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KEY FACTS

Terminology
- Ventriculomegaly caused by - Obstruction of CSF egress from ventricles: Intraventricular obstructive hydrocephalus - Decreased CSF resorption from subarachnoid space: Extraventricular obstructive hydrocephalus - Increased CSF production
- Use FOHR to track ventricle size over time - (Transverse diameter of frontal horns + transverse diameter of occipital horns) ÷ 2x transverse diameter of cranium
Imaging
- 3D FIESTA/CISS - Acquire in sagittal plane to outline aqueduct
- Limited shunt MR protocol - 30-second acquisition in each plane - Useful substitute for CT - Insensitive for parenchymal abnormalities
Pathology
- Extracellular, extravascular fluid in brain is managed by glymphatic system - CSF circulates from within ventricles → subarachnoid space → perivascular spaces ↔ interstitium ↔ perivenular spaces → subarachnoid space - CSF is resorbed from subarachnoid space at multiple sites - Along sheaths of cranial nerves (especially olfactory bulb) into head and neck lymphatics - Along sheaths of spinal nerves into perispinal lymphatics - Into meningeal (dural) lymphatics - Into dural sinuses via arachnoid granulations - Previously thought to be major site of resorption
- Hydrocephalus results from obstruction of CSF egress from ventricular system or reduced resorption from subarachnoid space

TERMINOLOGY

Abbreviations
- Intraventricular obstructive hydrocephalus (IVOH) - Acute IVOH (aIVOH) - Chronic "compensated" IVOH (cIVOH)
- Extraventricular obstructive hydrocephalus (EVOH)
- Frontal occipital horn ratio (FOHR) - (Transverse diameter of frontal horns + transverse diameter of occipital horns) ÷ 2x transverse diameter of cranium
- Subarachnoid lymphatic-like membrane (SLYM) - Recently discovered 4th meningeal membrane that divides subarachnoid space into inner and outer compartments
Definitions
- Ventriculomegaly caused by - Obstruction of CSF egress from ventricles: IVOH - Decreased CSF resorption from subarachnoid space: EVOH - Increased CSF production
- Ventriculomegaly secondary to loss of parenchyma, a.k.a. ex vacuo dilation, is nothydrocephalus

IMAGING

General Features

Best diagnostic clue

- Enlarged ventricles
        - With decreased extraaxial spaces (EAS): IVOH
        - With periventricular (transependymal) edema: Acute IVOH
        - With enlarged EAS: EVOH

Size

- FOHR > 0.33
- Temporal horn width > 3 mm

Morphology

- Ventricles proximal to obstruction enlarge, appear more rounded
- Trigones and occipital horns typically enlarge most
        - Wall pressure is proportional to diameter: Laplace law
        - Optic and infundibular recesses of 3rd ventricle may preferentially enlarge
        - Optic nerve sheaths may enlarge → papilledema

CT Findings

NECT

- Large ventricles proximal to obstruction
        - aIVOH
                  - "Ballooned" ventricles with periventricular low-density "halo"
        - cIVOH
                  - "Ballooned" ventricles, periventricular "halo"
- Basal cisterns, sulci compressed/obliterated

MR Findings

T1WI

- Lateral ventricles enlarged
- Corpus callosum (CC) thinned, stretched upward
        - May be impinged against falx
        - Impaction may cause pressure necrosis
- Fornix, internal cerebral veins (ICV) displaced downward
- Enlarged 3rd ventricle often herniated into expanded sella
- Funnel-shaped aqueduct of Sylvius in aqueductal stenosis

T2WI

- Acute obstruction
        - Transependymal edema extends into periventricular white matter (WM)
                  - Accentuated around frontal and occipital horns
- Chronic obstruction
        - Large ventricles without transependymal edema
        - CC may show hyperintensity after decompression (15% of shunted IVOH cases)

T1WI C+

- Diffuse leptomeningeal disease can cause EVOH
        - May only be apparent on postcontrast imaging

MRS

- Small lactate resonances can be detected in up to 20% of CSF spaces, even if no hydrocephalus

