eMedicine Specialties > Radiology > Brain/Spine

Normal Pressure Hydrocephalus

Author: James A Wilson, MD, MSc, FRCPC, BSc(H), Neurologist and Clinical Neurophysiologist, Oconee Neurology Services
Coauthor(s): Omar Islam, MD, FRCP(C), Assistant Professor of Diagnostic Radiology, Queen's University; Consulting Staff, Department of Diagnostic Radiology, Division of Neuroradiology, Kingston General Hospital
Contributor Information and Disclosures

Updated: Sep 11, 2007

Introduction

Background

First described by Hakim and Adams in 1965, normal pressure hydrocephalus (NPH) refers to a clinical entity consisting of the triad of gait disturbance, dementia, and incontinence, coupled with the laboratory findings of normal cerebrospinal fluid (CSF) pressures and radiographic findings of ventriculomegaly.¹ Although NPH is a relatively rare cause of dementia, identifying NPH is important because it is one of the few treatable entities.

See Normal Pressure Hydrocephalus in eMedicine's Neurology journal for an in-depth description of NPH as a clinical entity.

Pathophysiology

Although the pathophysiology of NPH has been under debate for decades, aberrations in CSF flow dynamics generally are considered to be central to the condition's development. One proposed mechanism involves a transmantle pressure gradient wherein CSF pressure in the ventricles is greater than CSF pressure in the subarachnoid space. Short-lasting CSF pulsations (B waves) periodically apply pressure to the ventricular walls and have a water-hammer effect that enlarges the ventricles. Abnormalities of ventricular wall compliance may contribute to ventricular dilatation.

Another mechanism involved in increasing transmantle pressure is impaired CSF flow, which may be at the level of the aqueduct (noncommunicating NPH) or distal to it (communicating NPH). In either case, flow is impaired enough to increase the transmantle CSF pressure gradient but not enough to raise intracranial pressure.

With respect to etiology of NPH, approximately 50% of cases generally are accepted to be idiopathic. In the other 50% of patients, there exists a history of events that can alter CSF flow dynamics, such as subarachnoid hemorrhage, trauma, meningitis, or surgery.

Frequency

United States

NPH is relatively rare, and the exact incidence and prevalence in the United States is not known.  However, some experts believe that NPH may cause up to 5% of cases of dementia. Thus, an estimated 750,000 Americans may have NPH, but this is probably an overestimation. Hospital discharge data suggest that annually around 11,500 patients in the United States are diagnosed with NPH.

International

Estimates have placed the frequency of NPH at approximately 1 case per 25,000 cases of dementia or as high as 6% of patients with dementia.^2,3,4 Vanneste and colleagues suggest that the incidence of shunt-responsive NPH represents approximately 0.4% of patients with dementia.⁵ These estimates have been derived from US and European data.

Mortality/Morbidity

The natural course of NPH appears to be a continual cognitive and motor decline, akinetic mutism, and eventual death, although this prognosis has been clouded by the tendency towards surgical intervention since the first description of NPH in the mid-1960s.

• To the authors' knowledge, no trials have utilized control subjects in studying the treatment of patients with NPH. Thus, morbidity and mortality information for NPH must largely be derived through analysis of the success and complications of surgical treatment. Further, due to the age group afflicted, other complicating medical issues often affect morbidity and mortality.
• Traditionally, the success of CSF shunting in NPH is reported as follows^6,7,8 :
  
  
  • One third of patients improve
  • One third of patients experience arrested symptom progression
  • One third of patients continue to deteriorate
  • Approximately one third of patients experience complications
• A further breakdown of treatment success by Vanneste and colleagues suggests that a 50-70% improvement may be seen in select patients with NPH with a known etiology.⁵ Postoperative complications still occurred in approximately one third, although only 5-15% of cases involved death or severe, residual morbidity. This resulted in a benefit-to-harm ratio of 3:1 in nonidiopathic NPH; the ratio was only 1.7:1 in idiopathic NPH.

Race

No racial predilection has been described in NPH.

Sex

NPH occurs with approximately equal frequency in males and females.

