The permissive effect of sebum in seborrhoeic dermatitis: an explanation of the rash in neurological disorders.

The hypothesis that sebum permits the growth of Pityrosporum ovale, and hence the development of seborrhoeic dermatitis, was tested by observing whether a reduction of sebum production by isotretinoin would improve the disorder. In 10 male patients with seborrhoeic dermatitis, treatment with isotretinoin for 6 weeks reduced the mean sebum excretion rate by 70% and improved the severity of the rash, but with a site difference in magnitude of response. It is concluded that the residual pool of sebum is important for the growth of P. ovale and that, within the physiological range, sebum has a permissive effect on the growth of this yeast. Variation in the pools of residual sebum explains a number of features of the disease such as site of involvement and greater prevalence in males than females. The pathological increase in the residual pool of sebum due to immobility explains the frequent occurrence of seborrhoeic dermatitis in patients with a variety of neurological disorders.