Normal pressure hydrocephalus
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Normal pressure hydrocephalus remains a controversial entity with often ambiguous imaging findings. It is classically characterized by the triad of gait apraxia/ataxia, urinary incontinence, and cognitive impairment, although not all patients with the condition have all three 31.
On imaging, it can be characterized both on CT and MRI by enlarged lateral and third ventricles out of proportion to the cortical sulcal enlargement, associated with Sylvian fissure widening and crowding at the vertex. This pattern has been termed disproportionately enlarged subarachnoid space hydrocephalus (DESH) and is helpful in distinguishing normal pressure hydrocephalus from other causes of hydrocephalus.
This article is focused on idiopathic normal pressure hydrocephalus (iNPH). For a discussion of secondary causes of communicating hydrocephalus, please refer to the article hydrocephalus.
It is important to note that there are many causes of communicating hydrocephalus without elevated opening CSF pressures, such as trauma, prior subarachnoid hemorrhage, and meningitis. This is sometimes confusingly referred to as secondary normal pressure hydrocephalus 3. Most clinicians will assume that one is referring to idiopathic normal pressure hydrocephalus if no qualifier is used.
The incidence of normal pressure hydrocephalus is much higher in elderly populations. Exact epidemiology is difficult to establish, as the diagnosis is often not made in nursing home patients as
20% of patients in nursing homes have gait impairment
10% of these patients also have dementia
15% have incontinence 19
but the prevalence is estimated at ~1.5% (range 0.1-2.9%) 35.
A large study in Norway found a striking increase in incidence of normal pressure hydrocephalus with increasing age from 3.3 per 100,000 people (50-59 years) to 181.7 per 100,000 people (70-79 years) 3.
The classical clinical findings of normal pressure hydrocephalus are 1-3:
congnitive impairment (ranging from mild to severe, i.e. dementia)
a classic neurological sign in normal pressure hydrocephalus is magnetic gait, where the patient's feet appear to be magnetically attached to the floor
These can be remembered with the unkind mnemonic wet, wacky, and wobbly.
As the name suggests, mean CSF opening pressure is within the normal range (<18 cmH2O or 13 mmHg) 3.
The underlying cause remains controversial. One theory is that the condition is an obstructive type of communicating hydrocephalus due to reduced CSF resorption. A second theory suggests it results from weakening of the ventricular wall due to periventricular white matter ischemic damage 3. The periventricular white matter ischemic change has also been hypothesized to slow the flow of CSF through the extracellular spaces, resulting in a "back-pressure" effect, leading to ventricular enlargement.
Shearing forces are exerted on the periventricular white matter as the ventricles enlarge. As the corticospinal tracts to the legs run medially, these tangential shearing forces cause gait disturbance.
Although CT is able to visualize the anatomical changes, it is inferior to MRI.
disproportionate changes in subarachnoid spaces
effacement of the sulci and subarachnoid spaces at the vertex and medial/parafalcine region 1,24,25,28
cingulate sulcus sign: posterior half of cingulate sulcus is narrower than the anterior half 1,17
focal/isolated dilation of individual sulci over the medial surface or convexity (sometimes called transport sulci) 1,24,28,29
The combination of ventriculomegaly, Sylvian fissure and insular cistern widening, and crowding at the vertex has been termed disproportionately enlarged subarachnoid space hydrocephalus (DESH), reflecting disproportionality between the superior and inferior CSF spaces 1,23,27. The finding of disproportionately enlarged subarachnoid space hydrocephalus without neurologic symptoms has been termed asymptomatic ventriculomegaly with features of idiopathic normal pressure hydrocephalus on MRI (AVIM) 1. It is felt by some authors that AVIM is actually a pre-clinical form of idiopathic normal pressure hydrocephalus 30.
Brain composition changes
Periventricular hypodensity (on CT) or high T2-FLAIR signal (on MRI) is supportive of changes in brain water content seen in normal pressure hydrocephalus, and is sometimes referred to as transependymal edema. It is worth noting, however, that these findings can be difficult to distinguish from microvascular ischemic changes and that, in fact, they may co-exist 2,25,35,26.
CSF flow changes
Although prominent aqueductal flow void due to increased CSF velocity across the aqueduct is often seen in patients with normal pressure hydrocephalus - best seen on T2 and PD spin-echo sequences 14 - it is no longer considered a useful sign, as in modern higher field strength MRI machines aqueductal CSF flow void is present in most of the normal subjects. Additionally, it is highly sequence-specific.
There is, however, a well-established correlation between higher flow velocities and the favorable outcome after ventriculoperitoneal (VP) shunting; hence, quantitative (rather than qualitative) methods have been developed which can be useful not only in patient selection for shunting but also in diagnosis and predicting prognosis 12-15
increased aqueductal CSF stroke volume
aqueductal CSF stroke volume is the average volume of CSF flowing through the cerebral aqueduct during both systole and diastole
increased aqueductal peak velocity
various publications have set various normal and abnormal ranges
flow rate of >24.5 mL/min 95% specific for normal pressure hydrocephalus 9,11
stroke volume of ≥42 μL shown in one paper to predict a good response to shunting 10, however, a more recent paper has cast serious doubt on the usefulness of aqueduct stroke volume in patient selection for VP shunting 21
stroke volume upper limit is now suggested to be variable between institutions due to intrinsic scanner differences; thus each center should obtain their own "normal values", with the upper limit being suggested as two times the normal value 16
studies have shown that aqueductal stroke volume decreases later in the disease process despite clinical progression
this has been theorized to be caused by cerebral atrophy, which indicates that the patient is unlikely to respond to shunt surgery 18
MR spectroscopy has not been shown to have any added value for differentiating idiopathic normal pressure hydrocephalus from other types of dementia, nor does it help in patient selection for VP shunting 22.
Nuclear medicine is less important in diagnosing normal pressure hydrocephalus. Some of the features described are 13:
early detection of the radiotracer into lateral ventricles giving a heart-shaped appearance of lateral ventricles rather than a normal trident pattern
persistence of radiopharmaceutical beyond 24-48 hours due to impaired absorption
radiotracer does not extend to the superior aspect of convexities of lateral ventricles
retrograde CSF flow into lateral ventricles
Treatment and prognosis
Treatment of normal pressure hydrocephalus, once the diagnosis is established, is with CSF shunting, usually a ventriculoperitoneal shunt (VP shunt). The challenge is identifying those patients who will benefit from shunting. Favorable prognostic factors include 3:
short duration of presurgical symptoms (less than 6 months)
onset of gait disturbance before cognitive impairment
temporary symptom relief from a CSF tap test (removal of 40 mL of CSF via lumbar puncture)
absence of significant cerebral vascular disease
presence of an aqueductal flow void on T2 imaging 10,14
When patients are selected on the basis of clinical and imaging features, 80% response favourably to CSF diversion 33.
History and etymology
It is thought to have been initially described by Colombian neurosurgeon Salomón Hakim and R D Adams in 1965, although it may actually have been described earlier under a different name by McHugh 4,6,7.
Clinical differential diagnosis includes vascular dementia and atypical parkinsonian disorders, e.g. progressive supranuclear palsy and multiple system atrophy - Parkinsonian type (MSA-P) 32.
The possible imaging differential spectrum includes:
normal aging brain
Alzheimer dementia: may show greater dilatation of perihippocampal fissures 2
due to mass lesion (e.g. pineal region, tectal plate, midbrain)
due to aqueductal stenosis
Lewy body dementia: visual hallucinations and delusions are more prominent
Parkinson disease: unilateral symptoms are important
AIDS-dementia complex: positive HIV serology