By Adriana Ramos Calvo
Hydranencephaly (HE) is a rare encephaloclastic congenital condition characterized by a membranous sac filled with cerebrospinal fluid replacing a big percentage of the corpus striatum and cerebral hemispheres, while normally not affecting the midbrain (Pant, Kaur and De, 1970). It is a relatively rare and mostly isolated disease since it is recorded in approximately 1 out of 5000 pregnancies. An accurate incidence rate of Hydranencephaly, however, has been almost impossible to calculate due to the lack of diagnostic techniques available in the past and the similarities it shares with other abnormalities. Hemihydranencephaly (HHE) is even rarer and affects only one hemisphere, though it can have a better prognosis (Pavone et al., 2014). Its bilateral counterpart, on the other hand, has a generally poor outlook, as many of its younger sufferers die before age 1.
Hydranencephaly happens as a consequence of destruction and resorption of preformed solid cerebral tissue commencing before birth. This process is thought to start as early as 8-12 weeks of gestation through a pathogenic mechanism which involves an in utero bilateral internal carotid artery obstruction (Malik, Ahmad, Khan and Ullah, 2013). Other etiologies include maternal exposure to carbon monoxide or butane gas-caused fetal hypoxia and tissue necrosis and cavitation followed by reabsorption of this tissue, as well as local destruction of the brain tissue by intrauterine infections like cytomegalovirus or congenital toxoplasmosis (Khalid et al., 2012). All of these possible etiopathogeneses can be grouped into two hypotheses. One of these is the destructive theory, which is in-utero destruction of the already formed brain and ventricles, after neural migration and before synaptogenesis. This would probably take place during the second trimester since it is during this phase that the fetal cerebral hemispheres transform, with wide proliferation into the mesencephalon, diencephalon and the entire brainstem. On the other hand, the second theory refers to a dysontogenic process with an early disruption of organogenesis. None of these hypotheses can be confirmed due to the lack of examples of HE in which a normal brain was observed in an ultrasound or MRI and was then destroyed (Pavone et al., 2014).
Typically, normal fetal movements are recorded during pregnancy. At birth children with HE show normal or slightly large head circumference, as opposed to the reduction that would be expected as consequence of the lack of cerebral cortex. This happens due to the cerebral spinal fluid still being produced by the choroid plexus within the lateral ventricles and said fluid not being adequately absorbed. Most affected children die before birth. In those who survive, normal leg and arms movements, sucking and swallowing reflexes and archaic reflexes are usually present at birth. More subtle clinical manifestations, however, are sometimes present, including difficulty with feeding, hypotonia or feeble crying. These signs rapidly become more pronounced leading to irritability, severe hypotonia and convulsive episodes. The latter is prevalently myoclonic and may derive from the rim of the preserved cortex or directly from the brainstem. Spastic diplegia, cognitive delay and visual impairment are common. Hearing is usually preserved with rare cases of bilateral sensorineural hearing impairment (Pavone et al., 2014).
In order to diagnosticate it, multiple techniques are used, including computed tomography (CT) of the head to check the absence of B/L cerebral hemispheres and their replacement with fluid, as well as the degree of preservation of the falx, pons, cerebellar hemispheres, medulla and thalamus. The falx is normally preserved as the disorder usually occurs after the formation of ventricles is completed. Additionally, somatosensory evoked potentials (SSEP) can be used to study the cortical activity, as well as Magnetic Resonance Imaging and ultrasound, both ante- and postnatal (Khalid et al., 2012).
Bearing in mind the severe brain impairment caused by Hydranencephaly, there is debate as to whether or not to perform surgical treatment on the children that survive. This treatment consists of the drainage of the fluid in order to reduce the cerebral tension and the progressive increase of the cerebral volume through the performance of a ventriculoperitoneal shunt. Other techniques have also been explored, including choroid plexus coagulation (CPC) which, in combination with endoscopic third ventriculostomy, may increase the shunt independent rate in order to prevent the shunt device-related complications. Drugs for epileptic seizures and physiokinesitherapy, as well as treatment of respiratory compromise and intervention in nutritional care are compulsory (Pavone et al., 2014).
To sum up, Hydranencephaly is a rare disease with a poor prognosis whose diagnosis can carry some ethical issues. These issues include the imposition of correct criteria to perform an abortion, since children affected by less severe forms of HE may survive long term and the importance of an early diagnosis which may help the family process emotional issues. The last ethical question is the correct therapy used for the disease, since surgical intervention may only offer a small hope of success that would not justify the procedure. Until ethical committees produce general guidelines to answer these questions, the weight of this decision falls on the parents (Pavone et al., 2014).
National Institute of Neurological Disorders and Stroke. 2020. Hydranencephaly Information Page | National Institute Of Neurological Disorders And Stroke. [online] Available at: <https://www.ninds.nih.gov/Disorders/All-Disorders/Hydranencephaly-Information-Page> [Accessed 6 December 2020].
Khalid, S., Zaheer, S., Redhu, N., Ekramullah and Khalid, M., 2012. Hydranencephaly: A rare cause of an enlarging head size in an infant. North American Journal of Medical Sciences, 4(10), p.520.
Malik, A., Ahmad, M., Khan, A. and Ullah, E., 2013. Hydranencephaly: a rare cause of delayed developmental milestones. Case Reports, 2013(apr30 1), pp.bcr2013009589-bcr2013009589.
Pant, S., Kaur, G. and De, J., 1970. Hydranencephaly. Kathmandu University Medical Journal, 8(1), pp.83-86.
Pavone, P., Praticò, A., Vitaliti, G., Ruggieri, M., Rizzo, R., Parano, E., Pavone, L., Pero, G. and Falsaperla, R., 2014. Hydranencephaly: cerebral spinal fluid instead of cerebral mantles. Italian Journal of Pediatrics, 40(1).