The brain contains a cerebral ventricular system that houses four ventricles. The third and fourth ventricles connect through the cerebral aqueduct, which serves as a conduit for cerebrospinal fluid. The cerebral aqueduct is a narrow channel around 1 to 2 millimeters in diameter and is particularly susceptible to occlusion. Occlusions in the cerebral aqueduct lead to aqueductal stenosis.
The cerebral aqueduct is located between the pons and cerebellum in the midbrain or mesencephalon. The cerebral aqueduct is a passage that connects the third and fourth ventricles. It acts as a go-between for cerebrospinal fluid to flow freely between these two ventricles.
This article seeks to define the cerebral aqueduct. It outlines the cerebral aqueduct, its function within the brain, and where you can find it. It explains the complications that result from occlusion within the cerebral aqueduct. Similarly, it summarizes the types, symptoms, and treatment of hydrocephalus. Hydrocephalus is a direct result of aqueductal stenosis.
What Is The Cerebral Aqueduct?
The cerebral aqueduct – also known as the aqueduct of Sylvius or the Sylvian aqueduct – is part of the ventricular system within the brain. The brain’s ventricular system contains four structures known as ventricles.
The four ventricles are open cavities or structures filled with cerebrospinal fluid or CSF. You can locate these fluid-filled cavities within functional brain tissue known as the parenchyma.
The brain has two lateral ventricles, the cavity of diencephalon, the rhombencephalic ventricle, and the cerebral aqueduct. The cavity of diencephalon and the rhombencephalic ventricles are known as the third and fourth ventricles, respectively.
Because these ventricles are full of cerebrospinal fluid, they provide cushioning and protection for the brain. They help to prevent significant brain injury and trauma.
The function of the cerebral aqueduct is as a channel that connects the third and fourth ventricles. It allows cerebrospinal fluid to flow between the two cavities. It is around 15 mm long and has a narrow diameter of 1 to 2 mm.
Where Is The Cerebral Aqueduct Located?
The cerebral aqueduct forms part of the ventricular system of the brain. It is located in the midbrain or the mesencephalon. Due to its location, some individuals refer to the cerebral aqueduct as the mesencephalic duct or the aqueductus mesencephali.
You can find the cerebral aqueduct between the third and fourth ventricles. It is also between the pons, which connects the brain and spinal cord, and the cerebellum. The cerebellum controls coordination and balance.
The brain contains gray matter that surrounds the cerebral aqueduct. This gray matter is also known as central or periaqueductal gray.
What Does The Cerebral Aqueduct Do?
The cerebral aqueduct forms part of the network of ventricles found in the brain. It connects the cavity of diencephalon (the third ventricle) and the rhombencephalic or fourth ventricle.
The function of the cerebral aqueduct is to allow cerebrospinal fluid to flow freely between the two ventricular cavities. The cerebrospinal fluid that fills these cavities use for cushioning to protect the brain from harm or injury.
The brain is also suspended in cerebrospinal fluid. These pockets of ventricles of CSF also allow the brain to be more buoyant.
Aqueductal Stenosis
The cerebral aqueduct is a narrow channel that allows cerebrospinal fluid to pass between the third and fourth ventricles in the brain. It is slim and only measures around one to two millimeters in diameter. Because it is so narrow, the cerebral aqueduct is particularly susceptible to occlusion or blockages. These blockages can lead to aqueductal stenosis.
'Aqueductal' refers to the brain channel facilitating cerebrospinal fluid flow between the cavity of diencephalon and the rhombencephalic ventricle. 'Stenosis' refers to the decrease in size within this channel. Therefore, aqueductal stenosis refers to the narrowing of the cerebral aqueduct within the brain’s ventricular system.
When this channel becomes narrower, or a blockage occurs, it can be difficult for the cerebrospinal fluid to pass through. This narrowing or obstruction can be present at birth or acquired. The blockages generally originate from infections, tumors, cysts, and hemorrhages.
Because the occlusion stops cerebrospinal fluid from flowing freely between the third and fourth ventricles, the fluid will begin to store in the brain. The excess buildup of cerebrospinal fluid is known as hydrocephalus.
