Cerebrospinal fluid (CSF) is a clear, colorless liquid that surrounds and cushions the brain and spinal cord within the skull and vertebral column. This fluid is constantly produced, circulated, and absorbed, maintaining a delicate balance necessary for neurological function. When the production, flow, or absorption of this fluid is disrupted, it leads to an abnormal buildup within the brain’s cavities, a condition medically known as hydrocephalus. This accumulation results in increased pressure on the delicate brain tissue, which can disrupt normal function and cause significant neurological symptoms. The proper dynamics of CSF are fundamental to the health and protection of the central nervous system.
The Role of Cerebrospinal Fluid
The main functions of cerebrospinal fluid are to provide mechanical protection, deliver nutrients, and remove waste products from the brain and spinal cord. One of its most important functions is to provide buoyancy, effectively reducing the net weight of the brain. This buoyancy prevents the brain from compressing its own delicate structures and blood vessels against the rigid bone of the skull.
CSF also acts as a hydraulic shock absorber, buffering the brain against sudden movements or trauma to the head. The fluid circulates throughout the four interconnected cavities deep within the brain, known as ventricles, and then flows over the surface of the brain and spinal cord in the subarachnoid space. Specialized tissue called the choroid plexus, located within the ventricles, produces this fluid by filtering blood plasma.
The body produces about 500 milliliters of CSF each day, even though the central nervous system only contains about 150 milliliters at any given moment. This continuous turnover means the entire volume of CSF is replaced multiple times daily, ensuring a consistent environment for brain cells. After circulating, the fluid is reabsorbed into the bloodstream primarily through small, valve-like structures called arachnoid granulations, which project into the large venous sinuses of the brain.
How Accumulation Occurs: Production, Flow, and Absorption Failures
The abnormal accumulation of cerebrospinal fluid results from an imbalance where the rate of production exceeds the rate of removal. This imbalance can be classified into three primary physiological mechanisms: excessive production, obstruction of flow, or failure of absorption.
The rarest mechanism is the overproduction of CSF, which can overwhelm the system’s ability to drain the fluid. This is typically caused by a tumor of the choroid plexus, known as a choroid plexus papilloma, which causes the fluid-producing tissue to secrete an abnormal volume of CSF. Although this mechanism is straightforward, it accounts for only a small percentage of hydrocephalus cases.
A far more common mechanism involves an obstruction that prevents the fluid from flowing freely through its normal pathways, leading to what is called non-communicating hydrocephalus. Blockages often occur at narrow passages within the ventricular system, such as the aqueduct of Sylvius, a small channel connecting the third and fourth ventricles. When flow is blocked, the fluid pools upstream, causing the ventricles to swell and exert pressure on the surrounding brain tissue. The continuous production of CSF behind the blockage causes the pressure to rise rapidly.
The third main mechanism is a failure of absorption, resulting in communicating hydrocephalus. In this scenario, the CSF can flow freely between the ventricles, but it cannot be properly reabsorbed into the bloodstream at the arachnoid granulations. This failure is often caused by scarring or inflammation that impairs the function of these absorption sites, preventing the fluid from draining away effectively.
Common Causes and Underlying Conditions
The underlying causes of abnormal CSF accumulation are broadly categorized as either congenital, meaning present at birth, or acquired, developing sometime after birth. These specific conditions and injuries directly lead to one of the three physiological failures discussed, most often obstruction or absorption failure.
A significant congenital cause is aqueductal stenosis, which involves the narrowing of the aqueduct of Sylvius. This condition acts as a physical bottleneck, severely limiting CSF flow from the third to the fourth ventricle and causing an obstructive type of hydrocephalus in infants. Other developmental issues, such as Arnold-Chiari malformations, involve structural defects at the base of the skull that can displace brain tissue and block the natural exit points of CSF from the fourth ventricle.
Acquired causes are numerous and often stem from trauma, infection, or internal bleeding. Intracranial hemorrhage, which is bleeding within the brain, is a frequent cause of acquired hydrocephalus. Blood products released during a subarachnoid or intraventricular hemorrhage can irritate and scar the arachnoid granulations, leading to a failure of CSF absorption.
Infections of the central nervous system, particularly bacterial meningitis, can also disrupt CSF dynamics. The resulting inflammation can thicken the membranes surrounding the brain and damage the arachnoid villi, creating resistance to reabsorption and causing communicating hydrocephalus.
Tumors or other space-occupying lesions are another major acquired cause, leading to obstructive hydrocephalus. A mass growing near the narrow pathways, such as the third or fourth ventricle, can physically compress the structures and block the flow of fluid. Head trauma and stroke are also recognized causes, as the resulting injury or bleeding can initiate the inflammatory and scarring processes that impair both flow and absorption.