The ventricles of the brain are a communicating network of cavities filled with cerebrospinal fluid (CSF) and located within the brain parenchyma. The ventricular system is composed of 2 lateral ventricles, the third ventricle, the cerebral aqueduct, and the fourth ventricle (see the images below). The choroid plexuses are located in the ventricles produce CSF, which fills the ventricles and subarachnoid space, following a cycle of constant production and reabsorption.
The ventricular system is embryologically derived from the neural canal, forming early in the development of the neural tube. The 3 brain vesicles (prosencephalon or forebrain, mesencephalon or midbrain, and rhombencephalon or hindbrain) form around the end of the first gestational month. The neural canal dilates within the prosencephalon, leading to the formation of the lateral ventricles and third ventricle. The cavity of the mesencephalon forms the cerebral aqueduct. The dilation of the neural canal within the rhombencephalon forms the fourth ventricle.
The lateral ventricles communicate with the third ventricle through interventricular foramens, and the third ventricle communicates with the fourth ventricle through the cerebral aqueduct (see the image below).  During early development, the septum pellucidum is formed by the thinned walls of the 2 cerebral hemispheres and contains a fluid-filled cavity, named the cavum, which may persist.
Tufts of capillaries invaginate the roofs of prosencephalon and rhombencephalon, forming the choroid plexuses of the ventricles. Cerebrospinal fluid (CSF) is secreted by the choroid plexuses, filling the ventricular system. CSF flows out of the fourth ventricle through the 3 apertures formed at the roof of the fourth ventricle by 12 weeks' gestation. 
The largest cavities of the ventricular system are the lateral ventricles. Each lateral ventricle is divided into a central portion, formed by the body and atrium (or trigone), and 3 lateral extensions or horns of the ventricles. [1, 2] The central portion or the body of the ventricle is located within the parietal lobe. The roof is formed by the corpus callosum, and the posterior portion of the septum pellucidum lies medially. The anterior part of the body of the fornix, the choroid plexus, lateral dorsal surface of the thalamus, stria terminalis, and caudate nucleus, form the floor of the lateral ventricle.  See the image below.
The interventricular foramen is located between the thalamus and anterior pillar of the fornix, at the anterior margin of the body. The 2 interventricular foramens (or foramina of Monro) connect the lateral ventricles with the third ventricle. The body of the lateral ventricle is connected with the occipital and temporal horns by a wide area named the atrium. [1, 2]
The anterior or frontal horn is located anterior to the interventricular foramen. The floor and the lateral wall are formed by the head of the caudate nucleus, the corpus callosum constitutes the roof and anterior border, and the septum pellucidum delineates the medial wall.  The posterior or occipital horn is located within the occipital lobe. The fibers of the corpus callosum and the splenium form the roof. The forceps major is located on the medial side and forms the bulb of the occipital horn. [1, 2]
The inferior or temporal horn is located within the temporal lobe. The roof is formed by the fibers of the temporal lobe; the medial border contains the stria terminalis and tail of the caudate. The medial wall and the floor are formed by the hippocampus and its associated structures. The amygdaloid complex is located at the anterior end of the inferior horn. [1, 2]
Capillaries of the choroid arteries from the pia mater project into the ventricular cavity, forming the choroid plexus of the lateral ventricle (see the image below). The choroid plexus is attached to the adjacent brain structures by a double layer of pia mater called the tela choroidea. The choroid plexus extends from the lateral ventricle into the inferior horn. The anterior and posterior horn have no choroid plexus.
The choroid plexus of the lateral ventricle is connected with the choroid plexus of the contralateral ventricle and the third ventricle through the interventricular foramen. The anterior choroidal arteries (branch of internal carotid artery) and lateral posterior choroidal arteries (branch of the posterior cerebral artery) form the choroid plexus. Venous supply from the choroidal veins drain into the cerebral veins. 
