Diagnosis and Classification of Normal Pressure Hydrocephalus

 Normal pressure hydrocephalus (NPH) is a clinical syndrome of gait abnormality, cognitive impairment, and urinary incontinence. Evaluation of CSF dynamics, patterns of fludeoxyglucose (FDG) uptake, and patterns of brain stiffness may aid in the evaluation of challenging cases that lack typical clinical and structural radiographic features.

In this episode, Katie Grouse, MD, FAAN, speaks with Aaron Switzer, MD, MSc, author of the article “Radiographic Evaluation of Normal Pressure Hydrocephalus” in the Continuum® June 2025 Disorders of CSF Dynamics issue.

Dr. Grouse is a Continuum® Audio interviewer and a clinical assistant professor at the University of California San Francisco in San Francisco, California.

Dr. Switzer is a clinical assistant professor of neurology in the department of clinical neurosciences at the University of Calgary in Calgary, Alberta, Canada.

Additional Resources

Read the article: Radiographic Evaluation of Normal Pressure Hydrocephalus

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Full episode transcript available here

Dr Jones: This is Dr Lyell Jones, Editor-in-Chief of Continuum. Thank you for listening to Continuum Audio. Be sure to visit the links in the episode notes for information about earning CME, subscribing to the journal, and exclusive access to interviews not featured on the podcast.

Dr Grouse: This is Dr Katie Grouse. Today I'm interviewing Dr Aaron Switzer about his article on radiographic evaluation of normal pressure hydrocephalus, which he wrote with Dr Patrice Cogswell. This article appears in the June 2025 Continuum issue on disorders of CSF dynamics. Welcome to the podcast, and please introduce yourself to our audience.

Dr. Switzer: Thanks so much for having me, Katie. I'm a neurologist that's working up in Calgary, Alberta, Canada, and I have a special interest in normal pressure hydrocephalus. So, I'm very happy to be here today to talk about the radiographic evaluation of NPH.

Dr Grouse: I'm so excited to have you here today. It was really wonderful to read your article. I learned a lot on a topic that is not something that I frequently evaluate in my clinic. So, it's really just a pleasure to have you here to talk about this topic. So, I'd love to start by asking, what is the key message that you hope for neurologists who read your article to take away from it?

Dr. Switzer: The diagnosis of NPH can be very difficult, just given the clinical heterogeneity in terms of how people present and what their images look like. And so, I'd like readers to know that detailed review of the patient's imaging can be very helpful to identify those that will clinically improve with shunt surgery.

Dr Grouse: There's another really great article in this edition of Continuum that does a really great job delving into the clinical history and exam findings of NPH. So, I don't want to get into that topic necessarily today. However, I'd love to hear how you approach a case of a hypothetical patient, say, where you're suspicious of NPH based on the history and exam. I'd love to talk over how you approach the imaging findings when you obtain an MRI of the brain, as well as any follow-up imaging or testing that you generally recommend.

Dr. Switzer: So, I break my approach down into three parts. First, I want to try to identify ventriculomegaly and any signs that would support that, and specifically those that are found in NPH. Secondly, I want to look for any alternative pathology or evidence of alternative pathology to explain the patient's symptoms. And then also evaluate any contraindications for shunt surgery. For the first one, usually I start with measuring Evans index to make sure that it's elevated, but then I want to measure one of the other four measurements that are described in the article, such as posterior colossal angle zed-Evans index---or z-Evans index for the American listeners---to see if there's any other features that can support normal pressure hydrocephalus. It's very important to identify whether there are features of disproportionately enlarged subarachnoid space hydrocephalus, or DESH, which can help identify patients who may respond to shunt surgery. And then if it's really a cloudy clinical picture, it's complicated, it's difficult to know, I would usually go through the full evaluation of the iNPH radscale to calculate a score in order to determine the likelihood that this patient has NPH. So, the second part of my evaluation is to rule out evidence of any alternative pathology to suggest another cause for the patient's symptoms, such as neurodegeneration or cerebrovascular disease. And then the third part of my evaluation is to look for any potential contraindications for shunt surgery, the main one being cerebral microbleed count, as a very high count has been associated with the hemorrhagic complications following shunt surgery.

Dr Grouse: You mentioned about your use of the various scales to calculate for NPH, and your article does a great job laying them out and where they can be helpful. Are there any of these scales that can be reasonably relied on to predict the presence of NPH and responsiveness to shunt placement?

