LAUSR

Normal pressure hydrocephalus is known as “curable dementia”, since patients might experience a significant improvement of their symptoms after shunt surgery. However, accurate prediction, which patients might benefit from surgery is still challenging due to several factors, i.e. comorbidities or misdiagnoses. A lumbar tap is often performed to evaluate the effects of cerebrospinal fluid (CSF) drainage preceding the shunt surgery. It exhibits a high specificity in the identification of therapy responders, but only a low sensitivity. Shunt surgery might lead to long-term advancements, which are not instantly reflected in short-term clinical symptom improvement after lumbar tap. To optimize the prognostic accuracy as well as the choice of appropriate treatment for patients with idiopathic normal pressure hydrocephalus, a valid imaging biomarker for beneficial shunt surgery would be highly desirable. A wide variety of predictive imaging biomarkers has been reported, such as the penumbra in stroke, diffusion metrics in intracerebral haemorrhage and traumatic brain injury, or tumor grading with magnetic resonance spectroscopy (MRS) and chemical exchange saturation transfer (CEST) imaging. This shows that by the use of advanced imaging methods, information on disease status and outcome can be obtained more and more noninvasively. In this issue of JMRI, Osawa et al. examine the dynamic changes in the apparent diffusion coefficient during the cardiac cycle (ΔADC) of the brain after lumbar tap and after shunt surgery in comparison to the ΔADC before intervention. ΔADC is defined as the difference between the maximum and the minimum of the ADC within a cardiac cycle in a dedicated region of interest; here, the frontal white matter. Previous publications have demonstrated that ΔADC is significantly higher in idiopathic normal pressure hydrocephalus than in controls. Therefore, ΔADC may have an important role in the causative mechanisms of the symptoms. It can be hypothesized that ΔADC normalizes after successful therapy. Regarding the lumbar tap, the authors categorized the patients into a “positive group” with symptomatic improvements and a “negative group” without symptomatic improvements and investigated them separately. ΔADC in the positive group decreased significantly after lumbar tap, while ΔADC in the negative group did not change. Regarding shunt surgery, only therapy-responders were investigated and ΔADC decreased in all seven patients. In summary, Osawa et al. nicely demonstrate that the ΔADC highly correlates with a beneficial outcome after lumbar tap and after shunt surgery. The noninvasive measurement of ΔADC might thus serve as a prognostic imaging biomarker to better select patients with idiopathic normal pressure hydrocephalus for subsequent shunt surgery. However, this concept has to be further validated in a larger patient population and especially in patients where lumbar tap provides negative or inconclusive results, since from a clinical point of view this “difficult” group of patients should be the primary target to address.