What fluid is found in the ventricles of the brain?

Pathogenesis

Cerebral ventricles are four interconnected cavities of the brain lined by ependymal cells and filled by the cerebrospinal fluid, a clear, colorless fluid that also surrounds the brain, spinal cord, and cauda equina. CSF is not a filtrate of the blood. It is produced by active secretion mainly at the choroid plexuses of the cerebral ventricles. It is formed at a rate of about 0.35 mL/min. The daily volume of CSF produced in adult humans is about 500 mL. Total CSF space in young adults is about 150 mL—that is, CSF is totally replaced about four times each day. Only about 25% of CSF volume lies within the ventricles. The rest resides in the cranial and spinal subarachnoid space.3

CSF circulates from the two lateral ventricles, through the interventricular foramen of Monro to the third ventricle, then through the cerebral aqueduct of Sylvius to the fourth ventricle. From the fourth ventricle, CSF passes to the subarachnoid space around the brain and spinal cord through the foramen of Magendie in the midline and the two foramina of Luschka laterally (Fig. 60.1). The CSF circulation comprises not only a directed flow of CSF but a pulsatile to-and-fro movement as well. It was thought that the dural venous sinus's arachnoid villi and granulations play an important role in CSF absorption. However, research has disputed that theory. The currently accepted theory is that CSF is cleared by bulk flow along sleeves of the subarachnoid space surrounding the olfactory and optic nerves as well as spinal nerve roots.3,4

CSF functions include protection of the brain acting as a cushion that lessens the impact of a blow. By keeping the brain buoyant, the net weight of the brain is reduced from about 1400 gm to about 50 gm, thereby eliminating pressure on the base of the brain and the important basal cerebral arteries. CSF also serves as a medium to transport hormones to other areas of the brain. CSF circulates around blood vessels penetrating from the subarachnoid space into the Virchow-Robin spaces. We are starting to understand that the clearance of waste products from the brain during sleep may depend on CSF circulation as part of the glymphatic system.5

The relationship between intracranial components' volume and intracranial pressure is important in the pathophysiology of many hydrocephalic syndromes. The Monro-Kellie doctrine states that the skull is a closed bony box with constant volume. An increase in volume of one of the cranial constituents, or the presence of a mass lesion (tumor or hematoma), would result in raised intracranial pressure (ICP).

v.intracranial(constant)=v.brain+v.CSF+v.blood+v.mass lesion

A physiologic buffer exists, where some compensation is possible as cerebrospinal fluid CSF and blood move into the spinal canal and extracranial vasculature, respectively. Beyond this point, ICP rises dramatically (Fig. 60.2). Increased ICP can lead to one of the life-threatening herniation syndromes (coning).6,7

The widely accepted value of normal ICP is 8 to 12 mm Hg in flat lateral position. This is derived from the CSF opening pressure on lumbar puncture done in flat lateral position. An important contributory factor to ICP is the CSF. Pressure is expected to be the same throughout the system (intracranial and spinal dural compartments) in supine position, but gravity will give rise to hydrostatic pressure gradients in upright or seated positions. Humans are upright most of the time. Normal ICP is thought to be negative in upright or seated positions. It is thought that CSF and ICP change in relation to posture change could be also influenced by posture-related change in intraabdominal or dural venous sinus pressures, which are, in turn, related to intrathoracic and spinal epidural venous plexus pressure.8,9

Ventricular System & Subarachnoid Space

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Cerebral ventricles consist of paired lateral, midline 3rd, & 4th ventricles

Communicate with each other as well as central canal of spinal cord & subarachnoid space

Direction of CSF flow

Lateral ventricles → foramen of Monro → 3rd ventricle → cerebral aqueduct → 4th ventricle → foramina of Luschka & Magendie → subarachnoid space

Bulk of CSF resorption through arachnoid granulations in superior sagittal sinus

Lateral ventricles

Paired, C-shaped, curve posteriorly from temporal horns, arch around/above thalami

Each has body, atrium, 3 horns (frontal, temporal, & occipital)

Occipital horn typically largest

Asymmetry is common, often L > R

Sizes change with maturity, more prominent in preterm infants

Atrium/trigone: Confluence of horns

Contains glomus of choroid plexus

Lateral ventricles communicate with each other & 3rd ventricle via Y-shaped foramen of Monro