The third ventricle is the narrow vertical cavity of the diencephalon. A thin tela choroidea supplied by the medial posterior choroidal arteries (branch of posterior cerebral artery) is formed in the roof of the third ventricle. The fornix and the corpus callosum are located superiorly. The lateral walls are formed by the medial thalamus and hypothalamus. The anterior commissure, the lamina terminalis, and the optic chiasm delineate the anterior wall. The floor of the third ventricle is formed by the infundibulum, which attaches the hypophysis, the tuber cinereum, the mammillary bodies, and the upper end of the midbrain. The posterior wall is formed by the pineal gland and habenular commissure. The interthalamic adhesions are bands of gray matter with unknown functional significance, which cross the cavity of the ventricle and attach to the external walls. [1, 2]
The fourth ventricle is connected to the third ventricle by a narrow cerebral aqueduct. The fourth ventricle is a diamond-shaped cavity located posterior to the pons and upper medulla oblongata and anterior-inferior to the cerebellum. The superior cerebellar peduncles and the anterior and posterior medullary vela form the roof of the fourth ventricle. The apex or fastigium is the extension of the ventricle up into the cerebellum. The floor of the fourth ventricle is named the rhomboid fossa. The lateral recess is an extension of the ventricle on the dorsal inferior cerebellar peduncle.
Inferiorly, it extends into the central canal of medulla. The fourth ventricle communicates with the subarachnoid space through the lateral foramen of Luschka, located near the flocculus of the cerebellum, and through the median foramen of Magendie, located in the roof of the ventricle. Most of the CSF outflow passes through the medial foramen. The cerebral aqueduct contains no choroid plexus. The tela choroidea of the fourth ventricle, which is supplied by branches of the posterior inferior cerebellar arteries, is located in the posterior medullary velum. [1, 2]
CSF is a clear, watery fluid that fills the ventricles of the brain and the subarachnoid space around the brain and spinal cord. CSF is primarily produced by the choroid plexus of the ventricles (≤70% of the volume); most of it is formed by the choroid plexus of the lateral ventricles. The rest of the CSF production is the result of transependymal flow from the brain to the ventricles. 
CSF flows from the lateral ventricles, through the interventricular foramens, and into the third ventricle, cerebral aqueduct, and the fourth ventricle. Only a very small amount enters the central canal of the spinal cord. CSF flow is the result of a combination of factors, which include the hydrostatic pressure generated during CSF production (known as bulk flow), arterial pulsations of the large arteries, and directional beating of the ependymal cilia. Hydrostatic pressure has a predominant role in the CSF flow within the larger ventricles, whereas cilia favor the movement of the CSF in the narrow regions of the ventricular system, such as the cerebral aqueduct. Immotile cilia syndrome is a rare cause of hydrocephalus in children. 
The ventricles constitute the internal part of a communicating system containing CSF. The external part of the system is formed by the subarachnoid space and cisterns. The communication between the 2 parts occurs at the level of fourth ventricle through the median foramen of Magendie (into the cistern magna) and the 2 lateral foramina of Luschka (into the spaces around the brainstem cerebellopontine angles and prepontine cisterns). The CSF is absorbed from the subarachnoid space into the venous blood (of the sinuses or veins) by the small arachnoid villi, which are clusters of cells projecting from subarachnoid space into a venous sinus, and the larger arachnoid granulations. [4, 5]
The total CSF volume contained within the communicating system in adults is approximately 150 mL, with approximately 25% filling the ventricular system. CSF is produced at a rate of approximately 20 mL/h, and an estimated 400-500 mL of CSF is produced and absorbed daily.
CSF absorption capacity is normally approximately 2-4 times the rate of production. The normal CSF pressure is between 5-15 mm Hg (65-195 mm H2 O) in adults. In children younger than 6 years, normal CSF pressure ranges between 10-100 mm H2 O. [4, 5]
CSF plays an important role in supporting the brain growth during evolution, protecting against external trauma, removal of metabolites produced by neuronal and glial cell activity, and transport of biologically active substances (like hormones and neuropeptides) throughout the brain. 
