Herpes Zoster — Shingles

• Latency establishment and maintenance: Following primary chickenpox infection, VZV establishes lifelong latency within sensory nerve ganglia (dorsal root, cranial nerve, and autonomic ganglia) through integration of viral DNA into the host genome. The virus remains dormant within infected neurons and satellite cells, largely sequestered from immune surveillance due to the blood-brain barrier and limited lymphocyte trafficking into neural tissue. VZV expresses latency-associated transcripts (LATs) that suppress lytic gene expression while maintaining the potential for reactivation. This state persists indefinitely unless specific conditions disrupt the carefully balanced immune homeostasis within ganglia.

• Immunological factors driving reactivation: VZV reactivation fundamentally represents failure of VZV-specific cell-mediated immunity, particularly CD8+ T cell-mediated cytotoxic responses that normally maintain VZV in latent state. Advancing age causes progressive decline in VZV-specific T cell proliferation and cytokine production (especially interferon-gamma), a phenomenon termed "immunosenescence." Loss of CD4+ T cell help, critical for maintaining CD8+ memory responses, contributes to reactivation risk in both aging and HIV/AIDS. Impaired dendritic cell function and reduced production of interleukin-2 and tumor necrosis factor-alpha further compromise ganglionic immune surveillance. The reactivation threshold appears to correlate with absolute VZV-specific CD4+ and CD8+ cell counts; in HIV patients, reactivation typically occurs when CD4 counts fall below 200-500 cells/μL.

• Viral replication and neuroinflammation: Once immune control falters, VZV reactivates within latently infected neurons and begins lytic replication. The virus spreads anterogradely along sensory nerve axons toward peripheral skin, triggering intense local and segmental inflammatory responses. Viral replication induces release of proinflammatory mediators (IL-1, IL-6, TNF-α) and chemokines from infected neurons, Schwann cells, and infiltrating immune cells. This neuroinflammation directly activates nociceptors and contributes to acute zoster pain. Simultaneously, retrograde spread to the dorsal root ganglion and spinal cord perpetuates ganglionic inflammation, explaining the characteristic dermatomal distribution following a single sensory nerve dermatome. The extensive viral burden within ganglia (estimated 10,000-100,000 infected cells per ganglion) correlates with severity of acute pain.

• Dermatomal distribution mechanism: The strictly segmental (dermatomal) distribution of zoster rash reflects the anatomical organization of sensory innervation and the path of viral spread. VZV replicates within a single dorsal root ganglion or cranial nerve ganglion, then spreads anteriorly along that nerve's sensory branches to innervate the corresponding dermatome. This contrasts with primary varicella (chickenpox), which spreads hematogenously and involves multiple non-contiguous dermatomes. The reason for localized reactivation to a single ganglion, rather than multiple simultaneous reactivations, remains incompletely understood but likely relates to spatial clustering of latently infected cells within individual ganglia and local variations in immune control.

• Postherpetic neuralgia pathogenesis: Chronic neuropathic pain in PHN results from multiple pathophysiological mechanisms, including primary sensory neuron injury and dysfunction. Autopsy studies reveal severe destruction of sensory nerve fibers within affected dermatomes, with loss of both unmyelinated C fibers and myelinated Aδ fibers; this degeneration persists indefinitely. Surviving neurons develop spontaneous ectopic firing due to altered ion channel expression (upregulation of sodium and calcium channels), contributing to burning pain and allodynia. Additionally, central sensitization occurs, with persistent abnormal firing from damaged peripheral nerves inducing maladaptive changes in dorsal horn neurons, including enhanced glutamatergic neurotransmission, loss of GABAergic inhibition, and expanded receptive fields. Neuroinflammation persists in affected ganglia and spinal cord due to ongoing infiltration by T cells and activation of microglia, perpetuating pain signal amplification. Older patients develop PHN more frequently, partly because age-related decline in nerve regenerative capacity limits recovery from viral-induced nerve damage.