Other Modality Findings
- Contrast-enhanced ventriculography - MR/CT used to identify site of obstruction, status of 3rd ventriculostomies - MR can be used for assessing CSF flow
- Cardiac-gated phase-contrast MR - May show loss of CSF flow through aqueduct - Useful for assessing status of endoscopic 3rd ventriculostomy (ETV)

Imaging Recommendations

Best imaging tool

- Sagittal T2WI MR
- Administer contrast if pattern suggests EVOH and no etiology is apparent

Protocol advice

- 3D FIESTA/CISS
        - Acquire in sagittal plane to outline aqueduct
        - Decreases CSF flow artifact
        - Allows better delineation of ventricular contour, septa
- SWI to assess for superficial siderosis in posthemorrhagic hydrocephalus
- Limited shunt MR protocol
        - Single-shot rapid technique with heavy T2 weighting
        - 30-second acquisition in each plane
        - Useful substitute for CT
                  - Reduce radiation exposure in frequently imaged population
                  - Rapid aquisition reduces need for sedation
        - Insensitive for parenchymal abnormalities
- Use FOHR to track ventricle size over time
        - Especially in younger children with open sutures
        - Accounts for increase in head circumference with increase in ventricular size

DIFFERENTIAL DIAGNOSIS

Ventricular Enlargement Secondary to Parenchymal Loss
- a.k.a. ex vacuo ventriculomegaly
- Age related (ventricular volume increases 1.2-1.4 mL after 60 years) - Ischemia/infarction, trauma, infection, toxic
- Obtuse frontal angle (> 110°)
- Associated enlargement of sulci, cisterns
- Normal lateral ventricles can be asymmetric
Benign Enlargement of Subarachnoid Spaces and Ventricles
- a.k.a. benign macrocrania
- Seen in association with macrocephaly in infants
- Transient and self-limited - Not associated with developmental delay - Does not require CSF diversion
- Likely reflects relative immaturity of glymphatic system - Diminished ability to resorb CSF as interstitial fluid compartment is shrunk by progressive myelination

PATHOLOGY

General Features

Etiology

- Extracellular, extravascular fluid in brain is managed by glymphatic system
        - Maintains homeostatic balance between interstitial fluid compartment, cellular compartment, and intravascular compartment
        - Analogous to lymphatic system in other organs
- While brain parenchyma does not have lymphatic vessels, they are present in dura mater and along sheaths of cranial nerves
- CSF circulates from within ventricles → subarachnoid space → perivascular spaces ↔ interstitium ↔ perivenular spaces → subarachnoid space
- CSF is resorbed from subarachnoid space at multiple sites
        - Along sheaths of cranial nerves (especially olfactory bulb) into head and neck lymphatics
        - Along sheaths of spinal nerves into perispinal lymphatics
        - Into meningeal (dural) lymphatics
        - Into dural sinuses via arachnoid granulations
                  - Previously thought to be major site of resorption
- SLYM is thought to facilitate CSF flow
        - Disruption after trauma may explain reduced glymphatic flow and posttraumatic cerebral edema
- Normal CSF production = 0.2-0.35 mL/min
        - Capacity of lateral, 3rd ventricles in adult = 20 mL
        - Total volume of CSF in adult = 120 mL
- Hydrocephalus results from obstruction of CSF egress from ventricular system or reduced resorption from subarachnoid space
- Obstruction within ventricular system results in IVOH
        - Ventricles expand, compress adjacent parenchyma; stretching may rupture/open ependymal cell junctions
        - Periventricular interstitial fluid increases → myelin destruction
- Etiology depends on site
        - Foramen of Monro
                  - Colloid cyst
                  - Subependymal giant cell astrocytoma in tuberous sclerosis
        - 3rd ventricle
                  - Craniopharyngioma, hypothalamic glioma, arachnoid cyst
        - Aqueduct of Sylvius
                  - Aqueductal stenosis, tectal glioma, pineal region tumor
                  - Obstruction by hemorrhage or inflammatory debris
                  - Enlarged vein of Galen due to arteriovenous fistula
        - 4th ventricle
                  - Medulloblastoma, ependymoma, pilocytic astrocytoma
                  - Chiari, Dandy-Walker malformation, Blake pouch cyst
        - Metastasis, neurocysticercosis, or meningioma can occur at multiple intraventricular locations
- Reduced resorption from subarachnoid space results in EVOH
        - Subarachnoid pathology may reduce absorptive capacity
                  - Hemorrhage or inflammation (acute or chronic)
        - Metabolic disorders may reduce resorptive capacity
- Overproduction of CSF may overwhelm ability of glymphatic system to manage and resorb CSF
        - Choroid plexus papilloma or carcinoma
                  - Focally enlarged and hyperenhancing choroidal mass
        - Choroid plexus villous hyperplasia
                  - Diffusely enlarged choroid

