Congenital Hydrocephalus / Ventriculomegaly
Authors: Susan Riordan, RN, RDMS
Jonathan Weeks, M.D.
Objectives: Upon the completion of this CNE article, the reader will be able to
Congenital Hydrocephalus / Ventriculomegaly
Authors: Susan Riordan, RN, RDMS
Jonathan Weeks, M.D.
Objectives: Upon the completion of this CNE article, the reader will be able to:
- Discuss the incidence and definition of hydrocephalus and ventriculomegaly, and how cerebral spinal fluid is formed, its flow, and its reabsorption.
- Describe how the ventricles can grow to an abnormal size and the three major types of hydrocephalus.
- Discuss the ultrasound findings and potential methods for making a diagnosis of hydrocephalus / ventriculomegaly.
- Discuss the prognosis and some of the management options that may be utilized once the diagnosis is made.
Using epidemiologic surveys, the incidence of congenital hydrocephalus ranges from 0.3 to 1.5 per 1000 births in the United States. Hydrocephalus refers to an increased intracranial content of cerebral spinal fluid (CSF). The term is generally used to refer to a situation in which an abnormal accumulation of cerebral spinal fluid results in an enlargement of the ventricular system.
The term "ventriculomegaly" is often used synonymously with hydrocephalus. However, ventriculomegaly means enlargement of the ventricles, which may be caused by increased intraventricular pressure secondary to obstruction of cerebral spinal fluid flow (for which "hydrocephalus" is the proper term), or it may be the result of passive enlargement caused by atrophy of the brain parenchyma. With atrophy of brain parenchyma, the ventricles basically enlarge to fill the voided space. Over time, it has become increasingly clear that a variable degree of enlargement of the ventricles is shared by a wide variety of anomalies different from obstructive hydrocephalus and that ventriculomegaly can be regarded as a potential marker of abnormal brain development.
Hydrocephalus almost always is due to an obstruction of cerebral spinal fluid flow, resulting in an increase in intracranial pressure. In rare instances, it may be the result of an increase in cerebral spinal fluid production or a relative decrease in the amount of CSF resorption. Hydrocephalus is said to be "isolated" when the fetus is free from anomalies that are not the direct result of the ventricular enlargement and intracranial pressure.
Cerebral spinal fluid is mainly formed by the choroid plexus, which are infoldings of blood vessels covered by a thin tissue called pia and are located in the four ventricles of the brain – the largest of which are found in the lateral ventricles. This CSF flows or communicates between the lateral ventricles and the third ventricle by way of the interventricular foramen (also called the foramen of Monro). The cerebral spinal fluid then slowly flows from the third ventricle to the fourth ventricle through the aqueduct of Sylvius. From the fourth ventricle, the CSF then passes through the foramen of Luschka and the foramen of Magendie to enter the subarachnoid space that externally bathes the cerebral structures of the brain and spinal cord. Flowing along the subarachnoid cisterns, the CSF is then reabsorbed by the arachnoid granulations (also called the granulations of Pacchioni) that are mainly distributed along the superior sagittal sinus.
Basically, there are four ways that the ventricles can grow to an abnormal size:
- Obstruction of CSF flow through the interventricular system and/or outflow obstruction into the subarachnoid space
- Impaired resorption of cerebral spinal fluid by the arachnoid granulations
- Overproduction of cerebral spinal fluid
- Underdevelopment or destruction of cortical tissue with a relative increase in the size of the ventricles
Hydrocephalus can be categorized into three major types:
- Aqueductal stenosis, which accounts for about 45% of cases.
- Communicating Hydrocephalus, which accounts for about 40% of cases.
- Dandy-Walker Malformation, which accounts for the remaining 15% of cases.
Aqueductal stenosis is an obstruction of the flow of CSF at the level of the aqueduct of Sylvius (between the third and fourth ventricles in the brain) (figure 1). It has numerous causes including genetic (especially the X-linked recessive form), infectious (toxoplasmosis, syphilis, cytomegalovirus) (figure 2), teratogenic (radiation), and neoplastic (tumors). Communicating hydrocephalus is caused by an obstruction of cerebral spinal fluid flow that is outside of the ventricular system. The most common cause for this form of hydrocephalus is spinal defects, such as meningomyelocoeles (figure 3). A meningomyelocoele can result in an Arnold-Chiari malformation (where the cerebellar vermis and fourth ventricle protrude down into the spinal canal thro