The ventricles are lined by a single layer of ciliated squamous or columnar ependymal cells. The ependymal cells develop from tanycytes, types of transitional cells with radially extending processes, which come in contact with the blood vessels, neurons, and glia.
The choroid plexus forms early in development, shortly after the closing of the neural tube. The ependymal cells coming in contact with the adjacent mesodermally derived tissue form pseudorosettes, which protrude within the neural tube at the sites of ventricular system formation. The differentiation of these cells with resulting development of the choroid plexus is largely completed by 22 weeks' gestation. 
The following 3 main barriers separate blood from the central nervous system (CNS) compartments:
The vascular endothelial barrier
The blood – cerebrospinal fluid (CSF) barrier
The subarachnoid barrier
The blood-brain barrier is formed by capillary endothelial cells, pluripotent pericytes, a dense basement membrane, and perivascular end-feet of astrocytes. The vascular endothelial barrier is formed by tight junctions and adherence junctions between endothelial cells. Cerebral capillary endothelial cells lack fenestrations, have fewer pinocytic vesicles, have an increased number of mitochondria, and have a thicker basement membrane (30-40 mm thick) and adjacent astrocytic end-feet relative to the systemic endothelial cells. A single cell usually spans the entire circumference of a cerebral capillary lumen.
In the blood, the CSF barrier and epithelial cells of the plexus are connected by tight junctions, forming a continuous layer that permits the passage of selected substances. The capillaries of the choroid plexuses have more fenestration than the brain capillaries. The choroid plexus capillaries are separated from the choroidal cells by a basement membrane and a layer of connective tissue. The ependymal cell form the lining of the ventricles and are continuous with the epithelium of the choroid plexus.
The main functions of the blood-brain barrier are to prevent the entry of potentially harmful substances into the CNS, to maintain ion and volume regulation, and to maintain metabolic as well as immunologic function. A dysfunction or disruption in the blood-brain barrier may be encountered in many disease states, such as infection, inflammation, presence of tumors, and hypoxic-ischemic events with potential severe neurologic sequelae. 
The blood-brain barrier is absent in several specialized areas of the brain, known as circumventricular organs. These are the area postrema of the fourth ventricle, the median eminence, basal hypothalamus/neurohypophysis, the pineal gland, subfornical and subcommissural organs, and lamina terminalis. In these regions, the ependymal lining has discontinuous gap junctions and few tight junctions, and the fenestrated capillaries are highly permeable. These areas have specific secretory function (neurohypophysis) or surveillance function (eg, area postrema). 
CSF is an ultrafiltrate of plasma. Sodium is secreted into the CSF by the sodium-potassium ATPase pump, followed by the passive transfer of water molecules. Intracellular carbonic anhydrase generates bicarbonate and hydrogen ions. Most proteins are excluded from the CSF by the blood-brain barrier. 
Intracranial pressure is the pressure within the closed craniospinal compartment, which encompasses 3 main components: brain parenchyma, intracranial cerebrospinal fluid (CSF), and cerebral blood volume.
An increase in CSF pressure happens as a result of an increase in the intracranial volume (eg, tumors), blood volume (with hemorrhages), or CSF volume (eg, hydrocephalus). Blocking the circulation of the CSF leads to dilatation of the ventricular system upstream to the level of obstruction, defined as hydrocephalus.
The old classification divides hydrocephalus into 2 types: noncommunicating and communicating. In noncommunicating or obstructive hydrocephalus, the CSF accumulates within the ventricles as a result of an obstruction within the ventricular system (most commonly at the level of cerebral aqueduct). In communicating hydrocephalus, the CSF flows freely through the outflow foramens of the fourth ventricles into the arachnoid space.
