Colloid Cyst

The classic cause of obstruction at the foramina of Monro is the colloid cyst (see Fig. 3-69). This is typically located in the anterior region of the third ventricle. On CT, the lesion is high in density before enhancement. MR often shows a lesion that is high intensity on T1-weighted (T1WI) and T2-weighted images (T2WI). The signal of colloid cysts is variable, depending on the protein concentration, hemorrhage, and other paramagnetic ion effects.

Congenital Aqueductal Stenosis

The cerebral aqueduct is one of the narrowest channels through which the CSF in the ventricles must flow. Congenital aqueductal stenosis is just one of the obstructors of the aqueduct. This is most commonly an X-linked recessive disorder seen in early childhood, although it can present at any age. Children typically have enlarging head circumferences and dilation of the lateral and third ventricles, but with a normal-appearing fourth ventricle. Gliosis of the aqueduct or forking, where the duct divides into blind-ending sacs, may cause the stenosis. Aqueductal webs, septa, or diaphragms may also obstruct the exit from the third ventricle. Aqueductal stenosis may be seen in neurofibromatosis type 1 (NF-1) and as a complication of inflammatory and neoplastic disorders.

CRASH syndrome refers to corpus callosum hypoplasia, retardation, adducted thumbs, spastic paraparesis, and hydrocephalus secondary to aqueductal stenosis. This is an X-linked disorder.

Sagittal MR is essential for distinguishing extrinsic mass compression from an intrinsic aqueductal abnormality (Fig. 8-4). Aqueductal stenosis may also be diagnosed on CSF flow imaging. The normal aqueductal signal in this case should be bright, signifying CSF flow. With aqueductal obstruction, a dark signal is seen on the flow scan (Fig. 8-5). Phase-contrast MR with a velocity encoding set to 10 to 15 mL/sec may be the best way to assess aqueductal patency. Interstitial edema, bright on a FLAIR scan, will also be present in the periventricular zone.

Figure 8-4 Aqueductal web. This high resolution sagittal constructive interference steady-state T2-weighted image shows a fine web (arrow) crossing the inferior aqueduct. The ventricles are not enlarged due to lateral ventriculostomy catheter (not shown).

Figure 8-5 Aqueductal gradient echo study. A, Axial gradient-echo scan with gradient moment nulling applied shows normal high signal intensity within the aqueduct (arrow), representing patent flow. A potential pitfall in this diagnosis would be a high signal intensity thrombus simulating flow. B, Aqueductal obstruction caused by an occult cerebrovascular malformation shows absence of the high signal intensity of flow within the aqueduct.

Clots or Synechiae

Other intrinsic causes of aqueductal obstruction include clots or synechiae resulting from trauma or chronic infection. Patients with a large amount of subarachnoid hemorrhage may demonstrate obstruction caused by clot formation in the aqueduct or in the foramina of the fourth ventricle. Synechiae may be due to fibrous adhesions of the third ventricle after ventriculitis or meningitis. A rare infectious cause of aqueductal obstruction is cysticercosis.

Masses or Tumors

Masses of the pineal gland are the most common causes of extrinsic obstruction of the aqueduct. These generally compress the aqueduct from posteriorly and cause dilatation of the lateral and third ventricular system. Tectal gliomas also obstruct the aqueduct early in their course (Fig. 8-6). Occult cerebrovascular malformations may occur there as well. The base of the aqueduct may be obstructed by tumors of the posterior fossa such as medulloblastomas (PNETs) or ependymomas.

Figure 8-6 Tectal glioma. A mass in the quadrigeminal plate of the tectum (arrows) occludes the aqueduct and causes hydrocephalus, seen as bowing of the corpus callosum and dilatation of the third ventricle’s anterior recesses.

Any of the pediatric and adult posterior fossa tumors may obstruct the fourth ventricle or lower portion of the aqueduct. Ependymomas are one of the classic intraventricular tumors to infiltrate the foramina of Magendie and Luschka and may cause hydrocephalus from outflow obstruction. Cerebellar astrocytomas, medulloblastomas, or hemangioblastomas may compress the fourth ventricle extrinsically.

Intraventricular tumors besides choroid plexus papillomas and ependymomas may obstruct parts of the ventricular system. The masses that can arise intraventricularly include meningiomas, neurocytomas, astrocytomas, choroid plexus papillomas, oligodendrogliomas, subependymal giant cell tumors, arachnoid ependymal cysts, craniopharyngiomas, and dermoids.

Hematomas and Infarcts

A well-placed hematoma can compress the ventricles and lead to occlusion at the foramen of Monro, aqueduct, or fourth ventricle. We have seen this in the settings of trauma, acute subdural or epidural posterior fossa hematomas, and with hypertensive bleeds. Posterior fossa strokes are notorious for bringing about the downfall of the patient by eliciting acute hydrocephalus as the fourth ventricle is compressed and obliterated by mass effect.

