This blog post is based on a webinar hosted by David Harker, MD. For upcoming webinars, click here.
Imagine a medicine resident shrugging and saying, “All EKGs look the same to me.” While this sounds ridiculous, the phrase “all rashes look the same to me” is frequently uttered by non-dermatologist clinicians at all levels of experience.
But to the trained eye, all rashes don’t look the same. Despite this, learning to observe and describe a rash gets little emphasis in general medical education. Consequently, it is a skill few clinicians outside of dermatology acquire.
This dearth of dermatology exposure in general medical education, combined with the relatively small number of dermatologists, leads to an unfortunate reality. While skin disease is very common, those adept at diagnosing skin disease are not.
What follows here is meant to provide insight into how dermatologists think and arrive at specific diagnoses. In medical school, the history is emphasized as an all-important tool and the first step of any patient encounter. While history still matters in dermatology, for most skin disease it is secondary to the physical exam in discerning the diagnosis. This reality is why learning how to observe the salient features of a rash and describe them is so critical. If you’ve correctly described a rash then you’ve identified the morphologic features that allow you to build a differential diagnosis. A rash of scaly papules and plaques has a completely different differential than a rash of vesicles and erosions. So, let’s start with some definitions you probably were exposed to briefly during your medical training.
Bullae, vesicles, and pustules
Blisters in the skin.
- Bulla: >1 cm
- Vesicle: <1 cm
- Pustule: blister filled with pus
A tense bulla is caused by subepidermal pathology (at the dermo-epidermal junction). Bullae that are easily ruptured (called flaccid bullae) are cause by intraepidermal pathology. Sometimes, in diseases like pemphigus vulgaris that cause flaccid bullae, there may not be any intact bullae on the skin—just erosions.
Erosions and ulcers
- Defects in the skin
- An erosion is a defect in skin involving part or all of the epidermis, but the dermis is preserved.
- An ulcer arises from loss of the entire epidermis and part or all of the dermis.
After you have determined the primary lesion(s), identifying the secondary features can help in making a diagnosis.
About Dr. Harker
Dr. Harker earned his doctor of medicine degree at The University of Texas Southwestern Medical School. In addition to excelling in his studies and clinical rotations, Dr. Harker invested significant time researching the immunology of melanoma, presenting posters at academic conferences, and contributing to publications in high-profile dermatology journals. After medical school, Dr. Harker completed an internship in internal medicine at Baylor University Medical Center. He then returned to The University of Texas Southwestern Medical Center for dermatology residency training. During residency, Dr. Harker served as chief resident over the VA dermatology clinic. As chief resident, he provided critical leadership through the COVID-19 crisis and was instrumental in establishing a teledermatology service for the clinic. Dr. Harker was recognized for his clinical acumen and surgical skills as a resident. In July 2020, Dr. Harker joined the Skin Surgery Center in Hickory, North Carolina.