Aqueductal Stenosis

Overview

A very rare disorder where there is a buildup of cerebrospinal fluid inside the skull due to a narrow canal that connects the third and fourth ventricles of the brain. Blockage of the aqueduct can lead tohydrocephalus, specifically as a common cause of congenital and/or obstructive hydrocephalus. Also referred to as: Hydrocephalus due to congenital stenosis of aquaduct of Sylvius.

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.

Symptoms

  • Enlarged brain ventricles
  • Mental retardation
  • Spastic paraparesis
  • Adducted thumbs
  • Large head
  • Hydrocephalus
  • Narrowed aqueduct of sylvius
  • Headache
  • Nausea and vomiting
  • Cognitive difficulty
  • Sleepiness
  • Seizures
  • Balance and gait disturbances
  • Visual abnormalities
  • Incontinence

Causes

Tumor compression:
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.

Narrow aqueduct:
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:
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.

Septum formation:
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.

Gliosis:
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.

Diagnosis

Diagnosis of hydrocephalus is determined by brain imaging, and may be suspected if macrocephaly is present. In a Dutch family described by Willems et al. (1987), progressive increase in head circumference led to the diagnosis in 3 patients; however, in 5 with normal head size, the initial diagnosis had been 'nonspecific' mental retardation. Moderate to severe hydrocephaly was present in all macrocephalic patients and in 3 of the 5 with normal occipital frontal circumference. CT scan of the brain did not show aqueductal stenosis in any of these patients.

Treatment

The general purpose of the following treatment methods is to divert the flow of CSF from the blocked aqueduct, which is causing the buildup of CSF, and allow the flow to continue. Another goal of these treatments is to reduce the stress within the ventricles. Studies have not shown that either of the following treatments results in a higher IQ of the patient, and there is not statistical difference in a patient's quality of life based on treatment method. The following treatment methods are not used for aqueductal stenosis caused by tumor compression; if the obstruction is a direct result of tumor compression, CSF flow may be normalized by the surgical removal of the tumor.