Trapped Ventricles

So-called trapping of the temporal horn may occur when the egress of CSF from this anteroinferior extension of the lateral ventricle is obstructed. This usually occurs because of a blockage at the atrium, either from intrinsic or extrinsic masses.

Selective enlargement of the third ventricle can be confusing. This must be distinguished from the presence of an intraventricular ependymal/arachnoid cyst and third ventricle squamopapillary craniopharyngiomas. Trapping of the third ventricle is uncommon.

Isolation of the fourth ventricle may occur when the aqueduct of Sylvius and the foramina of Magendie and Luschka are occluded. The fourth ventricle becomes “trapped” and will expand as CSF production by the choroid plexus continues unabated (Fig. 8-7). This expansion may compress the cerebellum and brain stem and lead to posterior fossa symptoms.

Figure 8-7 Trapped fourth ventricle. A, The fourth ventricle is markedly enlarged with effacement of cerebellar sulci. Note, however, how small the temporal horns and frontal horns are. B, A more superior cut shows the ballooning upward of the fourth. Did you catch the edge of the skull film finding? C, Yes, there was a shunt present that decompressed the ventricles above, but the aqueductal obstruction coupled with Magendie and Luschka outflow occlusion trapped the fourth ventricle.

Infection, Hemorrhage, Tumors

The arachnoid villi are sensitive, delicate structures that may get gummed up by insults of several causes, resulting in communicating hydrocephalus (Box 8-3). Think of them as the little fenestrations in your bathtub drain: The whole tub will overflow if these tiny conduits are obstructed. The most common causes of obstruction include infectious meningitis, ventriculitis, ependymitis, subarachnoid hemorrhage, and carcinomatous meningitis. As the CSF becomes more viscous with a higher protein concentration, the arachnoid villi lose their ability to reabsorb the fluid. This causes hydrocephalus with dilation of the ventricular system.

Do not let a normal appearance to the fourth ventricle dissuade you from considering communicating hydrocephalus or obstruction distal to the fourth ventricle. The fourth ventricle is the last ventricle to dilate, possibly because of its relatively confined location in the posterior fossa, surrounded as it is by the thick calvarium and sturdy petrous bones. Thus, it is not uncommon to see dilated lateral and third ventricles but a normal-sized fourth ventricle and have communicating hydrocephalus. Still, the most sensitive indicator will be the enlargement of the temporal horns or anterior recesses of the third ventricle—without that you probably do not have hydrocephalus. The hunt for a source of the ventricular dilation should not stop at the aqueductal level with this pattern.

As with any cause of hydrocephalus, there may be periventricular high-signal intensity on MR, very nicely demonstrated with FLAIR scanning. This is due to transependymal CSF migration into the adjacent white matter leading to interstitial edema (dark on diffusion-weighted imaging [DWI]). This is most commonly seen at the angles of the lateral ventricles and, because of its smooth and diffuse nature, can usually be distinguished from the focal periventricular white matter abnormalities associated with atherosclerotic small-vessel ischemic disease. Be aware that there may normally be mild high intensity at the angles of the ventricle (ependymitis granulosa) in middle-aged patients.

Normal-pressure Hydrocephalus /Adult Hydrocephalus

Normal-pressure hydrocephalus (NPH) has a classic triad of clinical findings: gait apraxia, dementia, and urinary incontinence (Box 8-4). We like to think of NPH as a treatable cause of a nontraumatic DAI (dementia, ataxia, and incontinence). Although NPH was initially described as being idiopathic, patients with a remote (cryptic) history of infection or hemorrhage are still lumped into this category when they have the clinical triad. This probably accounts for 50% of NPH cases. The patients have enlarged ventricles from communicating hydrocephalus with particular enlargement of the temporal horns. They may show evidence of transependymal CSF leakage on MR or CT. MR often shows accentuation of the cerebral aqueduct flow void (Fig. 8-8). These patients may respond to shunting procedures with amelioration of their clinical symptoms. The most accurate predictors of a positive response to shunting are (1) absence of central atrophy or ischemia, (2) gait apraxia as the dominant clinical symptom, (3) upward bowing of the corpus callosum with flattened gyri and ballooned third ventricular recesses, (4) prominent CSF flow void, and (5) a known history of intracranial infection or bleeding (nonidiopathic NPH). It is important to entertain this diagnosis because there is a chance at the possibility of return of function with a shunt.

