Hydrocephalus is a condition where cerebrospinal fluid (CSF) is abnormally accumulated within the brain. CSF is a liquid that is normally produced within chambers in the brain, which are called ventricles. This fluid travels through several ventricles until it exits the brain and circulates around the brain and spinal cord to cushion and support the central nervous system. This fluid is then reabsorbed at specialized locations at the top of the brain. CSF has many important functions, including acting as a cushion or shock absorber and helping in the delivery of nutrients to the brain and removing waste.
Since CSF is constantly produced, if its outward flow from the brain or its resorption is disturbed, an abnormal accumulation of this fluid within the brain can result. This, in turn, leads to build up of fluid inside the ventricles which causes them to become enlarged -- this condition is called hydrocephalus.
Since the skull can not expand in adults to compensate for the excess fluid buildup, if CSF accumulation occurs over a relatively short period of time, it can be associated with an increased pressure inside the brain which can lead to a variety of problems, and may even be life threatening. There are various conditions, however, which lead to a relatively slow rate of progression of hydrocephalus, thus allowing the brain to "adapt" to the slowly increasing pressure. Such conditions, instead of producing rapidly-progressive and life threatening problems, generally lead to chronic symptoms which are the result of stretching of nerve fibers in the walls of the ventricles as a result of enlargement of these chambers.
Hydrocephalus may be classified in various ways. One distinction is between congenital or acquired hydrocephalus. Congenital hydrocephalus is present at birth and may be caused by either environmental influences during fetal development or genetic predisposition. Acquired hydrocephalus develops at the time of birth or at some point afterward. This type of hydrocephalus can affect individuals of all ages and may be caused by injury or disease.
Hydrocephalus may also be classified as communicating or non-communicating. Communicating hydrocephalus occurs when the flow of CSF is blocked after it exits from the ventricles. This form is called communicating because the CSF can still flow between the ventricles, which remain open. Non-communicating hydrocephalus (also called obstructive hydrocephalus) occurs when the flow of CSF is blocked along one or more of the narrow pathways connecting the ventricles.
One of the most common causes of obstructive hydrocephalus is aqueductal stenosis. In this case, hydrocephalus results from a narrowing of the aqueduct of Sylvius, a small passageway between two of the ventricles in the middle of the brain.
There are two other forms of hydrocephalus which do not fit distinctly into the categories mentioned above and primarily affect adults: hydrocephalus ex-vacuo and normal pressure hydrocephalus. Hydrocephalus ex-vacuo occurs when there is damage to the brain caused by stroke or traumatic injury, which will lead to shrinkage of brain tissue over time and apparent enlargement of the ventricles. Normal Pressure Hydrocephalus (NPH) is a syndrome which is most commonly seen in older adults, and it is characterized by slow buildup of CSF over time leading to enlargement of ventricles, but without increased pressures inside of the brain -- hence the term, normal pressure hydrocephalus. Although NPH can occur after head trauma or certain brain infections, the cause of most cases of NPH is unknown.
Symptoms of hydrocephalus vary with type of hydrocephalus, age, disease progression, and individual differences in tolerance to CSF. For example, an infant's ability to tolerate CSF build-up and increased brain pressure differs from that of an adult. The infant skull can expand to accommodate the buildup of CSF because the joints that connect the bones of the skull have not yet closed in an infant. In such a patient, the most obvious indication of hydrocephalus is often the rapid increase in head circumference or an unusually large head size. Other symptoms may include vomiting, sleepiness, irritability, downward deviation of the eyes, and seizures.
Older children and adults may experience different symptoms because their skulls cannot expand to accommodate the buildup of CSF. In older children or adults, symptoms may include headache, vomiting, nausea, blurred or double vision, downward deviation of the eyes, problems with balance and coordination, gait disturbance, urinary incontinence, slowing or loss of development, lethargy, drowsiness, irritability, altered mental status, coma, or other changes in personality or cognition including memory loss.
Symptoms of NPH include progressive mental impairment and dementia, problems with walking, and impaired bladder control, leading to urinary frequency and/or incontinence. The patient may also have a general slowing of movements or may complain that his or her feet feel "stuck." These symptoms, however, can sometimes be due to the aging process or be associated with conditions such as Alzheimerís disease, rather than NPH. Therefore, proper diagnostic tests are needed to differentiate NPH from other similar conditions.
