Differential Diagnosis for Painful Rash

The following differential diagnosis is organized into categories to help guide the thought process.

• Single Most Likely Diagnosis

  • Herpes Zoster (Shingles): This is the most likely diagnosis given the description of a painful rash that started on the back and progressed to the arm and face. The rash is typically unilateral and follows a dermatomal distribution, which is consistent with the progression described.

• Other Likely Diagnoses

  • Contact Dermatitis: This could be considered if the rash is due to an allergic reaction or irritant, although the progression and pain might not fully align with typical presentations.
  • Eczema (Atopic Dermatitis): While eczema can cause a painful rash, its progression from the back to the arm and face is less typical.
  • Psoriasis: Although psoriasis can cause painful lesions, the pattern of spread described is not the most common presentation.

• Do Not Miss Diagnoses

  • Lyme Disease: Early stages can present with a rash (erythema migrans), which can be painful and expand. Given its potential for serious complications if untreated, it's crucial not to miss this diagnosis.
  • Rocky Mountain Spotted Fever (RMSF): This tick-borne illness can start with a rash that may be painful, although it typically starts on the wrists and ankles. The potential severity of RMSF makes it a "do not miss" diagnosis.
  • Meningitis: In rare cases, a rash can be associated with meningitis, especially in the context of certain bacterial infections like Neisseria meningitidis. The rash might be painful, and given the potentially fatal outcome of untreated meningitis, it's essential to consider this diagnosis.

• Rare Diagnoses

  • Erythema Multiforme: A skin condition characterized by target lesions, which can be painful. It's often associated with infections or medications.
  • Stevens-Johnson Syndrome/Toxic Epidermal Necrolysis: These are severe skin conditions usually triggered by medications or infections, characterized by blistering and peeling of the skin, which can be extremely painful. They are rare but life-threatening.
  • Dermatomyositis: An inflammatory disease characterized by skin rash and muscle weakness. The rash can be painful and may precede muscle weakness.

Each of these diagnoses has a different set of implications for treatment and prognosis, emphasizing the importance of a thorough diagnostic workup to determine the underlying cause of the painful rash.