Normal pressure hydrocephalus

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:

1. urinary incontinence
2. deterioration in cognition (dementia)
3. 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. 

A classic neurological sign in NPH is magnetic gait.

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 callosum¹⁴
• 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 sequences¹⁴
  • Not as useful on modern MR scanners¹⁵
  • favorable outcome if shunt surgery has been suggested but other work suggests that it is an unreliable marker of actual flow ¹².
• periventricular high signal on T2 weighted sequences
• MR spectroscopy - lactate peak in lateral ventricles
• CSF flow studies ³
  • increased aqueductal stroke volume
    • aqueductal stroke volume is the average volume of CSF (calculated by summing the absolute values of stroke volume in systole and diastole)
    • (|forward stroke volumne| + |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 predictsshown 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 2x the normal¹⁶

Nuclear medicine

Nuclear medicine is less important in diagnosing NPH .Some of the features described are 

• early detection of radiotracer in to lateral ventricles giving heart shaped appearance of lateral ventricles than a normal trident pattern¹³
• persistence of radiopharmaceutical beyond 24-48 hours due to impaired absorption¹³
• radiotracer doesnot extend to superior aspect of convexities of lateral ventricles¹³
• retrograde CSF flow in to lateral ventricles .¹³

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}

Etymology

It is thought ot have been initially described by the neuroradiologists 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.

Differential diagnsosis

Imaging imaging differential consideration include

• 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)