Normal pressure hydrocephalus (NPH) remains a controversial entity with often ambiguous imaging findings. It is classically characterised by the triad of gait apraxia, urinary incontinence, and dementia, although not all patients with NPH have all three.
On imaging, it can be characterised both on CT and MRI by enlarged lateral and third ventricles out of proportion to the cortical sulcal enlargement.
This article is focused on idiopathic NPH. 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 haemorrhage, and meningitis. This is sometimes confusingly referred to as secondary NPH 3. Most clinicians will assume that one is referring to idiopathic NPH if no qualifier is used.
The majority of cases of normal pressure hydrocephalus (NPH) are idiopathic. The incidence is much higher in elderly populations, although exact numbers are difficult to establish as the diagnosis is often not made in nursing home patients for the following reasons: approximately 20% of patients in nursing homes have gait impairment; approximately 10% of these patients also have dementia, and 15% have incontinence 19
In a large study in Norway found a striking increasing incidence with increasing age 3:
- 3.3 per 100,000 for people 50 to 59 years of age
- 49.3 per 100,000 for people 60 to 69 years of age
- 181.7 per 100,000 for people 70 to 79 years of age
The classical clinical findings of normal pressure hydrocephalus are 1-3:
- urinary incontinence
- deterioration in cognition (dementia)
- gait disturbances
These can be remembered with the unkind mnemonic Wet, Wacky and Wobbly.
As the name suggests mean CSF opening pressure in patients with NPH is within the normal range (<18 cmH2O or 13 mmHg) 3.
A classic neurological sign in NPH is magnetic gait.
The underlying cause of NPH remains controversial. One theory is that NPH is an obstructive type of communicating hydrocephalus due to reduced of CSF resorption. A second theory suggests that NPH results from weakening of the ventricular wall due to periventricular white matter ischemic damage 3. The periventricular white matter ischemic change has also been hypothesised 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 peri-ventricular 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 visualise the anatomical changes of NPH (see MRI section below), it is inferior to MRI.
- ventriculomegaly 1-3
- changes in sulcal size
- crowding of the gyri at the vertex (with small sulci)
- cingulate sulcus sign: posterior part of cingulate sulcus is narrower than the anterior part, the divider between the two being a line drawn parallel to the floor of the 4th ventricle17
- sylvian fissures out of proportion to sulcal enlargement (which is minimal) and hippocampus and mesial temporal lobe volumes (which are near normal)
CSF flow changes
- aqueductal flow void due to increased CSF velocity across the aqueduct in patients with NPH
- best seen on T2 and PD spin echo sequences 14 but it is no longer useful sign as in modern high strength MRI machines aqueductal CSF flow void is present in most of normal subjects. There is a well-established correlation between higher flow velocities and the favourable outcome after VP shunts; hence, quantitive rather than qualitative methods has been developed which can be useful not only in patient selection for shunting but also assist in diagnosis as well as predicting prognosis 12-15.
CSF flow studies 3
- increased aqueductal stroke volume
- aqueductal stroke volume is the average volume of CSF moving through the cerebral aqueduct
- calculated by summing the absolute values of stroke volume in systole and diastole and dividing by 2
- (forward stroke volume + reverse stroke volume)/2
- increased aqueductal peak velocity
- various publications have set various normal and abnormal ranges
- flow rate of > 24.5mL/min 95% specific for NPH 9,11
- stroke volume of > 42 microL shown on one paper to predict good response from shunting 10
- stroke volume upper limit is now suggested to be variable between institutions due to intrinsic scanner differences, thus each centre 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 theorised to be caused by cerebral atrophy, which indicates that the patient is unlikely to respond to shunt surgery18
- increased aqueductal stroke volume
Lactate peak in lateral ventricles has been reported.
Nuclear medicine is less important in diagnosing NPH. Some of the features described are 13:
- early detection of radiotracer into lateral ventricles giving heart shaped appearance of lateral ventricles than a normal trident pattern
- persistence of radiopharmaceutical beyond 24-48 hours due to impaired absorption
- radiotracer does not extend to 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 which will benefit from shunting. Favourable prognostic factors include 3:
- short duration of presurgical symptoms (less than 6 months)
- onset of gait disturbance before dementia
- temporary symptom relief from a CSF tap test (removal of 40ml of CSF via lumbar puncture)
- absence of significant cerebral vascular disease
- presence of an aqueductal flow void on T2 imaging 10,14
History and etymology
It is thought ot have been initially described by the neurosurgeon Salamon Hakim and R D Adams in 1965, although it may actually have been described under a different name earlier by McHugh 4,6-7.
The possible imaging differential spectrum includes
- normal ageing brain
- Alzheimer dementia: may show greater dilatation of perihippocampal fissures 2
- obstructive hydrocephalus: due to mass lesion (e.g. pineal region, tectal plate, midbrain)
- Lewy-body dementia: visual hallucinations and delusions are more prominent
- Parkinson disease: unilateral symptoms are important
- AIDS-dementia complex: positive HIV serology
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