Genetics

- Cell adhesion molecule L1 (*L1CAM*) recognized as cause of X-linked aqueductal stenosis
        - Located on X chromosome (Xq28)

Gross Pathologic & Surgical Features
- Focal/generalized ventricular enlargement
- Ependyma, adjacent WM are secondarily injured
- Variable pathology depending on causative factor

CLINICAL ISSUES

Presentation

Most common signs/symptoms

- Headache, papilledema (aIVOH)
- Nausea, vomiting
- Diplopia
        - 6th nerve palsy caused by compression of cisternal segment

Clinical profile

- Varies with etiology, severity, age of onset

Demographics

Age

- May be any age from in utero (congenital hydrocephalus) to adult

Epidemiology

- Epidemiological data varies widely, depending upon etiology and type of hydrocephalus

Natural History & Prognosis
- Usually progressive unless treated
Treatment
- Obstructive hydrocephalus is managed surgically - CSF diversion (shunt or ETV) - Resection of obstructing or hypersecreting lesion
- Most common neurosurgical procedure in children = CSF shunting for hydrocephalus
- CSF diversion is typically delayed in EVOH - Tendency to be much more shunt dependent - Less tolerant of minor pressure changes - Lesser degree of ventriculomegaly increases difficulty of surgery

DIAGNOSTIC CHECKLIST

Consider
- Longstanding aqueductal stenosis can be caused by slow-growing tectal tumor - Compensated IVOH
- CSF function and homeostasis are far more complex than previously thought - Free communication among anatomic/functional compartments is necessary for proper brain health
Image Interpretation Pearls
- Size of ventricles generally correlates poorly with intracranial pressure
- Pulsatile CSF may create confusing signal intensity, even mimic intraventricular mass

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References

Selected References

Images

Selected Images

Depiction of normal CSF flow through the glymphatic system. CSF descends along periarterial perivascular spaces (PVS) and through the interstitium before exiting along perivenular PVS, clearing macromolecules (black particles). The exchange between interstitium and PVS is modulated by astrocytic endfeet expressing AQP4 (pink channels).

An isoattenuating colloid cyst obstructs the foramina of Monro in this 7-year-old, causing obstructive hydrocephalus.

A large suprasellar arachnoid cyst balloons upward and obstructs the foramina of Monro in this 1-year-old with macrocrania.

This 3-month-old with hydrocephalus has a Blake pouch cyst obstructing outflow of CSF from the 4th ventricle. Note the membrane across the posterior foramen magnum and the uplifting of the vermis .

A medulloblastoma fills and obstructs the 4th ventricle in this 10-year-old, leading to supratentorial ventriculomegaly and papilledema . Papilledema visible on MR typically correlates to grade 3 on the Frisen scale (moderate edema).

Axial FLAIR MR through the lateral ventricles in the same child shows transependymal edema capping the frontal and occipital horns , reflecting the increased pressure in the ventricular system.

Bacterial meningitis (group A Streptococcus in this example) can restrict resorption of CSF but can also obstruct at the cerebral aqueduct and 4th ventricular outlets when complicated by ventriculitis, evident on this image by abnormal enhancement of the ependyma .