Figure 8-8 Normal-pressure hydrocephalus with accentuation of aqueductal flow void. Note the very low signal intensity of the aqueduct (open arrow) due to accelerated flow on this proton density-weighted image.

In patients with suspected NPH, an indium-111 DTPA (diethylene triamine penta-acetic acid) study is sometimes ordered. The agent is instilled in the CSF through a lumbar puncture. Normally, the tracer is resorbed over the convexities without ventricular reflux within 2 to 24 hours. In cases of communicating hydrocephalus and NPH, reflux of the tracer into the ventricles is seen with lack of tracer accumulation over the convexities 24 to 48 hours after instillation (Fig. 8-9). Patients who demonstrate this scintigraphic appearance allegedly have a better response to shunting than patients with normal or equivocal indium-111 DTPA findings.

Figure 8-9 Indium-DTPA study in a patient with normal-pressure hydrocephalus. Coronal indium-labeled study shows lack of ascension of the radiotracer over the convexities with reflux of the tracer into the lateral ventricles (V). This study was taken 48 hours after intrathecal tracer insertion.

The rate of clinical improvement after shunting in patients with NPH is still only 50%. Prominence of the CSF flow void in patients with this condition has led some investigators to use phase-contrast MR techniques to measure the flow through the cerebral aqueduct. A stroke volume of greater than 42 mL was predictive of better response to shunting. The specific parameters inherent in this measurement are related to scanner field strength and pulse sequences, so they are not necessarily transferrable to your own scanner, but the point made is that greater flow through the aqueduct means a better chance for shunt improvement. The rationale for this is that the brain is pushed centrifugally in NPH from enlarged ventricles (Fig. 8-10). As systole occurs, the blood being pumped into the closed space of the cranium forces CSF out of the lateral ventricles and into the aqueduct. Expansion of the lateral ventricles leads to shearing strains on the white matter and clinical symptoms.

Figure 8-10 Normal-pressure hydrocephalus (NPH). Ventricular enlargement out of proportion to sulcal dilatation without an obstructing lesion is the sine qua non of NPH. Minimal periventricular high signal on this FLAIR scan is of limited differentiating value because small-vessel ischemic changes abound in this age group.

External Hydrocephalus

Another benign cause of hydrocephalus from arachnoid villi malfunction is “external hydrocephalus,” also referred to as benign enlargement of the subarachnoid spaces in infants, benign extra-axial collections of infancy, extraventricular obstructive hydrocephalus, benign subdural effusions of infancy, and benign macrocephaly of infancy. This may be due to immaturity of the arachnoid villi with a decreased capacity to absorb CSF. External hydrocephalus is typically seen in children younger than age 2 years who have a rapidly enlarging head circumference, going off the scale on the pediatrician’s graph. Neurologically, these children are normal. Prematurity (immaturity), a history of intraventricular hemorrhage, and some genetic syndromes predispose to external hydrocephalus. CT or MR shows dilation of the CSF spaces over the frontal lobes and along the interhemispheric fissure but with relatively normal-sized ventricles (Fig. 8-11). The disorder usually resolves by the time the child is 3 to 4 years old and the head circumference returns to normal. The ventricles are usually normal in size (64% of cases)—thus the term external hydrocephalus. It is the CSF spaces external to the ventricles that are dilated. Why do the ventricles fail to dilate? Presumably because the cranial sutures are open, so the head enlarges instead. The differential diagnosis includes chronic subdural hygromas and atrophy caused by previous injury. When one sees sulcal dilatation and vessels coursing through the CSF collection, chronic subdural hygromas are much less likely (see Fig. 6-1). Atrophy is not usually associated with an enlarged head circumference.

Figure 8-11 External hydrocephalus. Note the dilatation of the interhemispheric fissure and the frontal cortical sulci in this infant with external hydrocephalus. Often the ventricles are not dilated.

Slit Ventricle Syndrome

Sometimes shunted patients are symptomatic but have tiny ventricles. These are usually patients who have had long-term shunt problems. The thought here is that the ventricles or brain lose their compliance and cannot expand despite the fact that they are under high pressure. This is probably due to the decreased compliance in the lateral ventricular walls owing to fibrosis in chronically shunted patients. Alternatively, they may be small owing to marked reduction in the pressure because of chronic overdrainage or leakage. The calvarium will thicken due to the overdrainage of the CSF. Then, if the cranial sutures close early, the brain has nowhere to go as it grows, being confined by the calvarium. This in turn leads to small ventricles and no capacity to accommodate normal variations in intracranial pressure. In this way, you can have small ventricles yet high pressure. The flow out the shunt will be limited.

