Hydrocephalus Causes

There are a number of hydrocephalus causes all of which are either developmental or genetic. Genes are not passed from generation to generation unchanged. They go through a process called meiosis, during which they divide to form gametes resulting in the mixing of genetic material between the genes. Genes can also be changed by external forces; some substances can cause alterations of the genes, as can radiation. These genetic mutations can lead to developmental changes that were not passed down from the parents. Most cases of genetically caused hydrocephalus lead to several malformations. One or more of these will affect the nervous system.

Blockage of cerebrospinal fluid (CSF) can be caused by a variety of conditions such as: spina bifida and other birth defects of the brain; certain brain infections like meningitis (pus can cause a blockage); haemorrhage within or around the brain, usually due to prematurity or a ruptured aneurysm; and brain trauma, or tumouhyrors. The blockage can occur within the ventricles themselves (obstructive hydrocephalus), or outside the brain in the areas where the spinal fluid is reabsorbed back into the blood stream (communicating hydrocephalus).

The term congential refers to cases where hydrecephalus is present at birth, but without any genetic factors. In cases of congential hydrecephalus it is usually not possible to determine the cause, and this is refered to as 'idiopathic'. In these cases one assumes that the condition arose before birth, in the form of developmental problems due to infections, problems with blood supply, etc.

Spina bifida

Spina bifida is a condition where the neural tube, which surrounds the nerves in the spine, does not develop correctly and close fully. The vertebrae of the spine will also fail to close around the affected area. This results in a space between one, or several, of the vertebrae. Spina bifida will in most cases affect the spine in the lower back.

Aqueductal stenosis

Aqueductal stenosis is a condondition where the Aqueduct of Sylvius, which connects the 3rd and 4th ventricles, becomes damaged and blocks the flow of CSF. There are several possible cause for stenosis, eg. the aqueduct may develop without having any openings to the ventricles, or it may have developed with a membrane of cell blocking the aqueduct.

Cases of hydrocephalus which are caused by aqueduct stenosis are usually present at birth, or develope soon there after. Chiari malformations are a possible cause of stenosis as this malformation will squeeze or bend the aqueduct.

Chiari malformations

The cerebellum is made up of two hemispheres, which are connected by the vermis. The cerebellar tonsis hang from each of the two hemispheres.

Chiari malformations (CM) are a group of disorders in which cerebellar tonsils protrude into the foramen of Magendie, thereby blocking the CSF flow between the ventricles and the spine.

There are four variants of the Chiari malformation, the first, refered to as CMI, involves only the cerebellar tonsils.

Type II also involves the brain stem tissue and the vermis may be malformed. This type may be accompanied by spina bifida and myelomeningocele.

In CM Type III both the brain stem and cerebellum herniate through the foramen magnum and into the spinal cord. Type III may also involve the fourth ventricle protruding into the spinal cord.

Types IV is where the cerebellum does not fully develope, or my be absent. The cerebellar tonsils will hang further down along the spinal canal. Areas of the spinal cord and skull may be exposed.

Cysts

Hydrocephalus causes also include various types of cysts. A cyst is a closed area of tissue forming a fluid filled pocket. A cyst will have its own membrane.

Porencephalic cysts are located within the brain (adjacent to the ventricle), and arachnoid cysts are located inside the ventricles or in the subarachoid space.

The arachnoid cyst may require a separate shunt if they go unnoticed before the original surgical proceedure.

The Dandy-Walker syndrome refers to cyst of the posterior fossa. This cyst affects the cerebellum, and may involve atrophy of the cerebellar vermis. Dandy-Walker syndrome also involves enlargement of the fourth ventricle, and increase the size of the arachnoid space.

Platybasia is a deformity of the base of the skull, and a frontwards displacement of the cervical vertebrae, and the brainstems bony protrusions. This is caused by the softening of skull or other developmental abnormality.

Childhood tumours

Obstructive hydrocephalus is caused by many childhood tumours, since these often occur in the mid-line posterior fossa; the suprasellar region; the 3rd ventricle; and the pineal region. Although controversial, it is not common practice to perform a preoperative shunt placement unless the child is very symptomatic from hydrocephalus and cannot be taken straight to surgery for tumour removal. In most childhood tumours, the hydrocephalus is treated preoperatively, blocking carbonic anhydrase and decreasing CSF production. This relieves elevated intracranial pressure.

The the main goal of the surgery to remove the tumour should be to open the blocked CSF pathways, frequently avoiding a shunt. Only 20% of children will need a shunt postoperatively. A temporary ventriculostomy is usually needed to control ventricular size in the immediate postoperative period.