3rd ventricle

Thin, usually slit-like, between thalami

May not see fluid, just bright echogenic line on US

80% have central adhesion between thalami (massa intermedia)

Communicates with 4th ventricle via cerebral aqueduct (of Sylvius), passing through dorsal midbrain

4th ventricle

Infratentorial, diamond-shaped cavity (rhomboid fossa) along dorsal pons & upper medulla

Fastigium: Blind ending, dorsally pointed midline outpouching from body of 4th ventricle

Important marker for true midline vermian plane on US

Communicates with subarachnoid space via foramina of Magendie & Luschka

Terminates inferiorly at obex, which communicates with central canal of spinal cord

Choroid plexus

Produces CSF

Glomus (enlargement of choroid plexus in atrium) thickest area

Tapers & extends anteriorly to foramen of Monro & roof of 3rd ventricle

Tapers laterally into roof of temporal horns

Present in roof of 4th ventricle but never extends into frontal or occipital horns

Subarachnoid space/cisterns

CSF spaces between pia & arachnoid

Numerous trabeculae, septa, membranes cross subarachnoid space & create smaller compartments termed cisterns

Supratentorial/peritentorial cisterns: Suprasellar, interpeduncular, ambient (perimesencephalic), quadrigeminal cistern, & cistern of velum interpositum

Infratentorial (posterior fossa) cisterns: Prepontine, premedullary, superior cerebellar, cisterna magna, & cerebellopontine

All cisterns communicate with each other & with ventricular system

Midline cystic structures (normal variants)

Cavum septi pellucidi: Anterior to foramen of Monro, between anterior horns of lateral ventricles

85% closed by 3-6 months after birth, some remain open into adulthood

Once closed called septum pellucidum

Cavum vergae: Posterior to foramen of Monro, interposed between bodies of lateral ventricles

Posterior extension of cavum septi pellucidi

Begins to close from posterior to anterior from 6-month gestation; 97% closed by full term

Cavum velum interpositum: Potential space above choroid in roof of 3rd ventricle & below fornices

Typically seen in premature infants

1. Hydrocephalus

Hydrocephalus refers to dilatation of the cerebral ventricles and is usually accompanied by an accumulation of cerebrospinal fluid within the dilated spaces (Fig. 12). Some cases of hydrocephalus in rabbits have been presumed to be related to a single autosomal recessive gene (hy/hy); however, occurrence with other abnormalities suggests that inheritance may be more complicated (Lindsey and Fox, 1994). In some cases, the condition appears to be inherited along with various ocular anomalies as an autosomal gene with incomplete dominance. In addition, hydrocephalus can occur in rabbits as a congenital condition related to hypovitaminosis A in pregnant does (Lindsey and Fox, 1994). In contrast, the condition may also be the result of an inherited underlying defect in vitamin A metabolism.

Infectious Agents

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Shunt nephritis (SN)

Cerebral ventricle CSF shunt to atrium, jugular vein or peritoneum

Bacteria adhere to plastic shunt and form biofilm, protecting them from antibiotics and immune system

6-27% of VA shunts have bacterial colonization

Low-grade bacteremia in 4-5% of patients

Typically low virulence bacteria

Staphylococcus epidermidis (75% of all cases)

Less often: Staphylococcus albus, Acinetobacter, Bacillus, Corynebacterium, Listeria, Propionibacterium, Pseudomonas, Peptococcus, and Micrococcus

Glomerulonephritis of CVI-GN

Visceral abscesses: Lung, rectum, appendix, septic abortion

Bone (osteomyelitis), subcutaneous, periodontal abscesses

Prostheses: Valvular, vascular, other

Vascular device-associated infection (e.g., injection reservoirs, indwelling catheters)

S. aureus most common pathogen

Portosystemic shunt immune complex GN not technically “SN”

TERMINOLOGY

Hydrocephalus

Enlargement of cerebral ventricles secondary to abnormal CSF formation, flow, or absorption resulting in ↑ CSF volume

IMAGING

Shunt failure → dilated ventricles + edema around ventricles, along catheter and reservoir

