Cerebrospinal fluid (CSF) circulates throughout and around the CNS. In other tissues, water and small molecules are filtered through capillaries as the major contributor to the interstitial fluid. In the brain, CSF is produced in special structures to perfuse through the nervous tissue of the CNS and is continuous with the interstitial fluid. Specifically, CSF circulates to remove metabolic wastes from the interstitial fluids of nervous tissues and return them to the blood stream. The ventricles are the open spaces within the brain where CSF circulates. In some of these spaces, CSF is produced by filtering of the blood that is performed by a specialized membrane known as a choroid plexus. The CSF circulates through all of the ventricles to eventually emerge into the subarachnoid space where it will be reabsorbed into the blood.

The Ventricles

There are four ventricles within the brain, all of which developed from the original hollow space within the neural tube, the central canal. The first two are named the lateral ventricles and are deep within the cerebrum. These ventricles are connected to the third ventricle by two openings called the interventricular foramina. The third ventricle is the space between the left and right sides of the diencephalon, which opens into the cerebral aqueduct that passes through the midbrain. The aqueduct opens into the fourth ventricle, which is the space between the cerebellum and the pons and upper medulla.

Figure 13.18 Cerebrospinal Fluid Circulation The choroid plexus in the four ventricles produce CSF, which is circulated through the ventricular system and then enters the subarachnoid space through the median and lateral apertures. The CSF is then reabsorbed into the blood at the arachnoid granulations, where the arachnoid membrane emerges into the dural sinuses.

As the telencephalon enlarges and grows into the cranial cavity, it is limited by the space within the skull. The telencephalon is the most anterior region of what was the neural tube, but cannot grow past the limit of the frontal bone of the skull. Because the cerebrum fits into this space, it takes on a C-shaped formation, through the frontal, parietal, occipital, and finally temporal regions. The space within the telencephalon is stretched into this same C-shape. The two ventricles are in the left and right sides, and were at one time referred to as the first and second ventricles. The interventricular foramina connect the frontal region of the lateral ventricles with the third ventricle.

The third ventricle is the space bounded by the medial walls of the hypothalamus and thalamus. The two thalami touch in the center in most brains as the massa intermedia, which is surrounded by the third ventricle. The cerebral aqueduct opens just inferior to the epithalamus and passes through the midbrain. The tectum and tegmentum of the midbrain are the roof and floor of the cerebral aqueduct, respectively. The aqueduct opens up into the fourth ventricle. The floor of the fourth ventricle is the dorsal surface of the pons and upper medulla (that gray matter making a continuation of the tegmentum of the midbrain). The fourth ventricle then narrows into the central canal of the spinal cord.

The ventricular system opens up to the subarachnoid space from the fourth ventricle. The single median aperture and the pair of lateral apertures connect to the subarachnoid space so that CSF can flow through the ventricles and around the outside of the CNS. Cerebrospinal fluid is produced within the ventricles by a type of specialized membrane called a choroid plexus. Ependymal cells (one of the types of glial cells described in the introduction to the nervous system) surround blood capillaries and filter the blood to make CSF. The fluid is a clear solution with a limited amount of the constituents of blood. It is essentially water, small molecules, and electrolytes. Oxygen and carbon dioxide are dissolved into the CSF, as they are in blood, and can diffuse between the fluid and the nervous tissue.

Cerebrospinal Fluid Circulation

The choroid plexuses are found in all four ventricles. Observed in dissection, they appear as soft, fuzzy structures that may still be pink, depending on how well the circulatory system is cleared in preparation of the tissue. The CSF is produced from components extracted from the blood, so its flow out of the ventricles is tied to the pulse of cardiovascular circulation.

From the lateral ventricles, the CSF flows into the third ventricle, where more CSF is produced, and then through the cerebral aqueduct into the fourth ventricle where even more CSF is produced. A very small amount of CSF is filtered at any one of the plexuses, for a total of about 500 milliliters daily, but it is continuously made and pulses through the ventricular system, keeping the fluid moving. From the fourth ventricle, CSF can continue down the central canal of the spinal cord, but this is essentially a cul-de-sac, so more of the fluid leaves the ventricular system and moves into the subarachnoid space through the median and lateral apertures.

