You evaluate a patient with a rash, and as you look, you notice small patches of purple lesions that are circular in distribution. You recognize what appears to be purpura. The differential diagnosis is long, and includes hematologic, infectious, and immune mediated conditions. How can we use physical diagnosis to narrow the differential diagnosis at the bedside?
Blanching: Intravascular vrs extravascular
First, apply pressure to see if it blanches. Dermatologists use glass and call it diascopy. In the ED we just press, let go, and we have a quick look to see if it had blanched. Blanching indicates the color is intravascular, like erythema from a rash. If it blanches, it is not purpura. Nonblanching indicates the problem is extravascular. Your patient’s lesion does not blanch.
Palpable purpura: Inflammation
Next, is it palpable? Nonpalpable purpura is caused by blood leaking out of a normal blood vessel, either because of trauma or a hematologic problem. The most common benign cause is senile or actinic purpura in the elderly, where a thin dermis renders the capillaries vulnerable to minor trauma. A normal bruise is not palpable, nor is the purpura from thrombocytopenia and coagulation defects.
In contrast, palpable purpura means there is inflammation around the blood vessels. Your patient’s lesions are palpable. You have now narrowed the differential diagnosis to a vasculitis, or an infection causing inflammation around the blood vessels. The most common cause in children is Henoch-Schonlein purpura. The most common in adults is leukocytoclastic vasculitis, a small vessel vasculitis with various causes.
Meningococcemia is palpable in later stages of the disease but in the early stages may not be palpable, and in very early stages may actually blanch (Riordan 1996). Petechial lesions restricted to the area above the clavicles generally signifies a benign cause, generally precipitated by coughing or vomiting (Wells 2001).
Physical diagnosis narrows the differential diagnosis of purpura to a more manageable degree. With the use of pressure to check for blanching, one can confirm whether the purple hue is intravascular or extravascular. If it is extravascular then this suggests purpura. One can then check for palpable purpura to decide if inflammation is present. In this manner, one can pursue this diagnosis to the highest degree at the bedside. Depending on the findings, a phone call to the local dermatologist, rheumatologist, or hematologist and oncologist can expedite this patient’s definitive care.
TAKE HOME POINTS
-Blanching suggests a vascular cause. Nonblanching supports purpura.
-Palpable purpura supports an inflammatory cause such as vasculitis or a systemic infection