Hydrocephalus and CSF Disorders

Main Text

Preamble

The brain CSF spaces include the ventricular system—a series of interconnected, CSF-filled cavities—and the subarachnoid space. This chapter begins with a brief discussion of the normal ventricles and CSF spaces.

We describe normal variants, which should not be mistaken for disease, then turn our attention to hydrocephalus and the manifestations of elevated CSF pressure, including idiopathic intracranial hypertension (IIH) (“pseudotumor cerebri”). We close the chapter with a discussion of CSF leaks and intracranial hypotension.

Normal Anatomy of Ventricles and Cisterns

Ventricular System

The ventricular system is composed of four interconnected, ependyma-lined cavities that lie deep within the brain (39-1) (39-3). Each lateral ventricle is C-shaped with a body, atrium, and three projections or “horns”: The frontal horns, temporal horns, and occipital horns. The paired lateral ventricles communicate with the third ventricle via the Y-shaped interventricular foramen known as the foramen of Monro. The third ventricle is a single, midline cavity that lies between the thalami. The anterior commissure lies along the anterior border of the third ventricle. The floor of the third ventricle is formed by the optic chiasm, hypothalamus, mammillary bodies, and roof of the midbrain tegmentum. The third ventricle communicates with the fourth ventricle via the cerebral aqueduct (of Sylvius). In turn, the fourth ventricle communicates with the subarachnoid space (SAS).

Choroid Plexus, CSF, and Brain Interstitial Fluid

The CSF space is a dynamic pressure system with a hydrostatic balance between CSF production and absorption. The choroid plexus (CP) has two major functions: CSF production and maintenance of the blood-CSF barrier. In adult humans, the CP epithelium forms CSF at a rate of ~ 0.4 mL per minute or ~ 500-600 mL every 24 hours. CSF is turned over about four times a day, allowing for the removal of waste products.

Brain interstitial fluid (ISF) in the extracellular spaces is also a significant extrachoroidal source of CSF.

The CP maintains the blood-CSF barrier via tight junctions between epithelial cells. The exchange of substances between the brain ISF and the CSF across the blood-CSF barrier is highly regulated. Specialized subpopulations of CP epithelial cells are responsible for the transfer of plasma proteins from blood to the CSF.

CSF Circulation and Homeostasis

Traditional Model of CSF Homeostasis

The longstanding, classic model of CSF homeostasis was based on the circulation theory, in which the majority of CSF is produced by the CP, then circulates from the ventricles into the SASs (39-2) (39-4). In this model, CSF flow in the ventricular cavities is unidirectional and rostrocaudal. CSF exits the fourth ventricle into the SASs through the medial foramen (of Magendie) and the two lateral foramina (of Luschka), which form the only natural communications between the ventricles and the SAS (39-4).

Updated Model of CSF and ISF Homeostasis

New evidence suggests that the traditional model of CSF production, circulation, and function is too simplistic and much more complex than previously thought. It is now recognized that CSF plays an essential role in the maintenance of brain ISF homeostasis and that the two are intimately interrelated in maintaining normal brain function. The main sources of ISF are the blood and CSF.

In the updated CSF-ISF model, the brain perivascular spaces (PVSs) (Virchow-Robin) and paravascular spaces play a critical role in CSF homeostasis. The PVSs form a key component of the brain’s “protolymphatic” or “glymphatic” system. The PVSs are lined with leptomeningeal (pial) cells that coat the PVSs as well as arteries and veins in the SAS, thus separating CSF in the SAS from the brain parenchyma and PVSs.

ISF diffuses through the brain’s extracellular spaces and then drains via bulk flow along the basement membranes of cerebral capillaries. ISF circulation likely occurs through the water-selective aquaporin (AQP) channels of the glymphatic system, key factors in regulating extracellular space water homeostasis. AQP4 is highly expressed in astrocytic end-feet and also appears to be crucial for fluid exchange between the CSF and ISF.

Finally, in this model of CSF and ISF homeostasis, drainage of extracellular fluids in the CNS and integrity of the brain glymphatic system is important for not only volume regulation, but also clearance of waste products, such as amyloid-β (Aβ), from the brain parenchyma.

