Time-of-Flight Effects

TOF effects can result in signal loss (dark CSF signal) or flow-related enhancement, which produces bright CSF signal. TOF signal loss is directly related to CSF velocity and most prominent where flow is accelerated through narrow confines. Typical locations for TOF signal loss are around the foramen of Monro and in the third and fourth ventricles (34-17).

Incomplete CSF nulling on FLAIR scans causes sulcal-cisternal CSF to appear spuriously bright, mimicking subarachnoid hemorrhage, infection, or metastatic disease (34-18).

Entry-slice phenomena are most striking on T1 scans. Bright signal is caused by inflow of unsaturated spins that have full longitudinal magnetization. The first slices of the imaging volume show the most prominent flow-related enhancement effects, which are most pronounced in the lower posterior fossa on axial sequences and around the foramen of Monro on coronal images. These entry-slice phenomena create artifacts that can mimic masses.

Turbulent Flow

Turbulent flow causes varied flow velocities and different directions with signal loss secondary to intravoxel spin dephasing. In the brain, turbulent flow with signal loss is common in the cerebral aqueduct, the fourth ventricle, and around pulsating vessels. This effect is especially pronounced around the basilar artery, where it can mimic aneurysmal dilatation.

Motion Artifacts

The most problematic artifact on MR is voluntary patient motion. Voluntary patient motion can be minimized with verbal reminders or mild sedation. Some patient motion is both intrinsic and involuntary, caused by pulsating arteries or CSF.

Pulsation artifacts along the phase-encoding direction cause propagation of "ghosting" artifacts in a straight linear band across the entire imaged plane. Phase-encoding artifacts are often seen as alternating foci of bright and dark signal (34-16).

Hydrocephalus

The term "hydrocephalus" literally means "water head." The term "ventriculomegaly" means enlargement of the ventricular system. Remember: the terms "hydrocephalus" and "ventriculomegaly" are descriptive findings, not a diagnosis! The role of imaging is to find the etiology of the ventricular enlargement.

Hydrocephalus has traditionally been regarded as an abnormality in the formation, flow, or resorption of CSF. If normal CSF flow is impeded by a blockage within the ventricular system, CSF production continues and the ventricles enlarge. In the classic model, hydrocephalus can also result from an imbalance between CSF production and absorption. When CSF absorption through the arachnoid granulations is compromised, the ventricles enlarge, and hydrocephalus results. Absorption can be blocked at any level within the subarachnoid cisterns, e.g., within the cisterna magna, at the basilar cisterns, or along the cerebral convexities.

In the newest attempt to understand the development of hydrocephalus, aquaporin (AQP)-mediated brain water homeostasis and/or clearance of both CSF and ISF into the PVSs and blood are compromised. The molecular mechanisms that drive AQP4 modifications in hydrocephalus that fail to facilitate removal of excess water are still relatively unknown.

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(34-16) Phase-encoding artifact propagates horizontally across ventricles and parenchyma.

(34-17) Axial T2WI shows hypointense CSF in the upper third ventricle caused by pulsatile CSF flow through the foramen of Monro.

(34-18) (Top) FLAIR with incomplete fluid suppression, CSF in sulci appears hyperintense. (Bottom) Corrected suppression looks normal.

(34-19) Triventricular IVOH shows markedly enlarged lateral, third ventricles, stretched corpus callosum, funnel-shaped cerebral aqueduct with distal obstruction. Note normal size of fourth ventricle and bulging floor of third ventricle .

Hydrocephalus is the most common disorder requiring neurosurgical intervention in children. Its treatment consumes a disproportionate share of healthcare dollars, approaching nearly a billion dollars a year in the United States alone. Once considered predominantly a disease of childhood, hydrocephalus is now increasingly recognized as a less common but still important cause of neurologic disability in adults.

Terminology

A rigorous definition of hydrocephalus is surprisingly difficult. Its terminology and classification are a matter of continuing debate. We follow the common approach of subclassifying hydrocephalus by the presumed site of CSF obstruction, i.e., inside [intraventricular obstructive hydrocephalus (VOH)] or outside the ventricles [extraventricular obstructive hydrocephalus (EVOH)]. The distinction is important, as treatment for IVOH (CSF diversion) differs from that of EVOH (membrane fenestration).

The outdated term "ex vacuo hydrocephalus" referring to ventricular and cisternal enlargement caused by parenchymal volume loss is no longer used.

Etiology

When abnormally large cerebral ventricles are identified on imaging studies, the diagnostic imperative is to find the cause of the hydrocephalus. The presence of enlarged ventricles with elevated intracranial pressure is only one presentation along a spectrum that ranges from idiopathic intracranial hypertension ("pseudotumor cerebri") to the recently recognized, enigmatic syndrome of low-pressure hydrocephalus.

(34-20) Sagittal autopsy case shows aqueduct stenosis , massively enlarged lateral ventricle , ballooned third ventricle, and normal fourth ventricle . (Courtesy Rubinstein Collection, AFIP Archives.)

In the pediatric age group, a majority of cases are caused by congenital defects of the CSF pathway. Adult-onset hydrocephalus is usually secondary to different pathologies that encompass a heterogeneous group of disorders. The most common is intracranial hemorrhage (45%, most often caused by aneurysmal subarachnoid hemorrhage) followed by neoplasm (30%) and head injury or infection (5% each). Normal pressure hydrocephalus (11%) and idiopathic intracranial hypotension (4%) together account for 15% of cases.

Intraventricular Obstructive

Hydrocephalus

Terminology

IVOH is used to designate physical obstruction at or proximal to the fourth ventricular outlet foramina. The term "noncommunicating hydrocephalus" is no longer used.

Etiology

General Concepts. IVOH can be congenital or acquired, acute (aIVOH) or chronic (cIVOH). Congenital IVOH occurs with disorders such as aqueductal stenosis.

