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Normal pressure hydrocephalus

Dr Jeremy Jones and Dr Gagandeep Singh et al.

Normal pressure hydrocephalus (NPH) remains a controversial entity with often ambiguous imaging findings.

Epidemiology

The majority of cases of normal pressure hydrocephalus (NPH) are idiopathic and is referred to as primary NPH. Many causes of secondary NPH exist including ³:

trauma
subarachnoid haemorrhage (SAH)
intracranial surgery
meningitis

Clinical presentation

The classical clinical findings of normal pressure hydrocephalus are ^1-3:

urinary incontinence
intellectual deterioration
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 cm H2O or 13 mm Hg) ³.

Pathology

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.

Radiographic features

CT

Although CT is able to visualise the anatomical changes of NPH, it is inferior to MRI.

MRI brain

MRI is the best modality to image anatomical changes and can also add support for the diagnosis with CSF flow studies and MRS.

ventriculomegaly ^1-3
- frontal and temporal horns of the lateral ventricles most affected
- upward bowing of the corpus callosum
crowding of the gyri at the vertex (with small sulci)
sylvian fissures out of proportion to sulcal enlargement (which is minimal) and hippocampus and mesial temporal lobe volumes (which are near normal)
aqueductal flow void
- best seen on T2 spin echo sequences
- favorable outcome if shunt surgery
periventricular high signal on T2 weighted sequences
MR spectroscopy : lactate peak in lateral ventricles
CSF flow studies ³
- increased aqueductal stroke volume
- increased aqueductal peak velocity
- NB : absence of a flow void does not exclude the diagnosis

Nuclear medicine

Content pending - if you are a bushy-tailed nuc-med physician, please please please help us out and add a little something here.

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

Differential diagnsosis

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)