Age

NPH occurs more frequently with age and typically is diagnosed in the sixth or seventh decade of life.

Anatomy

NPH involves ventricular enlargement without increased CSF pressure. A disproportionate lack of sulcal enlargement exists because of the proposed transmantle pressure gradient.

Anatomically, this means an enlarged third ventricle exists along with dilation of the occipital, frontal, and temporal horns of the lateral ventricles. Presumably, the periventricular white matter is stretched and dysfunctional as a result of inadequate perfusion without actually being infarcted.^9,10,11

The proposed dysfunctional periventricular white matter tracts are related to the clinical presentation of NPH, since observed dementia is of a subcortical nature. Disruption of periventricular white matter tracts may explain gait disturbance and incontinence. Further, a perfusion deficit without infarction can explain the partial success of CSF shunting, which may reduce the stretching of the periventricular white matter tracts and restore some perfusion.

Some functional and neuroimaging data suggest corpus callosum functioning is impaired in NPH.^12,13

Presentation

NPH classically presents with the clinical triad of gait disturbance, dementia, and urinary incontinence. Hydrocephalus, along with a normal CSF opening pressure, also is required to define NPH. The presence of hydrocephalus is ascertained via neuroimaging, while CSF pressures are determined by using lumbar puncture.

The gait disturbance in NPH has been described as appearing to be magnetic in nature, with the patient finding it difficult to initiate movement. The term gait apraxia frequently is used in NPH but is considered incorrect by some authors, since patients with NPH can exhibit near-normal walking movements when supported.^14,15 Patients with NPH demonstrate a short-stepped, shuffling gait with postural instability. Because this gait disorder is frequently the first clinical sign, the presentation may be confused with that of an extrapyramidal disorder, such as Parkinson disease.

The mental deterioration observed is frequently mild and is subcortical in nature. Memory problems, poor attention, and slowing of information processing are observed.

Urinary incontinence usually is present only in advanced cases and likely results from disruption of periventricular pathways to the sacral bladder center. This results in decreased inhibition of bladder contractions and, consequently, instability of bladder detrusors. Only in extremely advanced cases of NPH, with severe frontal lobe dysfunction, is incontinence a result of lack of concern for micturition.

Due to the high prevalence in the elderly of each independent feature of the NPH triad, paraclinical tests are paramount in attempting to predict who will benefit from neurosurgical CSF shunting. For example, clinical improvement resulting from large-volume CSF tapping may predict some success from shunting. Typically, the tapping procedure involves the removal of 40-50 mL of CSF, with monitoring of gait and memory. Unfortunately, this test is associated with a high rate of false negatives, and some authors have suggested that a more aggressive, continuous removal of CSF on the order of 150-200 mL be conducted daily for 3-5 days to better detect which patients will improve with shunting.¹⁶

Another invasive technique that has been utilized in an attempt to predict CSF-shunting success is lumbar infusion, wherein pressure dynamics are monitored following the infusion of saline. This test is rarely used in clinical practice, although it could be of reasonable value in certain circumstances.

A review of the above clinical tests and their value in predicting benefit from CSF shunting is provided by Marmarou and colleagues.¹⁷

Preferred Examination

Magnetic resonance imaging (MRI) of the brain is the preferred radiologic examination for the diagnosis of NPH, especially with T2-weighted images. Computed tomography (CT) scanning of the brain is useful if MRI is unavailable. Both radiologic techniques require clinical correlation.

Limitations of Techniques

The primary role of CT scanning and MRI is to assess for hydrocephalus with ventriculosulcal disproportion. This observation is a subjective assessment, and in patients with some sulcal widening or only minimal ventriculomegaly, the studies may not be sensitive or specific.

Patient Education: For excellent patient education resources, visit eMedicine's Dementia Center. Also, see eMedicine's patient education article Normal Pressure Hydrocephalus.

Differential Diagnoses

Alzheimer Disease
Periventricular Leukomalacia

Other Problems to Be Considered

Obstructive hydrocephalus
Cerebral atrophy with ex vacuo ventricular dilatation

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