Differential Diagnosis for Petechiae on One Arm Only

• Single most likely diagnosis
  • Trauma: The most likely cause of petechiae on one arm only is trauma, which could be due to a physical injury, tight clothing, or even a tourniquet effect from a blood pressure cuff. This would lead to localized damage to the small blood vessels, resulting in petechiae.
• Other Likely diagnoses
  • Vasculitis: Conditions like leukocytoclastic vasculitis can cause petechiae, and while they often present more diffusely, it's possible for them to start or be more pronounced in one area, such as an arm.
  • Infection: Certain infections, particularly those caused by bacteria or viruses, can lead to vasculitis or direct damage to blood vessels, resulting in petechiae. The localization to one arm could be due to the initial site of infection or a localized immune response.
  • Allergic reactions: Allergic reactions to substances like insect bites, certain medications, or contact dermatitis could potentially cause localized petechiae, although this is less common.
• Do Not Miss (ddxs that may not be likely, but would be deadly if missed)
  • Septic emboli: In the context of endocarditis or other severe infections, septic emboli could lodge in the small vessels of one arm, causing localized petechiae among other symptoms. This condition is life-threatening and requires immediate medical attention.
  • Malignancy: Certain types of cancer, such as lymphoma or leukemia, can cause petechiae due to bone marrow suppression or direct vascular invasion. While rare, missing a diagnosis of malignancy could have severe consequences.
  • Antiphospholipid syndrome: This autoimmune disorder can cause blood clots in small vessels, leading to petechiae, among other symptoms. It's crucial to diagnose and treat this condition to prevent further thrombotic events.
• Rare diagnoses
  • Fabry disease: A genetic disorder that leads to the accumulation of a particular type of fat in the body's cells, affecting many parts of the body including the skin, where it can cause petechiae.
  • Ehlers-Danlos syndrome: A group of disorders that affect connective tissues supporting the skin, bones, blood vessels, and many other organs and tissues. Some types can lead to easy bruising and petechiae due to fragile blood vessels.
  • Scurvy: Vitamin C deficiency can lead to weakened blood vessel walls, resulting in petechiae, among other symptoms like poor wound healing and joint pain.