Although aIVOH can occur suddenly (e.g., foramen of Monro obstruction by a colloid cyst), it usually develops over a period of weeks or even months. Any gradually expanding intraventricular mass (such as a neoplasm or cyst) can cause IVOH, as can an extraventricular mass of sufficient size to occlude a critical structure (e.g., the cerebral aqueduct).

When the ventricles become obstructed, CSF outflow is impeded. As CSF production continues, the ventricles expand.

(34-21A) Sagittal T1 C+ shows a heterogeneously enhancing mass expanding the 4th ventricle. The upper 4th ventricle and cerebral aqueduct are enlarged as are the third and lateral ventricles.

As the ventricles expand, increased pressure is exerted on the adjacent brain parenchyma. Increased intraparenchymal pressure compromises cerebral blood flow, reducing brain perfusion. The increased pressure also compresses the subependymal veins, which reduces absorption of brain interstitial fluid via the deep medullary veins and perivascular spaces. The result is periventricular interstitial edema. Whether the edema results from CSF extruding across the ventricular ependyma ("transependymal CSF flow") or accumulation of brain extracellular fluid is unknown.

In chronic "compensated" IVOH, the ventricles expand slowly enough that CSF homeostasis is relatively maintained. Periventricular interstitial edema is minimal or absent.

Pathoetiology. The general causes of obstructive hydrocephalus range from developmental/genetic abnormalities to trauma, infection, intracranial hemorrhage, neoplasms, and cysts.

The most common cause of acquired IVOH is intraventricular inflammatory or posthemorrhagic membranous obstruction. The most common sites of obstructing membranes are, in order, the foramina of Luschka, the cerebral aqueduct, and the foramen of Magendie. The foramen of Monro is a relatively rare location.

Intraventricular masses are the next most common cause of acquired IVOH. The prevalence of specific pathologies varies with location. Colloid cyst is the most common mass found at the foramen of Monro, followed by tuberous sclerosis (subependymal nodules and giant cell astrocytoma). After benign (membranous) obstruction, the most common lesions to obstruct the aqueduct of Sylvius are tectal plate glioma and pineal region neoplasms.

(34-21B) Coronal T1 C+ in the same case shows 4th ventricular mass extrudes inferiorly through the cisterna magna into the upper cervical canal . Note symmetrically enlarged lateral ventricles . Ependymoma was found at surgery.

The fourth ventricle is a common site for neoplasms that can cause obstructive hydrocephalus. In children, medulloblastoma is the most common tumor that causes IVOH, followed by ependymoma, pilocytic astrocytoma, diffusely infiltrating astrocytoma, and atypical teratoid/rhabdoid tumor (AT/RT).

In adults, metastases, hemangioblastoma, epidermoid cyst, and choroid plexus papilloma are fourth ventricular lesions that may cause hydrocephalus. Inflammatory cysts (e.g., neurocysticercosis) occur throughout the ventricular system and in patients of all ages.

Genetics. Congenital hydrocephalus can be syndromic or nonsyndromic. To date, only one gene—the neural cell adhesion molecule L1 (L1CAM)—has been recognized as a cause of congenital hydrocephalus. X-linked hydrocephalus (hereditary aqueduct stenosis) is caused by mutation in the

L1CAM gene.

Pathology

Grossly, the ventricles proximal to the obstruction appear ballooned (34-19) (34-20). The ependyma is thinned and may be focally disrupted or even absent. The corpus callosum (CC) is thinned and displaced superiorly against the rigid, unyielding falx cerebri. Focal encephalomalacic changes are common in the CC body.

Microscopic examination shows that the ependymal lining is discontinuous or inapparent. The periventricular extracellular space is increased, and the surrounding WM is rarefied and stains pale. The cortex is relatively well preserved.

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(34-22A) NECT in a 52y woman shows "tight" brain, large lateral ventricles , "blurred" margins , periventricular hypodense "halo .

Clinical Issues

Epidemiology and Demographics. IVOH can affect people at any age, from the fetus (in utero congenital hydrocephalus) to the elderly. There is no sex predilection except for primary congenital hydrocephalus, in which the M:F ratio is 2.6:1.0.

Presentation. The presentation of IVOH varies with acuity and severity. Headache is the most common overall symptom, and papilledema is the most common sign. Nausea, vomiting, and CN VI palsy are also common with aIVOH.

Natural History. The natural history of IVOH varies. Most cases are typically progressive unless treated. Untreated severe aIVOH can result in brain herniation with coma and even death. Some patients with slowly developing compensated IVOH may not present until late in adult life (e.g., the recently recognized syndrome of late-onset aqueductal stenosis).

Treatment Options. CSF diversion (shunt, ventriculostomy, endoscopic fenestration of the third ventricle floor) is common, often performed as a first step before definitive treatment of the obstruction (e.g., removal of a colloid cyst or resection of an intraventricular neoplasm).

Imaging

(34-22B) Axial T2WI in same patient, who had headaches, shows large lateral ventricles with extensive periventricular fluid accumulation .

(34-22C) Sagittal FLAIR shows hyperintense "fingers" extending along the entire margin of the lateral ventricle, acute IVOH (GBM of fornix).

General Features. A number of measurements have been devised to quantify hydrocephalus. These include indices such as diameter of the frontal horns in relation to the inner table of the skull (ventricular or Evans index), frontal horn radius, and ventricular angle. The utility of such twodimensional measurements versus visual judgment is uncertain. Computergenerated volume measurements have been proposed as providing better normative standards but are time-consuming and difficult to obtain.

Despite its acknowledged inaccuracies, subjective neuroradiologic evaluation remains the most common method of assessing ventricular size. Hydrocephalus is usually diagnosed when the ventricles appear disproportionately enlarged relative to the subarachnoid spaces.

Although NECT scans are often used as an emergent screening procedure in patients with headache and signs of increased intracranial pressure, MR is the procedure of choice. Multiplanar MR best delineates the hydrocephalus and often permits identification of its etiology.

