Overview of Plaque Psoriasis
SEPTEMBER 27, 2019
Mark Lebwohl, MD: Psoriasis can occur at any age. The youngest reported patient had it at birth. The oldest reported patient started with psoriasis at over the age of 100. So it can occur at any time. Typically, it’s characterized by waxing and waning—a little bit better, a little bit worse—with occasional flares sometimes induced by very specific triggers like a strep throat or some other infection, or sometimes induced by a medication. Only about 1 out of 10 patients spontaneously remit. The disease suddenly disappears often without a known cause and doesn’t come back for a long time, or ever. But that’s only true in about 1 out of 10 patients. The majority of patients have a relatively stable course of psoriasis.
What we’ve learned in recent years is that psoriasis isn’t just a disease of the skin. We’ve known for a long time that approximately 30% of patients have psoriatic arthritis where the joints are affected. It can be any joints, but particularly the small joints of the hands and feet. Some patients will have axial or spinal involvement, and others will have large joints. But psoriatic arthritis is a well-known comorbidity.
What we’ve learned in the past couple of decades is that patients with psoriasis have an increased risk of cardiovascular disease. For example, a patient with severe psoriasis at the age of 30 would have as much as a 3-fold increase in myocardial infarctions. Other comorbidities are hypertension, stroke, obesity. There are some papers that suggest that there’s an increase in certain cancers, particularly skin cancers and lymphoma. And other comorbidities like diabetes and hypertension occur as well.
Genetics plays a major role in the development of psoriasis. If you look at 2 parents who both have psoriasis, the chance of 1 of their children having psoriasis, or of a particular child having psoriasis, may be higher than 50%. If 1 parent has psoriasis, the chance of their child having psoriasis is approximately 1 out of 6. So genes certainly play a role. There are a lot of older twin studies that look at identical and fraternal twins and show that identical twins have a much higher likelihood of having psoriasis if 1 of the twins has psoriasis.
When we’ve looked for individual genes involved in the development of psoriasis, it’s become apparent very quickly that it is a multifactorial disease. It’s a combination of multiple genes with environmental factors that contribute to psoriasis—environmental factors being things like absence of sun exposure, which contributes to psoriasis. But there is more than 1 gene. There are many genes that have been implicated in the development of psoriasis.
There are other triggers for psoriasis in addition to the absence of sunlight, and I would say psoriasis is much more common at higher latitudes where there is less sunlight than equatorial latitudes where there’s more sun. But in addition, we know that in places where the seasons are more extreme, psoriasis tends to worsen in the winter and get better in the summer. That probably is because of sun exposure. So certainly, absence of sun is a trigger.
There are many other triggers. Guttate psoriasis, for example, is frequently triggered by strep infections, and that’s a kind of psoriasis that is more common in children, although it can occur in adults as well. Children will have a severe flare following a strep infection. And then, often following ultraviolet light treatments, they’ll clear and the condition might not ever come back. So that’s a well-known trigger of psoriasis.
There are a number of medications that are known to trigger psoriasis, the most common being the withdrawal of systemic steroids. So a patient who has mild psoriasis or doesn’t even know he or she has psoriasis, gets poison ivy, gets treated with prednisone, and when the prednisone is stopped, psoriasis flares badly. That is the most common medication trigger of psoriasis.
There are other well-known triggers. Lithium, for example, is a well-known trigger of psoriasis. There are some blood pressure medicines for which exacerbation of psoriasis can occur, although those tend to be more modest exacerbations, like beta blockers, ACE [angiotensin-converting enzyme] inhibitors. Antimalarials are known to trigger psoriasis. Interferon is known to trigger psoriasis. So there are a variety of medication triggers as well.
The severity of psoriasis is assessed by a number of measures. Typically, in a clinician’s office, they just look at a patient and make a judgment. Is it mild, moderate, or severe? Can I manage it with topical therapies? Do I need to go to systemic therapies because it’s too extensive? The impact that it has on a patient’s life factors in there as well. When we look at it scientifically in clinical trials, for example, the most common tool used to measure psoriasis is called the PASI score, the Psoriasis Area and Severity Index. What we incorporate into that score is a measure of the body surface area that’s affected combined with the severity measured on a 0 to 4 scale of plaque thickness, scaling, and redness. Each of those is assigned a 0 to 4. You end up with a score that’s somewhere from 0, which means no psoriasis, to 72, which means every spot on your body is covered, head to toe, with severe, thick, red scaly plaques. That would be a 72. And of course, patients with bad psoriasis are anywhere from 10 or higher.
The majority of patients measure 10 or lower. Roughly 80% of patients fall into that 0 to 10 measure where their psoriasis isn’t that bad, although it’s still bad enough to treat. But numbers higher than 10 tend to be more severe. Those are the numbers that usually require systemic therapy. Even in patients who have scores below 10, if you think of a PASI score, which incorporates the percentage of body surface area, the palm of the hand is 1%. Well, patients who have just their hands involved, both hands, that’s severe to them. Every time they shake someone’s hand, they have to be embarrassed about their psoriasis. Every time they button their shirt, they might not be able to button their shirt or their skin cracks and bleeds and they stain their clothes. So even just 2% that involves the hands is bad psoriasis. Severe psoriasis of the scalp that extends on to the face can be severe even though it’s a small percentage of the body surface area because people see it. And so, it has a dramatic impact on the patient’s quality of life.
Transcript edited for clarity.