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Adults usually develop heat rash in folds of skin and wherever clothing causes friction. In infants, the rash is mainly found on the neck, shoulders and chest, but it can also occur in the armpits, elbow creases and groin.

There are three types of miliaria, which are classified according to where the sweat ducts are blocked. Signs and symptoms for each type vary.

Miliaria crystallina

The mildest form of heat rash affects the sweat ducts in the topmost layer of skin. Miliaria crystallina is marked by:

  • Clear, fluid-filled blisters and bumps (papules) that break easily

The blisters that occur with miliaria crystallina aren't itchy or painful. This type of heat rash usually clears on its own but can come back if hot, humid weather persists. And though it's common in newborns, adults can develop it, too.

Miliaria rubra

Occurring deeper in the outer layer of skin (epidermis), miliaria rubra is sometimes called prickly heat. Adults usually develop miliaria rubra after they're exposed to hot, humid weather or if they're confined to bed rest. Infants usually develop this type of heat rash between the first and third weeks of life. Signs and symptoms typically include:

  • Red bumps
  • Itchy or prickly feeling in the affected area
  • Little or no sweating in the affected areas (anhidrosis)

Occasionally, miliaria rubra vesicles become pustular and then are called miliaria pustulosa.

Miliaria profunda

A less common form of heat rash, miliaria profunda occurs mainly in adults who have had repeat bouts of miliaria rubra. It affects the dermis, a deeper layer of skin, and appears soon after exercise or any activity that causes sweating. Signs and symptoms may include:

  • Firm, flesh-colored lesions that resemble goose bumps
  • A lack of perspiration, which may lead to symptoms of heat exhaustion, such as dizziness, nausea and a rapid pulse

When to see a doctor

Heat rash usually heals on its own and doesn't require medical care. See your doctor if you or your child has symptoms that last longer than a few days, the rash seems to be getting worse, or there are signs of infection, such as:

  • Increased pain, swelling, redness or warmth around the affected area
  • Pus draining from the lesions
  • Swollen lymph nodes in the armpit, neck or groin
  • A fever or chills
Mar. 07, 2012