On axial studies, helpful general imaging findings include enlarged temporal horns of the lateral ventricles (out of proportion to the basal subarachnoid spaces). The frontal horns assume a "rounded" appearance. The third ventricle—which usually appears slit-like on axial views—expands, losing its normal tapered appearance. The walls first become parallel, then expand outward so that the third ventricle appears oblong or ovoid. As the ventricles continue to enlarge, the subarachnoid cisterns and convexity sulci may become compressed, and gyri appear flattened against the calvaria.

Sagittal views show that the CC is thinned, stretched, bowed upward, and, in severe cases, even impacted against the falx cerebri. The anterior recesses of the third ventricle enlarge, losing their normal "pointed" appearance (3421).

In cases of cIVOH, pulsating CSF in the third ventricle pounds the central skull base relentlessly. The bony sella turcica gradually enlarges and assumes an "open" configuration. In severe cases, the anterior third ventricle may protrude into the sella itself.

If both lateral and third ventricles are enlarged but the fourth ventricle remains normal (e.g., as occurs with aqueductal stenosis), the condition is

termed triventricular hydrocephalus. If all four chambers of the ventricular system are enlarged, it is called quadriventricular hydrocephalus. Quadriventricular hydrocephalus is caused by a mass in the fourth ventricle or obstruction of the outlet foramina (typically infection or subarachnoid hemorrhage).

In approximately 0.5-1.0% of IVOH cases, just one lateral ventricle is enlarged ("unilateral hydrocephalus"). Most cases are acquired and associated with intraventricular neurocysticercosis or the presence of a membranous web at the junction of the inferior frontal horn with the foramen of Monro.

CT Findings. Imaging findings vary with acuity and severity. NECT scans in aIVOH demonstrate enlarged lateral and third ventricles, whereas the size of the fourth ventricle varies. The temporal horns are prominent, the frontal horns are "rounded," and the margins of the ventricles appear indistinct or "blurred." Periventricular fluid—whether from compromised drainage of interstitial fluid or transependymal CSF migration—causes a "halo" of low density in the adjacent WM (34-22A). The sulci and basal cisterns appear compressed or indistinct.

MR Findings. Axial T1WI shows that both lateral ventricles are symmetrically enlarged. On sagittal views, the CC appears thinned and stretched superiorly, whereas the fornices and internal cerebral veins are displaced inferiorly.

In aIVOH, T2 scans may demonstrate "fingers" of CSF-like hyperintensity extending outward from the lateral ventricles into the surrounding WM (3422B). Fluid in the periventricular "halo" does not suppress on FLAIR (3422C). In longstanding chronic "compensated" hydrocephalus, the ventricles appear enlarged and the WM attenuated but without a thick periventricular "halo" (34-23).

High-resolution thin-section T2WI, FIESTA, or CISS sequences exquisitely delineate the CSF spaces and may demonstrate subtle abnormalities not detected on standard sequences. 2D cine-phase contrast imaging is helpful to depict CSF dynamics in the aqueduct and around the foramen magnum.

Complications of Hydrocephalus. In severe cases of IVOH, the CC becomes compressed against the free inferior margin of the falx (34-24) (34-25). This can cause pressure necrosis and loss of callosal axons, the so-called corpus callosum impingement syndrome (CCIS). In acute CCIS, the CC may initially appear swollen and hyperintense on T2WI and FLAIR. Subacute and chronic changes are seen as encephalomalacic foci in a shrunken, atrophic-appearing CC. In 15% of treated IVOH cases, the CC shows T2/FLAIR hyperintensity after decompression. In rare cases, the hyperintensity extends beyond the CC itself into the periventricular WM (34-26).

Massive ventricular enlargement may weaken the medial wall of the lateral ventricle enough that a pulsion-type diverticulum of CSF extrudes through the inferomedial wall of the atrium (34-27). Such medial atrial diverticula may cause significant mass effect on the posterior third ventricle, tectal plate, and aqueduct. Large atrial diverticula can herniate inferiorly through the tentorial incisura into the posterior fossa, compressing the vermis and fourth ventricle (34-28).

In rare cases, the ependyma may actually rupture and spill CSF into the adjacent WM ("ventricular disruption"), creating a fluid-filled cleft in the hemisphere.

Differential Diagnosis

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(34-23A) T1WI in a 22y woman with longstanding aqueductal stenosis shows enlarged lateral, third ventricles with remodeled clivus .

(34-23B) Axial FLAIR shows enlarged third , lateral ventricles with minimal hyperintense rim .

The major differential diagnosis of IVOH is extraventricular obstructive hydrocephalus (see below). Patients often have a history of aneurysmal

(34-23C) More cephalad FLAIR shows marked symmetrically enlarged ventricles, thin fluid rim. This is chronic compensated IVOH.

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(34-24) (Top) IVOH, encephalomalacic CC are caused by falx impingement. (Bottom) T1WI shows CC impingement .

(34-25) Coronal T1 C+ of longstanding IVOH shows the lateral ventricles , corpus callosum are forced upward against the falx cerebri .

subarachnoid hemorrhage or meningitis. The lateral, third, and fourth ventricles are symmetrically and proportionately enlarged.

Parenchymal volume loss causes secondary dilatation of the ventricles (ventriculomegaly) with proportional enlargement of the surface sulci and cisterns. In infants with large ventricles, measuring head size is a critical component of the total evaluation. The finding of large ventricles in a large head favors hydrocephalus; seeing large ventricles with a normal to small head is more common with congenital anomalies or volume loss (atrophy).

A helpful feature to distinguish obstructive hydrocephalus from atrophy is the appearance of the temporal horns. In obstructive hydrocephalus, they appear rounded and moderately to strikingly enlarged. If the IVOH is acute, a periventricular "halo" is often present.

