Normal Pressure Hydrocephalus

Synonyms: chronic hydrocephalus, extraventricular obstructive hydrocephalus

Normal Pressure Hydrocephalus (NPH) describes the condition of ventricular dilatation in the absence of raised CSF pressure on lumbar puncture, characterised by a triad of gait abnormality, urinary (usually) incontinence and dementia. The importance of this diagnosis lies in the fact that it is a potentially reversible cause of dementia, accounting for up to 6% of dementias.¹

It was first described by Hakim and Adams in 1965.² 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.³

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.⁴

The cause of NPH is not always obvious. In 50% of cases, there is no known preceding cause (idiopathic NPH). In the remainder, it may be secondary to:

• Subarachnoid haemorrhage
• Meningitis
• Head injury
• Central nervous system (CNS) tumour

All of these conditions can cause hydrocephalus but the pathological process that leads to NPH is not clear. These patients must be differentiated from patients whose ventricular enlargement is a result of shrinkage of surrounding tissue, as in Alzheimer's disease for example.

Go to our record on Hydrocephalus for more detail about this condition.

The condition occurs mainly in elderly patients although some researchers maintain that similar CSF hydrodynamic changes are seen in some hydrocephalic children.^3,6 There is no predominance of gender.⁶

Symptoms

The (gradually progressive) classic triad of symptoms are:

1. 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. Freezing episodes can also occur. True ataxia and weakness is absent and the gait disturbance is referred to as gait apraxia.
2. Sphincter disturbance - this is also due to involvement of the sacral nerve supply. Urinary incontinence is predominant although bowel incontinence can also occur.
3. Dementia - this is due to distortion of the periventricular limbic system. The prominent features are memory loss, inattention, inertia and bradyphrenia (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 (but there has been found to be an association with glaucoma so glaucomatous optic disc changes may be noticed).⁷

• Neuroimaging - 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.
• Large volume lumbar puncture (spinal or CSF tap test)⁸ - CSF pressure will be normal, or intermittently raised. Furthermore, the effect of the lumbar puncture is assessed looking at improvement of the patient's symptoms which can last a period of days to weeks. The value of this test is limited in diagnosing NPH but may be useful in narrowing the differential diagnosis. If it is positive and symptoms do improve, it can be used as a predictor of positive operative outcome (see below).
• Intraventricular monitoring - in NPH, this may show a particular pattern characterised by beta waves - see management below.⁹
• Lumbar infusion test (intrathecal infusion test)⁸ - the CSF absorptive capacity is tested with a fluid challenge. An abnormal, sustained rise in CSF suggests NPH.

Although the invasive tests carry theoretical risks (infection, postprocedure headache, bleeding, localised pain and nerve root damage), evidence suggests that these are actually very safe procedures.⁸ In these patients, it is useful to also check the serum sodium as hyponatraemia has been reported.¹⁰

This is based on the clinical triad described above, in the absence of papilloedema, backed by neuroimaging (specific criteria are defined) without evidence of raised intracranial pressure and with evidence of symptom improvement on lumbar puncture.⁸ Accurate diagnosis is the key to treatment success.¹

• Alzheimer's 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's disease
• Parkinson-Plus syndromes
• Pick's disease
• Uraemic encephalopathy
• Wilson's disease

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. Currently, no definitive evidence exists that levodopa/carbidopa is an effective treatment for NPH - anecdotal reports of their success is probably explained by a misdiagnosis of Parkinson's disease.⁶

Surgical^3,6

The mainstay of treatment is surgical insertion of a CSF shunt.⁸ This could be to the peritoneum, the right atrium or, more recently, via external lumbar drainage. 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. The most reliable tests predicting positive operative outcome are:⁵

• Substantial improvement of one or more of the triad of features after CSF withdrawal; CSF tap test: lumbar CSF drainage of 200ml per day for 3-5 days.
• The presence of patterns (beta waves) evident on continuous intracranial pressure monitoring for >5% of a 24h period.

Other features suggesting a good outcome include:

• Presence of a clearly identified aetiology
• Predominant gait difficulties with mild cognitive impairment
• 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 variable, 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.⁶ Complications of shunt surgery include:

• Shunt occlusion
• Catheter breakage
• CSF hypotensive headaches
• Cerebral infarct
• Haemorrhage
• Infection
• Seizures
• Death

However, for those in whom surgery is successful, the benefits can be sustained for 5–7 years, even if shunt revision surgery is needed multiple times.¹