Differential Diagnosis for a Patient with Rheumatoid Arthritis Presenting with Rash and Burning Sensation

Single Most Likely Diagnosis

• Drug-induced rash or Stevens-Johnson syndrome due to methotrexate or hydroxychloroquine: This is a common side effect of these medications, especially methotrexate, which can cause a range of skin reactions from mild to severe, including Stevens-Johnson syndrome. The timing of the symptoms, starting after 6 months of treatment, fits with a potential delayed hypersensitivity reaction to one of the medications.

Other Likely Diagnoses

• Photosensitivity reaction: Both hydroxychloroquine and methotrexate can increase the risk of photosensitivity, leading to a rash and burning sensation, especially in sun-exposed areas.
• Methotrexate-induced folate deficiency: Methotrexate can lead to folate deficiency, which may cause a range of symptoms including skin problems. However, this is less directly linked to a rash and burning sensation compared to other options.
• Rheumatoid arthritis flare or associated condition: Although less likely given the description, it's possible that the symptoms could be related to a flare of rheumatoid arthritis or another condition associated with rheumatoid arthritis, such as a vasculitis.

Do Not Miss Diagnoses

• Toxic epidermal necrolysis (TEN): A severe skin condition usually triggered by medications, including those used to treat rheumatoid arthritis. It's a medical emergency and requires immediate attention.
• Severe cutaneous adverse reactions (SCARs): This includes conditions like Stevens-Johnson syndrome and toxic epidermal necrolysis, which are potentially life-threatening and require immediate medical intervention.
• Drug-induced lupus erythematosus: Hydroxychloroquine can rarely induce lupus-like symptoms, including a rash. Although the patient is already on treatment that includes hydroxychloroquine for rheumatoid arthritis, a drug-induced lupus could still be a consideration.

Rare Diagnoses

• Pellagra: A condition caused by niacin deficiency, which can be exacerbated by certain medications, including those used in the treatment of rheumatoid arthritis. It leads to dermatitis, diarrhea, dementia, and potentially death if not treated.
• Erythema multiforme: A skin condition characterized by target lesions, which can be triggered by medications or infections. It's less common but should be considered in the differential diagnosis of a patient with a new rash.

Treatment Approach

The treatment approach will depend on the specific diagnosis. For drug-induced reactions, discontinuation of the offending drug is usually the first step. Supportive care, including topical treatments for the skin and protection from the sun, may be necessary. In cases of severe reactions like Stevens-Johnson syndrome or toxic epidermal necrolysis, hospitalization and specialized care are required. For conditions like pellagra, supplementation with niacin is essential. Each potential diagnosis requires a tailored treatment plan, emphasizing the importance of a thorough diagnostic workup.