65 Otitis Externa
Otitis externa is an inflammation of the skin of the external auditory meatus (EAM). The commonest symptoms are otalgia, itching and otorrhoea. The mainstay of treatment is aural toilet with topical antibiotic/steroid preparations. The specific medical content of these preparations is an often-asked question in ENT examinations.
The skin of the EAM is comprised, in the outer third, of an epithelial layer containing hair follicles, ceruminous glands and sebaceous glands, lying on a thin dermal bed containing sweat glands. The skin of the medial bony ear canal lacks appendages and thins from lateral to medial. The secretions of the sebaceous glands keep the stratum corneum watertight and supple. Sweat gland secretions keep the secretion at a pH between 3 and 5 which is lethal for most human pathogens. Usually, the EAM is sterile or contains Staphylococcus albus commensals. Staphylococcus aureus and non-haemolytic streptococci are unusual.
In the acute phase of otitis externa, there are dilated dermal blood vessels of increased permeability which cause signs of a red, hot, oedematous and tender ear canal. The epithelial reaction consists of vesication, parakeratosis and spongiosis. It is usually a diffuse process but may be more localised such as in bullous myringitis or, the most extreme example, a furuncle, usually arising from a single hair follicle.
65.2 Predisposing Factors
1. Heat, humidity, bathing, swimming.
2. Trauma, especially from dirty fingernails, cotton buds and hairgrips.
3. Inherited—narrow ear canals.
4. Skin conditions—non-atopic eczema, psoriasis.
a. Bacterial diffuse otitis externa commonly caused by Pseudomonas aeruginosa, S. aureus and Bacillu sproteus.
Furunculosis, usually caused by S. aureus.
Malignant otitis externa, usually caused by P. aeruginosa or occasionally S. aureus.
Erysipelas caused by Streptococcus pyogenes. Perichondritis caused by P. aeruginosa (not, as is often thought, S. aureus).
Impetigo, an infection of the superficial layers of the epidermis, usually caused by S. aureus or occasionally S. pyogenes.
3. Viral herpes simplex
Presumptive in otitis externa haemorrhagica and bullous myringitis.
– Seborrhoeic dermatitis.
– Neuro dermatitis.
– Keratosis obturans.
– Secondary to discharge from an acute or chronic otitis media.
65.3 Clinical Features
The cardinal symptom of this condition is pain, often preceded by itching and/or a sense of blockage. Other symptoms include otorrhoea (the discharge may be waxy, watery or serosanguinous) and hearing loss, although this is usually a secondary symptom.
Otitis externa may be confined to a relatively small area of the meatus (localised) but is more commonly widespread (generalised). Localised infection can be circumscribed or diffuse while generalised infection can be either primary otological or primarily dermatological.