Even with relatively severe volume loss, the temporal horns retain their normal kidney bean shape and are only minimally to moderately enlarged. The lateral ventricle margins remain sharply defined. Periventricular hypodensity appears patchy and is caused by chronic microvascular ischemia, not interstitial edema or transependymal CSF migration.

Normal pressure hydrocephalus is typically a disorder of older adults and is typified clinically by progressive dementia, gait disturbance, and incontinence (see below). The ventricles often appear disproportionately enlarged relative to the sulci and cisterns.

Overproduction hydrocephalus is rare, associated with choroid plexus papilloma and the even rarer villous hyperplasia. The choroid plexus glomus is enlarged and avidly enhancing.

INTRAVENTRICULAR OBSTRUCTIVE HYDROCEPHALUS

Terminology, Etiology

•Proximal to 4th ventricle outlet foramina

•Can be congenital or acquired, acute or chronic

○Postinflammation/posthemorrhage

○Obstructing intraventricular mass

Acute Obstructive Hydrocephalus

•Ventricles proximal to obstruction are ballooned

•"Blurred" margins of ventricles

•Periventricular fluid accumulation (CSF, ISF, or both)

○"Halo" ± "fingers" of fluid around ventricles

○T2 hyperintense, does not suppress on FLAIR

Chronic Compensated Obstructive Hydrocephalus

•Large ventricles, no periventricular "halo"

•± Callosal impingement, atrial diverticula

Extraventricular Obstructive Hydrocephalus

Terminology

In extraventricular obstructive hydrocephalus (EVOH), the obstruction is located outside the ventricular system.

Etiology

(34-26) CCIS with decompression, postshunt FLAIR shows hyperintensity in CC, periventricular WM , disrupted fibers on DTI .

The obstruction causing EVOH can be located at any level from the fourth ventricular outlet foramina to the arachnoid granulations. Subarachnoid hemorrhage—whether traumatic or aneurysmal—is the most frequent cause. Other common etiologies include purulent meningitis, granulomatous meningitis, and disseminated CSF metastases.

EXTRAVENTRICULAR OBSTRUCTIVE HYDROCEPHALUS

Terminology

•Formerly called "communicating" hydrocephalus

•Obstruction outside ventricular system

○Any site from 4th ventricle foramina to arachnoid granulations

Etiology

•Most common

○Subarachnoid hemorrhage (aneurysm > trauma)

•Less common

○Meningitis (bacterial, granulomatous)

○Metastases

Imaging

•> 50% show no discernible etiology

•Use CISS to look for obstructing membranes

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Pathology

Gross pathology demonstrates generalized ventricular dilatation (34-30). The basal cisterns and convexity sulci may be filled with acute or chronic exudates (34-29), meningeal fibrosis, or arachnoid adhesions.

Clinical Issues

As with IVOH, the presentation of EVOH varies with acuity and severity. The most common symptom is headache followed by signs of increased intracranial pressure such as papilledema, nausea, vomiting, and diplopia.

Imaging

CT Findings. The classic appearance of EVOH on NECT scans is that of symmetric, proportionally enlarged lateral, third, and fourth ventricles. The basal subarachnoid spaces are hyperdense in acute subarachnoid hemorrhage and may

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appear isodense and effaced in pyogenic or neoplastic meningitis. CECT scans may demonstrate enhancement in cases of EVOH secondary to infection or neoplasm.

MR Findings. The same imaging sequences used in IVOH apply to the evaluation of EVOH (34-31). If the hydrocephalus is caused by acute subarachnoid hemorrhage or meningitis, the CSF appears "dirty" on T1WI and hyperintense on FLAIR. T1 C+ scans may demonstrate sulcal-cisternal enhancement.

In contrast to IVOH, more than half the cases of EVOH have no discernible cause for the obstruction on standard MR sequences. In such cases, it is especially important to identify subtle thin membranes that may be causing the extraventricular obstruction.

The CSF cisterns, ventricles, and outlet foramina are best demonstrated by special pulse sequences such as 3D constructive interference in the steady state (3D-CISS).

With the use of high-resolution 3D-CISS, thin membranous obstruction can be demonstrated in nearly 20% of patients with unexplained hydrocephalus. Even if the membrane is not visualized directly, differences in CSF signal intensity proximal and distal to the culprit membrane are helpful in localizing the obstruction.

Differential Diagnosis

The major differential diagnosis of EVOH is IVOH. In some cases—even with special sequences such 3D-CISS—it may be difficult, if not impossible, to localize the level of the obstruction.

Overproduction Hydrocephalus

Overproduction hydrocephalus is uncommon and results from excessive CSF formation. The choroid plexus epithelium is extremely efficient, having the highest rate of ion and water transport of any epithelium in the human body.

Approximately 80% of CSF is produced by the choroid plexus, but at least 20% is generated from the brain ISF. The flow of brain ISF is not unidirectional and can contribute to both net CSF production and reabsorption.

Some investigators believe that—at least in children—CSF overproduction is an underrecognized cause of hydrocephalus (34-32). Panventricular enlargement is the most common imaging finding but is not invariably present.

Choroid plexus papillomas (CPPs) are the most common cause of overproduction hydrocephalus (see Figure 18-39). CPPs account for 2-4% of childhood neoplasms and typically occur in children younger than 5 years. Some CPPs produce enormous amounts of CSF, overwhelming the capacity of the arachnoid villi and other structures to absorb the excess fluid.

Choroid plexus carcinomas (CPCas) can also cause overproduction hydrocephalus but are only a tenth as common as CPPs. Imaging findings of both CPP and CPCa are delineated in Chapter 18.

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Diffuse villous hyperplasia of the choroid plexus (DVHCP) is a rare cause of overproduction hydrocephalus. CSF production in DVHCP can exceed 3 L per day. DVHCP is histologically normal with little to no pleomorphism or hyperchromasia. Imaging studies in DVHCP show severe hydrocephalus with massive enlargement of the entire choroid plexus. The diffusely enlarged choroid plexus enhances strongly and often contains multiple nonenhancing cysts of varying sizes. DVHCP can be difficult to distinguish from rare bilateral CPPs, which typically cause focal—not diffuse—enlargement of the choroid plexus.

