Prevalence and Differential Diagnosis of Prurigo Nodularis (PN)

Raj Chovatiya, MD, PhD: Hello, and welcome to this HCPLive® Peer Exchange titled, “Advances in the Management of Prurigo Nodularis”[PN].My name is Dr Raj Chovatiya, and I’m an assistant professor of dermatology, director of the Center of Eczema and Itch, and medical director of the Clinical Trials unit in the Department of Dermatology at Northwestern University Feinberg School of Medicine in Chicago, Illinois. Joining me in this discussion are my colleagues. I’m pleased to have with me Shawn Kwatra, MD, associate professor of Dermatology at the Johns Hopkins University School of Medicine in Baltimore, Maryland, as well as Sarina Elmariah, MD, PhD, a dermatologist formerly at the Massachusetts General Hospital in Boston, and now at the University of California, San Francisco. Welcome to you both.

I’m really excited about our discussion today, which is going to focus on something we’ve been talking about a lot lately and something that’s gotten very exciting—the diagnosis and treatment of PN. The cool part of this is we’re going to get a chance to discuss data with the only Food and Drug Administration–approved treatment for PN, some implications for these results, and some future directions about where we might be headed. Thanks again, everyone. Let’s jump in.

Dr Kwatra, I can start with you. What do you think about when you hear the words PN? What is it? What are some of the symptoms and prevalence? What does everyone need to know?

Shawn Kwatra, MD: Thanks, Dr Chovatiya. When I think about PN, there are a few things patients must have. First, given by the name, is the presence of these nodules. They can be very large—up to a few centimeters—or they can be even just a few millimeters, so there’s a lot of heterogeneity and how these nodules may appear. However, some of the commonalities are that they tend to affect the trunk, the extremities, and these patients have a chronic itch. For at least 6 weeks, patients can suffer from itch, and there are signs of repetitive scratching on exams. Those are the core features of PN, and you can differentiate it from atopic dermatitis because you’ll often lack those intervening areas of eczema.

Raj Chovatiya, MD, PhD: It’s interesting because, with PN, I think it can be tough for us to really understand how common or uncommon it is. And I know there’s been a handful of stabs at trying to understand the prevalence. Maybe I can bounce over to you Dr Elmariah. Is this something we’re seeing all the time? Is this something that’s not popping into the clinic all that much? What are some of the open questions in terms of really understanding how common it is?

Sarina B. Elmariah, MD, PhD: I think there is a little bit of a difference between what’s reported and what’s out there in the literature in terms of the prevalence, which has been estimated to be approximately 72 patients per 100,000 adults in the United States. However, I think the issue is that there is more PN out there, and most dermatologists have seen it. I think a lot of primary care physicians and health care physician extenders have also encountered PN. It may not necessarily be that they’re diagnosed with PN; you may be diagnosed with eczema, idiopathic pruritus, or chronic pruritus of unknown origin, and sometimes even with delusions of parasitosis or other more psychiatric diseases. Often, PN may be mislabeled as something else, so I think the prevalence is a little bit higher than what we see. And certainly, once you specialize or you see a higher proportion of patients with inflammatory skin disease—and just itch in general—you’re going to see even more of those patients tracking through.

Raj Chovatiya, MD, PhD: I think you make a good point about the confusion and diagnosis, because I feel like if we were to rewind a little bit in terms of where eczema was as a field, I think people thought “Well, you were only seeing eczema in kids and not much in adults, so it must be something else.” Our estimates of prevalence have grown over time because we’ve understood that it presents in so many different ways. And, what Dr Kwatra brought up, PN can be quite heterogeneous.

Whenever someone walks into the clinic and is itchy, we all know, being in referral centers, that patients get labeled all sorts of things that may or may not be accurate. That’s one of the diagnostic boundaries when it comes to PN, because there are a lot of triggers, risk factors, and associated conditions that can cloud the picture, sometimes, in terms of what people are looking at. And as Dr Kwatra very nicely executed, there are definitely characteristic features you’re looking for when it comes to PN. However, that doesn’t mean there may not be underlying conditions or associated conditions that perhaps could be giving a ride to the initial state of itch that perhaps triggers off this entire situation.

This is where that debate has come into the field for those who are saying, “Well, PN was a primary disease, it’s a secondary disease—you have to have eczema, you can’t have eczema.” It’s been a fascinating debate. Dr Kwatra, how do you try to balance all that out when it comes to thinking about all these conditions that sometimes come along with PN?

Shawn Kwatra, MD: That’s a great question, Dr Raj Chovatiya. I love how you summarized that. And you’re right; this is a big question we have. The way I view PN is as people view pruritus in general. There’s a genetic impulse for patients who even develop a PN, and that’s the reason we do workup. We want to rule out liver disease, kidney disease, thyroid disease, whether it’s acute and onset, a malignancy. But what I would contend is there are other diseases that have comorbidities associated with them, so atopic dermatitis and psoriasis—we’re seeing all sorts of things—cardiometabolic diseases, and all those things. However, I think the end phenotype is conserved.

What we’re finding is that many of the therapies we’re using are going to be effective regardless of whether patients have X comorbidity or Y comorbidity. I think providers have given a disservice to patients who have PN, because some doctors have said, “Well, you have diabetes, so why don’t you just control your diabetes and your PN will go away?” Well, that’s not the case. Once you have the disease, you need to have a directed therapy in terms of the underlying etiology. So, I still believe PN is a distinct disease with a very distinct molecular etiology. Even though there may be many associated diseases, I think that’s retained throughout the different phenotypes.

Transcript edited for clarity