When two truths collide: overlapping herpes zoster and leukocytoclastic vasculitis in an atypical presentation

Diagnostic reasoning often favors simplicity, with clinicians seeking a single unifying explanation for a patient's presentation. However, in certain cases, a narrow approach can lead to diagnostic oversight and delay. In this case, an early attribution of symptoms to leukocytoclastic vasculitis (LCV) postponed the recognition and treatment of an underlying varicella-zoster virus (VZV) infection.

An 88-year-old woman presented with palpable purpuric macules in a linear distribution along the right pretibial region, dorsum of the foot, and toes, accompanied by severe unilateral pain radiating from the foot to the buttock. The pain preceded the rash by one day, and no vesicles were noted by the patient or clinician. Clinically, the morphology was consistent with leukocytoclastic vasculitis, but the pain followed a dermatomal distribution, raising concern for varicella-zoster virus (VZV) infection. A biopsy ultimately confirmed both leukocytoclastic vasculitis and viral cytopathic changes. The patient improved with a combination of valacyclovir, prednisone, and gabapentin.

This case illustrates a diagnostically challenging presentation in which two distinct pathologies mimicked and masked one another. Anchoring on a unifying diagnosis led to premature closure, delaying antiviral therapy. Herpes zoster without vesicles is uncommon but documented, and VZV-induced vasculitis is rare. Together, they created a deceptive clinical picture. The case reinforces several key teaching points: the value of maintaining diagnostic flexibility, the importance of empiric antiviral treatment when herpes zoster is suspected, and the need to recognize cognitive biases that can distort clinical judgment. When findings are discordant or incomplete, clinicians should resist diagnostic simplification and remain open to diagnosing coexisting conditions.