Symptoms Of Hydrocephalus
The symptoms of hydrocephalus vary and are primarily dependent on age. In infants, the condition is usually present at birth. Hydrocephalus is more commonly diagnosed in infants at birth. Still, it is also a common condition among adults over 60. In older adults, the condition is usually due to Normal pressure hydrocephalus or NPH.
Hydrocephalus Symptoms In Infants
The symptom that is most prevalent in infants is abnormal changes to the head. This symptom can include a protrusion at the top of the head, an oversized head, or a head that undergoes rapid growth.
Other symptoms that may present in conjunction with changes to the head include downturned eyes – also known as sunsetting of the eyes, poor muscle tonality, and seizures. Babies with hydrocephalus may also struggle to eat. Parents may notice their infant losing interest in food or refusing it completely. This lack of appetite is usually closely related to symptoms of nausea and vomiting.
Hydrocephalus Symptoms In Toddlers And Children
Much like infants, toddlers and children will experience physical changes to the structure or size of the head. They will exhibit poor bladder control and a loss of coordination and balance. Toddlers and children diagnosed with hydrocephalus may complain of a continual headache or migraine. These migraines may be severe and lead to blurry vision, nausea, and vomiting.
Children may also exhibit behavioral symptoms in conjunction with their physical symptoms. These behavioral changes often look like a complete change in personality, stunted development, irritability, and an inability to focus.
Hydrocephalus Symptoms In Adults
It's extremely rare for young and middle-aged adults to be diagnosed with hydrocephalus. However, a few symptoms can be noted in this age group.
Like children, adults can experience poor bladder control and a loss of coordination and balance. Adults may also experience extreme unexplained lethargy and blurry or impaired vision. Headaches are also a common symptom of hydrocephalus across all age groups.
The most notable symptoms in young and middle-aged adults are difficulty concentrating and poor memory. Poor memory symptoms include unexplained memory loss or difficulty with short-term memory.
Hydrocephalus Symptoms In Older Adults
Hydrocephalus is most commonly diagnosed at birth, before the child turns three, or in adults over 60. Older adults usually experience normal pressure hydrocephalus or NPH. It is mainly due to an expansion of the ventricles, which increases cerebrospinal fluid pressure on the brain.
People over 60 years old are can experience memory loss, poor bladder control or complete incontinence, and mobility issues. They may also suffer from a loss of general cognitive functioning and poor balance and coordination.
Types Of Hydrocephalus
Hydrocephalus is usually present at birth or can be acquired through occlusion of the cerebral aqueduct. There are various types of hydrocephalus. You can separate hydrocephalus into two categories: non-obstructive or communicating hydrocephalus and obstructive or non-communicating hydrocephalus.
Non-obstructive hydrocephalus happens when no occlusions or obstructions cause a buildup of CSF. Adversely, obstructive hydrocephalus occurs when the cerebral aqueduct is blocked or narrowed. This narrowing restricts the flow of cerebrospinal fluid, causing an accumulation of fluid in the brain.
Normal Pressure Hydrocephalus
Normal pressure hydrocephalus, or NPH, is caused by an expansion of the cerebral ventricles. This expansion can cause sporadic or irregular cerebrospinal fluid pressure levels within the brain. NPH is often misclassified in older adults. This misdiagnosis is because it does not always present the common physical symptoms of hydrocephalus.
Older adults may receive their diagnosis after receiving an MRI, CT scan, or a lumbar puncture. During a lumbar puncture, medical staff will check the pressure levels of the cerebrospinal fluid. Additionally, they will check for enlarged ventricles of cerebral edema in CT scans and MRIs, respectively. Cerebral edema increases pressure levels within the brain's ventricular system.
The treatment for normal pressure hydrocephalus includes implanting a hollow tube – also known as a hunt – into the brain. This shunt redirects the excess cerebrospinal fluid into the patient's abdomen, where the rest of the body can reabsorb it. The treatment can help relieve symptoms, and the enlarged ventricles will reduce over the days following surgery.
