What is responsible for this abnormality?
A vascular insult - which can be from various causes - results in an insult to the anterior cerebral circulation. The anterior and middle cerebral arteries are essentially deficient, resulting in an absence of the cerebral hemispheres. As the posterior circulation is unaffected the posterior fossa is normal.
What is the prognosis for these children?
Hydranencephaly is incompatible with a prolonged life after birth. The vast majority of live births dying prior to one year of age. If diagnoses on antenatal ultrasound, in some countries termination of pregnancy will be offered/performed.
Macrocephalic infant, with absent cerebral hemispheres, in keeping with hydranencephaly.
Note the thalami, cerebellum and brainstem are normal.
The falx is present as is a small amount of occipital lobe cortex.