Normal Pressure Hydrocephalus

Prevalence, Prognosis, and Definition

Normal pressure hydrocephalus is a relatively rare disorder characterized by enlargement of the ventricles, a gait disorder, incontinence, and cognitive impairment. Although some studies have suggested that up to 5% of patients with dementia have normal pressure hydrocephalus, other studies have found the prevalence to be closer to 1%, which is consistent with our experience of patients referred to a memory disorders clinic (for a review see ). It may, however, be underdiagnosed in the general population; one study showed that 5.9% of individuals 80 years and older met criteria for normal pressure hydrocephalus ( ). In Norway, the overall prevalence was found to be 21.9 per 100,000 and the incidence was 5.5 per 100,000 ( ). The Mayo Clinic Study of Aging found magnetic resonance imaging (MRI) scans consistent with normal pressure hydrocephalus in 5% of their study population ( ).

Criteria

Normal pressure hydrocephalus should be suspected when the so-called “triad” of symptoms (cognitive impairment, gait disorder, and urinary incontinence) is present in the setting of enlarged ventricles. However, it cannot be stated strongly enough that many disorders can cause cognitive impairment, a gait disorder, and urinary incontinence, and virtually all neurodegenerative diseases lead to ex vacuo dilatation of ventricles (enlarged ventricles caused by loss of brain tissue), so the differential diagnosis of these symptoms and signs must be carefully considered. Frequently used criteria for diagnosing normal pressure hydrocephalus are shown in Box 14.1 .

Risk Factors, Pathology, and Pathophysiology

Normal pressure hydrocephalus may result from low-grade scarring or obstruction of the ventricular system or subarachnoid pathways. Thus, although most cases are idiopathic, other causes include subarachnoid hemorrhage, head trauma, tumor, prior surgery, aqueductal stenosis, meningitis, and even lumbar puncture. The basic idea is that one of these etiologies causes enough scarring and/or obstruction to increase pressure and to damage the myelinated fibers surrounding the ventricles, but an equilibrium is reached such that the pressure is not raised enough to present as high-pressure hydrocephalus ( Fig. 14.1 ). Periventricular frontal cortical-basal ganglia-thalamocortical circuitry is disrupted in normal pressure hydrocephalus, which may be related to the observed cognitive deficits ( ).

Common Signs, Symptoms, and Stages

The gait disturbance in normal pressure hydrocephalus is typically the most prominent symptom and is usually the earliest in onset. The gait disorder is of a frontal type, and has been described as a “magnetic gait” or a “marche à petits pas” (walk of little steps), and is somewhat different from shuffling. It is very unsteady. There are a number of different problems that can occur with a frontal gait, including a gait apraxia, spasticity, and unsteadiness. Patients may describe a loss of strength in their legs that may not be the result of weakness of any individual muscles. Some patients may complain of difficulty lifting their feet off the ground, feeling like their feet are stuck to the floor.

Cognitive impairment generally presents next. A number of cognitive disturbances can occur, but the most common presentation is that of a frontal subcortical disturbance. Normal pressure hydrocephalus puts pressure on, and ultimately damages, periventricular white matter tracts. Because most of the brain’s white matter is involved in transferring information to or from the frontal lobes, patients with normal pressure hydrocephalus usually show signs and symptoms of frontal subcortical dysfunction. Attention is often severely affected. Most cognitive processes are slow, and some, such as memory, require additional trials to reach normal performance. Changes in behavior, including a slowing of thought and action that may progress to apathy and abulia, are often observed.

Although urinary urgency and frequency may be present early, frank incontinence generally occurs late in normal pressure hydrocephalus. Urge incontinence is usually seen, such that the patient only has a very short time between when he or she feels the need to empty their bladder and when it empties. Fecal incontinence may also be seen.