Drawing of the ventricular system from Gray”s Anatomy, with third and fourth ventricles and the aqueduct of Sylvius (cerebral aqueduct) labeled
Aqueductal stenosis is a narrowing of the aqueduct of Sylvius which blocks the flow of cerebrospinal fluid (CSF) in the ventricular system. Blockage of the aqueduct can lead to hydrocephalus, specifically as a common cause of congenital and/or obstructive hydrocephalus.
The aqueduct of Sylvius is the channel which connects the third ventricle to the fourth ventricle and is the narrowest part of the CSF pathway with a mean cross-sectional area of 0.5 mm2 in children and 0.8 mm2 in adults. Because of its small size, the aqueduct is the most likely place for a blockage of CSF in the ventricular system. This blockage causes ventricle volume to increase because the CSF cannot flow out of the ventricles and cannot be effectively absorbed by the surrounding tissue of the ventricles. Increased volume of the ventricles will result in higher pressure within the ventricles, and cause higher pressure in the cortex from it being pushed into the skull. A person may have aqueductal stenosis for years without any symptoms, and a head trauma, hemorrhage, or infection could suddenly invoke those symptoms and worsen the blockage.
Signs and symptoms
The enlarged skull of a person with hydrocephalus, which is a symptom of the excess CSF in the ventricular system. This may be caused by aqeuductal stenosis, and in some cases it is thought that hydrocephalus will cause aqueductal stenosis.
Many of the signs and symptoms of aqueductal stenosis are similar to those of hydrocephalus. These typical symptoms include: headache, nausea and vomiting, cognitive difficulty, sleepiness, seizures, balance and gait disturbances, visual abnormalities, and incontinence.
Signs of aqueductal stenosis other than those mentioned in “Causes of stenosis” include detection of an enlarged lateral and third ventricle in conjunction with a smaller fourth ventricle. This variation in ventricle size is indicative of a blockage in the aqueduct because it lies between the third and fourth ventricles. Another sign of stenosis is deformation of the midbrain, which can be severe. This is caused by the pressure gradient formed from a blockage in the aqueduct.
In cases of aqueductal stenosis caused by tumor compression, a brain tumor in the region of the midbrain forms. More specific anatomically, a tumor forms in the pineal region which is dorsal to the midbrain and is level with the aqueduct of Sylvius. As the tumor grows and expands, it compresses the aqueduct to eventually obstruct it.
A naturally narrow aqueduct allows for the it to be more easily obstructed. Narrow aqueducts have no unusual tissue characteristics, and ventricles are lined with normal epithelial cells. Narrowing can be a defect from birth which results in congenital aqueductal stenosis. Developmental errors that could result in this defect include abnormal folding of the neural plate which causes the neural tube to be narrowed from birth.
Forking refers to an aqueduct which has become split into multiple, separate channels as a result of incomplete fusion of the median fissure. These channels may connect back together to form a single aqueduct again, or they may abruptly stop and form a dead-end. Both of these deformations disrupt the laminar flow of CSF through the ventricular system, causing the force by the aqueduct on its surroundings to be lower than the compressive force being applied to the aqueduct. This greater compressive force could effectively stop the flow of CSF if the aqueduct closes due to the force.
Formation of a septum implies that through gliosis, a membrane of glial cells has developed across the aqueduct. This abnormal membrane most commonly forms at the lower and distal portion of the aqueduct, and completely obstructs the canal. This barricade causes the portion of the aqueduct above it to become dilated with the excess CSF which in turn applies more pressure to the cells in this upper part. This increased pressure amplifies the effects of gliosis, as described in the next section.
With this condition, the aqueduct begins as partially blocked. To compensate for the partial blockage and increase the CSF flow to normal rates, the pressure in the third ventricle is increased thereby also increasing the velocity of the CSF. This in turn creates more shear stress in the aqueduct, causing more damage to the epithelial cells lining the ventricle, and resulting in gliosis and a proliferation of glial cells. This increased number of cells thus causes the blockage to worsen, necessitating more pressure and velocity, and continuing the cycle of gliosis.
Other medical conditions
A genetic disorder called “Brickers-Adams-Edwards syndrome” or “X-linked hydrocephalus” has been discovered that leads to aqueductal stenosis. This disease is transmitted from mother to son. This disorder is caused by a point mutation in the gene for neural cell adhesion. Most males born with this have severe hydrocephalus, adducted thumbs, spastic motions, and intellectual problems. Females with this defect may have adducted thumbs or subnormal intelligence.
Bacterial meningitis can also result in gliotic blockage of the aqueduct. In utero infection or infection during infancy could both result in glial cell build up to make an obstruction.
Connection to hydrocephalus
It is generally considered that aqueductal stenosis is a precursor to non-communicating hydrocephalus, as the blockage of the aqueduct would result in the accumulation of CSF seen in hydrocephalus. However, some studies also argue that cases of aqueductal stenosis not involving a brain tumor are actually a result of communicating hydrocephalus, rather than a cause of it. When a patient has communicating hydrocephalus, the lateral ventricles and medial parts of the temporal lobes expand and compress the aqueduct. As a result, the pressure within the fourth ventricle drops and causes the aqueduct to close more tightly. This in effect could make aqueductal stenosis a byproduct of hydrocephalus. It is estimated that only 25% of males with X-linked hydrocephalus have aqueductal stenosis, which supports the theory that the stenosis may sometimes be a symptom of hydrocephalus.