Hydrocephalus is caused by an imbalance between the production and absorption of cerebrospinal fluid (CSF) or by a blockage in CSF flow, leading to an accumulation of fluid in the brain. It can be present at birth or develop later in life, and its underlying causes vary by type and age. Key causes and mechanisms

• Obstruction (non-communicating or obstructive): CSF flow is physically blocked within the ventricular system, often by aqueductal stenosis (narrowing of the cerebral aqueduct) or other structural anomalies. This prevents CSF from exiting the ventricles and leads to upstream dilation.

• Impaired absorption (communicating): CSF can be produced normally but is not properly absorbed into the bloodstream, typically due to scarring or dysfunction of the arachnoid granulations. This raises CSF pressure and causes ventricular dilation.

• Overproduction (rare): In some cases, CSF is produced in excess, though this is an uncommon mechanism compared with blockage or absorption problems.

• Congenital (present at birth): congenital hydrocephalus often results from genetic factors, neural tube defects (e.g., spina bifida), aqueductal stenosis, brain malformations, or conditions occurring in fetal development that disrupt CSF pathways.

• Developmental abnormalities: conditions such as Dandy–Walker malformation, Arnold–Chiari malformations, or other structural brain abnormalities can disrupt CSF dynamics and lead to hydrocephalus.

• Acquired (develops after birth): head injury, brain tumors, intraventricular hemorrhage, meningitis or other brain/spinal infections, and certain other illnesses can disrupt CSF flow or absorption and cause hydrocephalus.

• Normal pressure hydrocephalus (NPH): typically occurs in older adults and involves enlarged ventricles with little or no sustained increase in intracranial pressure, often due to impaired CSF absorption over time.

Clinical relevance

• The presenting symptoms and urgency depend on age and the rate of CSF accumulation. Infants may have a rapidly enlarging head circumference and bulging fontanelles, while older children and adults often experience headaches, cognitive changes, balance problems, or gait disturbances.

• Diagnosis combines clinical assessment with imaging (usually MRI or CT) to assess ventricular size and CSF pathways, along with evaluation of CSF production/absorption mechanisms.

• Treatment typically targets restoring CSF circulation or reducing pressure, commonly via surgical interventions such as ventriculoperitoneal (VP) shunting or endoscopic third ventriculostomy (ETV), among other approaches tailored to the cause and patient.

If you’d like, I can tailor this to a specific age group (infant, child, adult) or focus on a particular suspected cause (e.g., congenital aqueductal stenosis vs. post-infectious hydrocephalus) and provide a concise checklist for signs to watch for and questions to discuss with a clinician.