The Challenges of Normal Pressure Hydrocephalus: A Case-Based Review

CONFIRMATORY STUDIES
Beyond neuroimaging, a variety of specialty studies are used to increase diagnostic certainty and as predictors of outcome from surgical intervention. These include large-volume lumbar tap (“tap test”), external lumbar drainage, nuclear or CT cisternogram, and CSF flow imaging. Each of these tests has some risk, and no single test has been conclusively demonstrated by itself to be superior to one or a combination of the others. No CSF biomarkers have as yet been identified for the diagnosis of iNPH.¹⁷

The simplest supplemental test is the CSF tap test, which involves the removal of 40 to 50 mL of CSF via lumbar puncture. The patient is then assessed for improvement of symptoms by comparing gait and cognition prior to the test with that from 30 to 60 minutes after. Patients with significant symptomatic improvement (lasting at least a few weeks and up to months) have been found to be good candidates for shunt surgery.¹⁸ Patients who have high opening pressure (> 20 cm H₂O) require further investigation for secondary causes of NPH (eg, meningitis).¹⁸ Routine CSF analysis should be done (cell count, protein, glucose) to rule out chronic meningitis, which can mimic NPH.

The external lumbar drainage (ELD) test involves placement of an indwelling external lumbar catheter (lumbar drain) for external drainage of approximately 300 mL/d of CSF over one to five days. It is useful in patients who do not have a significant response to the tap test and for whom a high index of suspicion for iNPH remains. A positive response to ELD has been found to predict a potentially positive shunt response.¹⁹ The ELD test has a high positive predictive value (80% to 100%).¹⁸

Nuclear or CT cisternography has been used to evaluate CSF reabsorption. In the presence of iNPH, cisternography demonstrates ventricular reflux with slow cortical uptake.^20,21 A positive cisternogram combined with a radioisotope CT exam that shows normal cerebral blood flow is better than cisternography alone in predicting positive outcome from shunt surgery.²²

CSF flow studies utilize T2-weighted images on MRI to estimate CSF flow through the ventricles. In the assessment of iNPH, evaluation of CSF flow by MRI is used in the preoperative evaluation and also in post–shunt-placement follow-up. Slow-moving CSF has an increased signal, while regions of fast-moving CSF, such as in a narrow cerebral aqueduct, have no signal. In the presence of iNPH, the cerebral aqueduct shows an increased pulsatile flow void, and there is a hypointense or absent signal in the proximal fourth ventricle on proton density–weighted images. The presence of an increased CSF flow void has been found to be highly predictive of a positive outcome from ventriculoperitoneal (VP) shunt placement.²³

Another approach involves the direct measurement of the velocity of CSF stroke volume, which is the mean volume of CSF that passes through the aqueduct during systole and diastole. Studies have found that a CSF stroke volume of 42 µL or greater is an indicator for a good probability of improvement after VP shunt placement.²⁴

Continue for management >>