A small patch of dense itchy bumps on the arm may be related to contact dermatitis or eczema. 1. Contact dermatitis refers to the skin due to contact with a certain substance and the occurrence of perverse reactions, this substance is also called allergens, in the clinic can be manifested as local skin itching, can also be accompanied by some skin lesions. 2. Eczema belongs to a kind of inflammatory skin disease, which is also related to allergy. Itching is its main feature, in addition to a series of skin changes, such as small papules, macules, blisters and so on. Symptoms of contact dermatitis as well as eczema can be treated with targeted anti-allergy and anti-itching medications under medical supervision, such as topical compound dexamethasone cream and dieldrin cream. In addition to the above two causes, it may also be caused by other diseases, it is recommended to consult a doctor in time, after clarifying the cause of the disease, timely compliance with the doctor’s instructions for symptomatic treatment, do not self-medication.