Causes of Hydrocephalus

Causes of hydrocephalus.

There are a number of causes of hydrocephalus, these are either developmental or genetic. Genes are not passed from generation to generation unchanged. Genes go through a process called meiosis when they divide to form gametes, this process mixes the 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 changes can lead to developmental changes, which were not passed from their parrents. Most cases of genetically caused hydrocephalus lead to several malformations, one, or more, of these will affect the nervous system.

Blockage of CSF can be caused by a variety of conditions( spina bifida and other birth defects of the brain, certain brain infections (like meningitis, pus can cause a blockage), hemorrhage within or around the brain, usualy due to prematurity or a ruptured aneurysm, brain trauma, or tumors). The blockage can be 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).

In babies which are born with hydrocephalus, without there being genetic causes, the condition is said to be congenital. In congenital hydrocephalus the actual cause is usually impossible to determine, these cases are said to be idiopathic, it is assumed to be due to events during the baby's development before birth, such as damage to local blood supply or infection.

Spina Bifida

About 20 years ago the commonest form of hydrocephalus was that associated with spina bifida. Here the interference with CSF flow is due to abnormalities of the brain structure at the back of the head which develop at an early stage of the pregnancy. This is often called the Arnold-Chiari malformation.
As spina bifida has been of the decrease in recent years, and the largest number of cases of hydrocephalus in infancy occur in babies born prematurely. Even very small babies can now be made to survive, but their vital functions, normally taken care of during pregnancy by the mother, have to be controlled artificially. Unavoidable rapid changes in blood pressure can cause bleeding in the brain, and the blood from this haemorrhage blocks the sieve-like absorption system leading to post-haemorrhagic hydrocephalus. Brain haemorrhages in adults (stroke) is similar and can also lead to hydrocephalus in survivors.

Aqueductal stenosis

Aqueductal stenosis is the enlargement of the lateral and 3rd ventricles with a normal 4th ventricle. The aqueduct can be atretic(with-out natural openings) and subdivided into several small forked channels. A web or membrane composed of ependymal cells can run across the aqueduct. A third variety consists of two blind-ended channels with no true lumen. Rarely aqueductal stenosis can be familial with an X-linked recessive form of inheritance. The mumps virus can be associated with aqueductal stenosis.
The cerebral aqueduct can become secondarily stenotic (narrowed) in shunted hydrocephalus due to other causes, usually with obstruction of the 4th ventricle outflow pathways as the etiology(cause) of the hydrocephalus. This produces a situation where the 4th ventricle becomes trapped due to blockage of its outflow and inability of fluid to flow upward into the shunt. If symptomatic, the encysted, trapped 4th ventricle needs a second ventricular catheter which is connected into the existing shunt above the valve to ensure equal flow in both compartments.
The MRI scan is an excellent study to visualize the cerebral aqueduct and diagnose the stenosis. It will also differentiate aqueductal stenosis from a periaqueductal glioma(a malignant growth of nerve connective tissue), an indolent midbrain tectal tumor which is usually not seen on CT scanning. Aqueductal stenosis is seen in association with Chiari malformations either due to stenosis or narrowing due to the angulation and distortion of the brainstem with the Chiari malformation. The posterior fossa is small in hydrocephalus due to aqueductal stenosis.
Hydrocephalus due to aqueductal stenosis usually becomes manifest in-utero or by the first three months of infancy with typical signs of elevated intracranial pressure, bulging anterior fontanelle, splitting of the sutures, characteristic sound to head percussion and "setting-sun sign" due to tectal compression. Teenagers and young adults can present for the first time with aqueductal stenosis and hydrocephalus. This older group of patients usually presents with chronic headaches in the setting of having head which is large on the growth chart, and there may be history of school problems and learning disabilities throughout the learning years of childhood. Appropriate imaging studies diagnose the problem, and if symptomatic a VP shunt is recommended. Hydrocephalus in-utero is frequently noted by ultrasound studies during pregnancy. Aqueductal stenosis is the cause of hydrocephalus in about 40% of cases. Hydrocephalus due to aqueduct stenosis usually occurs in-utero or within 3 months after birth.

Chiari malformations

Type I Chiari malformations, with cerebellar tonsillar ectopia is usually seen without spina bifida and myelomeningocele. Symptoms can be referable to the hindbrain malformation with neck pain, vertical nystagmus, swallowing difficulty and voice abnormalities. There may be an associated syrinx of the spinal cord, with arm and hand weakness and atrophy, long tract signs in the legs, scoliosis and cape-like pain and sensory loss of the shoulders. Enlarged ventricles can accompany Chiari I syndromes, and must be recognized since treating the Chiari I malformation with untreated hyrocephalus can lead to complications, including pseudomengocele and wound leakage. Similarly, with a symptomatic cord syrinx and hydrocephalus, the hydrocephalus should be treated with a VP shunt prior to posterior fossa decompression.
In Chiari II malformations often the setting of spina bifida and myelomeningocele are seen. In children with myelomeningocele, 90% will have an associated Chiari II malformation and 70%-80% of the children will have hydrocephalus. The enlarged ventricles develop usually in the week or two following the closure of the back defect. Occasionally, there will be significant hydrocephalus seen at the time of birth or in-utero. This is a relatively poor prognostic sign in terms of intellectual outcome. The malformation consists of caudal displacement of the vermis of the cerebellum, along with the 4th ventricle into the cervical spinal canal, often down to C4 or C5. There is fusion of the displaced tissue to the underlying brainstem and cervical spinal cord. This blocks the foramen of Magendie, contributing to the hydrocephalus. There is kinking of the lower medulla due to the malformation, as well as aqueductal stenosis, polygyria, microgyria, abnormalities of brainstem nuclei and enlargement of the inferior commissure. The treatment of the hydrocephalus needs to be done early in children with myelomeningocele to prevent several complications. With symptoms referable to brain stem compression, a shunt is usually needed in addition to decompressing the Chiari. If there is cerebrospinal fluid (CSF) leakage after closing the back, a VP shunt should be performed.

