Developmental Hydrocephalus

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Pathogenesis and Pathophysiology. In contrast to normal CSF physiology, which is discussed in Chapter..26 , hydrocephalus is the result of a disturbance in the normal CSF fluid dynamics. Two terms frequently used when discussing hydrocephalus are obstructive (noncommunicating) and nonobstructive (communicating) hydrocephalus. These terms were first coined by Harvey Cushing to describe the results of injection of air (pneumoencephalography) into the subarachnoid space or dye into the ventricles. If the air was found to move into the ventricles or the dye out to the subarachnoid space, the cause of the hydrocephalus was considered nonobstructive. If no communication between the spaces was found, the cause of the hydrocephalus was considered obstructive. These terms were used to help define the pathophysiological mechanisms underlying the hydrocephalus. Although it is conceptually appealing, the value of this distinction has been challenged, because increased pressure leading to hydrocephalus is virtually always a result of blockage, y , y whether it is along the

pathway of CSF flow or at the site of resorption. Overproduction of CSF, as reported in rare cases of choroid plexus papillomas, is not likely to represent a common, if even plausible, cause of hydrocephalus. Thus, the entities included in this section, with the exception of the destructive lesions, are discussed, when known, in reference to where the blockage occurs (...Ta.(le ?8z1).

Hydrocephalus associated with congenital malformations and disruptions of development is associated with flattened gyri and reduced sulci. If hydrocephalus develops in utero, the cerebral hemispheres may show an abnormal gyral pattern characterized by multiple complex small gyri. This abnormal gyration has been called microgyria, polygyria, or stenogyria. It is important to distinguish this gyral abnormality from polymicrogyria (see later). The primary basis for this distinction relies on the abnormal cortical lamination in polymicrogyria and the normal cortical cytoarchitecture in hydrocephalus-related gyral abnormalities. Of course, it is equally important to remember that true polymicrogyria and hydrocephalus may co-exist in certain malformations.

When hydrocephalus occurs as a function of increasing intracranial CSF fluid volume, intracranial pressure will begin to increase as well. In an attempt to relieve the increased intracranial pressure, the child's head circumference will begin increasing by widening of the sutures. Although the cranial sutures begin fusion by 2 years of age, they can often be split open if the intracranial pressure

TABLE 28-1 -- OVERVIEW OF THE DIFFERENTIAL DIAGNOSIS OF CONGENITAL HYDROCEPHALUS

Etiology

Specific Causes*

Mechanism of Hydrocephalus

Tumors

Most types, benign and malignant

Blockage of CSF pathway

Malformations

Many:

Chiari II malformation

Unknown

Dandy-Walker syndrome

Unknown

Aqueductal anomalies

Obstruction of CSF flow at the level of the cerebral aqueduct

Forking/stenosis

X-linked hydrocephalus

Infectious/postinfectious

Abscess

Obstruction by a mass lesion

Meningitis/ventriculitis

Obstruction of CSF resorption and/or obstruction due to fusion of ventricular wall

Congenital infections CMV

Adhesions and fusion of ventricular wall

Toxoplasmosis

Ex vacuo or mass lesion

Vascular/hemorrhagic

Vascular malformation

Mass lesion of posthemorrhage

Posthemorrhagic

Germinal matrix origin

Intraventricular blood obstructs CSF flow

Choroid plexus origin

Destructive secondary

Hydranencephaly

Ex vacuo

Porencephaly

Ex vacuo

Perinatal leukomalacia

Ex vacuo

*The entities listed are only a partial list and are not meant to reflect a complete differential diagnosis. CMV, cytomegalovirus; CSF, cerebrospinal fluid.

*The entities listed are only a partial list and are not meant to reflect a complete differential diagnosis. CMV, cytomegalovirus; CSF, cerebrospinal fluid.

rises before the age of complete skull ossification (8 to 10 years). At the same time, the fontanelles, if still open, will become tense and bulging. The majority of conditions that are discussed in this chapter result in hydrocephalus before fusion of the cranial sutures; thus, an enlarging head occurs concurrently with the hydrocephalus.

Clinical Features and Associated Findings. Signs and symptoms of postnatal hydrocephalus depend on the expandability of the skull. Before fusion of the sutures, the skull is able to expand, resulting in an enlarging head circumference and tense fontanelles. In addition to the finding of a tense or bulging fontanelle, the separation of sutures can give rise to the so-called cracked pot sound on percussion of the skull. The scalp veins may be prominent, especially frontally. The absolute head circumference cannot be used as a strict diagnostic guide. Instead, it is important to follow the growth of the head circumference over a brief period of time. Any crossing of percentiles over a few weeks is potentially relevant and requires urgent investigation. Clinical signs and symptoms may be minimal or completely absent. [3 When present, irritability, frequent crying, unexplained vomiting (especially in the morning), and persistent or increasing headaches may all signal hydrocephalus. Alternatively, there may be developmental delay, disinterest in the environment with a dull affect, endocrine dysfunction, unsteady gait, and increased tone in the legs.y In the newborn period, bradyarrhythmia and apneic spells may be the only clinical sign of overt hydrocephalus.

