Why did the vesicular eruption occur only on the shoulder and hand, which were the areas of initial pain, and not on the other side?

Why Vesicular Eruption Occurred Only on Painful Shoulder and Hand

The vesicular eruption occurred only on the shoulder and hand because herpes zoster (shingles) follows a unilateral dermatomal distribution pattern, with the virus reactivating in a single dorsal root ganglion and traveling along specific sensory nerve pathways to produce skin lesions precisely where the prodromal pain was felt. 1

Pathophysiology of Dermatomal Distribution

• Herpes zoster is characterized by prodromal dermatomal pain occurring 24-72 hours before rash onset, followed by a unilateral vesicular eruption strictly confined to the dermatomal distribution of the affected nerve 1, 2
• The virus reactivates from a single dorsal root ganglion where it has remained latent since primary varicella infection, then travels along the sensory nerve fibers to the skin, explaining why lesions appear only in the specific dermatome where pain was initially experienced 1
• The unilateral dermatomal distribution is the hallmark feature that distinguishes herpes zoster from other vesicular conditions like herpes simplex virus (which lacks strict dermatomal distribution) or varicella (which presents with widespread bilateral eruption) 1, 2

Clinical Progression Pattern

• The lesion progression follows a predictable sequence: erythematous macules → papules → vesicles → pustules → ulcers, with lesions frequently coalescing within the affected dermatome 1, 2
• New lesions continue to erupt for 4-6 days within the same dermatomal distribution, but do not cross the midline or affect the contralateral side 1
• The duration of lesions in immunocompetent hosts is approximately 2 weeks, with all lesions remaining confined to the initially affected dermatome 1

Why the Contralateral Side is Spared

• The virus remains localized to the single affected dorsal root ganglion and does not simultaneously reactivate in multiple ganglia in typical cases 1
• The pain-then-rash sequence occurs because sensory nerve inflammation precedes viral migration to the skin, so only the dermatome supplied by the inflamed nerve develops lesions 1, 2
• Bilateral or non-dermatomal distribution would suggest an alternative diagnosis such as herpes simplex virus infection, varicella, or disseminated zoster (which occurs in only 10-20% of immunocompromised patients without prompt antiviral therapy) 1, 2

Critical Diagnostic Confirmation

• The pattern of dermatomal pain preceding rash by 1-3 days strongly confirms herpes zoster as the diagnosis 1, 2
• Laboratory confirmation through PCR testing of vesicle fluid is the most sensitive and specific method to definitively confirm varicella-zoster virus and exclude herpes simplex virus 1
• Direct fluorescent antibody testing or Tzanck smear showing multinucleated giant cells can confirm herpesvirus infection but cannot differentiate between HSV and VZV 1, 2

Common Pitfall to Avoid

• Do not expect bilateral involvement or symmetric distribution in herpes zoster—this would indicate either disseminated disease (requiring immediate evaluation for immunocompromise) or an alternative diagnosis entirely 1, 2
• The strict unilateral dermatomal pattern is so characteristic that deviation from this pattern should prompt reconsideration of the diagnosis and pursuit of laboratory confirmation 1, 2