About Hydranencephaly

This page is going to be more technical than the other chapters in this book. I want to give you a brief overview of brain development and what happens when there is a problem at different stages of development. There is also information from a radiologist on the diagnosis of Hydranencephaly. And, then is an overview of the brain and brainstem.

Before talking specifically about Hydranencephaly, here is a fairly good overview of brain development and what happens when something goes wrong at a specific stage of development.

Brain Development from Principal Health News http://www.principalhealthnews.com/topic/topic100586649

Brain development begins shortly after conception and continues throughout the growth of a fetus. A complex genetic program coordinates the formation, growth, and migration of billions of neurons, or nerve cells, and their development into discrete, interacting brain regions. Interruption of this program, especially early in development, can cause structural defects in the brain. In addition, normal brain formation requires proper development of the surrounding skull, and skull defects may lead to brain malformation. Congenital brain defects may be caused by inherited genetic defects, spontaneous mutations within the genes of the embryo, or effects on the embryo due to the mother's infection, trauma, or drug use.

Early on in development, a flat strip of tissue along the back of the fetus rolls up to form a tube. This so-called "neural tube" develops into the spinal cord, and at one end, the brain. Closure of the tube is required for subsequent development of the tissue within. Anencephaly (literally "without brain"), results when the topmost portion of the tube fails to close. Anencephaly is the most common severe malformation seen in stillborn births. It is about four times more common in females than males. Anencephaly is sometimes seen to run in families, and for parents who have conceived one anencephalic fetus, the risk of a second is as high as 5%. Fewer than half of babies with anencephaly are born alive, and survival beyond the first month is rare.

Encephalocele is a protrusion of part of the brain through a defect in the skull. The most common site for encephalocele is along the front-to-back midline of the skull, usually at the rear, although frontal encephaloceles are more common among Asians. Pressure within the skull pushes out cranial tissue. The protective layer over the brain, the meninges, grows to cover the protrusion, as does skin in some cases. Defects in skull closure are thought to cause some cases of encephalocele, while defects in neural tube closure may cause others. Encephaloceles may be small and contain little or no brain tissue, or may be quite large and contain a significant fraction of the brain.

Failure of neural-tube closure below the level of the brain prevents full development of the surrounding vertebral bones and leads to spina bifida, or a divided spinal column. Incomplete closure causes protrusion of the spinal cord and meninges, called meningomyelocele. Some cases of spina bifida are accompanied by another defect at the base of the brain, known as the Arnold-Chiari malformation or Chiari II malformation. For reasons that are unclear, part of the cerebellum is displaced downward into the spinal column. Symptoms may be present at birth or delayed until early childhood.

The Dandy-Walker malformation is marked by incomplete formation, or absence of, the central section of the cerebellum, and the growth of cysts within the lowest of the brain's ventricles. The ventricles are fluid-filled cavities within the brain, through which cerebrospinal fluid (CSF) normally circulates. The cysts may block the exit of the fluid, causing hydrocephalus. Symptoms may be present at birth or delayed until early childhood.

Soon after closure of the neural tube, the brain divides into two halves, or hemispheres. Failure of division is termed holoprosencephaly (literally "whole forebrain"). Holoprosencephaly is almost always accompanied by facial and cranial deformities along the midline, including cleft lip, cleft palate, fused eye sockets and a single eye (cyclopia), and deformities of the limbs, heart, gastrointestinal tract, and other internal organs. Most infants are either stillborn or die soon after birth. Survivors suffer from severe neurological impairments.

The normal ridges and valleys of the mature brain are formed after cells from the inside of the developing brain migrate to the outside and multiply. When these cells fail to migrate, the surface remains smooth, a condition called lissencephaly ("smooth brain"). Lissencephaly is often associated with facial abnormalities including a small jaw, a high forehead, a short nose, and low-set ears.

If damaged during growth, especially within the first 20 weeks, brain tissue may stop growing, while tissue around it continues to form. This causes an abnormal cleft or groove to appear on the surface of the brain, called schizencephaly (literally "split brain"). This cleft should not be confused with the normal wrinkled brain surface, nor should the name be mistaken for schizophrenia, a mental disorder. Generalized destruction of tissue or lack of brain development may lead to hydranencephaly, in which cerebrospinal fluid fills much of the space normally occupied by the brain. Hydranencephaly is distinct from hydrocephalus, in which CSF accumulates within a normally-formed brain, putting pressure on it and possibly causing skull expansion.

