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Aqueductus mesencephali – Anatomy and Function

The aqueductus mesencephali is a narrow canal in the midbrain connecting the third and fourth ventricles, playing a key role in cerebrospinal fluid circulation.

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The aqueductus mesencephali is a narrow canal in the midbrain connecting the third and fourth ventricles, playing a key role in cerebrospinal fluid circulation.

What is the Aqueductus mesencephali?

The aqueductus mesencephali – also known as the cerebral aqueduct or aqueduct of Sylvius – is a narrow, tube-shaped canal running through the midbrain (mesencephalon). It connects the third ventricle to the fourth ventricle and forms a critical part of the brain ventricular system. Cerebrospinal fluid (CSF) flows through this channel, providing mechanical protection, pressure regulation, and nutrient delivery to the central nervous system.

Anatomy and Location

The aqueductus mesencephali is situated centrally within the midbrain, surrounded by the periaqueductal gray (PAG) – a region of nerve cells involved in pain modulation and emotional processing. In adults, the aqueduct is approximately 1.5 to 2 cm long with a very narrow diameter of only a few millimeters, making it the narrowest point of the entire ventricular system.

Relationship to Adjacent Structures

Cranially (above): Opens into the third ventricle
Caudally (below): Opens into the fourth ventricle
Dorsally (behind): Tectum mesencephali (quadrigeminal plate) with superior and inferior colliculi
Ventrally (in front): Tegmentum mesencephali, containing structures such as the red nucleus and substantia nigra

Function

The primary function of the aqueductus mesencephali is to allow the continuous flow of cerebrospinal fluid from the third to the fourth ventricle. CSF is produced in the choroid plexus of the ventricles, passes through the ventricular system – including the aqueduct – and eventually reaches the subarachnoid space, where it is reabsorbed. This continuous CSF circulation is essential for:

Mechanical protection of the brain and spinal cord
Regulation of intracranial pressure
Removal of metabolic waste products
Nutritional support of the central nervous system

Clinical Relevance: Diseases and Disorders

Due to its narrow diameter, the aqueductus mesencephali is particularly vulnerable to narrowing or blockage, which can result in hydrocephalus – a pathological enlargement of the cerebral ventricles caused by accumulation of CSF.

Aqueductal Stenosis

Aqueductal stenosis (narrowing of the aqueduct) is the most common disorder associated with this structure. It may be congenital (present at birth) or acquired, and leads to obstructive hydrocephalus. Causes include:

Genetic factors (e.g., X-linked aqueductal stenosis associated with L1CAM gene mutations)
Inflammatory conditions (e.g., following meningitis or encephalitis)
Tumors in the midbrain or quadrigeminal region
Hemorrhage or scar tissue formation

Symptoms of Aqueductal Stenosis and Hydrocephalus

Headaches (especially in the morning)
Nausea and vomiting
Visual disturbances (e.g., double vision, Parinaud syndrome)
Gait disturbances and coordination problems
Cognitive impairment
In infants: abnormally large head circumference (macrocephaly)

Further Associated Conditions

Parinaud syndrome: An upward gaze palsy caused by pressure on the tectum mesencephali near the aqueduct
Sylvian aqueduct syndrome: A combination of vertical gaze palsy, convergence-retraction nystagmus, and pupillary abnormalities

Diagnosis

Disorders of the aqueductus mesencephali are primarily diagnosed using neuroimaging techniques:

MRI (Magnetic Resonance Imaging): The method of choice; provides detailed visualization of the aqueduct, ventricular system, and surrounding structures. Phase-contrast MRI can directly visualize CSF flow dynamics.
CT (Computed Tomography): Useful for rapid initial assessment of suspected hydrocephalus, particularly in emergency settings
Ultrasound: Available in newborns and infants through the open fontanelle

Treatment

Treatment depends on the underlying cause and severity of the condition:

Endoscopic third ventriculostomy (ETV): A minimally invasive neurosurgical procedure that creates an alternative CSF drainage pathway. It is the preferred treatment for obstructive hydrocephalus caused by aqueductal stenosis.
Ventriculoperitoneal (VP) shunt: A catheter system that drains excess CSF from the ventricle into the abdominal cavity
Treatment of the underlying cause: For tumors (surgical resection, radiotherapy), infections (antibiotics, corticosteroids), or other triggering conditions

References

Kahle W., Frotscher M. – Pocket Atlas of Human Anatomy: Nervous System and Sensory Organs. 5th Edition, Thieme Publishers, 2003.
Cinalli G., Sainte-Rose C., Chumas P. et al. – Failure of third ventriculostomy in the treatment of aqueductal stenosis in children. Journal of Neurosurgery, 1999; 90(3): 448–454.
Rekate H.L. – A contemporary definition and classification of hydrocephalus. Seminars in Pediatric Neurology, 2009; 16(1): 9–15.

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