Benign, nonneoplastic choroid plexus cysts have also been reported as another rare cause of overproduction and triventricular obstructive hydrocephalus in children.

Normal Pressure Hydrocephalus

There are no currently accepted evidence-based guidelines for either the diagnosis or treatment of normal pressure

(34-32A) Axial FLAIR in a 3y boy with progressively increasing head size shows enlarged third and lateral ventricles with unusually prominent choroid plexus glomi in the atria of the lateral ventricles.

hydrocephalus (NPH). In this section, we briefly review the syndrome and summarize the spectrum of imaging findings that—in conjunction with clinical history and neurologic examination—may suggest the diagnosis.

Terminology

NPH was first described by Hakim and Addams as "symptomatic occult hydrocephalus with 'normal' CSF pressure." NPH is characterized by ventriculomegaly with normal CSF pressure but altered CSF hydrodynamics.

NPH has also been called idiopathic adult hydrocephalus syndrome. Primary or idiopathic NPH (iNPH) is distinguished from secondary NPH (sNPH), in which there is a known antecedent such as subarachnoid hemorrhage, traumatic brain injury, or meningitis.

Etiology

The pathogenesis of NPH is poorly understood and remains controversial. Whether NPH reflects abnormal brain structure, disturbed blood flow, or altered CSF circulation (or a combination of all three) is unclear.

Animal studies have demonstrated that disruption of the periventricular matrix integrity could result in pressure gradients that favor progressive ventriculomegaly. Movement of CSF into the parenchyma in the form of interstitial edema may result.

Recent studies of NPH also suggest that CSF and ISF stasis with excess fluid in the interstitial spaces disrupts the balance between hydrostatic and osmotic pressures, reversing ISF flow. In turn, this could result in impaired or failed removal of

(34-32B) T1 C+ FS in the same case shows the enlarged, intensely enhancing choroid plexi . This is choroid plexus hyperplasia with overproduction hydrocephalus. Duplication of the short arm of chromosome 9 was found on genetic analysis.

neurotoxic compounds such as β-amyloid and tau from the brain parenchyma.

PET and SPECT studies all indicate widespread cortical and subcortical hypometabolism in NPH. Decreased global and regional cerebral blood flow and accelerated microvascular disease also contribute to the parenchymal degeneration that often accompanies NPH.

Other proposed etiologies invoke altered viscoelastic properties of the ventricular walls and adjacent parenchyma with a resultant "water hammer" effect of CSF pulsations. In this model, elevated arterial pulsations cannot be transmitted to the cortical veins and perivascular spaces due to reduced compliance. Intermittent high pressure "B" waves together with altered compliance of the venous system and craniospinal subarachnoid space have also been posited as potential etiologies for NPH.

Pathology

The ventricles appear grossly enlarged. The periventricular WM often appears abnormal with or without frank lacunar infarction. Neurofibrillary tangles and other microscopic changes typically found in Alzheimer disease are seen in 20% of cases.

Clinical Issues

Epidemiology and Demographics. NPH accounts for approximately 5-6% of all dementias. The reported prevalence of NPH is 0.5-3.0% in the elderly population. Although it is most common in patients older than 60 years, NPH also occasionally occurs in children following intraventricular

hemorrhage or meningitis. There is a moderate male predominance.

NPH is designated as "possible" or "probable" based on the combination of clinical findings, imaging studies, and response to high volume lumbar tap.

Presentation. The nature and severity of symptoms as well as the disease course vary in NPH. Impaired gait and balance are the typical initial symptoms. The classic triad of dementia, gait disturbance, and urinary incontinence is present in a minority of patients and typically represents advanced disease. While gait disturbances are seen in most cases, not all patients exhibit impaired cognition.

Natural History. The natural history of NPH has not been well characterized, nor is the tempo of progression uniform. Many patients experience continuing cognitive and motor decline.

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Treatment Options. Some patients initially respond dramatically to ventricular shunting ("shunt-responsive" NPH). The favorable response to shunting varies from about 35-40% in patients with clinically "possible" NPH to 65% in patients diagnosed with "probable" NPH.

Long-term outcome is more problematic. Although early gait improvement is common, only one-third of patients experience continued improvement 3 years after shunting. Cognition and urinary incontinence are even less responsive.

Imaging

General Features. Imaging studies in suspected NPH are necessary but insufficient to establish the definitive diagnosis of NPH. The goal of identifying patients who are likely to improve following ventriculoperitoneal shunting likewise remains elusive.

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The most common general imaging feature of NPH is a degree of ventriculomegaly (Evans index of at least 0.3) that appears out of proportion to sulcal enlargement ("ventriculosulcal disproportion").

CT Findings. NECT scans show enlarged lateral ventricles with rounded frontal horns. The third ventricle is moderately enlarged, whereas the fourth ventricle appears relatively normal.

The basal cisterns and sylvian fissures may be somewhat prominent, but, compared with the degree of ventriculomegaly, generalized sulcal enlargement is mild. Periventricular hypodensity is common and often represents a combination of increased interstitial fluid and WM rarefaction secondary to microvascular disease.

MR Findings. T1 scans show large lateral ventricles. The convexity and medial subarachnoid spaces may appear decreased or "tight," whereas the basal cisterns and sylvian

fissures are often enlarged. The corpus callosum is usually thinned. Most patients have a mild to moderate periventricular "halo" on T2/FLAIR (34-34). A prominent, exaggerated "hyperdynamic" aqueductal "flow void" may be present (34-33).

CSF flow studies are generally accepted as a supplementary tool for the assessment of shunt-responsive NPH. Either 2D or 3D phase-contrast studies may show hypermotile flow and markedly elevated aqueductal stroke volume. An aqueductal stroke volume greater than 42 μL has been associated with shunt responsiveness although a significant percentage of patients with lower stroke volumes also may respond to shunt surgery.