Dr. Switzer: I think the first thing to acknowledge is that predicting shunt response is still a big problem that is not fully solved in NPH. So, there is not one single imaging feature, or even combination of imaging features, that can reliably predict shunt response. But in my view and in my practice, it's identifying DESH, I think, is really important---so, the disproportionately enlarged subarachnoid space hydrocephalus---as well as measuring the posterior colossal angle. I find those two features to be the most specific.

Dr Grouse: Now you mentioned the concept of the NPH subtypes, and while this may be something that many of our listeners are familiar with, I suspect that, like myself when I was reading this article, there are many who maybe have not been keeping up to date on these various subtypes. Could you briefly tell us more about these NPH subtypes?

Dr. Switzer: Sure. The Japanese guidelines for NPH have subdivided NPH into three different main categories. So that would be idiopathic, delayed onset congenital, and secondary normal pressure hydrocephalus. And so, I think the first to talk about would be the secondary NPH. We're probably all more familiar with that. That's any sort of pathology that could lead to disruption in CSF dynamics. These are things like, you know, a slow-growing tumor that is obstructing CSF flow or a widespread meningeal process that's reducing absorption of CSF, for instance. So, identifying these can be important because it may offer an alternative treatment for what you're seeing in the patient. The second important one is delayed onset congenital. And when you see an image of one of these subtypes, it's going to be pretty different than the NPH because the ventricles are going to be much larger, the sulcal enfacement is going to be more diffuse. Clinically, you may see that the patients have a higher head circumference. So, the second subtype to know about would be the delayed onset congenital normal pressure hydrocephalus. And when you see an image of one of these subtypes, it's going to be a little different than the imaging of NPH because the ventricles are going to be much larger, the sulcal enfacement is going to be more diffuse. And there are two specific subtypes that I'd like you to know about. The first would be long-standing overt ventriculomegaly of adulthood, or LOVA. And the second would be panventriculomegaly with a wide foramen of magendie and large discernomagna, which is quite a mouthful, so we just call it PAVUM. The importance of identifying these subtypes is that they may be amenable to different types of treatment. For instance, LOVA can be associated with aqueductal stenosis. So, these patients can get better when you treat them with an endoscopic third ventriculostomy, and then you don't need to move ahead with a shunt surgery. And then finally with idiopathic, that's mainly what we're talking about in this article with all of the imaging features. I think the important part about this is that you can have the features of DESH, or you can not have the features of DESH. The way to really define that would be how the patient would respond to a large-volume tap or a lumbar drain in order to define whether they have this idiopathic NPH.

Dr Grouse: That's really helpful. And for those of our listeners who are so inclined, there is a wonderful diagram that lays out all these subtypes that you can take a look at. I encourage you to familiarize yourself with these different subtypes. Now it was really interesting to read in your article about some of the older techniques that we used quite some time ago for diagnosing normal pressure hydrocephalus that thankfully we're no longer using, including isotope encephalography and radionuclide cisternography. It certainly made me grateful for how we've come in our diagnostic tools for NPH. What do you think the biggest breakthrough in diagnostic tools that are now clinically available are?

Dr. Switzer: You know, definitely the advent of structural imaging was very important for the evaluation of NPH, and specifically the identification of disproportionately enlarged subarachnoid space hydrocephalus, or DESH, in the late nineties has been very helpful for increasing the specificity of diagnosis in NPH. But some of the newer technologies that have become available would be phase-contrast MRI to measure the CSF flow rate through the aqueduct has been very helpful, as well as high spatial resolution T2 imaging to actually image the ventricular system and look for any evidence of expansion of the ventricles or obstruction of CSF flow.

Dr Grouse: Regarding the scales that you had referenced earlier, do you think that we can look forward to more of these scales being automatically calculated and reported by various software techniques and radiographic interpretation techniques that are available or going to be available?

Dr. Switzer: Definitely yes. And some of these techniques are already in development and used in research settings, and most of them are directed towards automatically detecting the features of DESH. So, that's the high convexity tight sulci, the focally enlarged sulci, and the enlarged Sylvian fissures. And separating the CSF from the brain tissue can help you determine where CSF flow is abnormal throughout the brain and give you a more accurate picture of CSF dynamics. And this, of course, is all automated. So, I do think that's something to keep an eye out for in the future.

