Types (1) Restricted otitis externa. (2) Diffuse otitis externa. (3) Necrotizing otitis externa. (4) Fungal otitis externa. Etiology (1) Limited otitis externa, i.e., external auditory canal boils: Infection of the skin follicles in the cartilage of the external auditory canal, mostly solitary, but also multiple. (2) Diffuse otitis externa: Diffuse non-specific inflammation of the external auditory canal caused by bacterial infection, which can be divided into two categories: acute and chronic. (3) Fungal otitis externa, also known as external auditory canal fungal disease: caused by fungal infection. (4) The true etiology of necrotizing otitis externa has not been determined, but it is tentatively thought to be an immune-mediated disease that occurs mostly in elderly diabetic patients. This is because diabetic patients have poor leukocyte migration, delayed or defective phagocytosis, reduced lymphocyte reactivity and impaired regulatory antibodies. Malnutrition and anemia are predisposing factors, and the causative agent is often Pseudomonas aeruginosa.