Subarachnoid Spaces/Cisterns

The subarachnoid spaces (SASs) lie between the pia and arachnoid (39-2). The sulci are small, thin SASs that are interposed between the gyral folds. Focal expansions of the SASs form the brain CSF cisterns. Numerous pial-covered septa cross the SASs from the brain to the arachnoid, which is loosely attached to the inner layer of the dura. All SASs normally communicate freely with each other and the ventricular system.

Normal Variants

Age-Related Changes

Increase in lateral ventricular volume is a constant, linear function of age throughout life.

Asymmetric Lateral Ventricles

Asymmetric lateral ventricles can be identified on imaging studies in ~ 5-10% of normal patients. The asymmetry is typically mild to moderate. Bowing, deviation, or displacement of the septi pellucidi across the midline is common; by itself, it neither indicates pathology nor implicates an etiology for nonspecific headache.

Cavum Septi Pellucidi and Vergae

A cavum septi pellucidi (CSP) is a fluid-filled cavity that lies between the frontal horns of the lateral ventricles (39-5). A cavum vergae (CV) is an elongated, finger-like posterior extension from the CSP that lies between the fornices (39-6).

A CSP may occur in isolation, but a CV occurs only in combination with a CSP. When the two occur together, the correct Latin terminology is “cavum septi pellucidi et vergae.” In common usage, the combination is often referred to simply as a CSP.

The appearance of CSPs and CVs on CT and MR varies from an almost inapparent, slit-like cavity to a prominent collection measuring several millimeters in diameter. A CSP is isodense with CSF on NECT and follows CSF signal intensity exactly on MR. It suppresses completely on FLAIR.

Cavum Velum Interpositum

The velum interpositum (VI) is a thin, translucent membrane formed by two infolded layers of pia-arachnoid. The VI extends laterally over the thalami to become continuous with the CP of the lateral ventricles. Together with the fornices, the VI forms the roof of the third ventricle.

The VI is often CSF filled and open posteriorly, communicating directly with the quadrigeminal cistern. In such cases, it is called a cavum velum interpositum (CVI) (39-7). A CVI is considered a normal anatomic variant.

On imaging studies, a CVI appears as a triangular CSF space that curves over the thalami between the lateral ventricles. Its apex points toward the foramen of Monro (39-8). A CVI is isodense with CSF on NECT and isointense on all MR sequences. It suppresses completely on FLAIR, does not enhance, and does not restrict on DWI.

Enlarged Subarachnoid Spaces

Enlarged SASs occur in three conditions: Extraventricular obstructive (“communicating”) hydrocephalus (EVOH), brain atrophy, and benign enlargement of the SASs (BESS) (39-9). Obstructive hydrocephalus (both the intra- and extraventricular types) is discussed in the hydrocephalus section below. Brain volume loss is a manifestation of both normal aging and brain degeneration. In this section, we discuss benign physiologic enlargement of the SAS.

Terminology

Idiopathic enlargement of the SASs with normal to slightly increased ventricular size is common in infants. Large CSF spaces in developmentally and neurologically normal children with or without macrocephaly is termed BESS.

Etiology and Pathology

The etiology of BESS in infants is unknown but probably related to immature CSF drainage pathways. Approximately 80% of infants with BESS have a family history of macrocephaly.

Grossly, the SASs appear deep and unusually prominent but otherwise normal. There are no subdural membranes present that would suggest chronic subdural hematomas or effusions.

Clinical Issues

BESS typically presents between 3-8 months. There is a 4:1 M:F predominance. There are no findings indicative of elevated ICP or nonaccidental trauma.

BESS is reported on 2-65% of imaging studies for macrocrania in children under one year of age. BESS is a self-limited phenomenon that typically resolves by 12-24 months without intervention. The associated macrocephaly may resolve by two years, but it often levels off, remaining at the 98th percentile.

Imaging

Bifrontal and anterior interhemispheric SASs are > 5 mm in diameter (39-10). The suprasellar/chiasmatic cistern and sylvian fissures are also prominent. The lateral and third ventricles can be mildly enlarged. CECT scans demonstrate bridging veins traversing the SAS (39-10). There is no evidence of thickened enhancing membranes to suggest subdural hematoma or hygroma.

Fluid in the enlarged frontal SASs exactly parallels CSF on all MR sequences because it is CSF (39-11). The fluid suppresses completely on FLAIR. Bridging veins can be seen crossing the prominent SASs on T1 C+.

