Herpes Zoster (Shingles): A High-Yield USMLE Review
Herpes zoster is reactivation of latent varicella-zoster virus (VZV) that produces a painful, unilateral, dermatomal vesicular eruption. On the exam it appears as a classic rash to recognize, as a cause of cranial-nerve syndromes (Ramsay Hunt, herpes zoster ophthalmicus), and as a source of the major complication postherpetic neuralgia. Its prevention through Shingrix vaccination is a core adult health-maintenance point.
Pathophysiology
After primary varicella (chickenpox), VZV establishes lifelong latency in the dorsal root and cranial sensory ganglia — including the geniculate ganglion, which underlies Ramsay Hunt. Waning cell-mediated immunity from aging or immunosuppression permits reactivation; the virus travels down the sensory nerve to the skin, producing dermatomal pain and vesicles. In Ramsay Hunt the geniculate reactivation affects CN VII distal to the facial nucleus, so the entire hemiface (forehead included) is weak — unlike a central lesion, which spares the forehead due to bilateral cortical input to the upper face — and frequently spreads to the adjacent CN VIII, causing auditory and vestibular symptoms.
Presentation
- Classic picture: burning or neuropathic pain and paresthesias in a single dermatome, followed within days by a unilateral, non-crossing crop of grouped vesicles on an erythematous base that does not cross the midline
- Thoracic dermatomes are most commonly involved; unilateral dermatomal chest or trunk pain that precedes the rash is a recognized (non-lethal) mimic of other chest-pain causes
- Ramsay Hunt syndrome: reactivation in the geniculate ganglion causing a lower-motor-neuron CN VII palsy (entire hemiface including the forehead is weak) PLUS a vesicular rash in the external ear canal/pinna and often CN VIII involvement — sensorineural hearing loss, tinnitus, and vertigo
- Herpes zoster ophthalmicus: reactivation in the ophthalmic division of the trigeminal nerve (V1) with periorbital vesicles; Hutchinson's sign (vesicles on the nasal tip) predicts ocular involvement and threatens sight
- Rising incidence with advancing age and in immunocompromised states, reflecting the role of declining cell-mediated immunity
Diagnosis
- Primarily a clinical diagnosis based on the characteristic painful, unilateral, dermatomal vesicular rash
- PCR of vesicle fluid is the most sensitive and specific confirmatory test when the diagnosis is uncertain or the presentation is atypical/disseminated
- Direct fluorescent antibody (DFA) of a lesion scraping offers rapid confirmation and can distinguish VZV from HSV
- Tzanck smear shows multinucleated giant cells but cannot distinguish VZV from HSV (low specificity)
- In facial weakness, forehead involvement localizes to an LMN (peripheral) lesion; accompanying ear vesicles, hearing loss, tinnitus, or vertigo point to Ramsay Hunt rather than a stroke or isolated Bell's palsy
- In V1-distribution disease, perform a thorough eye exam and obtain urgent ophthalmology evaluation, especially with Hutchinson's sign
Management
- Start antiviral therapy — acyclovir, valacyclovir, or famciclovir — ideally within 72 hours of rash onset to shorten duration and reduce complications
- Add corticosteroids and eye protection for Ramsay Hunt; treat herpes zoster ophthalmicus with systemic antivirals plus urgent ophthalmology involvement
- Provide analgesia and manage postherpetic neuralgia — the most common complication — with gabapentin/pregabalin, tricyclic antidepressants, or topical agents (e.g., lidocaine, capsaicin)
- Prevention with the recombinant zoster vaccine (Shingrix): recommended for all immunocompetent adults ≥50 years and for immunocompromised adults ≥19 years (e.g., HIV, transplant recipients, hematologic malignancy, chemotherapy)
High-yield
- Herpes zoster = reactivation of VZV latent in dorsal root/cranial sensory ganglia, triggered by declining cell-mediated immunity
- Ramsay Hunt syndrome = geniculate ganglion reactivation → LMN CN VII palsy (whole hemiface, forehead weak) + ear canal/pinna vesicles + CN VIII signs (hearing loss, tinnitus, vertigo)
- Hutchinson's sign (nasal-tip vesicles) predicts ocular involvement in herpes zoster ophthalmicus (V1)
- PCR of vesicle fluid is the most sensitive/specific confirmatory test; Tzanck smear cannot separate VZV from HSV
- Antivirals work best within 72 hours of rash onset; postherpetic neuralgia is the key complication (gabapentin/pregabalin, TCAs)
- Shingrix: immunocompetent adults ≥50 and immunocompromised adults ≥19
Pitfalls
- Mistaking the LMN facial palsy of Ramsay Hunt for a stroke, or missing the ear vesicles and CN VIII symptoms that distinguish it from simple Bell's palsy
- Overlooking Hutchinson's sign and failing to arrange urgent ophthalmology in V1 (ophthalmic) zoster
- Assuming COPD or other chronic lung disease is a zoster-vaccine indication — it is not; Shingrix eligibility is defined by age ≥50 (immunocompetent) or immunocompromise ≥19
- Relying on a Tzanck smear to confirm zoster — it cannot distinguish VZV from HSV; use PCR or DFA
- Delaying antiviral therapy beyond the 72-hour window and neglecting proactive management of postherpetic neuralgia
- Anchoring on zoster as a cause of chest pain before excluding immediately life-threatening diagnoses
Don't just memorize Herpes Zoster (Shingles) — practice reasoning through it on branching cases where your decisions shape the patient.