Normal Pressure Hydrocephalus

Synonyms: chronic hydrocephalus, extraventricular obstructive hydrocephalus

Normal Pressure Hydrocephalus (NPH) was first described by Hakim and Adams in 1965.¹ The three patients identified in the original series had dilation of the cerebral ventricles without an increase in cerebrospinal fluid (CSF) pressure on lumbar puncture.

Further research has revealed that intracranial pressure (ICP) does indeed rise in the initial phase, and that subsequent ventricular enlargement results in the ICP returning to normal. CSF pressure as measured during an isolated lumbar puncture procedure is a poor assessment of CSF dynamics in these patients, many of whom do indeed exhibit a rise in pressure, albeit on an intermittent basis.²
Current studies suggest that CSF pressure is controlled by alterations in spinal compliance or decrease in CSF absorption at the sagittal sinus. Four sub-types of NPH have thus been identified depending on whether one or other or both of these mechanisms are operating. This may have important implications in terms of management.³

The cause of NPH is not always obvious. It may sometimes be associated with head injury, subarachnoid haemorrhage, meningitis, and central nervous system (CNS) tumour. All of these conditions can cause hydrocephalus, but the pathological process that leads to NPH is not clear.

The condition occurs mainly in elderly patients although some researchers maintain that similar CSF hydrodynamic changes are seen in some hydrocephalic children.^2,4
There is no predominance of gender.⁴ One Bavarian study of residents over the age of 65 found a prevalence of 0.41%.⁵

Symptoms

The classic triad of symptoms are:

• Gait disturbance This is due to distortion of the corona radiata by the dilated ventricles. This area contains the sacral motor fibres than innervate the legs. Movements are slow, broad-based and shuffling. The clinical impression is thus one of Parkinson's Disease, except that rigidity and tremor are less marked, and there is no response to carbidopa/levodopa. True ataxia and weakness is absent and the gait disturbance is referred to as gait apraxia.
• Sphincter Disturbance This is also due to involvement of the sacral nerve supply. Urinary incontinence is predominant, although bowel incontinence can also occur.
• DementiaThis is due to distortion of the periventricular limbic system. The prominent features are memory loss, inattention, inertia and bradyphenia (slowness of thought). The dementia progresses less rapidly than that seen with Alzheimer's disease.

Signs

• Pyramidal tract signs may be present.
• Reflexes may be brisk.
• Papilloedema is absent.

• Alzheimer Disease
• Aphasia
• Apraxia and related syndromes
• Confusional states and acute memory disorders
• Cortical basal ganglionic degeneration
• Dementia with Lewy Bodies
• Dementia in motor neurone disease
• EEG in dementia and encephalopathy
• Frontal lobe syndromes
• Frontal and temporal lobe dementia
• Hydrocephalus
• Marchiafava-Bignami Disease (gait disturbance and dementia, usually in alcoholics, thought to be due to bilateral reduction cerebral blood flow)
• Multi-infarct dementia
• Multiple system atrophy
• Paraneoplastic encephalomyelitis
• Parkinson Disease
• Parkinson-Plus syndromes
• Pick Disease
• Uraemic encephalopathy
• Wilson Disease

• Imaging MRI or CT scanning may show ventricular enlargement out of proportion to sulcal atrophy and periventricular lucency. Isotope cysternography may also be useful to demonstrate CSF dynamics, particularly when attempting to predict which patients will benefit from surgery.
• Lumbar Puncture CSF pressure will be normal, or intermittently raised. The value of this test is limited in diagnosing NPH but may be useful in narrowing the differential diagnosis.
• Intraventricular monitoring In NPH, this may show beta waves for more than 5% of a 24 hour period.⁶

The identification and treatment of NPH is worthwhile, providing patients are carefully selected, as it remains one of the truly reversible causes of dementia.

Medical

Medical treatment of NPH includes acetazolamide and repeated lumbar puncture. These methods are rarely successful long term, and are usually used as temporary deferment measures, or in patients too ill for surgery.

Surgical^2,4

Selection of patients for surgery is important as exposing patients to shunt-related complications such as mechanical failure or infection is unwarranted unless a good clinical outcome is expected. Various parameters are used to predict which patients will benefit from surgery. These include:

• Presence of a clearly identified aetiology
• Predominant gait difficulties with mild cognitive impairment
• Substantial improvement in gait or sphincter disturbance after CSF withdrawal (CSF tap test)
• Normal sized or occluded sylvian fissures and cortical sulci on CT or MRI
• Absent or moderate white matter lesions on MRI

Insertion of a ventriculoperitoneal shunt is the first line procedure, with ventriculoatrial shunting being used as an alternative.

Prognosis is poor, even after shunt surgery. One study found that 21% of patients improved after surgery, but 28% had complications, thus emphasising the importance of careful patient selection.⁴