• Advanced age (>50 years): This is the single most important risk factor, with incidence doubling per decade after age 50. Immunosenescence involves progressive decline in VZV-specific T cell responses, reduced interferon-gamma production, and impaired CD4+ helper function. Approximately 50% of people who live to age 80 will experience at least one episode of zoster. The age-related risk is dramatic enough that zoster incidence serves as a clinical marker of immune aging.

• HIV/AIDS infection: Zoster incidence increases 15-20 fold in HIV-infected patients compared to age-matched controls, with cumulative incidence approaching 20-30% over 10 years even with antiretroviral therapy. Risk is highest when CD4+ counts fall below 200-500 cells/μL, though zoster may occur at any CD4 count. Patients with HIV may experience severe disseminated zoster or recurrent episodes, features rare in immunocompetent individuals. Paradoxically, immune reconstitution inflammatory syndrome (IRIS) following antiretroviral therapy initiation can precipitate zoster.

• Solid organ and hematopoietic stem cell transplantation: Post-transplant immunosuppression dramatically increases zoster risk, with cumulative incidence of 10-15% within first year after transplant. Zoster occurs with highest frequency in hematopoietic stem cell transplant recipients, approaching 20-40% incidence. Both pharmacological immunosuppression and graft-versus-host disease contribute to risk. Zoster may appear years after transplantation during chronic immunosuppression.

• Hematologic malignancies: Patients with lymphomas (particularly Hodgkin lymphoma, non-Hodgkin lymphoma) and chronic leukemias face elevated zoster risk due to combined effects of malignancy-induced lymphopenia and chemotherapy-induced immunosuppression. Risk increases further with active chemotherapy. Zoster incidence in Hodgkin lymphoma patients exceeds 10% over 10 years.

• Solid organ malignancies: Advanced cancers and chemotherapy cause immune dysfunction predisposing to zoster. However, zoster risk is lower than with hematologic malignancies unless advanced metastatic disease with significant immunosuppression develops.

• Immunosuppressive medications: Corticosteroids (especially doses >20 mg prednisone daily or prolonged courses) impair cell-mediated immunity and increase zoster risk substantially. Biologic immunosuppressants including TNF-α inhibitors (infliximab, adalimumab, etanercept), methotrexate, azathioprine, and mycophenolate significantly elevate risk. TNF-α inhibitors merit particular attention as they increase zoster incidence 2-3 fold in rheumatologic patients. JAK inhibitors show emerging association with zoster risk. Among immunosuppressive medications, the zoster risk is highest with TNF-α inhibitors and lowest with selective immunosuppressants affecting specific cell populations.

• Chronic kidney disease and renal transplantation: Both conditions independently increase zoster risk, with cumulative incidence in transplant recipients reaching 15-20%. The combination of uremia-induced immune dysfunction and post-transplant immunosuppression substantially elevates risk.

• Diabetes mellitus: Type 1 and type 2 diabetes modestly increase zoster risk, potentially through hyperglycemia-induced impairment of T cell function and reduced VZV-specific cell-mediated immunity. The risk elevation is less dramatic than with other conditions but clinically meaningful, particularly in poorly controlled diabetes.

• Vaccination-related zoster: The live attenuated varicella-zoster vaccine (Zostavax) rarely precipitates zoster (0.01-0.04% of vaccinees) when the vaccine virus reactivates. The recombinant zoster vaccine (Shingrix) shows dramatically lower rates of vaccine-strain zoster due to its use of non-infectious recombinant antigen.

• Trauma and localized stress: Rarely, minor nerve trauma, surgery, radiation therapy, or even mechanical trauma to a dermatome may precipitate zoster in that specific area, suggesting that local factors within ganglia can influence reactivation risk.

• Prodromal phase (typically 2-7 days before rash): Patients experience dermatomal pain, dysesthesias, pruritus, or paresthesias strictly limited to one or occasionally two adjacent dermatomes (dermatomal distribution is a cardinal diagnostic feature). Pain quality varies: some describe burning or aching sensations, others report lancinating or electric-shock qualities. Associated systemic symptoms—malaise, low-grade fever, headache, fatigue—occur in minority of patients. Prodromal symptoms in the affected dermatome may be dismissed as musculoskeletal pain, cardiac ischemia (if thoracic), or abdominal pathology (if truncal), leading to diagnostic delays and unnecessary investigations.