Within the subarachnoid space, the CSF flows around all of the CNS, providing two important functions. As with elsewhere in its circulation, the CSF picks up metabolic wastes from the nervous tissue and moves it out of the CNS. It also acts as a liquid cushion for the brain and spinal cord. By surrounding the entire system in the subarachnoid space, it provides a thin buffer around the organs within the strong, protective dura mater. The arachnoid granulations are outpocketings of the arachnoid membrane into the dural sinuses so that CSF can be reabsorbed into the blood, along with the metabolic wastes. From the dural sinuses, blood drains out of the head and neck through the jugular veins, along with the rest of the circulation for blood, to be reoxygenated by the lungs and wastes to be filtered out by the kidneys

Components of CSF Circulation

Introduction

The colloid cyst is a benign growth usually located in the third ventricle and at or near the foramen of Monroe, which is at the anterior aspect of the third ventricle of the brain. The colloid cyst is an epithelial-lined cyst filled with gelatinous material. The gelatinous material commonly contains mucin, old blood, cholesterol, and ions.

Colloid cysts can cause various symptoms, including headaches, diplopia, memory issues, and vertigo. Rarely colloid cysts have been cited as a cause of sudden death. When colloid cysts are symptomatic, they most commonly cause headaches, nausea, and vomiting secondary to obstructive hydrocephalus. The obstructive hydrocephalus is precipitated by blocking the egress of cerebrospinal fluid (CSF) from the lateral ventricles at the foramen of Monro, which connects the lateral and third ventricles.

Etiology

The precise etiology of colloid cysts is unclear and still a topic of debate. In the early 20th century, the suggested etiology was that the cyst was a remnant of the paraphysis element. The paraphysis element is an embryonic structure located at the anterior portion of the diencephalon between the two hemispheres of the telencephalon.

As colloid cysts have also been found in the cerebellum, frontal lobe, and pontomesencephalon, there have been other theorized origins of the colloid cysts. Other etiologies include remnants of respiratory epithelium, an ependymal cyst from the diencephalon, and invagination of the neuroepithelium of the lateral ventricle, causing a cyst to form.

Epidemiology

Colloid cyst accounts for less than 2% of all primary brain tumors. More than 99% of all colloid cysts are reported to occur at the rostral end of the third ventricle, at or near the foramen of Monro. Colloid cysts account for approximately one in five intraventricular primary brain tumors. Most patients diagnosed with a colloid cyst are in their third through the seventh decade of life, but cases have rarely been reported as early as the first year of life.

Pathophysiology

Most colloid cysts identified are currently asymptomatic and identified incidentally on imaging. When a colloid cyst does cause issues, it most commonly causes obstructive hydrocephalus.

The colloid cyst is most commonly found in the rostral third ventricle at or near the foramen of Monro. The foramen of Monro is the conduit of cerebrospinal fluid (CSF) outflow from the lateral ventricles to the third ventricle. A colloid cyst can act as a ball valve, stopping CSF flow out of the lateral ventricles. If this occurs, CSF backs up into the lateral ventricles and causes ventriculomegaly and hydrocephalus. Some have posited that colloid cysts can cause intermittent obstructive hydrocephalus, thus causing intermittent symptoms.

Over time colloid cysts tend to grow very slowly. Some colloid cysts may never reach a size that will cause an issue and can be followed, whereas others grow more quickly and become symptomatic with time.

Histopathology

Grossly a colloid cyst is a unilocular round structure filled with a viscous material. Microscopically the cell wall is unicellular and typically composed of columnar epithelium, which may or may not be ciliated. The cyst contents are typically void of living cells and composed of desquamated ghost cells and filamentous material. The mucin stains are positive for periodic acid-Schiff (PAS). The epithelial cyst wall stains positive for keratin and epithelial membrane antigen (EMA).

