It is caused by the same virus which causes chicken pox (varicella zoster virus, V-Z virus) but occurs in individuals who are partially immune to this virus. Following an attack of chicken pox, the virus gets lodged in the posterior nerve root ganglia where it remains dormant.
Under certain circumstances, however, which lead to a depression of the cell mediated immunity, the virus gets reactivated and results in an attack of herpes zoster.
The eruption consists of grouped, tense vesicles or even bullae surrounded by a zone of erythema characteristically limited to a single neural segments is in trunk is very common, though less commonly, the lesions may occur on the upper or lower extremities.
The ophthalmic branch of the trigeminal nerve is commonly affected and this is often associated with involvement of the eye with corneal ulceration.
Involvement of the seventh and the eighth cranial nerves may lead to facial palsy and deafness respectively and this is called Ramsay Hunt syndrome.
Unilateral involvement of a neural segment is very diagnostic of herpes zoster.
The new lesions may continue to appear for the first three or four days, but ultimately all lesions tend to dry up to form crusts which fall off within seven to ten days. In severe cases, the vesicles may rupture to form ulcers which have the risk of secondary infection and scarring.
Formation of keloids can also occur in some cases.
Most cases of herpes zoster are associated with pain or paraesthesias in the involved neural segment. In a significant proportion of adult patients beyond 40 years in age, this pain or hyperesthesia persists after the skin lesions have healed.
This is called post-herpetic neuralgia and may last for several months.
In cases with immuno-suppression, isolated vesicles may also be found elsewhere on the body beyond the involved neural segment. This is called disseminated herpes zoster. In such cases, the internal organs such as the lungs, liver and the central nervous system may also be involved and the disease can be fatal.
The vesicles will dry up and heal in a week or so even if no treatment is given. vesicles should not be ruptured. Soothing lotions such as calamine lotion may be applied two times a day or even more frequently to give comfort to the patient.
Topical applications of anti-bacterial agents or even systemic antibiotics should be used to prevent superadded infection if ulceration of the lesions occur.
Analgesic drugs are effective for pain but once the neuralgia has developed, the only successful method of treatment lies in injecting a local anaesthetic or five percent aqueous solution of phenol into the posterior nerve root corresponding to the involved neural segment.
For disseminated herpes zoster, it is necessary to use acyclovir systemically.