Hydrocephalus is a pathologic state characterized by dilatation of the cerebral ventricles with an increase in volume of cerebrospinal fluid (CSF), almost always caused by an obstruction in the circulation of this fluid. In children, prior to fusion of the cranial sutures, there is enlargement of the skull. Hydrocephalus must be distinguished from other causes of irreproachably in infancy, including subdural hematoma. In older subjects, cranial enlargement cannot occur. Hydrocephalus is termed “active,” i.e.. progressive and associated with increased intraventricular pressure, or “arrested” when the intraventricular pressure has returned to normal and is no longer a stimulus for ventricular enlargement.
These forms of hydrocephalus are distinguished from hydrocephalus ex vacuo, which is characterized by an increase in the volume of CSF under normal pressure that is compensatory to a primary atrophy of the brain. “Normal” pressure occult hydrocephalus refers to the syndrome of ventricular dilatation associated with inadequacy of the subarachnoid spaces without evidence of increased intracranial pressure (vide infra).
Etiology and Pathophysiology.
There are three possible mechanisms for the development of hydrocephalus: overproduction of CSF, defective absorption of CSF, and obstruction of the CSF pathways. Overproduction of the fluid has not been documented, although it may occur with the rare papilloma of the choroid plexus. Defective absorption of the CSF at the arachnoid villi, where the fluid passes through valvelike structures to enter the venous sinuses, may occur with subarachnoid hemorrhage, meningitis, and with a very high CSF protein. Obstruction of the CSF pathways, which results in dilatation of the channels proximal to the site of obstruction, is the most common underlying mechanism in both the hydrocephalus of infancy and in adults.
When the block is within the ventricular system, the process is termed noncommunicating hydrocephalus, whereas communicating hydrocephalus describes ventricular dilatation in which there is free flow of fluid and air between the ventricular system and the spinal subarachnoid space. Communicating hydrocephalus is characterized by extraventricular obstruction of the CSF pathways, most commonly in the subarachnoid spaces about the brainstem at the incisura of the tentorium or in the subarachnoid spaces about the cerebral hemispheres, or both.
Ventricular dilatation develops in hydrocephalus because the intraventricular pressures (both the mean pressure and the pulsatile pressures, synchronous with cardiac systole) are pathologically increased; these give rise to a transmural pressure gradient across the cerebral mantle sufficient to cause characteristic compression of the adjacent white matter that has a loss of protein and lipid but an increase in water and sodium content. The magnitude and duration of the transmural pressure gradient necessary to induce ventricular dilatation have not been well defined. These changes may be reversible if surgical therapy can successfully relieve the increased intraventricular pressure (vide infra).
The major causes of obstruction in the CSF pathways that produce hydrocephalus are neoplasms, congenital malformations, and post-traumatic and postinflammatory lesions.
Neoplasms most likely to give rise to hydrocephalus are those that arise within or adjacent to the ventricular system and obstruct the flow of CSF. These include gliomas and ependymomas of the third and fourth ventricles and of the aqueduct (see Intracranial Tumors). Congenital malformations give rise to hydrocephalus by causing a variety of obstruction, the most common being “forking” or stenosis of the aqueduct.
This may give rise to symptoms early in life, or manifestations may be absent until adulthood. Congenital malformations of the craniovertebral junction also may be associated with hydrocephalus (see Arnold-Chiari Malformation). Most frequently, hydrocephalus is related to postinflammatory or post-traumatic obstruction of the basilar cisterns, particularly in the region of the tentorium. In infancy, this follows intracranial bleeding at the time of birth, at times unrecognized, or episodes of bacterial meningitis or toxoplasmosis. These processes lead to progressive fibrosis of the subarachnoid pathways at the base of the brain, with subsequent obstruction. In adults, postinflammatory hydrocephalus may develop with or after purulent, tuberculous and mycotic meningitis or with cysticercosis, and also may follow subarachnoid hemorrhage due to trauma or ruptured congenital aneurysm.
Clinical Manifestations of Hydrocephalus.
The major signs and symptoms of hydrocephalus are those of increased intracranial pressure (see Intracranial, Tumors). In infancy, this is manifest by a greater than normal growth rate and size of the head and by distended scalp veins; in severe cases there is downward displacement of the eyes and mental retardation. Although the head size precipitous- phlegmatic circumference) in comparison with the chest circumference . is of importance, repeated observations of the rate of head growth are more significant.
