Synopsis: Fact sheet examines Hydranencephaly a condition where the brain cerebral hemispheres are absent and replaced by sacs filled with cerebrospinal fluid.
Definition causes and treatment of Hydranencephaly, a condition where the brain cerebral hemispheres are absent and replaced by sacs filled with cerebrospinal fluid.
Hydranencephaly is defined as the congenital absence of the cerebral hemispheres in which the space in the cranium that they normally occupy is filled with fluid. Hydranencephaly is a neurologic disorder in which the cerebral hemispheres are lacking although the cerebellum, brainstem, and other central nervous system tissues may be intact. The newborn with hydranencephaly may have normal neurologic functions but does not develop, and computed tomographic scans indicate an absence of cerebral tissue. Conditions that can occur with hydranencephaly include deafness, blindness, paralysis, and cognitive impairments. Treatment is supportive. The condition should not be confused with hydrocephalus, which is an accumulation of cerebrospinal fluid in the ventricles.
Cephalic is the medical term for "head" or "head end of body".
Hydranencephaly, synonym hydroanencephaly (NORD, 2007), is one of the various cephalic classification types of disorders.
These disorders are congenital conditions that derive from either damage to, or abnormal development of, the fetal nervous system in the earliest stages of development in utero. Cephalic is the medical term for "head" or "head end of body." These conditions do not have any definitive identifiable cause factor; instead generally attributed to a variety of hereditary or genetic conditions, but also by environmental factors such as maternal infection, pharmaceutical intake, or even exposure to high levels of radiation (NINDS, 2007). This condition possesses isolated occurrences, affecting less than 1 in 10,000 births worldwide (Kurtz & Johnson, 1999) and officially classifying hydranencephaly as a rare disorder by affecting fewer than 1 in 200,000 in the United States (Rare, 2011). Even rarer are cases of hemihydranencephaly, in which the effects of damage are unilateral and existing only on one hemisphere of the brain (Dubey, Gobinathan, et.al, 2002).
Many children live for an extended period prior to diagnosis due to no obvious symptomatic presentation at birth.
Though hydranencephaly is typically a congenital disorder, it can occur as a postnatal diagnosis in the aftermath of meningitis, intra-cerebral infarction, and ischemia (stroke), or other traumatic brain injury (Dubey, Gobinathan, et.al, 2002).
Hydranencephaly is an extreme form of porencephaly (another rare disorder of the central nervous system which involves the existence of a cyst or cavity within the cerebral hemisphere) in which the cerebral hemispheres are absent and replaced by sacs filled with cerebrospinal fluid (NINDS, 2007) and remains the most severe form of bilateral cerebral cortical destruction. Differential diagnoses include bilaterally symmetric schizencephaly (a less destructive developmental process on the brain), severe hydrocephalus (cerebrospinal fluid excess within the skull), and alobar holoprosencephaly (a neurological developmental anomaly). Once destruction of the brain is complete, the cerebellum, mid-brain, thalami, basal ganglia, choroid plexus, and portions of the occipital lobes typically remain preserved to varying degrees. Though the cerebral cortex is absent, in most cases the fetal head remains enlarged due to the continued production by the choroid plexus of cerebrospinal fluid that is inadequately reabsorbed causing increased intracranial pressure (Kurtz & Johnson, 1999).
Although the exact cause of hydranencephaly remains undetermined in most cases, the most likely general cause is by vascular insult such as stroke or injury, intrauterine infections, or traumatic disorders after the first trimester of pregnancy (NINDS, 2007).
In a number of cases where intrauterine infection was determined the causing factor, most involved toxoplasmosis and viral infections such as enterovirus, adenovirus, parvovirus, cytomegalic, herpes simplex, Epstein-Barr, and syncytial viruses. Another cause factor is determined to be monochorionic twin pregnancies, involving the death of one twin in the second trimester, which in turn causes vascular exchange to the living twin through placental circulation through twin-to-twin transfusion, causing hydranencephaly in the surviving fetus (Kurtz & Johnson, 1999).
