Basic morphological elements
Primary elements of a skin rash are generally divided according to the nature of the lesions. The classification is based on the physical state of the lesion: whether it rises above the skin and whether it contains fluid inside.
The simplest element is a macule (macula)—an area that does not alter skin relief and lies on the same level with it, differing only in color. If an inflammatory spot has a diameter of up to 1 cm, it is called a roseola (roseola).
Rashes are divided into the following groups:
- Vascular and hemorrhagic changes:
- Erythema (erythema)—widespread redness (hyperemia) larger than 1 cm in size.
- Petechia (petechia)—a small, non-elevating punctate purplish hemorrhage (1–2 mm).
- Purpura (purpura)—an accumulation of numerous small hemorrhages.
- Ecchymosis (ecchymosis)—a large, bluish-purple irregularly shaped bruise.
- Solid (infiltrative) elements:
- Papule (papula)—a firm, elevated nodule up to 1 cm in diameter.
- Cystic/fluid-filled lesions (contain fluid):
- Vesicle (vesicula)—a blister up to 5 mm filled with serous fluid.
- Bulla (bulla)—a thin-walled blister larger than 5 mm containing exudate.
- Pustule (pustula)—a small pus-filled blister whose cavity contains purulent masses.
Differential diagnosis: hyperemia or hemorrhage?
Upon detecting a maculopapular/spotted rash, the physician's main task is to determine the pathogenetic mechanism of its appearance. A spot can form due to simple vasodilation (hyperemia) or damage to the vascular wall when erythrocytes extravasate into the surrounding tissues (hemorrhage).
Diascopy is used for differential diagnosis:
- The skin around the rash is stretched or pressed with a transparent glass slide.
- If the spot pales or temporarily disappears, it is a vascular reaction. Blood has simply left the dilated capillaries under pressure.
- If the color of the element does not change, it is a hemorrhage. Blood has already left the vascular bed and is present in the tissues.
The appearance of a hemorrhagic rash is an alarming symptom. The mechanism of its development is associated either with blood clotting disorders (thrombocytopenia, DIC) or vascular inflammation. Such a picture is typical of primary vasculitis (e.g., Henoch-Schönlein purpura), tumors, severe infections (endocarditis, septicemia), and chronic liver diseases.
Characteristics of erythema
Erythema refers to large spotted redness that may indicate specific pathological processes. Several specific variants are distinguished:
- Erythema nodosum (erythema nodosum). Manifests as painful indurations against a background of red spots, most commonly located on the anterior surface of the shins and near the joints of the lower extremities. The causes of its occurrence are diverse: from granulomatous processes (sarcoidosis, tuberculosis) and systemic diseases (rheumatism) to infections (chlamydiosis, salmonellosis) and side effects of hormonal contraceptives or antibiotics.
- Erythema migrans. A characteristic marker of borreliosis (Lyme disease).
- Fixed erythema. Occurs as a hypersensitivity reaction to the intake of certain medications. It presents as red spots or wheals. Its specific sign: upon re-exposure to the allergen, the rash recurs strictly in the exact same place. As the disease resolves, areas of persistent pigmentation remain on the skin, which can persist for months or a lifetime.
Specific local skin changes
Sometimes skin manifestations form a recognizable pattern that unambiguously points to the patient's condition or a chronic pathology:
- Pigmentation disorders. Include vitiligo (white depigmented areas surrounded by a zone of hyperpigmentation, prone to enlargement) and tinea versicolor. The latter is caused by the fungus Pityrosporum orbiculare and manifests as small spots (0.5–1 cm) in the axillary region and on the chest. On tanned skin they look light, and on pale skin they look dark.
- Oncological vigilance. Any fresh, newly appeared «mole» requires attention, as it may turn out to be a basal cell carcinoma, melanoma, or metastasis. The main signs of malignant transformation are a tendency to rapid growth and bleeding.
- Signs of alcohol intoxication. Patients with chronic alcoholism exhibit characteristic stigmata: a purplish-bluish face (especially cheeks and nose), dilated scleral vessels, and multiple telangiectasias («spider veins») on the upper body and face.
Briefly: answers to topical questions
What diseases cause a hemorrhagic rash?
A hemorrhagic rash is caused by conditions associated with hemostasis disorders, vasculitis, internal organ pathology, and severe infections. These include:
- DIC syndrome;
- Thrombocytopenic purpura (e.g., idiopathic thrombocytopenic purpura or Werlhof's disease);
- Hemorrhagic vasculitis (Henoch-Schönlein purpura) and drug-induced vasculitides;
- Chronic liver diseases;
- Tumor processes;
- Infectious diseases: septicemia, infective endocarditis, leptospirosis (most often the icteric form), meningococcemia (generalized form of meningococcal infection), and pneumococcal meningitis.
To quickly differentiate spots, remember the «glass rule» (diascopy): a dilated vessel under the glass «hides» (blanches), while a ruptured vessel (hemorrhage) «remains» (color does not change).
Class and exam questions
What is the difference between roseola and erythema?
The difference lies entirely in size. A roseola is a small inflammatory spot up to 1 cm. If the hyperemic area exceeds 1 cm in diameter, it is referred to as erythema.
How to distinguish a vesicle from a bulla?
Both elements are fluid-filled cavities, but a vesicle does not exceed 5 mm in diameter, while a bulla is a larger lesion over 5 mm in diameter with thin walls.
What does the appearance of a rash in the exact same spot after taking pills mean?
This is a classic manifestation of fixed erythema—a specific allergic reaction to a drug. Permanent pigmentation often forms at the site of the rash.
Which infections cause erythema nodosum?
Erythema nodosum can be triggered by yersiniosis, salmonellosis, and infections caused by Chlamydia (e.g., Chlamydia pneumoniae).
What else to study on this topic
- Pathogenesis of hemostasis disorders in DIC syndrome and thrombocytopenias
- Hemorrhagic vasculitis (Henoch-Schönlein purpura): clinical picture
- Differential diagnosis of skin manifestations in Lyme disease
- Systemic granulomatous diseases (sarcoidosis, tuberculosis) and skin lesions
- Infective endocarditis: mechanism of small vessel damage
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