Use CT or MR to evaluate ventricle size, plain radiograph shunt series to identify mechanical shunt failure

Baseline CT/MR following shunt insertion, follow-up at 1 year and as clinically needed

Shunt radionuclide studies: Used to confirm distal obstruction

TOP DIFFERENTIAL DIAGNOSES

Shunt failure with normal ventricle size or lack of interstitial edema

Noncompliant (“slit”) ventricle syndrome

Acquired Chiari 1 malformation/tonsillar ectopia

PATHOLOGY

Obstructive hydrocephalus: Secondary to physical blockage by tumor, adhesions, cyst

Communicating hydrocephalus: Secondary to ↓ CSF absorption across arachnoid granulations

CLINICAL ISSUES

Older children/adults: Headache, vomiting, lethargy, seizure, neurocognitive symptoms

Infants: Bulging fontanelle, ↑ head circumference, irritability, lethargy

DIAGNOSTIC CHECKLIST

Shunt + headache not always shunt failure

Confirm programmable shunt valve setting after MR

Compare current CT with prior studies to detect subtle changes in ventricle size

White Matter Incomplete Infarction

In the posterior hemispheric white matter surrounding the cerebral ventricles, there is often neuroradiologic and pathologic loss of tissue elements seen in elderly patients. Hypertension, intrinsic heart disease or carotid disease28 may be present and the term “leukoaraiosis” has been applied.29 These areas, when examined pathologically, show small vessel disease at the arteriolar level (Chapter 27) including arteriolar tortuosities.30 Subsequently, an occlusive, collagenizing process of arterioles may be seen in the periventricular white matter (see Figs. 4.5G and H) that may relate to the white matter change radiologically termed leukoaraiosis, a rarefaction seen around the ventricles.

Of the three branches of arterioles (see Fig. 4.6), it is the smallest that occlude first (see Fig. 4.5H) when collagen is laid down, due to their small diameter. Larger arterioles can be thickened by collagen without occluding.

Diffuse disease in these smallest of vessels gives rise to small infarcts/white matter ischemia and may contribute to vascular cognitive impairment (Chapter 18).31,32 The effect of ischemia in the white matter (Chapter 9) causes damage more to myelin than axons, due to axoplasmic flow continuously carrying axoplasm to ischemic areas from nonischemic zones. A selective demyelination thus represents the early ischemic lesion, but it is the harbinger of the axonal loss that follows with continued or more severe ischemia. The early demyelinative lesion can be thought of as the white matter analog of incomplete infarction of the gray matter. When it eventually gives way to total tissue breakdown involving axons as well, ischemic demyelination can give rise to a radiologic appearance that has been termed leukoaraiosis and Binswanger disease.33

Ventricles and Choroid Plexus

5.1.2 Cilia on Airway, Brain, and Sperm Cells

Motile cilia in mammals are observed in the tracheal lumen, cerebral ventricles, sperm, and the other tissues. Respiratory cilia and ependymal cilia generate fluid flow on the surface of cells and transport fluid containing pathogens and biological molecules. Alternatively, sperm cells move to an egg by moving through peripheral fluid using flagella. Therefore, defects in ciliary activity cause a number of diseases, including chronic bronchitis, hydrocephalus, and male/female sterility. Respiratory cilia contribute to mucociliary clearance in the airways. Ciliary beating transports mucous containing dust and viruses to the pharynx on the surface of the tracheal lumen. Ependymal cilia generate the flow of cerebrospinal fluid (CSF) on the surface of ependymal cells. Additionally, defects in ependymal cilia cause hydrocephalus (Chen et al., 1998; Taulman et al., 2001; Sapiro et al., 2002; Ibanez-Tallon et al., 2004) and alter the transport of neuroblasts toward the olfactory bulb (Sawamoto et al., 2006). These directions of ciliary beating are regulated and produce directional flow. The beating direction originates from the basal body orientation. Notably, the planar cell polarity of ciliary orientation is established by the direction of the basal foot associated with the basal body structure (Mitchell et al., 2007). For sperm flagella, the beating frequency must remain very high for long period of time in order for the sperm cell to reach the egg for fertilization. Therefore, a large amount of ATP is needed to maintain flagellum beating, and this ATP mainly is produced by glycolysis (Mukai and Okuno, 2004).

