Differential Diagnosis for Painful Rash

• Single most likely diagnosis
  • Herpes Zoster (Shingles): This is often the first consideration for a painful rash, given its prevalence and the distinctive pain that precedes the rash. The pain is typically unilateral and follows a dermatomal distribution.
• Other Likely diagnoses
  • Contact Dermatitis: This condition can cause a painful rash, especially if the offending agent is highly irritant or if the patient has a severe allergic reaction. The rash is usually confined to areas exposed to the irritant.
  • Eczema (Atopic Dermatitis): While not always painful, eczema can cause significant discomfort and, in severe cases, painful skin lesions. It's characterized by its chronic nature and association with atopy.
  • Cellulitis: An acute bacterial infection of the skin and subcutaneous tissue, cellulitis presents as a painful, erythematous, and warm area of the skin. It can be differentiated from other rashes by its rapid spread and systemic symptoms like fever.
• Do Not Miss (ddxs that may not be likely, but would be deadly if missed)
  • Necrotizing Fasciitis: A severe infection that destroys the tissue under the skin, including fat and the tissue covering the muscles (fascia). It's a medical emergency with high mortality if not promptly treated. Early signs can be subtle but include severe pain out of proportion to the appearance of the skin.
  • Stevens-Johnson Syndrome/Toxic Epidermal Necrolysis (SJS/TEN): These are severe skin and mucous membrane disorders, usually a reaction to medication or infection. They are characterized by blistering of the skin and mucous membranes and can be life-threatening.
  • Rocky Mountain Spotted Fever (RMSF): A tick-borne disease that can cause a painful rash, fever, and headache. It's critical to diagnose early as it can be fatal if not treated promptly.
• Rare diagnoses
  • Erythema Multiforme: An immune-mediated disorder that can cause a variety of skin lesions, including painful ones. It's often associated with infections (like herpes simplex) or medications.
  • Sweet Syndrome (Acute Febrile Neutrophilic Dermatosis): Characterized by the sudden onset of fever, leukocytosis, and tender, erythematous, well-demarcated papules and plaques. It's rare and can be associated with systemic diseases or medications.
  • Complex Regional Pain Syndrome (CRPS): A condition characterized by chronic pain, inflammation, and hypersensitivity after an injury or trauma, which can sometimes present with changes in skin appearance, including rashes.