When considering common causes of rash in children, a detailed history of the rash, associated symptoms and a careful examination will help to narrow the differential diagnoses.
This article focuses on the areas in the history and examination that will help with diagnosis of each condition.
Roseola infantum
Roseola infantum is caused by human herpes virus HSV6 and most commonly affects children aged six months to four years.
Parents will report a history of a high fever, followed by sudden appearance of a macular rash. The appearance of the rash coincides with the fever reducing.
The lesions appear on the trunk and neck and are pink almond-shaped macules. The macules may be discrete or may merge and can last a couple of hours to two days. There is no scaling found on examination.
Measles
Children with measles will usually have a history of two to three days of fever, general malaise and runny nose. The conjunctiva are injected and the child may be photophobic.
On examining the mouth, blue-white spots on a red background (Koplik's spots) can be found on the buccal mucosa opposite premolar teeth and are present for a couple of days before the rash appears.
The rash will usually appear by day four of the illness, starting on the forehead and behind the ears. It rapidly spreads within 24 hours to face, trunk and limbs, and while initially macular, develops into red papules that tend to coalesce.
The rash can last up to 10 days and may leave some brown discoloration on resolution.
Molluscum contagiosum
Molluscum contagiosum is caused by the pox virus and can occur at any age, but is most common in children aged three to nine years, with another peak in the late teens and early twenties.
It is characterised by discrete, pale, flesh-coloured papules that are dome-shaped and can be up to 5mm in diameter. The papule is umbilicated.
The patient may develop individual lesions, but they often occur in groups, commonly affecting the upper limbs and trunk. Some lesions may have an erythematous surrounding following trauma, suggestive of impending resolution.
Secondary impetiginisation may occur.
Rubella
A child with rubella will have a history of enlarged lymph nodes that have been present for up to a week before the appearance of the rash. Characteristically, the sub-occipital, post-auricular and cervical glands are affected.
Examination of the mouth will show dull red macules or petechiae on the soft palate. The rash appears first on the face and rapidly spreads to the trunk and limbs.
The rash is initially formed of discrete pink macules, which, starting on the face, become diffuse erythema. The face clears and the trunk becomes more erythematous as the macules coalesce. It generally fades by day four.
Herpes zoster
Herpes zoster generally involves a single dermatome reactivation of chickenpox virus. There is often a four-to-five day history of hypersensitivity or tenderness in the affected dermatome before the rash appears.
The rash begins as a swollen red plaque that spreads to affect part, or sometimes all, of the dermatome. Clusters of vesicles appear on the erythematous plaque; they are initially clear, but become purulent within three to four days.
Occasionally, the rash may affect two adjacent dermatomes, but generally the rash is limited to one dermatome that does not cross the midline, with the exception of a few vesicles. This is a classical feature of herpes zoster. In two-thirds of patients, a dermatome in the thoracic area will be affected.
Hand, foot and mouth disease
Hand, foot and mouth disease is caused by coxsackie virus, most commonly in the under-fives and in the spring months.
Patients often present with a painful stomatitis and pyrexia.
On examination, multiple aphthous ulcers can be found in the mouth.
In two-thirds of patients, the palms and soles will have small red macules, which develop into classical oval or square vesicles or blisters. The blisters and ulcers last three to five days and heal without scarring.
Herpes simplex rash
This rash is caused by HSV1 and HSV2 and can be a primary or secondary infection. There may be a history of tenderness or a burning sensation at the site of the rash before it appears.
Vesicles, which appear in groups on an erythematous background, erode and crust over a period of two to four weeks and will heal without scarring.
The primary episode tends to have more scattered vesicles, whereas a secondary infection has more localised vesicles.
A history of local skin trauma (UV light, chapping or abrasion) may be given, or the patient may have had a systemic upset. The reactivated virus travels down the peripheral nerve supplying the site of initial infection. The characteristic feature of herpes simplex infection is the focal recurrent infection affecting the same site.
Lesions in the secondary infection are similar to the primary infection and will crust and resolve within seven to 10 days.
Lesions appearing on the mucous membranes tend not to crust and appear as small aphthous ulcers.
Erythema infectiosum
Erythema infectiosum (Fifth disease) is caused by parvovirus B19 and mainly occurs in children aged five to 14 years. Recognition is important because it can cause serious complications in pregnant women, such as hydrops foetalis.
The classical facial erythema 'slapped cheek' starts as red papules that quickly coalesce to form red, slightly oedematous plaques. These spare the nasolabial folds and circum-oral region and fade within four days.
The patient may develop a fishnet/lace pattern of erythema on the extremities two days after the facial rash appears. This can extend to the trunk and buttocks and generally fades within one to two weeks.
Varicella
Varicella (chicken pox) is caused by varicella zoster virus. The child may have prodromal symptoms of a low-grade fever, general malaise or headache, but may often be asymptomatic until the appearance of the rash.
New lesions develop over the first four days and a mixture of red papules can be seen, which become vesicles and pustules before eroding and crusting. The rash classically begins on the trunk and spreads to the face and extremities.
When the vesicles appear, the child may become pyrexial and may find the rash itchy. The crusted lesions last about seven days and normally heal without scarring.
- Dr Dickson is a GP in Loanhead and hospital practitioner at the Royal Infirmary, Edinburgh
- This article was originally published in MIMS Dermatology www.healthcarerepublic.com/derm.
