been described, but the hypothesis of bilateral occlusion of the supraclinoid segment of the internal carotid arteries is the most accepted because it has been demonstrated by angiographic observation of aplastic or hypoplastic arteries.
Another hypothesis affirms that HE occurs after the brain and ventricles have formed, and are then destroyed in utero, as result of an encephaloclastic mechanism. The third hypothesis refers to a dysontogenetic process with an early disruption of organogenesis. Molecular dysfunctions are also being postulated (COL4A1 mutation or PI3K-Akt3-mTOR mutations)

Predisposing factors include intrauterine infections (toxoplasmosis) viral infections (Zika, rubella, cytomegalovirus, herpes simplex, Epstein-Barr, enterovirus, parvovirus), medications (warfarin, oestrogens, sodium valproate), maternal irradiation, Toxic exposures (cocaine, smoking) von Willebrand's disease, immune thrombocytopenias, congenital factor XII, X and V deficiencies and twin–twin transfusions and intrauterine death of a monozygotic co-twin. These entities could act on the internal carotid arteries causing the occlusion of the internal carotid arteries [2].
Differential diagnosis is difficult and includes alobar holoprosencephaly, severe hydrocephaly and porencephalic cyst.
In alobar holoprosencephaly there are usually coexisting midline malformations, no falx is observed, and the residual cortex has a pancake morphology. In severe hydrocephaly, a cortical rim is recognised, and the middle cerebral arteries are preserved. Porencephalic cysts are usually located in the middle cerebral artery territory with ischaemic infarcts that result in localised areas of cortical destruction.

Diagnosis of HE can be made by ultrasound, CT and MRI. MRI and MRA are the best diagnostic tests.
The prognosis of HE is usually poor. Affected patients mostly die in utero. For those who survive, death usually occurs within the first year of life. Prolonged survival is very rare [3].