Hydrocephalus is the abnormal enlargement of cerebral ventricles and/or subarachnoid space as a result of excess cerebrospinal fluid (CSF) accumulation. There are two types of true hydrocephalus: communicating hydrocephalus, which is caused by decreased CSF resorption or increased CSF production in the absence of CSF flow obstruction, and noncommunicating hydrocephalus, which is caused by the obstructed passage of CSF from the ventricles to the subarachnoid space. Both forms typically manifest with signs of elevated intracranial pressure (ICP). Other clinical manifestations may include changes in vital signs resulting from brainstem compression and, in congenital hydrocephalus, macrocephaly. Normal pressure hydrocephalus (NPH) is a gradual-onset chronic form of communicating hydrocephalus that primarily occurs in adults > 60 years of age. ICP is normal or mildly elevated; manifestations include a classic triad of gait apraxia, dementia, and urinary incontinence. All types of hydrocephalus are diagnosed using MRI brain or CT head; ultrasound is used in children < 18 months of age with an open anterior fontanelle. A lumbar puncture may be required (once intracranial lesions with mass effect have been ruled out) to measure opening pressure, obtain CSF samples for analysis, and, if NPH is suspected, perform a CSF tap test. Temporizing measures to reduce CSF volume, such as external ventricular drain placement or serial lumbar punctures, may be used in the acute setting. Definitive treatment often involves surgical insertion of a shunt that drains excess CSF into another area of the body. Hydrocephalus ex vacuo is the enlargement of the ventricles and subarachnoid space due to loss of brain tissue (e.g., cerebral atrophy). It is not considered a true hydrocephalus because ventricular enlargement does not result from CSF accumulation and it does not affect ICP or CSF flow.

Intracranial pressure (ICP) is the pressure that exists within the cranium, including its compartments (e.g., the subarachnoid space and the ventricles). ICP varies as the position of the head changes relative to the body and is periodically influenced by normal physiological factors (e.g., cardiac contractions). Adults in the supine position typically have a physiological ICP of ≤ 15 mm Hg; an ICP of ≥ 20 mm Hg indicates pathological intracranial hypertension. ICP may be elevated in a variety of conditions (e.g., intracranial tumors) and can result in a decrease in cerebral perfusion pressure (CPP) and/or herniation of cerebral structures. Symptoms of elevated ICP are generally nonspecific (e.g., impaired consciousness, headache, vomiting); however, more specific symptoms may be present depending on the affected structures (e.g., Cushing triad if the brainstem is compressed). Findings from brain imaging (e.g., a midline shift) and physical examination (e.g., papilledema) can indicate ICP elevation but may not be able to rule it out. Therefore, ICP monitoring and quantification are vital in at-risk patients. Management usually involves expedited surgery of resectable or drainable lesions, conservative measures (e.g., positioning, sedation, analgesia, and antipyretics), and medical therapy (e.g., hyperosmolar therapy such as mannitol or hypertonic saline, or glucocorticoids). Treatment options for refractory intracranial hypertension include temporary controlled hyperventilation, CSF drainage, and decompressive craniectomy (DC), as well as other advanced medical therapies (e.g., barbiturate coma, therapeutic hypothermia).