History and Physical

The majority of colloid cysts are found incidentally on imaging of the brain occurring for other reasons. When a colloid cyst is symptomatic, it most commonly causes non-communicating hydrocephalus. Symptoms of hydrocephalus can include headaches, nausea, vomiting, lethargy, coma, and death. If the hydrocephalus is slowly progressive, the patient can have more subtle findings, including urinary incontinence, trouble with walking, falls, altered mentation, and memory deficits.

For asymptomatic colloid cysts, the physical exam should be normal. If the patient has hydrocephalus from the colloid cyst, physical exam findings may include lethargy, failure of upward gaze, unsteady gait, ataxia, increased reflexes, and, if the hydrocephalus is chronic, papilledema and/or frontal release signs.

Evaluation

Immediate evaluation of suspected colloid cysts includes the airway, breathing, and circulation (ABCs) of emergency medical management if the patient may be at risk for acute hydrocephalus and neurologic deterioration. A thorough neurologic exam is important to identify any neurologic deficits, but imaging remains the cornerstone of evaluation for patients with a colloid cyst.

A colloid cyst is typically not visualized on plain radiographs of the head, and thus, computed tomography (CT) and magnetic resonance imaging (MRI) of the head are more important imaging studies.

CT Head

CT of the head can be quickly obtained to identify acute hydrocephalus. On CT imaging, the colloid cyst is typically a circular, hyperdense mass at or near the foramen of Monro. Rarely are colloid cysts isodense, hypodense, or calcified.

MRI Brain

MRI is the preferred method for imaging colloid cysts. On T1 sequencing, a colloid cyst can have variable characteristics and be either hyperintense, isointense, or hypointense. With gadolinium administration, the colloid cysts should not enhance. Rarely a peripheral enhancement will be noted around the colloid cyst, which most likely represents a vessel stretched over the colloid cyst.

On T2 sequence imaging, most colloid cysts are hypointense. They may also have a heterogeneous T2 signal. Low signal intensity on T2 imaging may suggest the contents of the colloid cyst are more viscous and thus harder to aspirate. On FLAIR sequencing, most colloid cysts have a similar intensity to the surrounding CSF. Most colloid cysts have decreased signal intensity on diffusion-weighted imaging.

Treatment / Management

Treatment and management depend if the colloid cyst is found incidentally or is symptomatic.

If the colloid cyst is symptomatic and causing hydrocephalus, the colloid cyst should be treated. For acute, life-threatening hydrocephalus, treatment of the hydrocephalus should be the next priority after ensuring adequate airway, breathing, and circulation. An external ventricular drain (EVD) can be placed to relieve acute hydrocephalus and may be a life-saving procedure.

After acute, life-threatening hydrocephalus has either been treated or ruled out, the clinician can deal with the colloid cyst. Current treatment options include craniotomy with excision via a transcallosal or transcortical route, endoscopic removal, and stereotactic aspiration.

Craniotomy for Colloid Cyst Removal

A colloid cyst can be removed with a craniotomy. A craniotomy is a surgery where an incision is made in the scalp, and part of the skull is removed for the duration of the surgery, then the skull is put back in place. Two separate routes exist to remove the colloid cysts: transcallosal and transcortical. In the transcallosal approach, the two frontal hemispheres are split apart, and a surgical corridor is created through the rostral end of the genu of the corpus callosum to access the colloid cyst. For the transcortical route, a surgical corridor is developed directly through the brain cortex, most commonly through the right frontal and middle gyrus, to access the lateral ventricle. The colloid cyst can then be removed through the lateral ventricle.

Removing a colloid cyst via a craniotomy has the highest up-front surgical risk but may have the lowest recurrence and reoperation rate. The open craniotomy provides more degrees of freedom for access to the colloid cyst and may be more suitable for larger colloid cysts but does have limitations based on the approach chosen.

