Hydrocephalus
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Hydrocephalus ('water-head', term derived from Greek) is an abnormal accumulation of cerebrospinal fluid in the ventricles of the brain. This increase in intracranial volume results in elevated intracranial pressure.
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History
Hydrocephalus was first described by Hippocrates, but it remained an intractable condition until the 20th century, when shunts and other neurosurgical treatment modalities were developed.
Causes
Hydrocephalus is caused by impaired cerebrospinal fluid (CSF) production, flow or resorption.
The most common cause of hydrocephalus is a flow obstruction, hindering the free passage of cerebrospinal fluid through the ventricular system and subarachnoid space (e.g. stenosis of the cerebral aqueduct, obstruction of the interventricular foraminae - foramen of Monro). This can be secondary to tumors, hemorrhages, infections or congenital malfomations. It can also be caused by overproduction of cerebrospinal fluid (relative obstruction).
Based on its underlying mechanisms, hydrocephalus can be classified into communicating, and non-communicating (obstructive).
Both communicating and non-communicating forms can be either congenital, or acquired.
Normal pressure hydrocephalus (NPH) is a particular form of communicating hydrocephalus, characterized by enlarged cerebral ventricles, with only intermittently elevated cerebrospinal fluid pressure. 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 (elasticity) of the ventricular walls, as well as increased viscosity of the cerebrospinal fluid may play a role in the genesis of normal pressure hydrocephalus. Brain atrophy, as it occurs in dementias, after traumatic brain injuries and even in some psychiatric disorders, such as schizophrenia, may also result in an enlargement of cerebral ventricles and subarachnoid spaces. 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.
Communicating hydrocephalus
Communicating hydrocephalus, a.k.a. non-obstructive or communicating hydrocephalus is caused by impaired cerebrospinal fluid resorption, in the absence of any CSF-flow obstruction. It has been theorized that this is due to functional impairment the arachnoid granulations (located along the superior sagittal sinus) - they represent the site of cerebrospinal fluid resorption back into the venous system). Various neurologic conditions may result in communicating hydrocephalus. These include subarachnoid/intraventricular hemorrhage, meningitis, [Chiari malformation]], and congenital absence of arachnoidal granulations, a.k.a. Pacchioni's granulations.
Non-communicating hydrocephalus
Non-communicating hydrocephalus, or obstructive hydrocephalus, is caused by a CSF-flow obstruction (either due to external compression or intraventricular mass lesions). In many cases, the flow obstruction is located at the level of the cerebral aqueduct (aqueduct stenosis), which connects the third and fourth ventricle. Third ventriculostomy (i.e. a surgical connection between the third ventricle and the subarachnoid space) should be considered as an alternative therapeutic option to traditional CSF-shunting procedures (ventriculo-peritoneal shunt, ventriculo-atrial shunt).
Congenital hydrocephalus
In newborns and toddlers with hydrocephalus, the head circumference is enlarged, since the skull bones are not yet firmly joined together. Bulging, firm fontanelles (soft spots) are an early sign of hydrocephalus in this age class.
About 80-90% of fetuses or newborn infants with spina bifida - often associated with myeloccelle or meningomyellocelle - develop hydrocephalus.
Acquired hydrocephalus
This condition is acquired as a consequence of CNS-infections, brain tumors, head trauma, intracranial hemorrhage (subarachnoid or intraparenchymal).
Clinical presentation
Like various other neurologic conditions (brain tumors, strokes, traumatic brain injury, etc.), hydrocephalus results in elevated intracranial pressure. Possible clinical manifestations include: headaches, vomiting (in some cases not accompanied by nausea), papilledema, somnolence, coma. Elevated intracranial pressure may result in uncal and/or cerebellar tonsill herniation, with resulting life threatening brain stem compression. Normal pressure hydrocephalus (NPH) is distinguished by a relatively typical clinical triad: gait instability, urinary incontinence and dementia. Focal neurologic deficits may also occur, such as abducens nerve palsy and vertical gaze palsy - Parrinaud syndrome (due to compression of the quadrigeminal plate, where the neural centers coordinating the conjugated vertical eye movement are located).
Treatment
State-of-the-art hydrocephalus treatment is surgical. It involves the placement of a ventricular catheter (a tube made of silastic), into the cerebral ventricles to bypass the flow obstruction/malfunctioning arachnoidal granulations and drain the excess fluid into other body cavities, from where it can be resorbed. Most shunts drain the fluid into the peritoneal cavity (ventriculo-peritoneal shunt), but alternative sites include the right atrium (ventriculo-atrial shunt), pleural cavity (ventriculo-pleural shunt), and gallbladder.