One medical journal reports hydranencephaly as an autosomal inherited disorder with an unknown mode of transmission, where an unknown blockage of the carotid artery where it enters the cranium causes obstruction and damage to the cerebral cortex. As a recessive genetic condition, both parents must carry the asymptomatic gene and pass it along to their child, a chance of roughly 25 percent. Despite determination of cause, hydranencephaly inflicts both males and females in equal numbers (NORD, 2007).
An accurate, confirmed diagnosis is generally impossible until after birth, though prenatal diagnosis using fetal ultrasonography (ultrasound) can identify characteristic physical abnormalities that exist.
Through thorough clinical evaluation, via physical findings, detailed patient history, and advanced imaging techniques, such as angiogram, computerized tomography (CT scan), magnetic resonance imaging (MRI), or more rarely transillumination (shining of bright light through the skull) after birth are the most accurate diagnostic techniques (NORD, 2007). However, diagnostic literature fails to provide a clear distinction between severe obstructive hydrocephalus and hydranencephaly, leaving some children with an unsettled diagnosis (Dubey, Gobinathan, et.al, 2002).
Unfortunately, contrary to many other associated diagnoses, hydranencephaly carries a lesser positive prognosis and method of effective management.
The outlook for children diagnosed with hydranencephaly is generally determined to be poor, with death occurring before the age of one (NINDS, 2007).
Medical text identifies that hydranencephalic children simply have only their brain stem function remaining (Kurtz & Johnson, 1999) thus leaving formal treatment options as symptomatic and supportive.
Severe hydrocephalus causing macrocephaly, a larger than average head circumference, can easily be managed by placement of a shunt (NINDS, 2007) and oftentimes displays a misdiagnosis of another lesser variation of cephalic condition (Kurtz & Johnson, 1999).
Plagiocephaly, the asymmetrical distortion of the skull, is another typical associated condition that is easily managed through positioning and strengthening exercises to prevent torticollis, a constant spasm or extreme tightening of the neck muscles (Parker & Parker, 2002).
Though medical research exclusive to hydranencephaly is limited, there are research efforts in the realm of neural tube defects and prevention of congenital neurological conditions. We encourage you to research all aspects of the condition prior to settling on one aspect of the diagnosis.
Dubey, AK Lt CoL, Gobinathan, Shalini, Gupta, RK Lt Col, Khurana, Simrita, and Singh, VP (2002). Is it Hydranencephaly-A VariantMJAFI 2002; 58:338-339. Retrieved from: medind.nic.in/maa/t02/i4/maat02i4p338g.PDF
Kurtz, Alfred B. MD and Johnson, Pamela, T. MD (1999). Case Number 7: Hydranencephaly. Radiology, 210, 419-422. Retrieved from: radiology.rsna.org/content/210/2/419.full#ref-3
NINDS: National Institute of Neurological Conditions and Stroke (2007). NINDS Hydranencephaly Information Page. Retrieved from: www.ninds.nih.gov/disorders/hydranencephaly/hydranencephaly.htm
NORD: National Organization for Rare Disorders (2007). Hydranencephaly. Retrieved from: www.rarediseases.org/rare-disease-information/rare-diseases/byID/369/viewFullReport
Parker, James, N. MD and Parker, Philip M. PhD (2002). The Official Parent's Sourcebook on Hydranencephaly: A Revised and Updated Directory for the Internet Age. A Reference Manual for Self-Directed Patient Research. San Diego: ICON Group International, Inc.
Rare Disease Day: February 28 (2011). What is a Rare Disease? Retrieved from: www.rarediseaseday.org/article/what-is-a-rare-disease
Hydranencephaly possesses isolated occurrences, affecting less than 1 in 10,000 births worldwide and officially classifying hydranencephaly as a rare disorder by affecting fewer than 1 in 250,000 in the United States.
Infants with hydranencephaly are presumed to have a reduced life expectancy, with a survival of several weeks to months. Rarely, patients with prolonged survival have been reported, but these infants may have had other neurologic conditions that mimicked hydranencephaly, such as massive hydrocephalus or holoprosencephaly.... Prolonged survival up to 19 years can occur with hydranencephaly, even without rostral brain regions, with isoelectric electroencephalograms, and with absent-evoked potentials. (www.ncbi.nlm.nih.gov/pubmed/10963978)