Additional information can be obtained from measurements of the anterior fon- tanelle, as active hydrocephalus does not occur in conjunction with a closing fontanel. On the other hand, a fontanel enlarging from month to month is evidence for increased intracranial pressure, and further investigations are warranted.
In older children and adults, the major manifestations include headache, vomiting, diplopia due to sixth nerve palsy, papilledema, visual blurring, nausea, and vomiting. Thus, occult hydrocephalus due to a benign process must be differentiated from intracranial mass lesions, including tumor without localizing signs and chronic subdural hematoma, as well as from the various forms of benign intracranial hypertension and the chronic meningitides, including fungal meningitis, sarcoidt»sis, and diffuse meningeal neoplasia (carcinomatous meningitis).
“Normal” Pressure-Occult Hydrocephalus.
In recent years, a new syndrome has been delineated as “normal” pressure-occult hydrocephalus. Typically, there is a gradual development over weeks or months of a mild impairment of memory with mental and physical slowness which progresses insidiously to a severe dementia with unsteady gait and urinary incontinence. The patients are usually headache-free and have no signs of increased intracranial pressure, e.g., they have normal fundi and normal CSF pressure at lumbar puncture.
Pneumoencephalography reveals enlarged ventricles and a lack of filling ‘of the subaracYvnoid space over the hemispheres. Isotope cisternography (intraspinal injection of RISA with serial scanning of the skull) has revealed a pathologic reflux of the isotope into the ventricular system with delayed and inadequate visualization of the cortical subarachnoid space. Some patients have a previous history of head injury or subarachnoid bleeding; in others, the etiology is obscure. Striking improvement in the mental state and gait has been noted to follow ventriculoatrial or other shunting procedures. This treatable syndrome is uncommon but must be differentiated from the more frequent forms of organic dementia as discussed in the article on Dementia.
Diagnosis of Hydrocephalus.
Detailed neurologic study is necessary to determine the cause of increased intracranial pressure and ventricular dilatation. Skull films show characteristic signs of increased intracranial pressure in young children; the signs in adults are generally less striking and may be absent. Electroencephalograms may be normal or may demonstrate* bilateral slowing of nonspecific nature, Carotid angiography will show displacement of the arteries and veins characteristic of ventricular enlargement. Air contrast studies are essential to establish the diagnosis of hydrocephalus in most cases. Ventriculography via a parietal burr-hole or lumbar pneumoencephalography, or both, will reveal the degree of ventricular enlargement, the presence of obstructive lesions in the cerebrospinal fluid pathways, and the adequacy of the subarachnoid spaces about the brainstem, base, and cerebrum.
Precise neuroradiologic analysis is essential to establish the diagnosis. The cerebrospinal fluid pressure is characteristically, but not invariably, increased in the cerebral ventricles and lumbar sac. The protein content is generally normal; elevated protein levels favor an underlying neoplastic process in the absence of bleeding or chronic meningitis.
Treatment of Hydrocephalus
The treatment of hydrocephalus is surgical; it is directed toward reducing the volume and pressure of cerebrospinal fluid by bypassing obstruction. A wide variety of shunting procedures has been used. The choice of procedure will, depend upon technical factors and the skill and experience of the operating surgeon. If the block exists in the third ventricle, aqueduct, or fourth ventricle (as with intraventricular tumors or with “aqueduct stenosis”), the Torkildsen procedure is often employed (ventriculocistemal shunt), wherein a catheter is placed from one or both lateral ventricles over the occiput into the cisterna magna to enable normal CSF reabsorption into the intracranial venous sinuses.
In patients with communicating hydrocephalus, direct shunting of the ventricular fluid into the venous system is generally used, particularly the ventriculoatrial shunt wherein the fluid exits from the ventricle through a valved tube, which extends into the superior vena cava and right atrium of the heart. There are many variations of these procedures in use, including shunting to other body cavities, e.g., ventriculopleural, ventriculoureteral, and ventriculoabdominal shunts. Infections and recurrent obstruction of the shunt are common complications of the various shunting procedures, and the results of treatment particularly in young children are frequently discouraging. Aceta- zolamide, a carbonic anhydrase inhibitor, reduces the rate of formation of CSF in laboratory animals, but there is no convincing evidence that it is useful in the management of the various forms of hydrocephalus. Some hydro-cephalic children will have spontaneous arrest and may function within the normal range with only a prominent forehead and large hat size as residual defects.
