Cerebrospinal fluid (CSF) is primarily produced in the choroid plexus which lines the ventricles of the brain. It is an ultrafiltrate of circulating blood and is crystal clear under normal circumstances. Once produced, the CSF makes its way out of the lateral ventricles, through the foramen of Monro, and into the third ventricle. Then, the CSF travels through the aqueduct of Sylvius to the fourth ventricle, and out a small foramen in the fourth ventricle, and into the subarachnoid space that surrounds the brain and spinal cord.
Cerebrospinal fluid fills the meninges (in the subarachnoid space), which act like a bag of fluid surrounding the brain and spinal cord. The CSF makes its way to the top of the brain, where it is absorbed back into the blood through the venous sinuses.
The processing of CSF can be compromised anywhere along this pathway, which leads to the accumulation of excess CSF in the ventricles. Accumulation of CSF is known as hydrocephalus. In other words, hydrocephalus is a disorder of CSF processing.
How does hydrocephalus develop?
Generally, CSF production is relatively constant at approximately 500 cc per day. However, if CSF is not removed from the system at a similar rate, it will back up in the ventricular system, causing the ventricles (particularly the lateral ventricles) to expand.
If a patient produces 500 cc of CSF per day but only removes 499.99 cc, they will develop hydrocephalus. If the patient removes none (due to an aqueduct of Sylvius obstruction), they will also develop hydrocephalus, only much more rapidly.
Infants with hydrocephalus will have expanded cranial vaults to accommodate the expanding brain. At this stage, their skull bones have not yet grown together, which facilitates the rapid growth of the brain. However, rapidly accumulating hydrocephalus will outstrip this form of compensation, injure the brain, and eventually kill the child.
If left untreated, slower accumulation of CSF can result in fantastically expanded heads and profound developmental delays. If treated appropriately, the life expectancy for a baby with isolated hydrocephalus should be normal. But in reality, life expectancy is often compromised due to management difficulties such as ventriculoperitoneal shunt failures, shunt infections, and other associated problems.
In children over a year old and in adults, the head will not expand with hydrocephalus. Therefore, with progressive hydrocephalus the ventricles will expand at the expense of the surrounding brain tissue and drive up the intracranial pressure.
The brain is compressed against the skull by the expanding ventricles and is further injured by the increased intracranial pressure. Eventually, herniation will occur. Interestingly, despite the increased intracranial pressure, CSF continues to be made and dumped into the expanding ventricles.
Depending on the imbalance between CSF production and removal, the patient may develop symptoms slowly or quite rapidly, but will eventually progress to coma and death if left untreated. Again, an imbalance of even a tenth of a milliliter a day will eventually expand the ventricles and drive up the intracranial pressure, but much slower than in a scenario where no CSF is being absorbed.
The rapid expansion of the ventricles and a rapid increase in intracranial pressure will lead to neurological signs such as headache, nausea, vomiting, slowing of cognition, lethargy, eventual coma, and death.
What to do if you suspect hydrocephalus in a child?
Obtain a good history
When evaluating a child for possible hydrocephalus, make sure to obtain a history of birth trauma, premature birth, central nervous system (CNS) infection, and intraventricular hemorrhage. All of these factors can lead to hydrocephalus.
Also, ask about specific syndromes such as myelomeningocele (a severe form of spina bifida), Crouzon syndrome (a genetic disorder where the sutures in the skull fuse early), and Dandy-Walker syndrome (a congenital disorder causing malformation of the cerebellum and fourth ventricle).
As you obtain a thorough medical history of the child, ask about feeding, vomiting, fussiness, milestones, head control, irregular breathing, and apneic spells. Again, slowly progressive hydrocephalus may present with subtle or no specific signs and symptoms.
In children over three (e.g., those who can speak reasonably well) and adult patients, ask specifically about headaches, visual changes, changes in mentation and processing, nausea, and vomiting.
Perform a general physical and neurological exam in all patients
When assessing for hydrocephalus in babies, look for lethargy and a lack of interest in their surroundings, as well as general hypotonia and poor head control. Also, notice if there is an obvious disproportion between the head in comparison to the face and body.
As you perform a physical exam on an infant, feel the anterior fontanelle. Is it bulging and firm? It should be flat and soft, although it may bulge with crying. Are the sutures widely split? Separation of the sutures indicates possible hydrocephalus. You shouldn’t be able to fit a fingertip into any of the sutures other than at the fontanelles.
Does the child have poor lateral gaze, suggesting a cranial nerve VI palsy? This is often seen with increased intracranial pressure.
As well, does the child have difficulty looking upwards? This is an indication of Parinaud’s syndrome (called sunset eyes) and is often seen in children with increased intracranial pressure from ventricular dilation. Interestingly, this syndrome is less evident in adults.
However, these findings may be very subtle, so the key consideration is the child’s head growth. Measure the head circumference at its greatest axial point (in the axial plane). Repeat the measurement several times until you find it to be consistent and plot the measurement on a formal head circumference chart.
Depending on your index of suspicion, you might bring the child back weekly, every couple of weeks, or monthly. Single, spot measurements are seldom definitive unless the child’s head is already well above the normal limits.
