Verification of the diagnosis of NPH, and the selection of patients for ventriculoatrial or ventriculoperitoneal shunt has presented difficulties. The CT scan, as shown in Fig. 29-4, displays enlarged ventricles without convolutional atrophy. This disproportionate enlargement of the ventricular system in comparison to the degree of cortical atrophy is judged by the CT and MRI appearance, but there is no broadly agreed upon method for its determination. Various unwieldy formulas have been designed to assess this ratio. As a group, patients who have a sustained response to drainage of CSF by shunting, as described below, have had the first two elements of the clinical triad (fewer than half of our successfully treated patients have reached the point of incontinence) and their lateral ventricular span at the level of the anterior horns has been in excess of approximately 39 mm (a true dimension calculated from CT or MRI scans). MRI may show some degree of transependymal egress of water surrounding the ventricles, but this is not usually the case, and this sign is sometimes difficult to differentiate from the periventricular white matter change that is ubiquitous in the elderly. There may be an indication of inadequate pulsatile flow of CSF through the aqueduct as appreciated on T2-weighted MRI.
A lumbar puncture is often performed for diagnostic purposes and the pressure measured carefully but here too, there is no uniformly agreed upon approach. In most cases of NPH, the CSF pressure is above 150 mm H2O with the patient fully relaxed, but the disorder has occurred, at least as judged by improvement with shunting, with lower pressures, in a few instances as low as 120 mm H2O. Drainage of large amounts of CSF (20 to 30 mL or more) by lumbar puncture often results in clinical improvement in stance and gait for a few days, usually with a delay of hours or a day after the puncture so the patient and family must be depended upon to report these changes and these reports are prone to excessive optimism. Objective improvement in gait after spinal drainage, measured by reduced time to walk a predetermined distance and fewer steps than prior to drainage, is one way to select patients for shunt operations when the clinical picture is not entirely clear, but the even this test is not infallible. Several small series suggest that a negative test does not preclude benefit from shunting (see Walchenbach et al). However, in these same series, improvement after removal of CSF has had a high predictive value for success of shunting.
When there has been doubt as to the effects of lumbar puncture, one appropriate course is to admit the patient to the hospital, and insert a lumbar drain for up to 3 days, removing approximately 50 mL of CSF daily in order to observe the response in gait and mentation. It is worthwhile to quantify the speed and facility of gait two or three times before the lumbar puncture or drainage and to perform this testing at periodic intervals for several days after the procedure in order to be certain that improvement is genuine. Even more persuasive is a definite improvement followed days later by worsening of gait.
Monitoring of CSF pressure over a day or more may show intermittent rises of pressure, possibly corresponding to the A-waves of Lundberg, but this undertaking is not generally practical and is now done by only a few centers. According to Katzman and Hussey, the infusion of normal saline into the lumbar subarachnoid space at a rate of 0.76 mL/min in NPH provokes a rise in pressure to greater than 300 mm H2O that is not observed in normal individuals. Theoretically, this test or any of its derivatives, such as the one proposed by Børgesen and Gjerris, should reflect the adequacy of CSF absorption, but they too, have yielded unpredictable results. Radionuclide cisternography had been used in the past to demonstrate persistence of CSF labeling in the ventricles and, although we still use it occasionally in uncertain cases of NPH, it is no longer considered a compelling test.
Treatment of NPH in Adults