How can I differentiate between herpes zoster and other dermatoses in a patient presenting with a unilateral vesicular rash?

To differentiate herpes zoster from other dermatoses in a patient presenting with a unilateral vesicular rash, consider the following key clinical features:

• Distribution: Herpes zoster typically presents as a painful, unilateral vesicular rash confined to a single dermatome, often thoracic or cranial, without crossing the midline.
• Prodromal symptoms: Patients often report localized pain, burning, or tingling in the affected dermatome before rash onset.
• Vesicle characteristics: The rash consists of grouped vesicles on an erythematous base that evolve to pustules and crusts over 7-10 days.
• Associated signs: Involvement of the ophthalmic branch of the trigeminal nerve (e.g., rash on the tip or side of the nose known as Hutchinson's sign) suggests herpes zoster ophthalmicus and requires urgent specialist assessment.
• Systemic symptoms: Fever and malaise may accompany the rash but are not always present.
• Exclusion of other causes: Other dermatoses such as contact dermatitis, eczema herpeticum, or bullous impetigo usually have different distribution patterns, lack dermatomal confinement, and may not present with the characteristic grouped vesicles and prodromal pain.

Clinical judgment supported by these features helps distinguish herpes zoster from other vesicular dermatoses. If diagnosis is uncertain or complications are suspected, referral to dermatology or specialist advice is recommended.

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