Current imaging techniques, including CT scanning and MRI (see the image below), make inferences about the level of obstruction, depending on the presence or absence of ventriculomegaly, especially fourth ventricle dilatation. Fourth ventricle dilatation implies obstruction distally, usually at the level of the subarachnoid space. A small fourth ventricle suggests obstruction proximal to the fourth ventricle. [2, 3]
Intraventricular obstructive hydrocephalus refers to hydrocephalus resulting from an obstruction within the ventricular system (eg, aqueductal stenosis). The continuous production of the CSF leads to dilatation of one or more ventricles, depending on the site of obstruction. In the acute obstruction phase, transependymal flow of CSF may occur. The gyri are flattened against the skull. If the skull sutures are not calcified, such as in children younger than age 2 years, the head may enlarge.
Extraventricular obstructive hydrocephalus indicates an obstruction outside the ventricles (eg, at the level of arachnoid villi, as a result of previous bleeding, infection, or inflammation, which results in thickening of the arachnoid and decreased absorption of the CSF). [3, 7]
Hydrocephalus causes symptoms mainly due to increased intracranial pressure. The symptoms and findings vary with age. Clinical features of hydrocephalus in infants include irritability, lethargy, poor feeding, vomiting, and failure to thrive. In older children and adults, morning headache associated with vomiting, diplopia, gait dysfunction as a result of stretching of the paracentral corticospinal fibers, coordination problems, and impairment in the higher functions are seen.
Macrocephalus, cracked pot sound with percussion, separation of sutures, frontal bossing, or occipital prominence is usually seen in children with hydrocephalus that developed before the closing of the cranial vault. Papilledema, exudates or hemorrhages, and optic atrophy may be seen upon funduscopic examination in children or adults. Enlargement of the blind spot is also noted.
Diplopia is usually caused by bilateral sixth nerve palsy due to increased intracranial pressure. A paralysis of the upgaze or partial Parinaud syndrome (setting sun sign) is seen as a result of pressure on the superior colliculus or tectum. Other findings include hormonal changes as a result of third ventricle dilatation and pressure on the hypothalamic-pituitary structures, cognitive dysfunction, changes in personality may be seen, and, occasionally, seizures. Posterior fossa tumors may cause transforaminal herniation of the cerebellar tonsils with neck stiffness. 
The etiologies and pathogenesis of hydrocephalus include overproduction, blockage, or diminished absorption. The only known etiology of excess production is choroid plexus papilloma, which accounts for less than 2% of childhood tumors.
Etiologies of hydrocephalus secondary to blockage or diminished absorption include developmental abnormalities, trauma, tumors, infectious, inflammatory, and idiopathic. Solid tumors produce hydrocephalus by obstruction of the ventricles, whereas nonsolid tumors (eg, leukemia, carcinomatous infiltration) impair CSF absorption within the subarachnoid space. [3, 7]
The following are some causes of obstruction at specific locations in the ventricular system:
Foramen of Monro obstruction may be caused by a suprasellar mass (eg, glioma, arachnoid cyst, craniopharyngioma), septum pellucidum tumor, colloid cyst, or tuberous sclerosis
Third ventricle obstruction may result from a colloid cyst, large hypothalamic-optic or thalamic glioma, or suprasellar mass
Cerebral aqueduct obstruction may be the result of aqueductal stenosis, vascular malformations (eg, arteriovenous malformations or vein of Galen aneurysm), ventriculitis, ependymitis, or tumors (eg, pineal, brainstem, cerebellar, or mesencephalic)
Obstruction at the level of fourth ventricle may be caused by posterior fossa tumors, hemorrhage, or ventriculitis
Obstruction of the fourth ventricle foramina of Luschka and Magendie may be due to a Dandy-Walker malformation, arachnoid cyst, infection (eg, ventriculitis, meningitis), or cerebellar tumors
Obstruction at the level of subarachnoid space is usually caused by hemorrhage (subarachnoid or subdural), meningitis, and, rarely, by Chiari malformation
Congenital hydrocephalus has an incidence of 0.4-0.8 per 1000 live births and stillbirths; noncommunicating hydrocephalus is the most common form of hydrocephalus in fetuses. Aqueductal stenosis is the most common cause of congenital hydrocephalus, whereas mass lesions are the most common cause of aqueductal obstruction during childhood.  Other causes of congenital noncommunicating hydrocephalus include the following:
Dandy-Walker malformation, which consists of a markedly dilatated fourth ventricle associated with failure of the foramen of Magendie to open, aplasia of the posterior cerebellar vermis, heterotopias of the inferior olivary nuclei, pachygyria, agenesis of the corpus callosum, and other abnormalities 
Klippel-Feil syndrome, defined by obstructive hydrocephalus at the level of fourth ventricle associated with malformation of the craniocervical skeleton (This condition may be associated with Chiari malformation and basilar impression.)