Ventriculomegaly

Fetal ventriculomegaly (VM) is an enlargement of the lateral cerebral ventricle caused by an excess of cerebrospinal fluid (CSF). The incidence of VM ranges from 0.3 to 1.5% life births. Unilateral VM occurs in 60% of the cases, leaving 40% bilateral. There is a male predominance (70%). VM may result from obstructive malformations, destructive lesions and abnormal development of the brain (Table 28.2).

The diagnosis is made with the measurement of the lateral ventricle in a strict sagittal plane on ultrasonography. The measurement is performed opposite to the internal parieto-occipital sulcus, putting the callipers on the inner wall at its widest part and aligned to the long axis8 (Fig. 28.8). An atrial width between 10 and 15 mm is considered mild VM; a width greater than 15 mm constitutes severe VM (Figs. 28.9 and 28.10). Fetal MRI is helpful to diagnose additional abnormalities in 5% to 50% of the cases.9,10

Fetal VM is often associated with CNS anomalies (agenesis of the corpus callosum [CC], spina bifida [SB]). In 30% of cases, severe VM is associated with non-CNS anomalies. Chromosomal anomalies are detected in more than 15% of cases when VM is associated with other fetal anomalies.11 In mild VM, structural anomalies range from 10% to 76%. However, even in apparently isolated VM, malformations are found in 13% of the cases after birth.12

Ventriculomegaly associated with other brain anomalies carries a high mortality rate (60%–70%). Early detection and progression of the VM are bad prognostic factors. Isolated mild VM has a poor outcome in 20% of the cases with perinatal death in 1.4%. In 3% of cases with isolated mild VM chromosomal abnormalities are seen (mainly T21: 9 times more).12 Prenatal stabilisation in size or regression is a favourable sign. Recently, isolated mild VM documented by a normal fetal MRI has been associated with a normal neurodevelopment outcome when evaluated between 18 and 36 months of age by the Vineland Adaptive Behavior Scales. The same study demonstrated a similar result for moderate VM but on a rather small sample size and should be confirmed on a larger population.13

VENTRICULOMEGALY

The term ventriculomegaly is commonly used to indicate enlargement of the lateral cerebral ventricles. The incidence of this finding is unclear. Severe ventriculomegaly or hydrocephalus is found in less than 1 per 1000 births. Ventriculomegaly may be the consequence of cerebral malformations, chromosomal abnormalities or congenital infection. Genetic factors play an important role. About 25% of severe ventriculomegaly occurring in males is due to X-linked transmission.

Fetal ventriculomegaly is diagnosed sonographically, by the demonstration of abnormally dilated lateral cerebral ventricles. A transverse scan of the fetal head at the level of the cavum septum pellucidum will demonstrate the dilated lateral ventricles, defined by an internal diameter of the posterior horn (or atrium) of 10 mm or more.13 The choroid plexuses, which normally fill the lateral ventricles, are surrounded by fluid. A diameter of 10–15 mm indicates mild ventriculomegaly. A diameter greater than 15 mm indicates moderate to severe ventriculomegaly.33 Certainly before 24 weeks and particularly in cases of associated spina bifida, the head circumference may be small rather than large for gestation.

Fetal or perinatal death and neurodevelopment in survivors are strongly related to the presence of other malformations and chromosomal defects.34 Isolated severe ventriculomegaly is associated with an increased risk of perinatal death and a 50% chance of neurological sequelae in survivors. Although isolated mild ventriculomegaly (atrial width of 10–15 mm) is generally associated with a good prognosis, it is also the group with the highest incidence of chromosomal abnormalities (often trisomy 21). In addition, in a few cases with apparently isolated mild ventriculomegaly there may be an underlying cerebral maldevelopment (such as lissencephaly) or destructive lesion (such as periventricular leukomalasia). It has been suggested that ventricles of 10–12 mm, which represent the bulk of these cases, tend to have a good prognosis, with neurological compromise in the range of 4%, while those cases in which the measurement is 13–15 mm are associated with a greater probability of handicap, in the range of 12%.