An infiltrating tectal glioma obstructs the cerebral aqueduct in this 10-year-old. Absence of transependymal edema suggests a compensated hydrocephalus.

A papilloma of the choroid plexus in the occipital horn of the left lateral ventricle causes moderate hydrocephalus by excessive CSF production in this 2-month-old.

CSF overproduction can rarely be nonneoplastic in nature, as in this 6-month-old with villous hyperplasia of the choroid plexus in each lateral ventricle . Note the preservation of peripheral sulci, as the unfused sutures of the infant can widen in response to increased intracranial volume.

Additional Images

Sagittal T1 MR shows a large mass within the 4th ventricle causing intraventricular obstructive hydrocephalus or noncommunicating hydrocephalus.

Sagittal T2 MR in the same patient shows transependymal CSF flow, seen here as "fingers" extending into white matter around the enlarged lateral ventricle. The case was medulloblastoma with acute IVOH.

Coronal T1 C+ MR shows IVOH with a large, enhancing intraventricular mass causing marked enlargement of the lateral ventricles .

Axial NECT in the same patient shows the large intraventricular mass within the 4th ventricle. Note the dilated temporal horns .

Sagittal T1 MR shows IVOH secondary to aqueductal stenosis and distal stenosis of cerebral aqueduct . Note the enlarged lateral and 3rd ventricles.

Axial FLAIR MR shows neurosarcoidosis and EVOH secondary to diffuse meningeal disease. Periventricular white matter hyperintensities are also present, as well as choroid involvement .

Coronal T1 C+ MR shows neurocysticercosis involvement within the 3rd ventricle and aqueduct , causing IVOH. The lateral ventricles are dilated.

Axial FLAIR MR shows neurocysticercosis resulting in IVOH. Large intraventricular cysts are present in the lateral vents , obstructing the foramina of Monro.

Axial T1 MR shows a well-defined, hyperintense lesion at the foramen of Monro in a patient with headaches, most consistent with a colloid cyst. Note the enlargement of the lateral ventricles due to obstruction at the foramen of Monro.

Sagittal T1 C+ MR shows a homogeneously enhancing mass in the posterior 3rd ventricle , which causes obstruction and dilatation of the lateral and 3rd ventricles. On pathology, this was an astrocytoma.

Coronal T2 MR shows a pilocytic astrocytoma centered in the right thalamus , causing severe mass effect on the 3rd ventricle and resultant obstructive hydrocephalus .

Axial T2 MR demonstrates a well-defined CSF intensity cyst with the left temporal horn most consistent with an ependymal cyst . Note the dilated and trapped left temporal horn .

Sagittal T1 C+ MR shows an enhancing mass in the pineal region causing mass effect on the tectal plate and aqueductal obstruction. Note the extensive leptomeningeal enhancement due to CSF spread of tumor. CSF cytology showed a primitive neuroectodermal tumor.

A medulloblastoma fills and obstructs the 4th ventricle in this 10-year-old, leading to supratentorial ventriculomegaly and papilledema.

FIESTA shows pineal parenchymal tumor of intermediate differentiation obstructing the cerebral aqueduct in this 8-year-old boy.

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KEY FACTS

Terminology

Imaging

Pathology

TERMINOLOGY

Abbreviations

Definitions

IMAGING

General Features

Best diagnostic clue

Size

Morphology

CT Findings

NECT

MR Findings

T1WI

T2WI

T1WI C+

MRS

Other Modality Findings

Imaging Recommendations

Best imaging tool

Protocol advice

DIFFERENTIAL DIAGNOSIS

Ventricular Enlargement Secondary to Parenchymal Loss

Benign Enlargement of Subarachnoid Spaces and Ventricles

PATHOLOGY

General Features

Etiology

Genetics

Gross Pathologic & Surgical Features

CLINICAL ISSUES

Presentation

Most common signs/symptoms

Clinical profile

Demographics

Age

Epidemiology

Natural History & Prognosis

Treatment

DIAGNOSTIC CHECKLIST

Consider

Image Interpretation Pearls

References

Selected References

Images

Selected Images

Additional Images

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