Congenital brain tumors, most common being astrocytoma, medulloblastoma, teratoma, and choroid plexus papilloma (These tumors are more often supratentorial and midline, usually compressing the cerebral aqueduct.)
Vein of Galen malformation
Walker-Warburg syndrome, a congenital syndrome characterized by hydrocephalus, agyria, and retinal dysplasia, with or without encephalocele, associated with congenital muscular dystrophies 
Hydrancephaly, porencephaly, and schizencephaly
Hydranencephaly results from replacement of the brain parenchyma by the CSF. Causes include a failure in normal brain development, intrauterine disease destroying the normal brain tissue, or untreated progressive obstructive hydrocephalus. 
Porencephaly refers to hemispheric cysts resulting from the destruction of immature brain parenchyma, which may or may not communicate with the lateral ventricle and subarachnoid space.
Normal pressure and arrested hydrocephalus
The uniformly dilatated ventricles with normal CSF pressure are classified as normal pressure hydrocephalus (NPH). Arrested hydrocephalus may represent a form of normal pressure hydrocephalus. Normal pressure hydrocephalus may be accompanied by gait disorder, incontinence, and dementia in elderly patients. The etiology is presumed to be idiopathic, resulting in increased resistance to CSF absorption across the arachnoid villi. A remote history of trauma, infection, or subarachnoid hemorrhage may be elicited occasionally. CT scanning or MRI reveals uniform ventricular dilatation out of proportion to the cortical atrophy, with periventricular lucencies. 
Idiopathic intracranial hypertension
Idiopathic intracranial hypertension (IIH) (also known as pseudotumor cerebri) is a diagnosis of exclusion. Predominantly seen in young, obese women (age 20-40 y; female-to-male ratio, 3:1), it manifests with headaches and visual disturbances; in the most severe cases, visual loss may result. The eye examination findings are related to increased intracranial pressure and include papilledema, retinal hemorrhages, exudates, enlargement of the blind spot, and sixth cranial nerve palsies. On CT scan or MRI, the ventricular system appears normal. Empty sella may be seen in a small percentile of patients. Lumbar puncture reveals elevated CSF pressure greater than 250 mm H2 O, with normal CSF composition. 
In patients with normal pressure hydrocephalus, large-volume lumbar puncture with removal of 40-50 mL of CSF is followed by clinical improvement and high convexity tightness, as seen on CT scan or MRI, indicate a potential benefit with shunting procedures.  Isotope cisternography and perfusion tests are additional tests used in selecting surgical candidates. 
For idiopathic intracranial hypertension, the treatment is directed at lowering CSF pressure and volume. The mainstays of medical treatment include weight reduction, low sodium diet, and diuretics (acetazolamide). [9, 12] A surgical approach is recommended in the setting of failure of standard medical treatment, including shunting, optic nerve fenestration, and, more recently, venous sinus stenting. [13, 14] In obese patients with idiopathic intracranial hypertension, there have been reports that suggest a potential benefit in resolution of symptoms after bariatric surgery. 
CSF leak and low pressure may occur after lumbar puncture, dural surgical procedures, or as a spontaneous thecal tear. A headache that worsens in the upright position is the clinical hallmark of CSF leaks. Treatment depends on the etiology and includes bedrest, hydration, and an autologous blood patch.