Seborrhoeic dermatitis

What it is

A chronic dermatitis of the sebum-rich skin. The etiology is not exactly known, but it is an inflammatory reaction to Malassezia spp., a normal skin inhabitant (Malassezia furfur, formerly known as Pityrosporum ovale). It appears in two forms:

• Cradle cap in infants and children, on the scalp
• Dandruff in adults, on the scalp

It is common, affecting around 2% of the general population. A table setting it beside dandruff puts it at up to 40% of infants within 3 months of age and 1-3% of the general adult population, and puts dandruff at 50% of the adult population.

Causes and risk

The predisposing factors and causes are sebaceous gland activity, fungal colonization, and individual susceptibility, which covers epidermal barrier integrity, host immune response, genetic factors, neurogenic factors and stress, and nutrition. Associated conditions include HIV and Parkinson's disease, so it should be considered in a Parkinson's patient with a recurrent rash.

Put another way, the underlying cause is chronic inflammation due to overgrowth of Malassezia, a yeast, in sebaceous-rich areas, and it is also written as sebaceous gland hyperactivity with that overgrowth. The key associations are Parkinson's, HIV and immunocompromised states, and the sites named with them are the nasolabial folds, scalp, eyebrows and chest.