Differential Diagnosis for Skin Darkening Under One Eye

• Single Most Likely Diagnosis
  • Allergic contact dermatitis or atopic dermatitis: This is a common condition that can cause skin darkening due to inflammation and post-inflammatory hyperpigmentation, especially if the individual has allergies or is prone to eczema.
• Other Likely Diagnoses
  • Melasma: Although more common on the face, melasma can sometimes appear under the eyes due to hormonal changes or sun exposure, leading to patches of darker skin.
  • Periorbital hyperpigmentation: This condition is characterized by dark circles under the eyes and can be due to various factors including genetics, thin skin, and increased visibility of blood vessels.
  • Eczema (atopic dermatitis): Similar to allergic contact dermatitis, eczema can lead to darkened skin under the eyes due to chronic inflammation and scratching.
• Do Not Miss Diagnoses
  • Basal cell carcinoma or other skin cancers: Although rare, skin cancers can present as pigmented lesions and must be considered, especially if there's a history of sun exposure or if the lesion is growing or changing.
  • Sarcoidosis: This systemic disease can cause skin lesions, including those under the eyes, and is important to diagnose due to its potential systemic implications.
• Rare Diagnoses
  • Dermatomyositis: An autoimmune condition that can cause skin changes, including darkening, especially on the eyelids.
  • Addison's disease: A rare endocrine disorder that can cause hyperpigmentation of the skin, including under the eyes, due to increased levels of adrenocorticotropic hormone (ACTH).
  • Neurofibromatosis: A genetic disorder that can lead to cafe-au-lait spots or other pigmentation changes, potentially under the eyes, although this would typically be part of a broader constellation of findings.