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If an exanthem is defined as any cutaneous rash with (or without) fever or other symptoms, the list would be very long. We believe that this definition should be further narrowed to include only those rashes presenting the following characteristics :
— rapid onset;
— purely erythematous cutaneous lesions possibly very slightly vesicular, purpuric, or squamous, etc;
— an acute course.
This chapter deals with viral exanthems only.
Before an exanthem can be diagnosed, the symptoms and appearance of the rash must be taken into consideration. The basic rash can be described as morbilliform, scartlatiniform or rubelliform. A morbilliform rash consists of erythematous macules that tend to merge and are separated by patches of non pruriginous, healthy skin. The scarlatiniform rash presents as tiny, sometimes infiltrated macules converging rapidly to form broad expanses of bright or dark red skin with rare patches of healthy skin in between, particularly in the skin folds ; it is rarely pruriginous. The rubelliform rash consists of pale pink, more or less widespread, tiny, flat macules separated by areas of healthy skin ; it is not pruriginous. The pattern of the rash on the body and its course must also be taken into consideration.
A morbilliform rash may be part of the symptoms and signs of many viral infections. Ten to 15 percent of patients with infectious mononucleosis present with a rash affecting mainly the trunk and arms between the fourth and sixth day, but it is rarely detected and often nonspecific. Virtually all patients who are prescribed ampicillin develop an exanthem. Morbilliform exanthems include Gianotti-Crosti syndrome or papular acrodermatitis of childhood. Asymmetric periflexural exanthem of childhood (APEC) is characterised by an asymptomatic or moderately pruriginous scarlatiniform exanthem generally starting in an armpit and radiating outwards to the trunk and proximal part of the limbs. Careful examination will find more modest contralateral lesions ; the general condition of the patient tends to be unaffected and the rash is usually self-limiting within 4 to 5 weeks.
Human herpes virus 6 (HHV6) is the causative agent of exanthema subitum or roseola infantum. In addition, several viruses – particularly the arboviruses and enteroviruses - can cause roseoliform rashes. Other unusual exanthems include Kawasaki syndrome and gloves and socks syndrome, which presents as scarlatiniform lesions of the hands and feet, sharply marginated on the wrists and ankles. The lesions often become purpuric, tend to converge and are sometimes tender and cause irritation. Dengue’s typical cutaneous feature is a transient mild flush-like macular eruption over the nape of the neck and face. The eruption evolves to become more maculopapular in nature and will last up to 5 days. Petechiae or purpura may sometimes be seen. In Yellow fever, besides an intense jaundice, facial flushing, conjunctival hyperemia and petechiae can be observed. West Nile virus infection, together with fever, malaise, anorexia, photophobia, myalgia, lymphadenopathy, arthralgia, and neurologic dysfunction producing an encephalitic picture can provoke punctate, erythematous, maculopapular eruptions most pronounced on the extremities. Lassa fever, besides constitutional symptoms such as fever, chills, headache and myalgia, can cause generalized petechial eruption and facial edema.
Due to high vaccination coverage, e.g., for measles and rubella, some classical exanthems are now rarely seen in Western Countries. In addition, for this reason and because the widespread use of antibiotics, measles, rubella, scarlet fever, erythema infectiosum, and roseola infantum can be difficult to differentiate. Moreover, measles can also occur in people who have been immunized, as a result of vaccination failure.
It is also necessary to note that many virus capable to cause exanthematic rashes may also provoke systemic symptoms without any skin lesions. For instance, parvovirus B19, besides erythema infectiosum, can trigger also aplastic crisis in patients with chronic haemolytic anaemia, rash, hydrothorax/ascites, arthralgia/arthritis, pregnancy complications, fever, fetal hydrops or fetal death and liver transplant rejection.
Today, it is important to stress that a single virus may give rise to a variety of rashes (for example, parvovirus B19) and that the same rash can be induced by different viruses (for instance, Gianotti-Crosti syndrome).
INFORMATION FOR PATIENTS
Both patients and parents must be reassured and informed about the generally benign and transient nature of these rash-inducing viruses ; children with rubella and erythema infectiosum must be kept away from pregnant women. Children with measles and chickenpox should avoid contact with others and be kept home from school. In all cases – particularly with infectious mononucleosis – patients should be advised to avoid strenuous activities and to rest as much as possible.
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