Differential Diagnosis

The major differential diagnoses of BESS are atrophy, EVOH, and nonaccidental trauma. Occasionally, infants with BESS have minor superimposed hemorrhagic subdural collections similar to those sometimes observed with arachnoid cysts. In such infants, abusive head trauma must be a consideration until careful screening discloses no substantiating evidence of inflicted injury.

Hydrocephalus

Preamble

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 &/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.

Terminology

We follow the common approach of subclassifying hydrocephalus by the presumed site of CSF obstruction, i.e., inside [intraventricular obstructive hydrocephalus (IVOH)] or outside the ventricles [extraventricular obstructive hydrocephalus (EVOH)]. 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.

In the pediatric age group, a majority of cases are caused by congenital defects of the CSF pathway or as a complication of hemorrhage, infection, or neoplasms. Adult-onset hydrocephalus is usually secondary to different pathologies that encompass a heterogeneous group of disorders, such as intracranial hemorrhage (most commonly aneurysmal subarachnoid hemorrhage; ~ 45%) and neoplasms (~ 30%). Normal pressure hydrocephalus (NPH) accounts for ~ 11% 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 like aqueductal stenosis.

When the ventricles become obstructed, CSF outflow is impeded. As CSF production continues, the ventricles expand. 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 ISF via the deep medullary veins and PVSs. 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.

Causes of obstructive hydrocephalus include developmental/genetic lesions, intracranial hemorrhage, infection, trauma, neoplasms, and cysts. The most common cause of acquired IVOH is intraventricular inflammatory or posthemorrhagic membranous obstruction. The next most common cause of acquired IVOH is intraventricular masses. Colloid cyst is the most common mass found at the foramen of Monro (39-12). These benign masses may result in aIVOH. Other masses in this location include subependymal giant cell astrocytomas in tuberous sclerosis patients. Other common neoplasms to cause obstruction at the cerebral aqueduct include tectal plate gliomas and pineal region neoplasms, such as pineal parenchymal tumors or germ cell tumors (39-13). Masses that may cause obstruction at the fourth ventricle level include medulloblastoma and ependymoma in children and metastases and hemangioblastomas in adults (39-14). Inflammatory cysts, including neurocysticercosis, may occur throughout the ventricular system at any age (39-15).

Pathology

Grossly, the ventricles proximal to the obstruction appear ballooned (39-16). The ependyma is thinned and may be focally disrupted or even absent. The corpus callosum (CC) is thinned and compressed superiorly against the falx cerebri. The ependymal lining is discontinuous or inapparent, the periventricular extracellular space is increased, and the surrounding white matter (WM) is rarefied and stains pale.

Clinical Issues

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 CNVI palsy are also common with aIVOH.

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).

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

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 ISF or transependymal CSF migration—causes a “halo” of low density in the adjacent WM. 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 (39-18A), 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 (39-17). Fluid in the periventricular “halo” does not suppress on FLAIR (39-17B). In longstanding chronic “compensated” hydrocephalus, the ventricles appear enlarged and the WM attenuated but without a thick periventricular “halo”(39-18).

Complications of Hydrocephalus

In severe cases of IVOH, the CC becomes compressed against the free inferior margin of the falx (39-19) (39-20). This can cause pressure necrosis and loss of callosal axons, the so-called CC impingement syndrome (CCIS) (39-26B). 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 (39-21).

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. 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.

Differential Diagnosis

The major differential diagnosis of IVOH is EVOH. Parenchymal volume loss causes secondary dilatation of the ventricles (ventriculomegaly) with proportional enlargement of the surface sulci and cisterns.

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. Even with relatively severe volume loss, the temporal horns retain their normal kidney bean shape and are only minimally to moderately enlarged.

NPH is typically a disorder of older adults and is typified clinically by progressive dementia, gait disturbance, and incontinence. The ventricles often appear disproportionately enlarged relative to the sulci and cisterns.

Extraventricular Obstructive Hydrocephalus

Terminology

In EVOH, the obstruction is located outside the ventricular system.

Etiology

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

As with IVOH, the presentation of EVOH varies with acuity and severity. Headache is the most common symptom followed by signs of increased ICP, including nausea, vomiting, papilledema, and diplopia.