Dr Grouse: I wanted to ask a little more about the CSF flow dynamics, which I think may be new to a lot of our listeners, or certainly something that we've only more recently become familiar with. Can you tell us more about these advances and how we can apply this information to our evaluations for NPH?

Dr. Switzer: So currently, only the two-dimensional phase contrast MRI technique is available on a clinical basis in most centers. This will measure the actual flow rate through the cerebral aqueduct. And so, in NPH, this can be elevated. So that can be a good supporting marker for NPH. In the future, we can look forward to other techniques that will actually look at three-dimensional or volume changes over time and this could give us a more accurate picture of aberrations and CSF dynamics.

Dr Grouse: Well, definitely something to look forward to. And on the topic of other sort of more cutting-edge or, I think, less commonly-used technologies, you also mentioned some other imaging modalities, including diffusion imaging, intrathecal gadolinium imaging, nuclear medicine studies, MR elastography, for example. Are any of these modalities particularly promising for NPH evaluations, in your opinion? Do you think any of these will become more popularly used?

Dr. Switzer: Yes, I think that diffusion tract imaging and MR elastography are probably the ones to keep your eye out for. They're a little more widely applicable because you just need an MR scanner to acquire the images. It's not invasive like the other techniques mentioned. So, I think it's going to be a lot easier to implement into clinical practice on a wide scale. So, those would be the ones that I would look out for in the future.

Dr Grouse: Well, that's really exciting to hear about some of these techniques that are coming that may help us even more with our evaluation. Now on that note, I want to talk a little bit more about how we approach the evaluation and, in your opinion, some of the biggest pitfalls in the evaluation of NPH that you've found in your career.

Dr. Switzer: I think there are three of note that I'd like to mention. The first would be overinterpreting the Evans index. So, just because an image shows that there's an elevated Evans index does not necessarily mean that NPH is present. So that's where looking for other corroborating evidence and looking for the clinical features is really important in the evaluation. Second would be misidentifying the focally enlarged sulci as atrophy because when you're looking at a brain with these blebs of CSF space in different parts of the brain, you may want to associate that to neurodegeneration, but that's not necessarily the case. And there are ways to distinguish between the two, and I think that's another common pitfall. And then third would be in regards to the CSF flow rate through the aqueduct. And so, an elevated CSF flow is suggestive of NPH, but the absence of that does not necessarily rule NPH out. So that's another one to be mindful of.

Dr Grouse: That's really helpful. And then on the flip side, any tips or tricks or clinical pearls you can share with us that you found to be really helpful for the evaluation of NPH?

Dr. Switzer: One thing that I found really helpful is to look for previous imaging, to look if there were features of NPH at that time, and if so, have they evolved over time; because we know that in idiopathic normal pressure hydrocephalus, especially in the dash phenotype, the ventricles can become larger and the effacement of the sulci at the convexity can become more striking over time. And this could be a helpful tool to identify how long that's been there and if it fits with the clinical history. So that's something that I find very helpful.

Dr Grouse: Absolutely. When I read that point in your article, I thought that was really helpful and, in fact, I'm guessing something that a lot of us probably aren't doing. And yet many of our patients for one reason or other, probably have had imaging five, ten years prior to their time of evaluation that could be really helpful to look back at to see that evolution.

Dr. Switzer: Yes, absolutely.

Dr Grouse: It's been such a pleasure to read your article and talk with you about this today. Certainly a very important and helpful topic for, I'm sure, many of our listeners.

Dr. Switzer: Thank you so much for having me.

Dr Grouse: Again, today I've been interviewing Dr Aaron Switzer about his article on radiographic evaluation of normal pressure hydrocephalus, which he wrote with Dr Patrice Cogswell. This article appears in the most recent issue of Continuum on disorders of CSF dynamics. Be sure to check out Continuum Audio episodes from this and other issues, and thank you to our listeners for joining today.

Dr Monteith: This is Dr Teshamae Monteith, Associate Editor of Continuum Audio. If you've enjoyed this episode, you'll love the journal, which is full of in-depth and clinically relevant information important for neurology practitioners. Use the link in the episode notes to learn more and subscribe. AAN members, you can get CME for listening to this interview by completing the evaluation at continpub.com/audioCME. Thank you for listening to Continuum Audio.