Postherpetic neuralgia (PHN)
Epidemiological features: PHN occurs in about 10-20% of patients with herpes zoster. rare in children. The risk of occurrence is mainly associated with increasing age. Female patients, as well as patients with ocular and ear herpes zoster, have a higher likelihood of developing PHN, while immunocompromised individuals have a very low risk of developing chronic pain.
Pathological manifestations: degeneration of axons and cell bodies, atrophy of the dorsal horn of the spinal cord, scar formation in the dorsal root ganglion, and loss of epidermal innervation in the affected area. The cause of nerve damage may be progressive viral replication.
Duration of pain: weeks, months, occasionally years.
Nature of pain: may range from mild to extreme; constant, intermittent, or induced by minimal stimulation. In patients with herpes zoster, PHN can be predicted based on the patient’s age, severity of prodromal and post-rash pain, extent of the rash, trigeminal nerve and eye involvement, and viraemia.
Treatment: Treatment of herpes zoster includes herpes treatment and neuralgia treatment. The goals are to relieve pain in the acute phase, limit the spread of lesions, shorten the duration of lesions, and prevent or mitigate PHN and other acute or chronic complications (Table 1). It is important to emphasize that ophthalmologists should be consulted as soon as possible for ocular comorbidities and that other cranial nerve complications, such as ear herpes zoster, also require specialist consultation.
1. Indications for antiviral treatment of herpes zoster
Herpes zoster is a self-limiting disease, and even without antiviral therapy, herpes zoster of the trunk and extremities in young patients without risk factors usually resolves spontaneously and without complications. However, in patients outside the above range, antiviral therapy can shorten the course of the disease and reduce the incidence, severity, and duration of PHN. The indications for early systemic antiviral therapy are: older than 50 years, immunocompromised or deficient, malignant primary disease, cranial nerve involvement (especially ocular herpes zoster and ear herpes zoster), and severe atopic dermatitis or severe eczema. In addition, systemic antiviral therapy is indicated if the rash occurs in more than one dermatomal area, has hemorrhagic lesions and/or mucosal involvement
Indications for systemic antiviral therapy for herpes zoster
Emergency indications
Herpes zoster at any site in patients older than 50 years of age
Herpes zoster of the head/neck in patients of all ages
Severe herpes zoster of the trunk/extremities
Herpes zoster in immunocompromised or deficient patients
Herpes zoster in patients with severe atopic dermatitis or severe eczema
Relative indications
Herpes zoster of the trunk and extremities in patients younger than 50 years of age
Timing of antiviral therapy Systemic antiviral therapy should be initiated as early as possible, i.e., within 48 to 72 hours of the onset of cutaneous symptoms. Effective concentrations of antivirals must be achieved and maintained rapidly to obtain optimal treatment results. Systemic antiviral therapy may be initiated even 72 hours after the onset of cutaneous symptoms in patients with disseminated herpes zoster, persistent ocular and ear herpes zoster, and immunodeficient patients with visceral involvement. Antivirals are beneficial in preventing PHN even when administered 72 hours after the onset of symptoms.
There are three systemic antivirals that can be used in the treatment of herpes zoster: acyclovir, famciclovir, and famciclovir. All three drugs are guanine adenosine analogues with specific affinity for the virus but low toxicity to the host cells of mammals. Upon entry into virus-infected cells, acyclovir competes with deoxyribonucleoside for viral thymidine kinase or cellular kinase and is phosphorylated to activated acyclovir triphosphate, which then inhibits viral replication in two ways: (1) by interfering with viral DNA polymerase, which inhibits viral replication; and (2) by binding to the growing DNA strand under the action of DNA polymerase, which causes disruption of DNA strand extension. Acyclovir can be administered both orally and by intravenous infusion. Oral administration is 400 mg five times a day for seven days. Intravenous acyclovir is the standard therapy for the treatment of herpes zoster in immunocompromised patients at a dose of 5-10 mg/kg IV 3/day. The patient should be given adequate water during administration to prevent acyclovir from precipitating in the renal tubules and causing damage to renal function.
2. Indications and timing of herpes zoster hormone application
In the early treatment of acute attacks of herpes zoster, the systematic application of high-dose glucocorticoids can suppress the inflammatory process and shorten the duration of acute pain and healing of lesions, but is largely ineffective in chronic pain (PHN). Corticosteroids alone are not recommended in the absence of systemic antiviral therapy. Prednisone (30 mg/day for 7 days) is generally used. In relatively healthy patients over 50 years of age with localized herpes zoster, the combination of antivirals and glucocorticoids can improve the quality of life of patients.
3. Treatment of neuralgia.
A stepwise treatment plan should be used. Individualized differences and adverse drug reactions should be noted during treatment. If necessary, the patient should consult a pain clinic.
1) First step: non-steroidal analgesics. For example, paracetamol (acetaminophen) 1.5-5g/day. Aspirin is of limited use for PHN and ibuprofen is ineffective.
2) Step 2: Add low potency narcotic analgesics (e.g. tramadol, 200-400mg/day, codeine 120mg/day)
3) Step 3: In addition to “peripheral” analgesics, high potency central opioid-like substances (e.g. buprenorphine 1.5-1.6mg/day; oral morphine 30-360mg/day) may be given. This last step is indicated for patients who do not respond well to basic therapies. For severe neuropathic pain, step 1 or step 2 can be combined with an antiepileptic drug (e.g., carbamazepine 400-1200 mg/day, gabapentin 900-2400 mg/day). Antiepileptic drugs can reduce pins and needles pain, but are not effective for persistent pain. Antidepressants (e.g., amitriptyline 10-75 mg) and neuroleptics (e.g., methotrexate 20-150 mg/day) may also be effective, especially in older patients. Amitriptyline is the standard treatment for PHN and can be started at 25 mg and tapered to 50-75 mg over 2-3 weeks in patients over 60 years of age with herpes zoster. nortriptyline has similar analgesic effects to amitriptyline, but has fewer adverse effects. In addition to oral medications, topical lidocaine gel can be used topically for the treatment of acute herpes zoster pain and PHN, which is easy to use and has no systemic adverse effects. Capsaicin can affect the release synthesis and storage of substance P, a pain transmission factor. Capsaicin ointment applied topically achieves analgesic and antipruritic effects by reducing substance P. In addition, treatments such as sympathetic nerve blockade with local anesthetics and transcutaneous electrical nerve stimulation can be tried. Individual cases can be treated neurosurgically (e.g., spinal cord gray matter colloid Rolandi thermal coagulation).