Imaging

The classic appearance of EVOH on NECT scans is that of symmetric, proportionally enlarged lateral, third, and fourth ventricles. CECT scans may demonstrate enhancement in cases of EVOH secondary to infection or neoplasm.

The same imaging sequences used in IVOH apply to the evaluation of EVOH (39-22). 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 (39-23).

In contrast to IVOH, > 1/2 of EVOH cases 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.

Overproduction Hydrocephalus

Overproduction hydrocephalus is uncommon and results from excessive CSF formation. CP papillomas (CPPs) are the most common cause of overproduction hydrocephalus (see Fig. 22-8). CP carcinomas may also result in overproduction hydrocephalus. Diffuse villous hyperplasia of the CP (DVHCP) is a rare cause of overproduction hydrocephalus. Imaging studies in DVHCP show severe hydrocephalus with massive enlargement of the entire CP.

Normal Pressure Hydrocephalus

There are no currently accepted evidence-based guidelines for either the diagnosis or treatment of normal pressure 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 is characterized by ventriculomegaly with normal CSF pressure but altered CSF hydrodynamics. 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. Recent studies of NPH suggest altered CSF dynamics (production, kinetics, reabsorption), and ISF stasis disrupts the balance between hydrostatic and osmotic pressures, reversing ISF flow and causing ventricular enlargement. Newer studies suggest involvement of the glymphatic system in 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

NPH accounts for ~ 5-6% of all dementias. The nature and severity of symptoms and the disease course vary. 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.

Some patients initially respond dramatically to ventricular shunting (“shunt-responsive” NPH). The favorable response to shunting varies from ~ 35-40% in patients with clinically “possible” NPH to 65% in patients diagnosed with “probable” NPH. The responsiveness is often improvement in gait and incontinence. The predictors of positive response to shunting remain elusive.

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.

The most common general imaging feature of NPH is ventriculomegaly that appears out of proportion to sulcal enlargement (“ventriculosulcal disproportion”) (39-24).

Additional helpful imaging findings include a prominent cerebral aqueduct flow void (39-24A)(39-25) and enlarged lateral and third ventricles with the fourth ventricle relatively normal. Disproportionately enlarged subarachnoid space hydrocephalus (DESH), particularly involving the sylvian fissures and basal cisterns, as well as a high tight convexity with effacement of sulci at the brain vertex have also been described as imaging features suggestive of NPH.

CT Findings

NECT scans show enlarged lateral ventricles with rounded frontal horns (39-28). The third ventricle is moderately enlarged, whereas the fourth ventricle appears relatively normal. Compared with the degree of ventriculomegaly, generalized sulcal enlargement is mild. Periventricular hypodensity is common and often represents a combination of increased ISF and WM rarefaction secondary to microvascular disease.

MR Findings

T1 scans show large lateral ventricles. The convexity and medial SASs may appear decreased or “tight,” whereas the basal cisterns and sylvian fissures are often enlarged. The CC is usually thinned. Most patients have a mild to moderate periventricular “halo” on T2/FLAIR (39-26A).

A prominent, exaggerated “hyperdynamic” aqueductal “flow void” may be present (39-24A)(39-25). Recent studies show some NPH patients have hyperdynamic CSF flow with increased velocity and volume in both systole and diastole. Net flow direction is caudocranial, the reverse of normal.

Two useful measurements to help evaluate for the presence of NPH on CT or MR include the Evans index, which is a ratio of the widest diameter of the frontal horns:the widest diameter of the transverse internal skull measured on the same axial slice with a value of ≥ 0.3 considered abnormal (39-27). The callosal angle is the angle between the lateral ventricles on coronal imaging at the level of the posterior commissure with a value of < 90° considered abnormal (39-28). Together, the Evans index and the callosal angle have shown a high accuracy rate for identifying those patients with NPH.

Nuclear Medicine

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

Spine Intervention

Prior to CSF diversion surgery, patients will often undergo a large-volume lumbar puncture (LP) to assess the patient’s response to CSF removal. Typically, during the LP, ~ 30-50 mL of CSF are removed. The patient’s gait and cognitive function are evaluated before the LP and again at 3-4 hours after the LP. This “tap procedure” has a high positive predictive value (73-100%) for those patients who might benefit from CSF shunt placement.

