Normal pressure hydrocephalus and polyneuropathy.

Normal Pressure Hydrocephalus (NPH) is a treatable form of dementia and therefore important. It was described by Hakim and coworkers in 1965 as a clinical syndrome occurring in patients with idiopathic or "occult" hydrocephalus with normal IC pressure (ICP) who could benefit from shunt surgery. NPH is characterized by the clinical triad of gait and balance disturbance, mental deterioration and urinary incontinence. Previously, hydrocephalus had been strongly associated with increased ICP and symptoms such as headache, papillary edema and impaired wakefulness.

It is important to bear in mind that many patients with NPH do not present with the whole classic triad. Thus, gait disturbance, which often is the dominating symptom in older NPH patients, might be absent in younger patients in whom postural disturbance is frequently seen. There is a strong association between dysfunction in postural and motor tasks, and it is thus conceivable that younger patients have less affected gait due to better compensatory mechanisms. The diagnostic challenge of NPH is well illustrated in the current case, which describes a patient with polyneuropathy and hydrocephalus that "mask" the traditional symptoms.

The pathophysiology and symptoms of normal pressure hydrocephalus are still not well understood. The cerebrospinal fluid (CSF) tap test, as described in the case study, mimics the effect of the shunt by removing CSF. Interestingly, the improvement after a tap test regularly can be observed some hours after the tap when the removed CSF already has been replaced with newly formed CSF, and the IC pressure has returned to baseline. This indicates that reversible metabolic disturbances in the brain cause the hydrocephalic symptoms. NPH symptoms have been found to be linked to several regions of the brain, such as the basal parts of the frontal lobes, the mesial part of the temporal lobes, the periventricular region and the upper brainstem. Some recent studies on lumbar and ventricular CSF markers suggest that reversible neuronal damage in hydrocephalus patients causes the NPH symptoms, but the true mechanisms behind hydrocephalus symptoms remain to be studied.

What is the relationship between the polyneuropathy, described in this case, and NPH? Most likely they are two, coexisting diseases, since it is difficult to find any causal connection between the hydrocephalic state and peripheral nerve damage. Weakness in the legs can be related to atrophy of the muscles caused by inactivity in a patient with disabled gait and balance. However, signs of polyneuropathy are very seldom found in patients with NPH, and weakness and atrophy related to inactivity are likely to improve after shunt surgery.

The peak age of onset of adult hydrocephalus is in the 70s; and in this age group, there are various disorders with central brain atrophy that clinically and radiologically resemble NPH.

To predict the outcome of shunt surgery in NPH, there are a number of complementary investigations, including the tap test, infusion manometry, radionuclide cisternography, measurement of cerebral blood flow, analyses of ICP waves, CSF analyses and specialized clinical examinations. Currently, most researchers recommend the CSF tap test or the external lumbar drainage procedure for prediction. Most authors agree that there is a need for improved diagnostic and predictive methods, especially since shunt surgery has a rather high complication rate. However, the perioperative mortality rate in modern series is close to zero. The relative improvement rate among the oldest operated group has been reported to be equivalent to that of other age groups, and thus age should not exclude patients from surgery.

Hydrocephalus is not an uncommon affliction and causes a significant proportion of new cases of dementia. In Sweden, the annual incidence of surgery for hydrocephalus is 3.4 per 100,000 adult inhabitants. Furthermore, there are significant differences in the incidence of surgery between different regions of the country. Since NPH is a chronic disease with a slow onset, atypical cases in elderly patients easily go unnoticed. It is likely that an increased awareness of the condition will lead to an increased referral rate. Since hydrocephalus is a treatable condition, it is important that knowledge about its symptoms and basic diagnostics be widely disseminated, particularly because the treatment available to these patients often drastically improves their quality of life.

Magnus Tisell, MD, PhD

From the Institute of Clinical Neurosecience, Goteborg, Sweden.

Reprint requests to Magnus Tisell, MD, PhD, Institute of Clinical Neuroscience, Van 5, Bla Straket 7, 413 45 Goteborg, Sweden.

Accepted March 30, 2006.