Rash morphology

Key criteria for exanthem evaluation

To make an accurate differential diagnosis in conditions accompanied by skin rashes, it is necessary to analyze a set of five basic parameters:

1. Timing of onset. The time from the onset of the disease to the first elements of the rash.
2. Morphology. The character of the elements (macules, papules, wheals, hemorrhages).
3. Topography and stages. Favorite localization and sequence of spread across the body.
4. Dynamics of fading. The presence or absence of pigmentation and subsequent epidermal desquamation (peeling).
5. Associated syndromes. Intoxication, mucous membrane lesions, lymphadenopathy, or a clear association with an allergen.

Morphology and pathogenesis of scarlet fever rash

Scarlet fever is accompanied by pronounced pharyngeal involvement ("fiery pharynx", "strawberry tongue") and the formation of a primary infectious complex due to lymphogenic spread of the pathogen. However, the exanthem itself is caused by the hematogenous route — toxemia.

The absorption of bacterial toxins into the blood triggers systemic microvessel vasculitis. Morphologically, this manifests as marked fullness of blood vessels, erythrocyte stasis, and sludging. Lymphomacrophagal infiltrates accumulate around the affected vessels (especially in the dermis), ultimately leading to necrotic changes in the epidermis.

• Localization: The rash starts on the face, leaving only the nasolabial triangle intact. Then the elements spread to the neck, trunk, and limbs.
• Resolution: After the fading of the elements, lamellar desquamation begins, which is most pronounced on the palms and soles.

Differential diagnosis of common infections

The morphological pattern of the rash is strictly specific to various infectious agents:

• Measles. Characterized by a late onset of the rash (on days 3–5). The elements have a maculopapular character. The most important marker is strict downward staging: on the first day, the face is affected; on the second, the trunk; on the third, the extremities. Resolution is accompanied by pigmentation and desquamation. The disease proceeds with pronounced catarrhal syndrome and pathognomonic enanthem on the buccal mucosa.
• Rubella. The exanthem is finely maculopapular, appearing early (days 1–2 of illness). Favorite localization includes the face, back, and extensor surfaces of the arms and legs. The elements disappear without a trace. A key concomitant sign is regional lymphadenopathy (enlargement of the posterior cervical and occipital nodes).
• Enteroviral exanthem. A macular rash that occurs on days 1–3. It appears predominantly on the face and trunk at the peak of the temperature reaction or as it drops. It is distinguished by being ephemeral—elements can completely disappear within a day.
• Allergic exanthem. Develops within the first day after contact with a trigger. The rash is polymorphic, often urticarial (itchy wheals), and has no regular patterns of localization or staging. It disappears without a trace upon elimination of the allergen.

Specific vascular signs and localization

In some cases (especially with scarlatiniform rashes), the morphology of the rash can vary from punctate to hemorrhagic. Typical localization includes the lateral surfaces of the trunk, lower abdomen, inguinal, and axillary regions.

Of particular diagnostic significance are the following signs:

• Clustering of the rash: elements concentrate on the flexor surfaces of the extremities, in natural skin folds, and around affected joints, often merging with each other.
• "Gloves", "socks", and "stockings" sign: pronounced hyperemia and edema of the hands and feet, which appear simultaneously with the rash. This phenomenon is caused by toxic damage to autonomic nerve ganglia.
• Character of desquamation: in the 2nd–3rd week of the disease, the skin of the trunk and auricles becomes covered with fine branny desquamation, while large-lamellar detachment of the epidermis occurs on the hands and feet.

Class and exam questions

What else to study on this topic

• Pathomorphology and stages of development of scarlet fever tonsillitis
• Early purulent complications of infectious exanthems
• Mechanisms of development of cross-autoimmune reactions
• Morphological pattern of post-infectious glomerulonephritis
• Systemic internal organ damage in severe intoxication

Other topics in the "Additional Topics" section

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