Differential Diagnosis

A major difficulty in diagnosing NPH is distinguishing it from other neurodegenerative disorders, such as Alzheimer disease, vascular dementia, and age-related atrophy. Recent studies suggest CSF biomarkers, such as AQP4, T-tau, and Aβ40, may be helpful in separating NPH from cognitive and movement disorder mimics.

Arrested Hydrocephalus

Arrested hydrocephalus (AH) has also been called compensated hydrocephalus, ventriculomegaly of adulthood, and late-onset idiopathic aqueductal stenosis. Patients rarely present with symptoms or stigmata of elevated ICP, and the diagnosis of hydrocephalus is often incidental and unexpected. Moderate to severe triventricular enlargement without evidence for periventricular fluid accumulation is present on imaging studies and may remain stable for years.

Idiopathic Intracranial Hypertension

Terminology

Idiopathic intracranial hypertension (IIH) 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 a known cause of elevated ICP (e.g., dural venous sinus stenosis) are nonetheless often 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. The venous sinus stenosis is typically at the junction of the transverse and sigmoid sinuses and may be seen on CTV, MRV, or conventional angiography (39-32)(39-33).

Clinical Issues

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 (39-29). Cranial nerve deficits, usually limited to CNVI, are common. Patients may also present with transient vision loss, diplopia, or vertigo.

The definitive diagnosis of IIH is established by LP, which demonstrates elevated ICP (> 20 cm H₂O in adults or 28 cm H₂O in children) with normal CSF composition.

Visual loss is the major morbidity in IIH. In fulminant IIH, visual loss can progress rapidly and become irreversible (39-29). Serial CSF removal (10-20 mL at initial LP) often temporarily ameliorates IIH-associated headache. Venous sinus stenting in patients who have transverse sinus stenosis has been successful in improving symptoms and reducing papilledema in some cases, though this remains controversial (39-32). Other treatments include weight loss, carbonic anhydrase inhibitors, and therapeutic LP. Surgical treatment, including lumboperitoneal shunt and optic nerve sheath fenestration, is reserved for patients who continue to experience vision loss despite conservative management and those initially presenting with rapid vision loss.

Imaging

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

The most significant imaging findings of IIH include flattening of the posterior globes, distention of the perioptic SAS with or without a tortuous optic nerve, intraocular optic nerve protrusion, partial empty sella, and transverse venous sinus stenosis on CTV/MRV or DSA (39-30)(39-32)(39-34)(39-33)(39-31). 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.

The prevalence of other reported findings, such as slit-like or “pinched” ventricles (10%), “tight” SASs (small sulci and cisterns), and inferiorly displaced tonsils, may be present. Cerebellar tonsillar ectopia (39-31)may be present and sometimes even peg-like in configuration, mimicking a Chiari 1 malformation or intracranial hypo tension. It is key to make the correct diagnosis and not confuse these other entities with IIH, as the treatments are vastly different.

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

Differential Diagnosis

The most important differential diagnosis in patients with suspected IIH is secondary intracranial hypertension (i.e., increased ICP with an identifiable cause). Ventriculomegaly is more common in secondary intracranial hypertension, whereas the ventricles are usually normal to small in IIH.

Dural sinus thrombosis is a major consideration. T2* (GRE, SWI) shows “blooming” thrombus in the affected sinuses. Dural venous sinus thrombosis most commonly involves the transverse sinuses. Noncontrast CT usually shows a hyperdense sinus, > 70 HU. A filling defect is typically seen on CTV or MRV. Only ~ 50% of venous thrombosis patients will have an associated venous infarct.

Other differential considerations include secondary intracranial hypertension from vascular etiologies, including bilateral jugular vein thrombosis, superior vena cava syndrome, or arteriovenous fistula. Medications, including tetracycline, minocycline, vitamin A, lithium, retinoids, and anabolic steroids, have been associated with intracranial hypertension. Elevated ICP may also result from endocrine disorders, including Addison disease and hypoparathyroidism.

IDIOPATHIC INTRACRANIAL HYPERTENSION

Terminology and Etiology

• a.k.a. benign intracranial hypertension

• Often no cause identified (idiopathic)

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