Obstructive Hydrocephalus
Obstructive hydrocephalus can occur when the cerebral aqueduct is injured and diminished due to tumors, hemorrhages, and other lesions. This narrowing leads to aqueductal stenosis. Aqueductal stenosis contributes to the development of hydrocephalus. However, aqueductal stenosis is not the only cause. Hydrocephalus can also occur due to occlusion in other parts of the brain.
The interventricular foramina connect the lateral ventricles. It also connects to the cavity of diencephalon. Blockages in this brain area can lead to an enlargement of the lateral ventricles, which causes increased levels of cerebrospinal fluid pressure.
Similarly, blockages in the rhombencephalic or fourth ventricle can cause an expansion of the other ventricles that make up the cerebral ventricular network. The fourth ventricle connects to the cavity of diencephalon or the third ventricle through the cerebral aqueduct.
Hydrocephalus Ex Vacuo
Unlike other types of hydrocephalus, this condition does not result from a buildup of cerebrospinal fluid. Instead, it is the brain's way of making up for a loss of functional brain tissue. This tissue loss is generally due to cerebral atrophy in older adults or as a result of traumatic brain injuries and other disorders.
Congenital Hydrocephalus
Congenital hydrocephalus is usually present at birth, but it can affect toddlers up to three years old. This type of hydrocephalus occurs before the infant's skull has completely fused and formed.
Congenital hydrocephalus may be genetic or a result of infections, malformations, and aqueductal stenosis. Premature infants may also be at risk of developing congenital hydrocephalus due to an increased risk of a brain hemorrhage.
Treating Hydrocephalus
After successfully diagnosing hydrocephalus, there are two main treatments available to patients. These treatments include an endoscopic third ventriculostomy, an external ventricular drain, and shunt surgery.
Once these treatments have been completed successfully, the patient should be relieved of their symptoms.
Endoscopic Third Ventriculostomy
An endoscopic third ventriculostomy or ETV is a surgery that is usually undertaken as a treatment for obtrusive hydrocephalus. During an endoscopic third ventriculostomy, the surgeon makes a small hole in the cavity of diencephalon. This hole in the third ventricle allows the excess cerebrospinal fluid to drain away from the brain's surface.
Once the excess fluid has been drained, it can be reabsorbed into the body. Draining will help to decompress the ventricles and relieve symptoms.
External Ventricular Draining
External ventricular draining works similarly to an endoscopic third ventriculostomy. It helps to reroute the additional fluid away from the blockage.
An external ventricular drain helps drain the cerebrospinal fluid away from the brain through an incision in the scalp. The excess fluid drains and collects into an external system. Patients who undergo this treatment are usually kept in a medical suite or hospital for their medical team to assess their cerebrospinal fluid pressure levels.
Cerebral Shunt Surgery
The surgeon makes a small perforation into the skull during cerebral shunt surgery. The perforation allows for inserting a tube into the brain and the necessary ventricle to help redirect excess spinal fluid away from the brain. The surgeon then positions the line in the abdominal cavity, where the body can reabsorb the additional fluid.
Conclusion
The cerebral aqueduct is a narrow channel that connects the cavity of diencephalon and the rhombencephalic ventricle. It allows cerebrospinal fluid to pass freely between the third and fourth ventricle. You can locate this structure in the midbrain, between the pons and cerebellum. Periaqueductal gray matter surrounds the cerebral aqueduct.
When the cerebral aqueduct becomes blocked, excess cerebrospinal fluid builds up around the brain. The buildup causes increased levels of cerebrospinal fluid pressure. This pressure can lead to hydrocephalus, which is generally treated with surgery. Hydrocephalus is most common in infants and adults over the age of 60.
References
https://www.ncbi.nlm.nih.gov/books/NBK540988/
https://www.sciencedirect.com/topics/neuroscience/cerebral-aqueduct
https://emedicine.medscape.com/article/1923254-overview
https://radiopaedia.org/articles/cerebral-aqueduct-of-sylvius