Ophthalmological signs often occur early and are important to recognize. Disturbances of extraocular movements include a supranuclear upgaze paresis underlying the setting-sun sign in infants. In this condition, the eyes appear driven downward and the white of the sclera is seen above the iris. This phenomenon is probably related to aqueductal distention with compression of periaqueductal structures, or by pressure on the quadrigeminal plate by a dilated third ventricular subpineal recess. Unilateral or bilateral abducens or trochlear nerve paresis can occur. Swelling of the optic nerve head (papilledema) occurs before closure of the sutures only with very rapidly progressive hydrocephalus. On the other hand, chronic and longstanding hydrocephalus can lead to optic atrophy, so that papilledema would no longer be apparent. In neonatal hydrocephalus, stretching of the optic radiations and cortices can also result in a clinical picture of cortical blindness. Another unusual neurological manifestation of hydrocephalus is the so-called bobble-head doll syndrome, in which the patient has rhythmical to-and-fro movements of the head in the upright position. It is related to obstructive lesions in and around the aqueduct as well as to cystic lesions of the third ventricle.

Longstanding untreated hydrocephalus has been associated with a neuropsychological profile referred to as the cocktail-party syndrome. [5 Characteristic manifestations include a fluent and profuse speech pattern that lacks content and is pragmatically deficient. Chronic hydrocephalus may also result in precocious puberty. Earlier detection, primarily as a result of improved imaging of the CNS, along with advances in treatment and management, have resulted in these manifestations of chronic hydrocephalus being relatively rare.

A special case in the pathogenesis of hydrocephalus occurs in the case of familial aqueductal stenosis, most frequently inherited as an X-linked trait. Mutations in the gene for L1-CAM, a neuronal surface glycoprotein implicated in neuronal migration and axon fasciculation '6 are identified in one form (linked to chromosome Xq28). How mutations in this gene result in aqueductal stenosis is unclear. In the case of Xq28-linked familial aqueductal stenosis, mental retardation, spastic paraparesis, and adducted thumbs in about 50 percent of affected males may become apparent later in life even without the presence of overt hydrocephalus.

Differential Diagnosis. Hydrocephalus does not necessarily imply an enlarged head, although an enlarged or enlarging head can be the presenting feature. The converse is also not necessarily true--an enlarged head (macrocephaly) is not always equivalent to hydrocephalus. Several metabolic, hamartomatous, and other genetic conditions can be associated with macrocephaly independent from hydrocephalus. M Examples include Alexander's disease, Canavan's disease, several of the gangliosidoses, glutaricaciduria type I, neurofibromatosis type I, and several of the overgrowth syndromes such as Sotos' syndrome. If the brain itself is enlarged without a significant component of hydrocephalus, the term megalencephaly is appropriate. Malformations and disruptions with enlarged ventricles without CSF flow obstruction have to be differentiated by careful imaging. Holoprosencephaly and hydranencephaly provide examples. A special case is raised by the differential diagnosis of an isolated patient, often a newborn, with aqueductal stenosis. In the newborn, congenital infections, especially toxoplasmosis, must be considered, as does the possibility of preceding intracerebral hemorrhage. Space- occupying lesions around the midbrain, tectum, and pineal region have to be ruled out by appropriate imaging.

Evaluation. Fetuses and infants with hydrocephalus often have an increased head circumference, tense and bulging fontanelles, and widened sutures. Plain radiographs of the skull may show an irregular, shallow scalloping of the inner bone table.

The diagnosis of hydrocephalus relies on the application of neuroimaging techniques combined with the clinical features outlined in the beginning of this section. Although hydrocephalus is obvious on a computed tomographic (CT) scan or cranial ultrasound, precise anatomical definition of the level of blockage can best be achieved by magnetic resonance imaging (MRI). Assessment of the cerebral aqueduct in the sagittal plane, including evaluation of a CSF flow void through the aqueduct, is also possible via this technique. Evaluation must take into account whether the cranial sutures are open or closed. The ventricular size with an expandable skull versus a fixed-volume skull can be dramatically different even with an equal increase in intraventricular pressure. In the case of open sutures, even moderate increases in pressure can expand the ventricles enormously and the surrounding brain may appear compressed to a thin band. A dramatic reconstitution of the cerebral mantle may be seen after shunting, sometimes with reversion of some preexisting neurological deficits, in particular impaired visual function. On the other hand, in the fixed-volume skull, even enormously elevated intraventricular pressures may give rise to only moderate expansion of ventricular size, especially with so-called communicating hydrocephalus. On CT or MRI, transependymally extruded CSF in the periventricular white matter may be apparent.