• Acute phase with vesicular eruption: Within days of prodrome, the characteristic dermatomal vesicular rash emerges in erythematous plaques. Lesions progress from macules to papules to fluid-filled vesicles over 3-5 days. Vesicles are typically 2-5 mm, clear initially then become turbid, and coalesce into larger bullae in severe cases. The classic teaching is that lesions in different stages of evolution (macules, papules, vesicles, crusts) appear simultaneously within the same dermatome—a feature distinguishing zoster from varicella (which shows crops of lesions over several days affecting multiple dermatomes). Crusting begins within 7-10 days and complete scab formation takes 2-3 weeks. Pain severity is typically most intense during the vesicular phase and early crusting phase, sometimes before lesions appear (prodromal pain).

• Thoracic zoster (most common location): Accounts for approximately 50% of cases. Vesicles respect midline, appearing in one thoracic dermatome from posterior aspect around chest wall to anterior midline. Severe thoracic zoster may cause pleuritic chest pain that can mimic acute coronary syndrome or pulmonary embolism, warranting careful evaluation to avoid unnecessary testing.

• Trigeminal zoster (second most common, 20% of cases): Typically involves ophthalmic (V1) division, followed by maxillary (V2) then mandibular (V3). Ophthalmic zoster carries highest risk of serious complications (see complications section). Lesions appear on forehead, upper eyelid, and bridge of nose (V1 distribution). Hutchinson sign—vesicles on the nasal tip, indicating involvement of the external nasal branch—predicts increased ocular involvement risk with up to 50% developing keratitis.

• Cervical zoster: Affects upper back, neck, and occipital scalp in C2-C4 distribution. Posterior cervical lymphadenopathy commonly accompanies cervical zoster.

• Lumbosacral zoster (15% of cases): Affects lower back and buttocks, sometimes extending to lower extremity in a dermatomal pattern. May cause motor involvement affecting lower extremity function. Dysuria and urinary retention may complicate sacral zoster due to autonomic involvement.

• Disseminated zoster: Defined as rash extending beyond single or two adjacent dermatomes (affecting ≥3 dermatomes) or vesicles appearing on non-contiguous areas. Occurs in approximately 2-5% of immunocompetent hosts but up to 20-30% of HIV/AIDS patients and other severely immunocompromised individuals. Disseminated zoster signals systemic VZV viremia and substantially increases risk of serious complications including disseminated infection and CNS involvement. Vesicles in disseminated zoster may appear identical to varicella lesions when distributed widely.

• Physical examination findings: Beyond the characteristic dermatomal vesicular rash, key physical exam findings include regional lymphadenopathy (in dermatome drainage sites), edema and erythema surrounding vesicles, and in severe cases confluent bullae or hemorrhagic vesicles. Assessment for complications is essential: examine for eye involvement (conjunctivitis, keratitis, anterior uveitis signs), test motor function to assess myositis or motor nerve involvement, palpate abdomen for tenderness in affected dermatome, and carefully examine otologic and facial nerve function if Ramsay Hunt syndrome suspected.

• Immunocompromised presentations: In severely immunocompromised patients (CD4 <50 cells/μL in HIV, post-bone marrow transplant), zoster may present atypically: vesicles may be large and hemorrhagic, healing is prolonged, and chronic verrucous or hyperkeratotic lesions may persist for months. Immunocompromised patients may develop disseminated cutaneous zoster, disseminated visceral zoster, or progressive zoster with failure of vesicles to heal.

• Ramsay Hunt syndrome (zoster oticus): Facial nerve (CN VII) involvement manifests as ipsilateral facial paralysis with vesicles on auricle, external auditory canal, and soft palate. Associated features include ipsilateral ear pain, hearing loss, vertigo, and taste disturbance (anterior 2/3 tongue via chorda tympani). This represents zoster reactivation within geniculate ganglion. Facial paralysis develops abruptly over hours to days, sometimes progressing for up to 2 weeks.

