Textbook Question
The innermost layer of the meninges, delicate and adjacent to the brain tissue, is the
a. Dura mater
b. Corpus callosum
c. Arachnoid mater
d. Pia mater
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12. The Central Nervous System
Introduction to the Central Nervous System
2:13 minutesProblem 29
Textbook Question
A 10-month-old infant has an enlarging head circumference and delayed overall development. Her CSF pressure is elevated, and she has a bulging anterior fontanelle (fontanelles are the as yet unossified fibrous membranes connecting skull bones in infants). Based on these findings, answer the following questions:
a. What are the possible cause(s) of an enlarged head?
b. Which tests might be helpful in obtaining information about this infant's problem?
c. Assuming the tests conducted showed the cerebral aqueduct to be constricted, which ventricles or CSF-containing areas would you expect to be enlarged? Which would likely not be visible? Respond to the same questions based on a finding of obstructed arachnoid granulations.
Verified step by step guidance1
Step 1: Understand the possible causes of an enlarged head in an infant. This condition is often associated with hydrocephalus, which is an abnormal accumulation of cerebrospinal fluid (CSF) in the brain. Hydrocephalus can result from either an obstruction in CSF flow (non-communicating hydrocephalus) or impaired absorption of CSF (communicating hydrocephalus). Other potential causes include congenital malformations, infections, or tumors.
Step 2: Identify the diagnostic tests that could provide more information. Useful tests include imaging studies such as an ultrasound (if the fontanelle is still open), magnetic resonance imaging (MRI), or computed tomography (CT) scans to visualize the brain and ventricles. Additionally, measuring CSF pressure via lumbar puncture and analyzing CSF composition can help identify the underlying issue.
Step 3: Analyze the findings of a constricted cerebral aqueduct (aqueductal stenosis). The cerebral aqueduct connects the third and fourth ventricles. If it is constricted, CSF flow is obstructed, leading to enlargement of the lateral ventricles and the third ventricle. The fourth ventricle and areas beyond it (e.g., subarachnoid space) would likely not be enlarged because the obstruction prevents CSF from reaching these areas.
Step 4: Consider the findings of obstructed arachnoid granulations. Arachnoid granulations are responsible for reabsorbing CSF into the venous system. If they are obstructed, CSF absorption is impaired, leading to a buildup of CSF throughout the ventricular system. In this case, all ventricles (lateral, third, and fourth) and the subarachnoid space may be enlarged due to the generalized increase in CSF pressure.
Step 5: Summarize the implications of the findings. For aqueductal stenosis, treatment might involve surgical intervention such as ventriculoperitoneal shunting or endoscopic third ventriculostomy to bypass the obstruction. For obstructed arachnoid granulations, treatment may focus on addressing the underlying cause (e.g., infection or inflammation) and managing CSF pressure through shunting or other means.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Hydrocephalus
Hydrocephalus is a condition characterized by an accumulation of cerebrospinal fluid (CSF) within the ventricles of the brain, leading to increased intracranial pressure. In infants, this can manifest as an enlarged head circumference and developmental delays. The condition can be caused by various factors, including congenital malformations, infections, or obstruction of CSF pathways.
Cerebral Aqueduct
The cerebral aqueduct is a narrow channel in the brain that connects the third and fourth ventricles, allowing CSF to flow between them. Constriction or obstruction of the cerebral aqueduct can lead to hydrocephalus, as it impedes normal CSF circulation, causing the ventricles upstream to enlarge while those downstream may remain unaffected.
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Arachnoid Granulations
Arachnoid granulations are small protrusions of the arachnoid membrane into the dural sinuses, facilitating the absorption of CSF into the bloodstream. Obstruction of these granulations can lead to increased CSF pressure and hydrocephalus, as the fluid cannot be adequately reabsorbed, resulting in potential enlargement of the ventricles and other CSF-filled spaces.
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