Head circumferences crossing multiple percentile lines are very suspicious for hydrocephalus, as is head growth of more than 1.25 cm per week, or a head circumference that is well out of proportion to the child’s height and weight.
Note that a relatively common entity seen in the clinic is a normally appearing and functioning baby whose head is large on measurement. The child may simply have a large head, a finding you may also notice is present in the child’s parents.
On the other hand, the child may be big overall. Check the child’s height and weight measurements to see if it corresponds with the head size. You may also want to measure the parents’ heads to see if they have large heads.
When in doubt, consult a pediatric neurosurgeon or neurologist. It is acceptable to move ahead with imaging if you are concerned, or refer the patient to the neurological physicians.
Some babies will have large heads, and on imaging scans they are found to have expanded spaces around the brain and mildly enlarged ventricles. This is a phenomenon known as benign external hydrocephalus. The head growth in these cases can be rather impressive for the first year of life, but the child shows no evidence of functional compromise.
Usually, benign external hydrocephalus will completely resolve by 2–2.5 years of age. Eventually, head growth begins to follow normal growth curves—although the head size still tends to be on the large side. The extra-axial spaces will shrink, and the ventricles may or may not become smaller. These babies need monthly checks of their head circumference until the growth rate follows normal curves. Confirm with a pediatric neurosurgeon or neurologist before dismissing the baby’s scenario as benign external hydrocephalus.
Order diagnostic imaging for hydrocephalus in infants and children
If you suspect hydrocephalus, go ahead with imaging of the brain. Very young babies can have their brain imaged with ultrasound through the anterior fontanelle. However, this option is quickly lost as the fontanelle closes.
Magnetic resonance imaging (MRI) is the preferred method of imaging due to its lack of ionizing radiation, but it may require sedation in infants and children and can be a challenge to schedule.
A computed tomography (CT) scan is equally adept at defining ventricular size. Note that children with hydrocephalus often undergo frequent imaging over the years. Repeated CT scans of the brain result in considerable radiation doses to their developing brains and can cause cognitive loss.
If a patient shows signs of increased intracranial pressure such as a firm and bulging fontanelle, papilledema, vomiting, lethargy, severe irritability, or severe headache, admit the patient for observation while awaiting imaging.
How do you make a definitive diagnosis of hydrocephalus?
Due to the variability of ventricular size and anatomy in people, there is no definitive set of measurements that unequivocally determines hydrocephalus. The diagnosis is made by combining clinical findings and imaging. Ideally, you would have serial images that demonstrated definitive expansion of the ventricles. However, there are five features you can look for on imaging that increase the suspicion of hydrocephalus:
- The temporal horns of the lateral ventricles are greater than 2 mm in diameter.
- There is a ballooning or wasp-wasting of the third ventricle (which should be slit-like).
- The maximal frontal horn diameter takes up more than 50% of the intracranial diameter in the same plane on axial sections.
- There is ballooning of the frontal or occipital horns of the ventricles.
- The Evans ratio is greater than 0.3.
The Evans ratio is the diameter across the frontal horns divided by the largest intracranial biparietal diameter. An Evans ratio greater than 0.3 points towards hydrocephalus.
How to treat a pediatric patient with hydrocephalus
If you suspect definitive hydrocephalus, consult neurosurgery for care. Keep in mind that the treatment of hydrocephalus is evolving.
A ventriculoperitoneal (VP) shunt can be placed, which is designed to shunt CSF from the ventricles into the abdomen (where it can be easily absorbed). Ventriculoperitoneal shunt systems are quite varied, and many are now programmable.
In essence, a VP shunt is a one-way system of tubing that persistently drains fluid from the ventricles and diverts it to the abdominal cavity. The peritoneal space of the abdomen is usually quite adept at absorbing CSF back into the bloodstream.
With VP shunts, there is almost always an interposed valve mechanism that regulates how rapidly CSF is allowed out of the head. The valves are usually a little more palpable than the rest of the tubing and are often tucked behind one ear or the other.
Ventriculoperitoneal shunting has been the historical workhorse for the treatment of hydrocephalus—and is incredibly effective. Thousands of children have been saved from developmental delay and death with VP shunt systems.
Unfortunately, VP shunt systems are not problem-free. They are prone to blockage, breakage, and infection. A VP shunt malfunction can lead to the backup of CSF in the ventricles which causes ventricular dilation, increased intracranial pressure, and can result in death. Shunt malfunctions are considered surgical emergencies. Some children undergo dozens of shunt revisions over the years. Interestingly (and thankfully), shunt failures are much less common once the child reaches adulthood.
Endoscopic third ventriculostomy is gaining popularity in the treatment of hydrocephalus, particularly in older children with aqueductal stenosis. The procedure involves creating a hole in the bottom of the third ventricle to give intraventricular CSF access to the subarachnoid space. This is done through an endoscope. The ventricles frequently do not decrease substantially in size, which can be confusing when evaluating for failure. Keep in mind that failure rates for this procedure are not insignificant, and VP shunting may still be required.
There is also renewed interest in choroid plexus cauterization via endoscopic methods, particularly when combined with the third ventriculostomy. The safety and long-term efficacy of this procedure are still being evaluated.
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