Small Animal Dermatology, 3rd edition

542 DISEASES/DISORDERS

Epithelial migration moves epithelial cells, cerumen, and canal contents up and

out of the external canal

Tufts of hair may be present on the ventral surface of the external ear canal prox-

imal to the opening of the acoustic meatus near the ventral attachment of the

tympanic membrane.

 Tympanum (Figures 37.1, 37.2):
Pars flaccida: alternatively called the dorsal membrane; small portion dorsal to

the osseous ring of the external acoustic meatus; may swell with inflammation

or increased pressure in the middle ear

Pars tensa: thin firm structure attached to the osseous ring of the external

acoustic meatus; manubrium of the malleus on the medial surface curves

rostrally.

 Middle ear canal: medial surface of the tympanic membrane, bulla (tympanic cavity

• normally air filled) and auditory ossicles and associated structures; dog – incom-
  plete septum (bulla ridge) and promontory (contains cochlea and communicates with
  the bulla through the oval and round windows); cat – bulla divided by a nearly
  intact septum (associated with postganglionic sympathetic nerves); three auditory
  ossicles transfer movement from the tympanum to the inner ear; bulla connected to
  the nasopharynx by the auditory tube.

 Inner ear: anatomic structures of the cochlea, semicircular canals, and associated

nerves (cranial nerves VII and VIII).

 Otitis externa:
Common disease in dogs; frequent in cats
Chronic inflammation results in alterations of the normal environment of the

canal

With inflammation, glands enlarge and produce excessive wax

Epidermis and dermis thicken and become fibrotic

Thickened canal folds effectively reduce canal width

End-stage calcification of the auricular cartilage results in permanent changes.

 Causes of otitis externa are frequently classified as primary, secondary, predisposing,

or perpetuating:

Primary causes directly initiate or cause inflammation in a normal ear canal (e.g.,

parasites, hypersensitivity, endocrinopathy)

Secondary causes initiate or cause inflammation in an abnormal ear canal (e.g.,

bacterial or yeast infection, medication reaction)

Predisposing causes change the environment of the normal ear canal, facilitating

inflammation and encouraging secondary infection

Perpetuating causes result from changes in the external ear canal from inflam-

mation that prevent resolution of the inflammation and/or infection of the ear

canal.

 Otitis media: most often an extension of otitis externa through a ruptured tympanum;

may occur without membrane rupture; may occur from polyps or neoplasia within

the middle ear or auditory tube.

 Otitis interna: extension of otitis media or hematogenous spread of infection; exten-

sion of neoplasia from surrounding tissue.