Pathophysiology
Summary
Cerebrospinal fluid (CSF) is produced in the choroid plexus, primarily located within the lateral ventricles of each cerebral hemisphere. These ventricles communicate with the third ventricle, located between the two thalami in the midbrain, via the interventricular foramina. Subsequently, the third ventricle connects to the fourth ventricle—a diamond-shaped space anterior to the cerebellum—through the cerebral aqueduct or aqueduct of Sylvius. CSF exits the fourth ventricle into the subarachnoid space via the foramen of Magendie and the paired foramina of Luschka. Eventually, CSF is reabsorbed into the systemic circulation through arachnoid granulations, which are specialized projections of the arachnoid mater that extend into the dural venous sinuses.
Hydrocephalus is the pathological enlargement of the ventricles due to an increased volume of CSF. It can be categorized into communicating and noncommunicating (obstructive) types. In communicating hydrocephalus, dysfunction in the arachnoid granulations impairs the drainage of CSF. Conditions like meningitis, subarachnoid hemorrhage, & other inflammatory states can cause fibrosis of the arachnoid granulations, resulting in communicating hydrocephalus. On the other hand, noncommunicating hydrocephalus results from obstruction in the flow of CSF out of the ventricles. Causes include Chiari malformation (especially type II), congenital ToRCH infections leading to meningeal scarring, blockage by a colloid cyst in the third ventricle, pineal gland tumors (pinealoma) compressing the cerebral aqueduct, and tumors in the posterior fossa and fourth ventricle. Both forms of hydrocephalus can lead to intracranial hypertension.
The cerebral aqueduct runs along the tectum of the midbrain. Damage to the superior colliculus, located within the tectum, can result in Parinaud's syndrome or the ‘setting-sun sign,’ characterized by vertical gaze palsy. In Parinaud’s syndrome, pupils have a bilateral light-near dissociation, meaning they do not constrict well with light but will constrict with accommodation.
In infants, hydrocephalus manifests differently. Congenital hydrocephalus leads to macrocephaly and a bulging fontanelle, with symptoms including irritability, poor feeding, and developmental delays.
Normal pressure hydrocephalus (NPH), a subtype of communicating hydrocephalus, predominantly affects the elderly and develops over several years. Despite its name, ventricles slowly enlarge, leading to hydrocephalus with normal CSF pressure. NPH is often idiopathic, though impaired CSF absorption by the arachnoid granulations may be a cause. Clinically, it presents a triad of symptoms: gait instability, termed a ‘magnetic’ gait (‘wobbly’); urinary incontinence (‘wet’); and dementia (‘wacky'). NPH is distinctively marked by ventriculomegaly without sulci enlargement.
Hydrocephalus ex vacuo is characterized by the appearance of enlarged ventricles, mainly a result of cerebral atrophy. This enlargement is caused by a loss of brain tissue, typical of normal aging or neurodegenerative conditions. Unlike NPH, hydrocephalus ex vacuo also presents with enlarged sulci.
Lesson Outline
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FAQs
CSF is produced by the choroid plexus. It originates in the lateral ventricles, which are located in each cerebral hemisphere. From there, CSF flows through the interventricular foramina to the third ventricle, situated between the two thalami. It then travels via the cerebral aqueduct, also known as the aqueduct of Sylvius, to the fourth ventricle, which is anterior to the cerebellum. CSF exits the fourth ventricle through the foramen of Magendie and the foramina of Luschka, entering the subarachnoid space. Finally, CSF returns to the systemic circulation through arachnoid granulations, which project into the dural venous sinuses.
Communicating hydrocephalus arises from dysfunction of the arachnoid granulations, leading to decreased CSF drainage. Conditions like meningitis, inflammation, and subarachnoid hemorrhage can cause fibrosis of the arachnoid granulations, resulting in this type of hydrocephalus. On the other hand, noncommunicating hydrocephalus, also known as obstructive hydrocephalus, is caused by an obstruction of CSF flow within the ventricles. This can occur in conditions like Chiari malformation, congenital ToRCH infections, or tumors. Both types can lead to intracranial hypertension.
Parinaud's syndrome, also known as the ‘setting-sun sign’, is characterized by a vertical gaze palsy, making it challenging for individuals to look upwards. This condition is caused by compression of the superior colliculus in the tectum, where the cerebral aqueduct runs. In Parinaud's syndrome, the pupils do not constrict well with light, but do constrict well with accommodation. Hydrocephalus, tumors, and strokes affecting the tectum can lead to this syndrome.
Normal pressure hydrocephalus (NPH) is a condition that predominantly affects elderly individuals and is often idiopathic. It may be a variant of communicating hydrocephalus caused by impaired CSF absorption by the arachnoid granulations. Over time, the ventricles slowly enlarge, leading to hydrocephalus while maintaining normal CSF pressure. Clinically, NPH is characterized by a triad of symptoms: gait instability described as a wide-based, ‘magnetic' gait (‘wobbly’), urinary incontinence (‘wet’), and dementia (‘wacky').
Hydrocephalus ex vacuo is ventricular enlargement with a proportionate increase in the sulci due to the loss of brain tissue in neurodegenerative diseases or with normal aging. In contrast, other forms of hydrocephalus, such as communicating or noncommunicating, involve an accumulation of CSF leading to ventricular enlargement without a proportionate increase in the size of the sulci.
