Estimated incidence of 1/10,000 births. Result of the lack of development of both cerebral hemispheres that are replaced by one or more pockets containing C.S.F. The  skull  and  meninges are intact as well as the brainstem,  cerebellum  and falx cerebri, which explains the maintenance of basic vital functions.

It differs from major hydrocephalus  by the total absence of the cortex  and accounts for about 1 % of cases clinically diagnosed as congenital hydrocephalus.

The most likely cause is the bilateral occlusion of the internal carotid arteries between the 8th  and 12th  week of gestation. It is sometimes found in trisomy 13, in the context of in utero infections (rubella, toxoplasmosis, CMV, herpes), in combination with heart defects, in case of agnathia or renal dysplasia.

There is often macrocephaly that can become monstruous and justify a ventricular derivation. Some cases present as extreme microcephaly. Survival rarely exceeds more than a few months; death is usually due to aspiration pneumonia, convulsions or thermoregulation disorders.

Anesthetic implications: 

difficult positioning of the head and difficult mask ventilation and intubation; risk of hypothermia; undernutrition; convulsions. Risk of intracranial hypertension.

References : 

-        Tovar AR, Thoeny AL.
Unveiling what is absent: illustrating anesthetic considerations in a patient with hydranencephaly. A case report.
BMC Anesthesiology 2020; 20:224. doi/10.1186/s12871-020-01142-3

Updated: October 2020