Excessive brain size is termed megalencephaly (literally "big brain"). Megalencephaly is defined as any brain size above the 98th percentile within the population. Some cases are familial, and may be entirely benign. Others are due to metabolic or neurologic disease. The opposite condition, microcephaly, may be caused by failure of the brain to develop, or by intrauterine infection, drug toxicity, or brain trauma. This information is from Principal Health News http://www.principalhealthnews.com/topic/topic100586649

Radiological Diagnosis of Hydranencephaly
Jim Barkovich, MD is one of the best-known pediatric neuroradiologists in the country. His book, Pediatric Neuroimaging, is one of the most used references on the subject. He defines hydranencephaly as "...a condition in which most of the brain mantle (cortical plate and hemispheric white matter) has been damaged, liquified, and resorbed." This condition may be the end result of more than one causative event. Infection and massive vascular occlusion involving the carotid arteries are the two most often cited. Because hydranencephaly may be a result of different processes the imaging can be somewhat variable. The "classic" appearance is complete replacement of the cerebral hemispheres with fluid. No cortical mantle is visible on MRI. The thalami and cerebellum are usually spared. Some sparing of the inferior frontal and inferior temporal lobes may be present. Roger Harned

The Brain In Hydranencephaly
Basically Hydranencephaly indicates that a child is missing much or most of their cerebral hemispheres, that is, the two masses of folded brain tissue (cortex) that surround the brain stem. Literally "anencephaly" means "without brain", but this is technically incorrect as a term for the cases to which it is applied, which almost invariably have a brain stem. The brain stem is most definitely a part of the brain, and a very important part of the brain.. However, many children have some of their cerebral hemispheres so can use these and learn to do more than would be expected by this diagnosis. Just as all children are different, all children with Hydranencephaly are different as well. What may be a major difficulty for one child (like seizures) may not even be present in another.

A brief orientation about the brainstem by Dr Bjorn Merker

In the figure we see the outline of the human brain in its place in the head. It is seen as if it were divided down the middle and viewed from the exposed side. If we did not open it up this way along the middle, but simply viewed the whole brain from the side, we would not see much of the brain-stem, because its upper parts would lie buried under the cortex which surrounds it on all sides except below.

The gray shading indicates the brain stem. It has several parts, each of which includes numerous neural systems with complicated connections with one another. At the top we see the "tweenbrain." It is more often called the diencephalon, which means the same thing, namely that it is located "between" two other things. What it is sitting between is the brainstem and the cortex, because the whole cortex (the outer, folded part of the brain) is attached to the brainstem only at the tweenbrain. This means that the tweenbrain is well connected with the cortex, and the part of it, which is particularly, so is called the thalamus, which is the upper part, bulging upwards. The remaining, lower part of the tweenbrain is called the hypothalamus ("below the thalamus"). It is a vital integrating center for hormonal, visceral and motivational regulation. You can see its glandular appendage, the pituitary gland (also called the hypophysis), extending down from it to the right.

The next part of the brainstem is the midbrain, technically known as the mesencephalon. On the left you can see two small bumps. These are the colliculi, or "little hills," where vision, hearing and the sense of touch are integrated. Below them is a region of neural tissue surrounding a canal called "the aqueduct." If you look closely the aqueduct is indicated by dotted lines. It carries a fluid - cerebrospinal fluid - which bathes the brain. When the flow of this fluid is obstructed the fluid builds up in the brain, causing hydrocephalus. This must be corrected by a shunt, which allows the fluid to drain. The neural tissue surrounding the aqueduct is called the periaqueductal grey matter, which plays an important role in emotional expression.

Then we come to the large part of the brainstem called the hindbrain. It has several major parts. On the left is the cerebellum, the "little brain." It got its name because like the cortex it has a folded surface, and looks like a small brain in its own right. It has a role in the adjustment of movement patterns based on learning, and is well connected with both brainstem and cortex. The bulge that you see extending to the right, opposite the cerebellum, is called the pons, the "bridge." It contains a mass of nerve fibers connecting the cerebellum with itself and the rest of the brain. The border region between the pons and the midbrain houses important systems for regulating cycles of sleep, wakefulness and alertness.

Finally, the lower part of the hindbrain, called the medulla, or "bulb," extends on into the spinal cord. It contains in a sense the basic wiring of the brain. It houses numerous system - both sensory and motor - which maintain vital functions such a breathing, heart rate and balance and organized reflexes such as swallowing and coughing at an automatic level of functioning.

The above is only a thumb-nail sketch of the most essential parts of the brainstem. With regard to Hydranencephaly it is important to remember that function is determined not only by which parts of the brain are missing and which are present, but also by how the parts that are present are affected by the loss of connections from the parts that are missing. Here there is much that we do not yet know, and some of this we might learn from children with Hydranencephaly.
Dr B. Merker is a neuroscientist in Sweden who is a friend of the families of children with Hydranencephaly.

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August 16, 2001- January 12, 2005

This website is funded in loving memory of Jason S. by his mother Kammy

The information on this site is provided by families, caregivers, and professionals who are or have been caring for a child with Hydranencephaly.

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