Postmeningitic and postinflammatory hydrocephalus causes

Postmeningitic and postinflammatory hydrocephalus are usually a communicating hydrocephalus due to obstruction at the basal cisterns. E. Coli meningitis and Hemophilus influenza meningitis are the usual infectious agents. Moderate ventricular enlargement is very common. This will either resolve itself; progress into hydrocephalus which requires a VP shunt; or evolve into a case of atrophy with hydrocephalus-ex-vacuo. The finding of progressive ventricular enlargement and an enlarging head circumference confirms the need for a shunt.

Intraventricular haemorrhage

In premature infants weighing less than 1500g, 50-60% will develop intraventricular haemorrhage (IVH). Shortly after the haemorrhage, nearly 3/4 of infants will develop ventricular enlargement. Signs that are indicative of bleeding include: stupor, respiratory difficulty, seizures, unstable vital signs, and a bulging fontanelle. Several pharmacological agents can be used to prevent intraventricular haemorrhage.

When IVH occurs, and is accompanied by hydrocephalus, the initial treatment is to relieve the hydrocephalus without a shunt. This is done because: the blood would occlude (block) the catheter; the infant is usually too small to support a shunt; and the hydrocephalus may resolve itself over time. While the incidence of acute hydrocephalus is up to 60%, the long term incidence of progressive hydrocephalus requiring a shunt has fallen to 10 - 20%.

Vascular lesions

Vascular lesions can cause childhood hydrocephalus. In particular, vascular malformations of the vein of Galen can be a cause of hydrocephalus in infancy. The pathophysiology involves blockage of the cerebral aqueduct by the enlarged Vein of Galen, as well as elevated venous pressure due to the arteio-venous shunting which reduces CSF absorption. Hydrocephalus as a presenting sign of a vein of Galen AVM (arteriovenous malformation) is usually not seen in the neonatal period, where the leading cause is high flow cardiac failure. Shunting should be performed in cases of progressive hydrocephalus, although some children show progressive thrombosis of the malformation and resolution of the ventricular enlargement. When a VP shunt is required, a catheter trajectory should be designed which is kept as far away as possible from the AVM and dilated vessels. This is usually done by placing a frontal ventricular catheter.

Thrombosis of the dural sinuses

Thrombosis of the dural sinuses can lead to hydrocephalus. The condition "otitic hydrocephalus" is seen in children with middle ear infection with thrombosis of the lateral sinus adjacent to the petrous bone. Sagittal sinus thrombosis can occur in children due to direct extension of an infection, or in cases of severe hypernatremic dehydration. This can lead to a pseudotumour-like picture with a swollen brain. However, enlarged ventricles are also seen in this disorder. Thrombosis of the superior vena cava, in cases of mediastinal tumour or long standing indwelling catheter, can lead to a progressive communicating hydrocephalus. Tissue Plasminogen Activators (TPA) have been used in a variety of cerebrovascular disorders to dissolve clotted blood. TPA may play a role in these thrombotic disorders and avoid progressive hydrocephalus.

Choroid pluxus papilloma

A papilloma of the choroid plexus will increase the amount of CSF being produced. They usually occur after infancy and are associated with signs of inter-cranial pressure.

Absorptive blockage of the subarchnoidal space

Meningeal scarring can be caused by arachnoidal haemorrhage or bacterial meningitis. If this scarring blocks the exits from the cisterns, or affects the arachnoidal villi, the CSF-flow will be impeded. This will cause enlargement of the ventricles and the subarachnoidal spaces.

Infection

In-utero infection, which involves the central nervous system, can lead to impairment of CSF flow. Mumps meningoencephalitis and bacterial meningitis are also believed to be possible hydrocephalus causes.

Genetic or family hydrocephalus

Bickers-Adams syndrome is transmitted by a recessive gene of the X-chromosome. It is characterised by stenosis of the aqueduct of Sylvius, and severe mental retardation. This accounts for ca. 4% of hydrocephalus cases.

Achondroplasia dwarfism

Achondroplasia dwarfism can also lead to hydrocephalus due to the fact that the skull develops abnormally, leading to the foramen magnum and spinal canal being abnormally narrow. The narrowing of the foramen magnum may restrict the CSF flow between the brain and the spinal cord. Narrowing of the foramen magnum, due to stenosis of the sigmoid sinus, may also be the cause of increased intracranial venous pressure.



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