• Post-herpetic neuralgia: Continuing beyond acute phase, this complication manifests as burning pain, lancinating pain, allodynia (pain from non-painful stimuli), and hyperesthesia persisting in affected dermatome for weeks to years after lesions heal. PHN prevalence is 10-15% in patients over 60 years old infected with zoster, rising to 50% in those over 80 years. Pain severity in PHN may be as severe or more severe than acute zoster pain.

• Clinical diagnosis (history and physical examination): Classic presentation of dermatomal vesicular rash in various stages of evolution is diagnostic. Key history elements include prodromal unilateral dermatomal pain or dysesthesias, clear temporal progression of rash, and unilateral distribution strictly within one or occasionally two adjacent dermatomes (not crossing midline for truncal distribution). Physical exam findings characteristic of zoster include sharply demarcated erythematous plaques with vesicles, crusting within 1-2 weeks, and regional lymphadenopathy. Sensitivity of clinical diagnosis approaches 95% for typical presentations. Clinical diagnosis allows immediate antiviral therapy initiation without waiting for test results, which is critically important since antiviral efficacy is highly time-dependent.

• Tzanck smear (rapid, low-cost confirmation): Scrapings from unroofed vesicle base (scrape the base, not surrounding skin) are fixed and stained with Giemsa, Wright-Leishman, or methylene blue stain. Positive findings show multinucleated giant cells and ballooning keratinocytes characteristic of herpes virus infection (both HSV and VZV produce identical changes). Sensitivity is 50-70% (varies by operator technique and stain quality), while **specificity is

Immediate triage decisions

• Time from rash onset: antiviral therapy is most effective when started within 72 hours of rash onset; the CDC still advises treating beyond that window if new vesicles are still forming, since ongoing vesiculation implies continued viral replication.
• Route decision: localized zoster in an immunocompetent host is treated orally; disseminated, visceral, or CNS disease and severe disease in the profoundly immunocompromised require IV acyclovir (10 mg/kg q8h) with generous hydration.

First-line therapy (oral nucleoside analogues — guanosine analogues requiring viral thymidine kinase for activation)

• Valacyclovir (prodrug of acyclovir, superior bioavailability): 1 g PO three times daily for 7 days — the usual first choice per CDC guidance.
• Famciclovir: 500 mg PO every 8 hours for 7 days; equivalent efficacy.
• Acyclovir: 800 mg PO five times daily for 7 days — effective but the dosing burden limits adherence. All three require renal dose adjustment.

Analgesia, layered by severity

• Acetaminophen/NSAIDs for mild pain; short-course opioids for severe acute pain.
• Gabapentinoids (gabapentin, pregabalin) and TCAs (nortriptyline) target ectopic neuronal firing and central sensitization; start early in older patients with severe pain.
• Topical lidocaine 5% patch only after lesions have fully crusted; capsaicin 8% patch is an option for established postherpetic neuralgia.

Special situations

• Herpes zoster ophthalmicus: same-day ophthalmology referral plus systemic antivirals (American Academy of Ophthalmology Preferred Practice Pattern); topical steroids only under ophthalmologic supervision.
• Ramsay Hunt syndrome: antiviral plus systemic corticosteroid, started as early as possible.
• Acyclovir-resistant VZV (thymidine-kinase mutants in advanced HIV/transplant): switch to foscarnet, per CDC/NIH/IDSA opportunistic infection guidance.

Contraindicated or discouraged

• Corticosteroid monotherapy — never give steroids without an antiviral.
• Live attenuated zoster vaccine in immunocompromised patients (no longer distributed in the US); ACIP recommends the recombinant zoster vaccine (Shingrix), two doses, for all adults ≥50 and for immunocompromised adults ≥19, including those with prior zoster.