Recent studies show NPH patients have hyperdynamic CSF flow with increased velocity and volume in both systole and diastole. The inflow during diastole exceeds that of systole, so the net flow direction is caudo-cranial, the reverse of normal.

(34-35A) Axial T1WI in a patient with ventriculoperitoneal shunt and headache shows shunt catheter in a collapsed, slit-like left lateral ventricle . The right lateral ventricle is moderately enlarged; the sulci appear normal.

This hyperdynamic flow reversal may play a key role in the development of ventriculomegaly in NPH patients.

DTI is a good marker of WM pathology and shows increased FA in the posterior limb of the internal capsule. Increased diffusivity in the same tract can be seen as early as 2 weeks following shunting.

Nuclear Medicine. Prominent ventricular activity at 24 hours on In-111 DTPA cisternography is considered a relatively good indicator of NPH. 18F FDG PET shows decreased regional cerebral metabolism.

Differential Diagnosis

The major difficulty in diagnosing iNPH is distinguishing it from other neurodegenerative disorders. Up to 75% of patients with NPH have another neurodegenerative disorder, most commonly Alzheimer disease and vascular dementia. In age-related atrophy, both the ventricles and the subarachnoid spaces are proportionately enlarged.

Syndrome of Inappropriately Low-

Pressure Acute Hydrocephalus

Most patients with acute obstructive hydrocephalus have ventriculomegaly and elevated intracranial pressure (ICP). However, a small subset of patients with acute obstructive hydrocephalus have ventriculomegaly with inappropriately low ICP.

Terminology

The syndrome of inappropriately low-pressure acute hydrocephalus (SILPAH) has sometimes been called "negative-

(34-35B) The shunt was replaced. The patient had acute neurologic deterioration 10 days later. NECT shows "ballooned" ventricles, periventricular edema , small sulci. EVD showed unexpectedly low pressure, consistent with SILPAH.

pressure hydrocephalus." As opening pressures are not always "negative" (i.e., subzero), the terms SILPAH or "very lowpressure hydrocephalus" are more accurate.

Etiology

Once thought to occur only in patients with a preexisting ventriculoperitoneal shunt, SILPAH is now known to occur in other patients, too. The common factor is isolation of the ventricular system from a subarachnoid space that leaks (or is drained of) CSF, resulting in low brain turgor and decreased ICP. CSF production continues, builds up, and expands the ventricles.

Clinical Issues

SILPAH is both uncommon and—because of its enigmatic and counterintuitive nature—often unrecognized.

Patients with SILPAH present with progressive neurologic deterioration, acute progressive ventriculomegaly, and ICP that is inappropriately low when an external ventricular drain (EVD) is inserted (34-35). SILPAH affects patients of all ages; 20% are children.

Shunted patients with SILPAH typically have opening pressures less than 0 mm H O. Patients without a shunt typically have much lower than expected pressures that rapidly become even lower. In both scenarios, ICP is too low to allow CSF drainage with normal EVD protocols.

Treatment by neck wrapping with a tensor bandage and/or lowering the EVD to negative levels typically results in clinical improvement and resolution of the ventriculomegaly. Reestablishing communication between the ventricular system and the SAS may be required to correct ICP dynamics.

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OTHER HYDROCEPHALUS

Overproduction Hydrocephalus

•Rare

•Results from CSF overproduction

○Choroid plexus tumor > > hyperplasia

•Imaging shows panventricular enlargement

Normal Pressure Hydrocephalus

•Ventriculomegaly with normal CSF pressure, altered fluid dynamics

•Accounts for ≈ 5% of dementias

•Dementia, gait apraxia, incontinence (minority)

•Imaging diagnosis difficult

○Disproportionately enlarged ventricles vs. sulci

○MR may show exaggerated aqueductal "flow void," elevated stroke volume

○In-111 DTPA cisternography, intraventricular tracer at 24 hours

Syndrome of Inappropriately Low-Pressure Acute

Hydrocephalus

•Progressive neurologic deterioration

○Acute progressive obstructive hydrocephalus

○CSF opening pressure very low or negative

•Imaging

○Like acute obstructive hydrocephalus with elevated ICP

○Quadriventricular enlargement

○Small/inapparent sulci common

•Differential diagnosis

○Acute obstructive hydrocephalus with elevated ICP

○Idiopathic intracranial hypotension

○Critical postcraniotomy CSF hypovolemia

Arrested Hydrocephalus

•No/minimal overt symptoms

○No stigmata of elevated ICP

○Ventriculomegaly often incidental, unexpected MR finding

○May be stable for years

•May cause subtle neurologic deterioration

○Rare = sudden, fatal decompensation

Imaging

Imaging findings are identical to those of acute severe obstructive hydrocephalus. "Quadriventricular" enlargement, "halos" of periventricular interstitial edema, and small—sometimes almost inapparent—subarachnoid spaces are present (34-35).

Differential Diagnosis

The major differential diagnosis of SILPAH is the much more common syndrome of acute obstructive hydrocephalus with elevated ICPs. Imaging findings are identical, so the definitive diagnosis is established only when EVD discloses unexpectedly low ICP.

SILPAH must be differentiated from idiopathic intracranial hypotension, a disorder also characterized by low ICP (see below). Intracranial hypotension is characterized by downward

displacement of the central core brain structures, midbrain sagging, tonsillar descent, and dural thickening/enhancement.

Critical postcraniotomy CSF hypovolemia can cause marked cerebral hypotension with dramatic downward migration of intracranial structures. In both idiopathic intracranial hypotension and postcraniotomy CSF hypovolemia, the ventricles are usually small, not large.

