Herpes zoster, also known as shingles, is caused by the human herpes virus type 3 (HHV3).
Primary infection with the virus occurs in childhood and produces a varicella (chickenpox) infection. After this, the virus lays dormant in the sensory nervous system, usually in the geniculate, trigeminal or dorsal root ganglion.
It can lie dormant for years or decades before flaring up and causing the typical infection of herpes zoster.
Herpes zoster cannot be caught from anyone, particularly anyone with chickenpox; this is a common misconception. It is always due to reactivation of the dormant virus. Rarely, chickenpox can be caught from a patient with active shingles due to shedding of live viruses.
About 20 per cent of people will develop herpes zoster at some stage in their lives.
The incidence and severity of herpes zoster increases with age. In the under-20s, the incidence is 0.4-1.6 cases per 1,000; in the over-80s, it is 4.5-11 cases per 1,000 in the US.
Immunocompromised patients, patients with HIV, lymphoma and those with bone marrow transplants, are also at increased risk. If herpes zoster affects more than one dermatome, it suggests the patient may be immunocompromised. Multi-dermatomal herpes zoster is seen in HIV infection.
Before the rash erupts, the patient will complain of burning, itching or paraesthesia in the affected area, lasting up to a week.
When the lesions appear, most patients have an acute neuritic pain. In rare cases, known as zoster sine herpete, there is no rash and the patient just has severe neuralgic pain.
The rash affects one or more adjacent dermatomes in the thoracic or lumbar region. It consists of blisters and surrounding erythema; the blisters scab over and heal within 14-21 days. Because the rash is strictly dermatomal, it never crosses the midline.
Patients are contagious until the lesions have dried (as in chickenpox). There may be malaise or low-grade fever after this, but usually no abnormalities on laboratory investigation. Secondary infection with Staphylococcus aureus can occur. Immunosuppressed patients are at risk.
Many patients experience postherpetic neuralgia, a severe, intractable pain that persists once the rash has healed. Postherpetic neuralgia can persist for months, years or permanently, and is more common in the elderly. Pain lasting more than one year occurs in roughly half of patients over 70 years of age.
Oral antiviral therapy may prevent postherpetic neuralgia if started within 72 hours of the rash appearing. Aciclovir is the most well known antiviral, and has to be given at a dosage of 800mg five times daily for seven days in adults.
Prompt treatment with an antiviral is indicated in the elderly, but probably not in the young, given the low incidence of postherpetic neuralgia in this population. Antivirals are mandatory in the immunocompromised or in ophthalmic zoster. Analgesia is required for pain.
A systematic review of treatment for established postherpetic neuralgia found that tricyclic antidepressants, topical capsaicin, gabapentin and oxycodone were effective for alleviating pain, but long-term benefits are uncertain and side-effects are common. Intrathecal steroids and nerve blocks are sometimes used.
A vaccination against chickenpox is available for at-risk groups.
Variations on presentation
Trigeminal herpes zoster occurs in 7-10 per cent of cases. If the ophthalmic branch of the trigeminal nerve is involved, it may affect the eye. Lesions on the sclera or cornea require same-day referral to an ophthalmologist. About 25 per cent will develop long-term sequelae.
Ramsay Hunt syndrome is herpes zoster of the geniculate ganglion. The infection affects the motor fibres of the facial nerve or auditory nerve and can present as Bell's palsy. Involvement of the auditory nerve can impair hearing, and paralysis of the tensor tympani can cause hyperacusis.
Motor zoster is characterised by weakness and paralysis and is caused by involvement of anterior horn cells. This can cause monoparesis of the upper or lower limb or diaphragmatic palsy. Neurogenic bladder and pseudobstruction of the colon are seen in some patients.
Dr Warburton is a GP in Ironbridge, Shropshire
Tyring S, Barbarash R A, et al; Famciclovir for the treatment of acute herpes zoster effects on acute disease and post-herpetic neuralgia. Ann Int Med 1995; 123: 89-96.
Alper B S, Lewis P R. Does treatment of acute herpes zoster prevent or shorten postherpetic neuralgia? J Fam Pract 2000; 49: 255-64.