Yesterday afternoon I checked in with one of my Facebook hydrocephalus support group and the question came up whether it is possible for the aqueduct of Sylvius can be completely absent from the brain. After some early morning research, the answer to that question is "no". Let me explain what it does and you will better understand why this is the case.
The aqueduct of Sylvius serves as the communicator between the third and fourth ventricles of the brain. Located just above the hypothalamus (pictured at left) in the mid-brain, the Aqueduct of sylvius measures 15 to 18 millimeters long by 1 to 2 millimeters in diameter and is shaped like the neck of a swan. It contains the nuclei of the third and fourth cranial nerves.
What can go wrong?
In 1935, Lysholm suggested that a kink in the aqueduct of Sylvius was considered pathogonomic of space occupying lesions of the posterior fossa. Three years later, in 1938, Hyndman wrote about four (4) cases where tumors were located either in or pressing on the cerebellar vermis causing a kinked aqueduct of Sylvius. This was later reenforced by another researcher who hypothesized that a kink could be a variation of the normal. Below is a case study that seems to agree with that finding.
Case study # 1
While hospitalized for an unrelated issue, a 70 year-old man presented with the clinical triad of dementia, gait instability, and urinary incontinence. A radionuclide cisternagram strongly pointed to normal pressure hydrocephalus since the test demonstrated entrance of the nuclide into the ventricles with no flow over the cerebral convexities. pneumoencephalography indicated a kinked aqueduct, however, there was no displacement of the fourth ventricle. It also showed gross enlargement of the lateral ventricles.
Case study # 2
A 16 year-old boy presented with headache, double vision, nausea, and vomiting. Examination discovered bilateral papiledema (swelling of the optic nerve) accompanied by paresis of the upward gaze. (Just as an FYI, papiledema and paresis of the upward gaze are also characteristic symptoms in hydrocephalus patients -- especially those, such as myself who are unshunted.) A radiograph of the skull identified a Calcified pineal gland and a pneumoencephalagram indicated a kink in his Aqueduct of Sylvius. Based on these findings - and the lack of displacement of the fourth ventricle (which would be indicative of a tumor), he was diagnosed with Pinealoma -- a type of tumor that develops on the pineal gland.
Conclusion
Modern day research has disproved Lysholm's hypothesis that a kink in the aqueduct of Sylvius is pathognomonic - meaning that it is indicative - of space occupying lesions of the posterior fossa. It is now evident that a kink can also occur in persons with other cranial abnormalities such as those pointed out in the case studies cited above.
Bibliography
Lysholm, E. (1935) Acta Radiology, Supplement 26, 65 - 79 (1935)
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