Arrested Hydrocephalus

Arrested hydrocephalus (AH) has also been called asymptomatic hydrocephalus, occult hydrocephalus, compensated hydrocephalus, long-standing overt ventriculomegaly of adulthood, and late-onset idiopathic aqueductal stenosis. Moderate to severe triventricular enlargement without evidence for periventricular fluid accumulation is present on imaging studies and may remain stable for years (34-36) (34-37).

Patients with AH rarely present with symptoms or stigmata of elevated ICP, and the diagnosis of hydrocephalus is often incidental and unexpected. As many patients with AH have no overt symptoms or evidence of neurologic deterioration, some clinicians have advocated a conservative approach with serial imaging and "watchful waiting."

Whether AH patients benefit from ventriculo-peritoneal shunt or third ventriculostomy remains unresolved with limited data to guide clinical decision making. Improvement in headaches and neuropsychiatric outcomes has been reported in some cases following CSF diversion. Despite clinical and imaging stability, some investigators believe longstanding ventriculomegaly is not benign and may be associated with cognitive decline and even sudden, fatal decompensation.

Idiopathic Intracranial Hypertension

Terminology

Idiopathic intracranial hypertension (IIH), also known as benign intracranial hypertension, is preferred to the term pseudotumor cerebri. IIH is characterized by unexplained elevation of ICP not related to an intracranial mass lesion, a meningeal process, or cerebral venous thrombosis. Patients with elevated ICP secondary to certain medications or transverse dural venous sinus stenosis are nonetheless still classified as having IIH.

Etiology and Pathology

The precise etiology of IIH is unknown. An obese phenotype with elevated body mass index is common. Disturbed CSF-ISF drainage through the "glymphatic pathway" has been invoked by some investigators.

It is unclear whether the dural venous sinus stenosis found in the vast majority of IIH patients is a cause (from venous outflow obstruction) or an effect (from extrinsic compression) of elevated ICP, or both.

(34-36) Longstanding "compensated" IVOH from aqueductal stenosis shows symmetrically enlarged lateral ventricles and dilated foramen of Monro . WM is severely reduced in volume, but cortex appears normal. (Courtesy R. Hewlett, MD.)

Clinical Issues

Epidemiology and Demographics. IIH is rare, but the incidence is rising with the worldwide obesity epidemic in industrialized nations. A recent epidemiology study reported an incidence of 23/100,000/year when stratified for reproductive age, female sex, and weight.

Presentation. Classically, IIH presents in overweight women who are 20-45 years of age although recent studies have confirmed a rising incidence in obese children, especially girls.

Headache is the most constant symptom (90-95%) followed by tinnitus and visual disturbances. Papilledema is the most common sign on neurologic examination. Cranial nerve deficits, usually limited to CN VI, are common.

Comorbidities are common and include—among others—polycystic ovarian syndrome, metabolic syndrome, obstructive sleep apnea, and hypervitaminosis A.

The definitive diagnosis of IIH is established by lumbar puncture, which demonstrates elevated ICP (>200 mm H O in adults or 280 mm H O in children) with normal CSF composition.

Natural History. Visual loss is the major morbidity in IIH. In fulminant IIH, visual loss can progress rapidly and become irreversible (34-38).

Treatment Options. A 2015 Cochrane review concluded that there is no current consensus on the best management strategy for IIH. The two key approaches are to preserve visual function and reduce long-term headache disability.

(34-37) NECT in a cognitively normal 73y man with headaches, normal neurological examination shows markedly enlarged lateral ventricles without periventricular fluid accumulation. Serial examinations showed no change; arrested hydrocephalus.

Serial CSF removal (10-20 mL at initial LP) often temporarily ameliorates IIH-associated headache. Weight reduction and pharmacologic intervention can be effective in some patients. Acetazolamide is the mainstay of medical therapy for IIH, lowering ICP by its effects on choroid plexus CSF secretion.

Most recently, venous sinus stenting in patients who have transverse sinus stenosis has been successful in improving symptoms and reducing papilledema. Occasionally, spontaneous resolution of the stenosis occurs in obese patients with nonsurgical weight loss or following bariatric surgery.

Imaging

Neuroimaging is used to (1) exclude identifiable causes of increased ICP (e.g., neoplasm or obstructive hydrocephalus) and (2) detect findings associated with IIH.

The most significant imaging findings of IIH include flattening of the posterior globes, distension of the perioptic subarachnoid space with or without a tortuous optic nerve, intraocular optic nerve protrusion, partial empty sella, and transverse venous sinus stenosis (34-39) (34-41). The presence of one or a combination of these signs—especially transverse sinus stenosis—significantly increases the odds of IIH, but their absence does not rule out IIH. Prepubescent children have significantly lower frequencies of these findings compared with adults and adolescents!

MR Findings. Sagittal scans show a partial empty sella. Here the pituitary gland occupies less than 50% of the pituitary fossa, and its superior surface appears concave. Dilated optic nerve sheaths are common, often appearing kinked and tortuous. The posterior globe is flattened or concave, and

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(34-38) Funduscopic image shows findings of severe papilledema with elevated, blurred optic disc (From K. Digre, MD, in Imaging in Neurology.)

(34-39A) Sagittal T1WI in a 33y obese woman shows excessive subcutaneous fat and a partial empty sella .

intraocular protrusion of the optic nerve may be visible (34-40). In some patients, the optic nerve head enhances on T1 C+ FS sequences.

The prevalence of other reported findings such as slit-like or "pinched" ventricles (10%), "tight" subarachnoid spaces (small sulci and cisterns), and inferiorly displaced tonsils may be present. Cerebellar tonsillar ectopia may be present and sometimes even "peg-like" in configuration, mimicking Chiari I malformation.

Meningoceles or cephaloceles protruding through osseous defects in the skull base are common, especially in extremely obese patients. CSF leaks are common (34-43), and CT or gadolinium MR cisternography may identify which of several bony defects is leaking.

