Dr. Siegfried: So how does that differ from the eruptive keratin cysts that happen with isotretinoin?
Dr. Leyden: You can have keratinous cysts as part of the attempt to heal disrupted follicular epithelium that occurs in acne. Occasionally with the higher doses of 1 mg/kg, you can get eruptive keratinous cysts. A keratinous cyst is another imperfect healing.
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You can recognize them in a couple of ways. First of all, these areas are constantly recurring areas of inflammation in the exact same spot. If you suspect it clinically or you hear that history, feel around the obvious area of inflammation under the surface. You can feel the rest of the lesion as something you can define and get your fingers around. They can occur eruptively with the 1 mg/kg dose.
Dr. Siegfried: That reminds me of John Strauss' teaching when I was an early resident. He actually palpated everyone's acne and sometimes with teenagers they are mortified when that happens. But it was an incredible, valuable clinical tool and you can learn a lot by feeling it as well as looking at it. He couldn’t really judge cystic acne alone without palpating it.
Leyden: I could not agree more. Hoffmann-La Roche had a new formulation of isotretinoin that got approved by the FDA and then they decided not to market it because the FDA wanted a phase IV study that would have taken three generations of dermatologists to be involved in. They were concerned about, for one thing, whether or not ideation of suicide could be induced by the drug. But I looked at the baseline photographs of all the people in the study and I identified 37 patients who had lesions that I said would not go away with the drug, because I thought they were either a sinus tract or keratinous cyst, and I was right with 37 out of 37.
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Once you have learned to recognize these things clinically and to distinguish them from nodular areas of inflammation that you can’t get your hands around and feel, they’re pretty easy to recognize. But until you do, it is a little bit like listening to mitral murmurs; somebody can hear them, but you’re having trouble hearing them.