Cysts

Hydrocephalus can be caused by either porencephalic cysts within the brain adjacent to the ventricle, or arachnoid cysts in the subarachoid space or in the ventricle.
Arachnoid cysts can enlarge due to a one way ball valve effect, and compress brain or block the CSF pathways. There is no evidence that the arachnoidal membrane is capable of secreting CSF on its own.
When there is an arachnoid cyst causing the hydrocephalus, if it is not appreciated pre-shunt, then it is seen on the CT or MRI scan after the ventricles are decompressed. Symptoms referable to direct compression by the cyst such as Parinaud's syndrome with quadrigeminal cysts will not improve after VP shunting. This leads to the need for further shunting of the arachnoid cyst. In infants and young children, it may be possible to use the ultrasound to direct a shunt catheter into both the arachnoid cyst and the ventricles. The use of a fiberoptic ventriculoscope can also be used to communicate the cyst with the ventricle and then shunt this combined space.
The Dandy-Walker cyst is a posterior fossa CSF cyst in communication with an enlarged 4th ventricle, associated with aplasia of the vermis of the cerebellum. The posterior fossa is large, and the Torcula Herophili and inion are elevated. There is absence or blockage of the foramen of Magendie resulting in the encysted 4th ventricle and progressive hydrocephalus in some children. When symptomatic with advancing ventricular enlargement, the treatment is to place a shunt. Both the ventricle and the posterior fossa cyst need to be shunted with a single distal shunt system. Shunting is the procedure of choice compared to craniotomy and fenestration of the cyst.
Platybasia is a deformity where the angle formed by the basisphenoid and the clivus, normally 130-140 degrees, is increased with flattening of the skull base. There is associated shortening of the basi-occipital bone and several malformations around the foramen magnum and cervical spine, such as Klippel-Feil syndrome. The shortening of the skull base can lead to compression at the foramen magnum and cervical spine. Deformity of the skull base in achondroplasia contributes to the development of hydrocephalus in these children.

Childhood tumors

Obstructive hydrocephalus is caused by many childhood tumors, since the midline posterior fossa, suprasellar region, 3rd ventricle and pineal region are common sites in children for tumors to occur. Although controversial, our practice will not perform a preoperative shunt unless the child is very symptomatic from hydrocephalus and cannot be taken right to surgery for tumor removal. In most childhood tumors, the hydrocephalus is treated preoperative, blocking carbonic anhydrase and decrease CSF production, which improves signs of elevated intracranial pressure.
The surgery to remove the tumor should have as a major goal 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

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, which will either resolve, progress into hydrocephalus which requires a VP shunt, or a picture of atrophy may evolve with hydrocephalus-ex-vacuo. The finding of progressive ventricular enlargement and an enlarging head circumference confirms the need for a shunt.

Intraventricular hemorrhage

In premature infants weighing less than 1500 gms, 50-60% will develop intraventricular hemorrhage(IVH). Shortly after the hemorrhage, nearly 3/4 of infants will develop ventricular enlargement. Signs which herald a bleed include; stupor, respiratory difficulty, seizures, unstable vital signs and a bulging fontanelle. Several pharmacological agents can be used to prevent intraventricular hemorrhage.
When IVH occurs, and is accompanied by hydrocephalus, the initial treatment is to relieve the hydrocephalus without a shunt, since the blood would occlude(block) the catheter, the infant is usually too small to support a shunt and the hydrocephalus may resolve over time. While the incidence of acute hydrocephalus is up to 60%, the long term incidence of progressive hydrocephalus requiring a shunt falls to 10 - 20%.

Vascular lesions

Vascular lesions can cause childhood hydrocephalus. In particular, vascular malformations of the vein of Galen can present with 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 by bulk flow. Hydrocephalus as a presenting sign of a vein of Galen AVM(arteriovenous malformation) is usually not seen in the immediate newborn period, where high flow cardiac failure predominates the mode of presentation. 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 away from the AVM and dilated vessels should be designed, usually by placing a frontal ventricular catheter.

Thrombosis of the dural sinuses

Thrombosis of the dural sinuses can lead to hydrocephalus. The condition "otitic hydrocephalus" described by Symonds, 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 pseudotumor-like picture with a swollen brain. Enlarged ventricles, however, are also seen in this disorder. Thrombosis of the superior vena cava in cases of mediastinal tumor or long standing indwelling catheter can lead to a progressive communicating hydrocephalus. The use of Tissue Plasminogen Activator(TPA) has 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. Thet usually occus after infancy and are assiciates with signs of intercranial pressure.

Absorptive Blockage of the Subarchnoidal Space

Meningeal scarring can be caused by arachnoidal hemarrhage ro 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 anf the subarachnoidal spaces.

Infection

In utero infection which involves the cental nervous system can lead to imparement of CSF flow. Mumps meningoencephalitis, and bacterial meningitis are also believes to be a possible cause of hydrocephalus.

Genetic or family hydrocephalus

Bickers-Adams syndrome is a transmited by recessive gene of the X-chromosome. It is characterised by stnosis of the aqueduct of Sylvius and severe mental retardation.