Differentials
Rubella
SIGNS / SYMPTOMS
Typically, non-specific exanthem of rose-pink macules that spread cephalocaudally.
Joint involvement is common.
Tender cervical, occipital and/or posterior auricular lymphadenopathy is common.[28]
INVESTIGATIONS
Serology shows presence of anti-rubella IgM or a 4-fold increase in anti-rubella IgG antibodies.[28]
Measles infection
SIGNS / SYMPTOMS
Prodrome typically includes cough, coryza and conjunctivitis.
There is an exanthem consisting of grey-white papules on the buccal mucosa (Koplik's spots) during the prodrome.
The exanthem is an erythematous maculopapular eruption that spreads cephalocaudally and usually begins to clear within 1 week.[28]
Roseola infantum
SIGNS / SYMPTOMS
Typical prodrome of sudden high fever that lasts for 3 to 5 days.
Non-specific exanthem appears at the time of defervescence.
The disease usually affects younger children (6 months to 3 years) than erythema infectiosum.[30]
INVESTIGATIONS
Serological studies and polymerase chain reaction are all available to identify human herpesvirus type 6.
Culture may be available but is rarely necessary.[30]
Scarlet fever
SIGNS / SYMPTOMS
Typically preceded by pharyngitis and fever as opposed to the relatively asymptomatic prodrome of erythema infectiosum.
The eruption associated is papular (a sandpaper texture) and spreads centripetally.
Other cutaneous findings include petechial streaks in skin folds (Pastia's lines) and circumoral pallor.
Mucosal signs include exudative pharyngitis and a white or beefy red tongue.
Rash usually followed by desquamation.
INVESTIGATIONS
Rapid antigen detection test (RADT) for group A streptococcus (GAS) for patients ≥3 years of age, with culture of throat swab for children and adolescents aged 3-15 years with suspected Scarlet fever who have a negative RADT result for GAS.
Erysipelas
SIGNS / SYMPTOMS
Involved area usually a unilateral plaque as opposed to the bilateral macular cheek erythema seen in erythema infectiosum.
Rash commonly warm, tender and indurated.
Regional lymphadenopathy is typical.[31]
INVESTIGATIONS
Occasionally, a left shift and elevated leukocyte count will be seen.
Cultures of the affected area rarely yield the aetiological organism, Streptococcus pyogenes or Staphylococcus aureus, but anti-steptolysin O titres and antideoxyribonuclease-B titre may indicate cutaneous infection.[31]
Blood cultures may be helpful in delineating the causative organism.
Drug hypersensitivity
SIGNS / SYMPTOMS
History and temporal association with a causative pharmacological agent frequently identified.
May be associated with a generalised maculopapular eruption.
INVESTIGATIONS
No specific tests.
CBC may reveal an eosinophilia. Liver function tests or renal function tests may demonstrate end organ dysfunction.
Systemic lupus erythematosus
SIGNS / SYMPTOMS
Persistence of cutaneous and systemic symptoms with multi-organ involvement.
The classic malar eruption of systemic lupus erythematosus involves the nasal bridge as opposed to the rash of erythema infectiosum.
INVESTIGATIONS
Positive antinuclear antibody titres and pathological examination of skin biopsy will differentiate lupus from erythema infectiosum.
Juvenile dermatomyositis
SIGNS / SYMPTOMS
Characterised by progressive weakness rather than true arthritis as seen in erythema infectiosum.
Calcinosis is a feature not seen in erythema infectiosum.
The eruption of dermatomyositis often involves the face and eyelids but often has scale, in contrast to erythema infectiosum.
Photo-sensitivity, nailfold telangiectasias and a scaling eruption over the knuckles, are highly characteristic of dermatomyositis.
INVESTIGATIONS
Elevated serum creatine kinase, aldolase, lactate dehydrogenase and positive serum antinuclear antibody often seen in dermatomyositis.
Skin biopsy can also help differentiate these two entities.
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