Neurologic

• Postherpetic neuralgia: the most common complication; sensory neuron destruction with ectopic firing and dorsal horn central sensitization produces burning pain, allodynia, and hyperesthesia persisting after lesions crust. Signalled by dermatomal pain outlasting rash healing; risk rises steeply with age.
• VZV vasculopathy/stroke (emergency): transmural arterial infection after transaxonal viral spread to cerebral vessels. Suspect when focal deficits, hemiparesis, or aphasia follow zoster — classically weeks to months after ophthalmic zoster. CSF PCR/anti-VZV IgG and vessel imaging are the next steps.
• Meningoencephalitis, myelitis, Guillain–Barré-like polyradiculitis (emergency): headache, altered mentation, or a sensory level with bowel/bladder dysfunction; requires IV acyclovir.
• Segmental motor paresis: anterior horn/ventral root extension of ganglionic inflammation — weakness in the myotome matching the rash.
• Sacral zoster with urinary retention: autonomic S2–S4 involvement; check post-void residual.

Ocular (herpes zoster ophthalmicus)

• Keratitis, anterior uveitis, scleritis: red eye, photophobia, decreased acuity; Hutchinson sign predicts involvement. Pseudodendrites on fluorescein staining distinguish VZV from HSV's true dendrites.
• Acute retinal necrosis / progressive outer retinal necrosis (emergency): painless vision loss with peripheral retinal whitening; sight-threatening, needs same-day ophthalmology and IV antivirals.

Systemic and cutaneous

• Disseminated cutaneous or visceral zoster (emergency): viremic spread in the immunocompromised; ≥3 dermatomes or non-contiguous lesions, hepatitis, or pneumonitis. Requires airborne plus contact precautions and IV therapy.
• Bacterial superinfection: Staphylococcus aureus or group A Streptococcus entering through eroded vesicles; expanding erythema, purulence, fever.

Treatment-related

• Acyclovir crystal nephropathy and neurotoxicity: rising creatinine or confusion/myoclonus, especially with IV dosing or unadjusted renal dosing — prevent with hydration and dose adjustment.
• Gabapentinoids: sedation, ataxia, falls in the elderly. TCAs: anticholinergic effects, orthostasis, QT prolongation. Corticosteroids: hyperglycemia, insomnia, and worsened immunosuppression.

• The 72-hour rule: the single best next step in a patient with a fresh dermatomal vesicular rash is start an oral antiviral (valacyclovir) — do not wait for confirmatory testing. Board stems reward treating on clinical grounds.
• Hutchinson sign (vesicles on the tip of the nose, nasociliary branch of V1) predicts ocular involvement: the next step is urgent ophthalmology referral, not topical antibiotics.
• Tzanck smear cannot distinguish HSV from VZV — both show multinucleated giant cells. When the distinction matters, the answer is PCR of vesicle fluid, the most sensitive and specific test.
• Zoster in a patient under ~50 with no obvious immunosuppression should trigger HIV testing; multidermatomal, recurrent, or chronic verrucous zoster is the examiners' favorite clue to advanced HIV.
• Ramsay Hunt syndrome = facial palsy + vesicles in the ear canal/auricle ± hearing loss and vertigo (geniculate ganglion). Distinguish from Bell palsy, which has no vesicles; treatment adds an antiviral to corticosteroids.
• Contagion nuance: a patient with shingles transmits VZV via vesicle fluid and gives a susceptible contact chickenpox, not shingles. Localized zoster in an immunocompetent host needs only lesions covered and standard precautions; disseminated zoster or any zoster in an immunocompromised host requires airborne plus contact precautions.
• Vaccination: per ACIP, the recombinant (non-live) zoster vaccine, two doses, is indicated for adults ≥50 and immunocompromised adults ≥19 — including patients who already had shingles or previously received the live vaccine. Vaccinating during the acute episode is not the answer; defer until the rash resolves.
• Common distractor: corticosteroids may modestly ease acute pain but are not given to prevent postherpetic neuralgia, and never as monotherapy. Likewise, a new LBBB-style trap here is calling thoracic zoster prodrome "atypical angina" — remember unilateral pain that stops at the midline.