The causes of congenital hydrocephalus are not all well understood; and are thought to involve genetic inheritance (aqueductal stenosis) or developmental disorders such as those associated with neural tube defects, including spina bifida and encephalocele. Other possible causes include complications of premature birth, such as intraventricular hemorrhage, or diseases such as meningitis, tumors, or traumatic head injury.
Acquired hydrocephalus may be caused by a variety of disorders that block the flow or absorption of CSF such as tumors (see Brain and Spinal Cord Tumors), meningitis, head trauma (see Traumatic Brain Injury), or subarachnoid hemorrhage (see Cerebral Aneurysms or Traumatic Brain Injury).
Hydrocephalus ex-vacuo occurs as a result of brain shrinkage in response to stroke or traumatic injury to the brain and the causes of normal pressure hydrocephalus are at this time unknown.
Hydrocephalus is diagnosed through clinical neurological evaluation and by using cranial imaging techniques such as ultrasonography (in infants), computed tomography (CT), magnetic resonance imaging (MRI), and by brain pressure monitoring techniques. The imaging modalities (CT and MRI) can often demonstrate the various sources of hydrocephalus, such as blockage by tumor or hemorrhage, aqueductal stenosis, or congenital abnormalities. A physician selects the appropriate diagnostic tool based on the patient's age, clinical presentation, and the presence of known or suspected abnormalities of the brain or spinal cord.
With regard to NPH, the diagnostic studies are slightly more complicated. In addition to brain imaging to rule out other conditions and to evaluate ventricular size, patients suspected of having NPH based on clinical symptoms often undergo a procedure called a high-volume lumbar puncture (HVLP). During this procedure, a spinal tap is performed by inserting a needle through the skin in the lower back into the CSF space within the lower spinal column. A large volume of CSF is then withdrawn after which the needle is removed and the patient's symptoms are monitored for the next several hours to days. Since the main problem with NPH is a backup of CSF, the drainage of a large volume of this fluid should lead to relief of the symptoms, at least partially. This relief, if present, will be transient since the constant production of CSF will replace the drained fluid and cause recurrence of symptoms.
In patients who respond well to the HVLP, a second test is performed which is called a lumbar drain trial. This is a similar procedure to the HVLP, except that this time a thin catheter is inserted through the needle into the CSF space, which remains in place for several days and drains CSF constantly. In patients with NPH, this should lead to a significant improvement of symptoms; and those who respond positively will be good candidates for surgical placement of a shunt for long-term treatment of NPH symptoms. Those patients whose symptoms do not improve with either the HVLP or the lumbar drain trial are not good candidates for this surgery and may have a condition other than NPH.
In cases where hydrocephalus is due to an obstructive source, such as a tumor, treatment of the primary source, such as removal of the tumor, may relieve the hydrocephalus. In those cases, however, where no clearly identifiable or treatable source exists, such as in NPH, hydrocephalus is treated with the surgical placement of a ventricular shunt system. This system diverts the flow of CSF from the ventricles to another area of the body (usually the abdominal cavity) where it can be absorbed. A shunt consists of a thin silastic tube and a valve. One end of the tube is placed in the ventricle through a hole in the skull, while the other end is tunneled under the skin to the abdomen where it is usually placed inside of the abdominal cavity, but may also be placed at other sites within the body, such as a chamber of the heart or a cavity around the lungs. A valve located along the catheter maintains one-way flow and regulates the rate of CSF flow. Adjustments can be made to the shunt valve after surgery based on the patientís symptoms -- several adjustments of the valve may be needed before the most appropriate setting is found.† Adjusting a shunt valve, which is done through the use of a special magnet, is easy and painless, and takes only a few seconds.
In certain patients who have obstruction of the flow of CSF at certain places along the CSF flow pathway (such as in aqueductal stenosis), a procedure called an endoscopic third ventriculostomy may be performed for relief of hydrocephalus, instead of shunt placement. This procedure involves the use of a thin and flexible camera, called an endoscope, which is inserted into the ventricular system of the brain through a small hole in the skull. The camera is then used to navigate instruments that will create a hole in the floor of one of the brain ventricles (called the third ventricle) which will allow CSF to bypass the remainder of the ventricular system (including the site of blockage) and flow directly into the area outside of the brain and spinal cord. This avoids the need for placement of a shunt system which is favorable since the shunt system is prone to malfunction or infection, often requiring surgical replacement over the years. There is, however, a considerable failure rate involved with an endoscopic third ventriculostomy. Therefore, patients with either a shunt placement or this procedure need to be followed closely over time for signs of recurrence of hydrocephalus.