Migraines

Punctate white matter high-intensity foci occur in migraineurs, usually in the subcortical white matter. They do not arise in the periventricular zones and hence should not be confused with multiple sclerosis. Rarely one can see frank strokes. Remember that migraines are vascular in etiology—usually due to hypersensitivity of meningeal vessels. A transient enhancement of the cisternal segment of the third cranial nerve has been reported in patients with ophthalmoplegic migraines.

Alzheimer Disease

Of the disorders in Table 8-2, Alzheimer disease (dementia Alzheimer type, or DAT) is one of the most notorious and common, accounting for 60% to 90% of the dementing disorders with progressive memory loss. DAT affects 2 to 4 million Americans; 8% of the population older than age 65 years and 30% of those older than age 85 years. Women are more commonly affected by a 2:1 margin. With DAT, life can be fascinating or petrifying, because memory is severely affected. DAT usually occurs in the late middle-aged adult, with the major dysfunction noted in memory, personality, and thought. Olfaction is one of the first senses to show some effects of the disease, but this is rarely tested. The patient’s social skills, personality, and speech pattern are also affected early. Depression often coexists. Late in the course the patient becomes severely impaired, myoclonic, vegetative, and weak. Current treatments are variably effective.

Senile plaques, seen as amorphous material in the cerebral cortex and neurofibrillary tangles in the nerve cells in the form of tangled loops of cytoplasmic fibers are the diagnostic pathologic features of DAT. Disease progression from the entorhinal cortex to the hippocampus to the neocortex is the rule. Curiously, these same pathologic findings are seen in adult patients with Down syndrome, Parkinson disease, and “punch-drunk” fighters (dementia pugilistica).

The main finding on CT and MR scanning of DAT is diffuse cortical atrophy, often more prominent in the temporal lobes. Temporal horn dilation more than 3 mm in diameter is seen in more than 65% of patients with DAT. Increases in ventricular size, sulcal size, sylvian fissure size, and total CSF volume are noted in patients with DAT compared with age-matched control subjects. On longitudinal studies, the rate of atrophic change in patients with DAT is much faster than in normal persons. Atrophy increases over time. The subiculum of the hippocampal region appears to be most severely affected in DAT.

One study found that the measure with the best sensitivity in discriminating DAT patients from control subjects was the width of the temporal horn. If one combines the measure of width of the temporal horn, width of the choroid fissure, height of the hippocampus, and the interuncal distance into a compound factor, one can discriminate patients with mild DAT from control subjects with 86% sensitivity (Fig. 8-12). There may, in fact, be a continuum of progressive hippocampal atrophy between normal elderly, those with mild cognitive impairment (MCI), and those with DAT. Yet another volumetric study showed that the best correlation with progression to Alzheimer disease is the change in volume of the entorhinal cortex and inferior parietal lobule over time (even more so than the hippocampus).

Figure 8-12 Dementia, Alzheimer type. Axial unenhanced computed tomography scan shows dilation of the choroidal-hippocampal fissure complex (arrows) with dilation of the adjacent temporal horns caused by temporal lobe atrophy.

Considerable investigation has been done concerning the presence of deep white matter and periventricular white matter areas of high signal intensity on T2WI in patients with DAT. There is a nonstatistical trend toward more small foci of white matter abnormality in patients with clinically diagnosed probable DAT.

On MR spectroscopy (MRS) reduced levels of N-acetyl aspartate (NAA) and increased levels of myoinositol characterize DAT. The NAA levels are significantly reduced in the frontal, temporal, and occipital cortex of DAT patients, presumably due to neuronal loss. Being able to distinguish between normal aging and DAT with MRS runs at the mid-80% range; distinguishing between DAT and other dementias drops the accuracy to the mid-70% range. Myoinositol elevation has also been reported in AIDS-related dementia and Pick disease.

Positron emission tomography has demonstrated decreased oxygen utilization and decreased regional cerebral blood flow in frontal, parietal, and temporal lobes in patients with DAT. The findings are most striking in the posterior temporoparietal lobes. On single-photon emission computed tomography (SPECT) brain studies, patients with DAT have reduced cerebral blood flow as measured by parietal-to-cerebellar and parietal-to-mean cortical activity. The severity of symptoms may correspond to the reduction in uptake of technetium hexamethyl propyleneamine oxime (HMPAO).

Recently, some investigators have used dynamic susceptibility contrast-enhanced MR perfusion imaging to try to duplicate the nuclear medicine flow studies. Indeed, they have found that relative values of temporoparietal regional cerebral blood volume (as a percentage of cerebellar relative cerebral blood volume) were reduced by a factor of 20% bilaterally in the patients with DAT compared with normal controls. Using left and right temporoparietal relative cerebral blood volume as index measures, specificity was 96% and sensitivity was 95% in moderate DAT and 88% in mild DAT.