Hydrocephalus

==Type==
The cause of hydrocephalus is not known with certainty and is probably multifactorial. It may be caused by impaired CSF flow, reabsorption, or excessive CSF production.<ref>{{Cite journal | vauthors = Nelson Jr SL, Espay AJ, Hord ED | veditors = Talavera F |date=2022-02-02 |title=Hydrocephalus: Practice Essentials, Background, Pathophysiology |url=https://emedicine.medscape.com/article/1135286-overview | journal = Medscape }}</ref>
* Obstruction to CSF flow hinders its free passage through the ventricular system and [[subarachnoid space]] (e.g., [[stenosis]] of the [[cerebral aqueduct]] or obstruction of the [[interventricular foramina (neural anatomy)|interventricular foramina]] secondary to [[tumor]]s, [[hemorrhage]]s, [[infection]]s or [[congenital]] malformations) and can cause increases in ICP.<ref>{{Cite journal | vauthors = Nelson SL, Murro AM, Espay AJ, Hord ED |date=2022-03-11 | veditors = Talavera F |title=Ventricles of the Brain: Overview, Gross Anatomy, Microscopic Anatomy |url=https://emedicine.medscape.com/article/1923254-overview | journal = Medscape }}</ref>
* Hydrocephalus can also be caused by overproduction of CSF (relative obstruction) (e.g., [[choroid plexus papilloma]], villous [[hypertrophy]]).<ref name="AdunkaBuchman2010">{{cite book | vauthors = Adunka O, Buchman C |title=Otology, Neurotology, and Lateral Skull Base Surgery: An Illustrated Handbook |url=https://books.google.com/books?id=2kv-Z-L5UUAC&pg=PT353 |access-date=12 August 2013 |date=11 October 2010 |publisher=Thieme |isbn=978-3-13-149621-8 |pages=353– |url-status=live |archive-url=https://web.archive.org/web/20140705102607/http://books.google.com/books?id=2kv-Z-L5UUAC&pg=PT353 |archive-date=5 July 2014}}</ref><ref name="pmid20367337">{{cite journal | vauthors = Nimjee SM, Powers CJ, McLendon RE, Grant GA, Fuchs HE | title = Single-stage bilateral choroid plexectomy for choroid plexus papilloma in a patient presenting with high cerebrospinal fluid output | journal = Journal of Neurosurgery. Pediatrics | volume = 5 | issue = 4 | pages = 342–345 | date = April 2010 | pmid = 20367337 | doi = 10.3171/2009.10.peds08454 }}</ref>
* Bilateral [[ureter]]ic obstruction is a rare, but reported, cause of hydrocephalus.

Hydrocephalus can be classified into communicating and noncommunicating (obstructive). Both forms can be either congenital or acquired.<ref>{{Cite web |title=Different Types of Hydrocephalus |url=https://ana-neurosurgery.com/hydrocephalus/different-types/ |access-date=2022-05-17 |website=Advanced Neurosurgery Associates |language=en-US}}</ref>

===Communicating===
Communicating hydrocephalus, also known as nonobstructive hydrocephalus, is caused by impaired CSF reabsorption in the absence of any obstruction of CSF flow between the ventricles and subarachnoid space. This may be due to functional impairment of the [[arachnoid villi|arachnoidal granulations]] (also called arachnoid granulations or [[Pacchioni's granulations]]), which are located along the [[superior sagittal sinus]], and is the site of CSF reabsorption back into the venous system. Various neurologic conditions may result in communicating hydrocephalus, including subarachnoid/intraventricular hemorrhage, meningitis, and congenital absence of arachnoid villi. Scarring and fibrosis of the subarachnoid space following infectious, inflammatory, or hemorrhagic events can also prevent reabsorption of CSF, causing diffuse ventricular dilatation.<ref>{{Cite book| vauthors = Kaye A, Fox C, Diaz J |title=Essentials of Pediatric Anesthesiology |publisher=Cambridge University Press |year=2014 |pages=106 }}</ref>
[[File:Hydrocephalus_cranial_deformity_2.jpg|75px|thumbnail|right|An adult  with congenital hydrocephalus in the [[Philippines]]]]
===Noncommunicating===
Noncommunicating hydrocephalus, or obstructive hydrocephalus, is caused by an obstruction to the flow of CSF.<ref>{{Cite web |date=2020-02-21 |title=Communicating and Non-communicating Hydrocephalus {{!}} Helpful |url=https://www.hydroassoc.org/communicating-non-communicating-hydrocephalus/ |access-date=2022-05-17 |website=www.hydroassoc.org |language=en-US}}</ref>
* [[Interventricular foramina (neural anatomy)|Foramen of Monro]] obstruction may lead to dilation of one, or if large enough (e.g., in [[colloid cyst]]), both lateral ventricles.
* The [[Cerebral aqueduct|aqueduct of Sylvius]], normally narrow, may be obstructed by a number of genetic or acquired lesions (e.g., atresia, [[ependymitis]], hemorrhage, or tumor) and lead to dilation of both lateral ventricles, as well as the [[third ventricle]].
* [[Fourth ventricle]] obstruction leads to dilatation of the aqueduct, as well as the lateral and third ventricles (e.g., [[Chiari malformation]]).
* The [[foramina of Luschka]] and [[Median aperture|foramen of Magendie]] may be obstructed due to congenital malformation (e.g., [[Dandy–Walker malformation]]).

===Other===
[[File:BrainAtrophy(exvacuo).png|thumb|Hydrocephalus ''ex vacuo'' from vascular dementia as seen on MRI]]
* [[Normal pressure hydrocephalus]] (NPH) is a particular form of chronic communicating hydrocephalus, characterized by enlarged cerebral ventricles, with only intermittently elevated cerebrospinal fluid pressure. Characteristic triad of symptoms are; dementia, apraxic gait and urinary incontinence. The diagnosis of NPH can be established only with the help of continuous intraventricular pressure recordings (over 24 hours or even longer), since more often than not instant measurements yield normal pressure values. Dynamic compliance studies may be also helpful. Altered compliance ([[wikt:elastic#Adjective|elasticity]]) of the ventricular walls, as well as increased [[viscosity]] of the cerebrospinal fluid, may play a role in the pathogenesis.<ref>{{cite journal | vauthors = Martin BA, Loth F | title = The influence of coughing on cerebrospinal fluid pressure in an in vitro syringomyelia model with spinal subarachnoid space stenosis | journal = Cerebrospinal Fluid Research | volume = 6 | issue = 1 | pages = 17 | date = December 2009 | pmid = 20043856 | pmc = 2806373 | doi = 10.1186/1743-8454-6-17 | doi-access = free }}</ref>
* ''Hydrocephalus ex vacuo'' also refers to an enlargement of cerebral ventricles and subarachnoid spaces, and is usually due to brain [[atrophy]] (as it occurs in [[dementia]]s), post-[[traumatic brain injury|traumatic brain injuries]], and even in some psychiatric disorders, such as [[schizophrenia]].<ref name=":0">{{Cite book|title=Complications in neuroanesthesia| vauthors = Hemanshu P |isbn=9780128040751|oclc=939553425|date = 2016-02-29| publisher = Elsevier Science }}</ref> As opposed to hydrocephalus, this is a compensatory enlargement of the CSF-spaces in response to brain [[parenchyma]] loss; it is not the result of increased CSF pressure.<ref name=":0" />