Endoscopic Removal of a Colloid Cyst

An endoscopic surgery consists of making a small incision in the scalp and a small hole in the bone. A small tube, typically called a sheath, is advanced through the brain to get access to the lateral ventricle. An endoscope can then be passed into the lateral ventricle to remove the colloid cyst. An endoscopic, in its simplest form, is a tube with a light, camera, and working channel. The light provides illumination for the camera to see what is going on. The working channel provides the surgeon with a way to get instruments and tools in front of the camera to perform surgery.

Endoscopic removal of a colloid cyst tends to have less up-front risk than open surgery but also may have a slightly higher reoperation rate than open surgery. The endoscopic approach may not be suitable for all colloid cysts depending on the size and location of the cyst.

Stereotactic Aspiration of a Colloid Cyst

A third option to treat a colloid cyst is a stereotactic aspiration. This is performed by making a small incision in the scalp and then a small hole in the bone. The surgeon then advances a needle through the brain and into the cyst using some variety of either frame-based or frameless neuronavigation. The contents of the colloid cyst may be able to be aspirated, decreasing its size.

Aspiration of a colloid cyst may not be achievable if the contents of the colloid cyst are particularly thick or if there is no safe corridor to the colloid cyst. Stereotactic aspiration of a colloid cyst has less relative surgical risk than an endoscopic or open resection of the colloid cyst but has the highest reoperation rate compared to the other two treatment modalities. With the aspiration of the colloid cyst, the cyst is left in place and simply decompressed. The cyst may reexpand over time and become symptomatic again.

Asymptomatic Colloid Cyst

An asymptomatic colloid cyst does not necessarily warrant treatment. If there is hydrocephalus, all surgeons would agree that surgery is warranted, but if a colloid cyst is found incidentally, then surgery is not necessarily warranted. Colloid cysts that are smaller than 10 mm or more centrally located in the third ventricle are less likely to obstruct the near term. Such colloid cysts may be monitored over time with serial imaging looking for colloid cyst size and location as well as any evidence of hydrocephalus. There have been infrequent reported cases of colloid cysts which were followed clinically but caused acute hydrocephalus and death.

Differential Diagnosis

There are other lesions that can appear similar to a colloid cyst on imaging, including:

• Craniopharyngioma

• Ependymoma

• Germinoma

• Giant cell astrocytoma

• Hemorrhage

• Lymphoma

• Meningioma

• Metastasis

• Pilocytic astrocytoma

• Pituitary tumor

Prognosis

Some colloid cysts can be watched for years to decades without any issue. Others can slowly grow in size or cause subacute or acute hydrocephalus. With complete surgical resection, the prognosis is good, and colloid cysts are rare to recur after complete resection. Rare cases of sudden death have been reported with colloid cysts, which are usually attributed to acute obstructive hydrocephalus.

Complications

• Hydrocephalus

• Brain herniation

• Death

• Intralesional hemorrhage

Deterrence and Patient Education

Patients kept on observation should be instructed to report to the nearest emergency department should they will develop severe headaches or vomiting.

Enhancing Healthcare Team Outcomes

The CNS colloid cyst is best managed by an interprofessional team that also includes neurosurgeons, neurologists, radiologists, and neuroscience nurses. When a colloid cyst is diagnosed, the biggest dilemma is how to manage it. The minimally invasive approaches have less morbidity but have a higher rate of recurrence and reoperation. Removing a colloid cyst via a craniotomy has the highest up-front surgical risk but may have the lowest recurrence and reoperation rate. The open craniotomy provides more degrees of freedom for access to the colloid cyst and may be more suitable for larger colloid cysts but does have limitations based on the approach chosen. A thorough discussion should be undertaken with the patient and let him or her decide which approach they favor. Specialty-trained nurses in perianesthesia, operating room, and critical assist in the care monitor patients and report changes in status to the team. An interprofessional team approach will result in gest outcomes.