CTV/MRV. The single most sensitive finding in IIH is a thinned, narrowed transverse-sigmoid sinus. Contrast-enhanced MRV has a pooled sensitivity of 97% in discriminating IIH patients and is helpful in differentiating a hypoplastic sinus segment from thrombosis (which is rare in IIH).

CT Findings. Solitary or multiple skull base osseous-dural defects are common. These appear as thinned, deossified, and/or dehisced bone with "sagging" of meninges through the bony defect. "Pits" around the margins of the temporal bones are also frequent.

Differential Diagnosis

The most important differential diagnosis in patients with suspected IIH is secondary intracranial hypertension (i.e., increased ICP with an identifiable cause). Although dilated optic nerve sheaths ("hydrops") and flattened posterior globes indicate elevated ICP, they can be seen in both secondary and idiopathic intracranial hypertension. Ventriculomegaly is more common in secondary intracranial hypertension, whereas the ventricles are usually normal to small in IIH.

Dural sinus thrombosis is another important consideration. T2* (GRE, SWI) shows "blooming" thrombus in the affected sinuses. MRV and CTV demonstrate a cigar-shaped, long-segment clot.

IDIOPATHIC INTRACRANIAL HYPERTENSION

Terminology and Etiology

•Also known as benign intracranial hypertension

•Often no cause identified (idiopathic)

Clinical Issues

•F > M; obesity = definite risk

•Peak 20-45 years

•Headache, tinnitus, vision loss, LP > 200 mm H O

Imaging

•Dilated optic nerve sheaths

•Intraoptic disc protrusion

•Flat posterior globe

•Partial empty sella

•"Tight" brain, ± tonsillar herniation

•± Dural sinus thrombosis, stenosis

CSF Shunts and Complications

(34-39B) T2WI shows flattening of the globes, intraocular protrusion of optic nerve heads . At LP the OP was 440 mm H O. This was IIH.

Although endoscopic third ventriculostomy is gaining acceptance, the standard treatment for all types of obstructive hydrocephalus remains placement of a shunt for CSF diversion.

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Placement of ventricular shunts are one of the most common of all neurosurgical procedures. Multiple surgeries are the rule, not the exception; approximately 50% of ventricular shunts in children fail in the first 2 years, and the vast majority have failed by 10 years after insertion.

The costs and lifelong morbidity associated with shunt placement to treat both childhood and adult hydrocephalus are substantial. More than half of all pediatric and adult patients require shunt revision. Almost 55% of children have four or more shunt revisions, and nearly 10% experience three or more shunt infections. Direct treatment-related costs for patients of all ages with hydrocephalus exceed $1 billion annually in the USA alone.

Imaging is a key component in evaluating patients with CSF diversions. Shunt failure can result in either enlarging or collapsing ventricles. The most common imaging manifestation of shunt failure is enlarging ventricles. CT is generally the preferred technique to assess patients with intracranial shunt catheters. Alternative modalities include transfontanelle ultrasound and new rapid MR techniques such as fast steady-state gradient-recalled-echo (SS-GRE) sequences.

We now briefly examine some of the most common causes of shunt malfunction.

Mechanical Failure

Mechanical failures represent nearly 75% of shunt malfunctions. Disconnection or fracture account for another 15% or so.

Most ventriculoperitoneal shunts have several components. The commonly used systems consist of three pieces: (1) a ventricular catheter connected to (2) an inline valve and (3) a distal peritoneal catheter. Shunt discontinuity can occur at any site, but disconnection is most common at the junctions of the various components.

The most common imaging modality to assess mechanical failure is a shunt series. Although some evidence suggests only a small number (less than 1%) of shunt series help in surgical decision making, shunt series are still frequently requested studies.

Standard shunt series are composed of skull (two views), neck, chest, and abdomen/pelvis radiographs to track shunt trajectory and integrity. Accurate diagnosis of shunt fractures and disconnections is complicated by three factors: (1) the wide variety of systems used, (2) the accumulation of residual "abandoned" catheter fragments in patients who have undergone multiple shunt revisions, and (3) nonradiopaque shunt segments. Careful comparison of current and prior studies is essential to determine whether the "active" shunt system is intact.

settings. (In up to 50% of cases, the opening pressure of an implanted valve has to be changed, sometimes months or even years later.)

Imaging is often performed to assess valve settings. Interested readers are referred to the comprehensive guide to valves and their radiographic appearances by Lollis et al. 2010.

Slit Ventricle Syndrome

Some shunted hydrocephalus patients exhibit clinical signs of shunt failure without evidence of ventricular enlargement, a condition called slit ventricle syndrome (SVS) (34-42).

The etiology of SVS is controversial. Some patients have scarred ventricular walls with decreased compliance and reduced tolerance for the normal fluctuations in intracranial pressure. Others may have low pressure with collapsed ventricles secondary to overdrainage or CSF leak. Intermittent or partial shunt obstruction may be a contributory factor.

Comparison to prior imaging studies is essential. NECT scans show that one or both lateral ventricles are small or slit-like. Functional studies show that the shunt may fill slowly but still functions, although flow is often reduced.

Miscellaneous Complications

Decompression of longstanding hydrocephalus and CSF overdrainage both increase the risk of subdural hematoma. Intraventricular scarring and adhesions can block CSF flow from one compartment to another, causing an encysted "trapped" (isolated) ventricle. Continued CSF production can result in massive enlargement of the affected ventricle. Infection is a relatively uncommon complication but can result in meningitis, ventriculitis, and pyocephalus.

Abdominal complications from ventriculoperitoneal shunts include loculated CSF collections ("pseudocysts"), ascites, and bowel perforations. Distal shunt obstruction can cause shunt failure and hydrocephalus.

COMMON CSF SHUNT COMPLICATIONS

Mechanical

• Shunt discontinuity, fracture

Programmable Valve

• Valve setting too high, low

Miscellaneous

•Overdrainage (slit ventricle syndrome)

•Ventricles trapped, encysted