9 Ear
9.1 External ear
9.1.1 Otitis externa
Otitis externa will be one of the most common conditions you encounter in small animal clinical practice, and is most frequently see in dogs. It will no doubt have been covered extensively in classes on dermatology, and will likely have been considered in the sections on dermatopathology as well. It is a complex problem, and one that can be frustrating for both owners and veterinarians alike.
The external ear is essentially a modified extension of the skin, and conditions that affect the skin can equally affect the ear. Atopy or food allergy are the most common primary causes of otitis externa. There are many other factors, however, that predispose dogs to otitis externa:
- pendulous ears (e.g. cocker spaniel)
- hairy ear canals (e.g. poodles)
- excess cerumen production (e.g. German shepherds)
- stenotic ear canals (e.g. Shar peis, or as a result of chronic otitis externa)
- ear mites
- aural tumours
- foreign bodies
Grossly, ears with otitis externa are inflammed (as demonstrated by warm, red auricles), and the ear canal often contains abundant discharge, with or without hemorrhage. The ears are frequently quite painful. The environment of the external canal becomes warm and moist, favouring the growth of bacteria normally found on the skin. The inflammation, which in acute cases is predominantly neutrophilic, promotes epidermal and glandular hyperplasia. In chronic cases, macrophages, lymphocytes, and plasma cells tend to dominate, and the hyperplastic changes become more dramatic. In the most severe cases, fibrous connective tissue replaces adnexa, markedly distorting the internal environment of the ear canal. The additional tissue, along with marked epidermal hyperplasia and hyperkeratosis, results in a stenotic ear canal, which in turn limits treatment and resolution of the condition.
Otitis media and Aural hematoma are relatively common complications of otitis externa.
9.1.2 Aural hematoma
The constant head shaking of dogs with Otitis externa can cause small fractures to the auricular cartilage. These fractures may produce sharp edges that lacerate capillaries, leading to hemorrhage that accumulates, presenting as a grossly swollen, warm ear. Left to heal on its own, the ear will fibrose and contract, resulting in a cosmetically damage, but otherwise functional, ear. Dogs presenting with aural hematomas should be checked for Otitis externa.
9.1.3 Parasitic diseases
There are a number of ectoparasites that affect the ears. Of the most important is Otodectes cynotis, especially in cats, in which they often cause a secondary otitis externa. Infection with Otodectes results in a thick, waxy, brown discharge that obstructs the ear canal. Auricles are often alopecic.
9.1.4 Neoplasia
Unlike the inflammatory conditions of the ear, neoplasms are typically unilateral. Several of the common dermal neoplasms, including histiocytomas, sebaceous gland tumours, mast cell tumours, and plasma cell tumours, can occur in the ear.
9.1.4.1 Squamous cell carcinoma
Squamous cell carcinoma of the ear is an invasive and malignant neoplasm most commonly seen in white cats, and is believed to be the result of UV radiation. They are typically raised, nodular, and ulcerated. Metasasis through vascular or lymphatic invasion is relatively common.
9.2 Middle ear
9.2.1 Otitis media
Otitis media affects all species, but is least common in the cat. It is usually bacterial. Access to the middle ear can be either through rupture of the tympanic membrane or through ascension up the auditory tube.
In dogs, otitis media is most frequently due to perforation of the tympanic membrane secondary to otitis externa. Note that the tympanic membrane can heal very quickly, so the presence of an intact tympanum does not rule out otitis media.
In cats, otitis media is usually thought to be the result of migration of bacteria up the auditory tube. It is not usually associated with otitis externa.
In dairy calves, Mycoplasma bovis is frequently found alongside concurrent pneumonia.
In pigs, impaired function of the auditory tube can lead to ascension of Mycoplasma hyorhinis, Pasteurella multocida, or Truperella pyogenes.
Otitis media may be unilateral or bilateral. Inflammation is typical, as is exudate within the tympanic bulla, either purulent or caseous. Severe inflammation may lead to lysis of the bony ossicles.
9.2.2 Inflammatory polyps
These are extremely common in cats, and are typically found in young animals. They are thought to originate from the middle ear, and can exit either through the external ear (‘aural’ polyps) or pharynx (‘nasopharyngeal’ polyps). The clinical signs, and associated pathology, depends to some degree on where they grow.
Polyps are typically composed of fibrous connective tissue with abundant small blood vessels, and are lined by a ciliated epithelium. The cell of origin and their pathogenesis remain uncertain, however. It is thought they may be granulation tissue arising from a chronic infection either to the auditory tube or middle ear. Complete removal of polyps is curative; however, if some of the polyp is left behind, it may regrow.