This symptom often appears between the ages of 20 and 40 among men and women alike, and although its causes are still unclear, it is thought to be due to the body's autoimmunity and in a minority of cases is accompanied by pathologic changes to the immune system such as leukoderma, diabetes, and disturbance of the thyroid function.
這種病症常常發生在廿歲到四十歲之間，男女病例都有，雖然成因尚不明朗，但推測極有可能是由「自體免疫」引起，少數伴有白斑症、糖尿病、甲狀腺功能障礙等免疫系統的病變。