Arrested, or inactive, hydrocephalus with apparently normal pressure measurements may pose a special problem for management decisions, because spot check measurements of CSF pressure by lumbar puncture may not accurately reflect chronic CSF dynamics. Longer continuous monitoring of intracranial pressure using an epidural or subepidural device has been advocated for such cases. Obviously, these investigations are not without risk for the patient and are not universally recommended. Meticulous attention to clinical details in order to decide whether or not the hydrocephalus is symptomatic should guide treatment decisions.

Early prenatal detection of hydrocephalus is possible. It should be kept in mind, however, that hydrocephalus may still develop after 16 to 18 weeks' gestation. Of paramount importance is the detection of additional CNS and extracerebral malformations, because these problems may significantly influence the prognosis (see later).

Management and Prognosis. Shunting of the CSF from the ventricles is the mainstay of therapy for hydrocephalus. y The predominant shunt systems currently in use are ventriculoperitoneal shunting (VP) devices with pressure-controlled valves under the scalp, close to the burr hole. The major complications associated with shunt treatment can be broken down into mechanical problems, shunt-related infections, and functional problems. Signs of increased intracranial pressure with headache, lethargy, and vomiting are apparent in the case of shunt malfunction. Functional overshunting can also occur, albeit less frequently. Again, headache is the most common clinical symptom, but in contrast to the headache associated with increased intracranial pressure, headache resulting from overshunting tends to be relieved when the patient is placed in the recumbent position. Shunt infections are a serious complication and are most often caused by Staphylococcus aureus. Unexplained fever or frank meningitis usually is the presenting feature. The condition is treated with penicillinase-resistant antibiotics, but culturing the pathogen for determination of drug susceptibility should be attempted in all cases. Replacement of all hardware is frequently necessary. Abdominal complications of VP- shunting include peritonitis, perforation of an abdominal organ, peritoneal cysts, and the development of hydroceles in boys. Seizures, especially during the first year after shunting, occur in approximately 5.5 percent of patients. The incidence of developing seizures declines after the first year. In the Xq28-linked familial aqueductal stenosis, VP-shunting is indicated as well. Unfortunately, the development of mental retardation and spastic paraparesis appears to be independent of the ventricular dilatation per se and does not improve with the treatment.

Under certain circumstances such as neonatal posthemorrhagic hydrocephalus, in which some patients may show resolution of the hydrocephalus without intervention, temporizing measures may be useful. Daily lumbar punctures removing substantial amounts of CSF have been used and are under further clinical investigation. Possible pharmacological intervention consists of a combination of high-dose acetazolamide with furosemide, which can reduce CSF

production considerably. Careful monitoring for the development of nephrocalcinosis (a problem particularly in premature infants treated with this combination of drugs) and electrolyte imbalances is essential. The management of neonatal posthemorrhagic hydrocephalus is very complex and beyond the scope of this chapter (see reference 10 for a review).

The detection of a fetal ventriculomegaly by ultrasound has created a new problem in the management of hydrocephalus that requires insight into the natural history of fetal ventriculomegaly. [ii] , y The prognosis is strongly influenced by the co-existence of major neural and extraneural abnormalities. In one large series, y 50 percent of diagnosed fetuses died in utero or the pregnancy was electively terminated, mainly because of the detection of major additional anomalies. From the remaining 50 percent, only a small percentage of fetuses were found to develop progressive ventriculomegaly in utero on follow-up investigations. These infants were shunted immediately after delivery and had relatively good outcomes. In an even smaller percentage of infants, the ventriculomegaly resolved, also with a favorable outcome. Of those who were unchanged on follow up in utero, a little more than half remained stable postnatally, their ultimate prognosis again being dependent on the presence of additional neural abnormalities. The other group developed postnatal progression of the hydrocephalus, requiring shunting. The outcome was good in the infants without additional anomalies. The same conclusion holds true for hydrocephalus detected at birth. y Given the importance of additional anomalies for the ultimate prognosis, once fetal ventriculomegaly is identified, a complete workup should be conducted at an experienced center. At present, no definite benefit to intrauterine shunting has been conclusively demonstrated.

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