Ears, the basics all practitioners should know (Proceedings)
Ear disease is a very common problem presented to veterinarians. It may be broken down into two major components, pinnal disease and otitis, though the strict definition of otitis would include any inflammatory pinnal disease. Otitis is defined as inflammation of the ear and is subdivided into inflammation of three main anatomic regions, externa, media and interna. Chronic otitis externa is a difficult and frustrating problem often referred to dermatology specialists. It is an important problem that veterinary dermatologists must understand and be able to manage. A good approach and understanding of ear disease requires the understanding of what really happens in a case of ear disease and some basic rules that must be followed to achieve a successful outcome.
Otitis cases are often complex and involve more than one etiologic component. For example most plant awns do not just present for foreign body induced inflammation of the ear canal. By the time the case is presented to a veterinarian there is usually inflammation from the plant awn as well an infection from organisms present in the normal ear or introduced by the plant awn. In addition there may be damage to the ear canal skin resulting in abnormal physiology of the ear or damage to the tympanum and extension of the infections into the middle ear. If the plant awn is removed the case may not get better until the infection is treated. Even if the infection is effectively treated the ear may still develop another infection if treatment is stopped before the skin is totally healed and normal or the middle ear inflammation is resolved. This means the diagnosis and management of otitis externa is often much more complex than just recognizing what " caused" the ear disease.
Anatomy and PhysiologyRecognizing the components of otitis requires an understanding of the normal anatomy. For a more complete description and anatomic photos the reader is referred to references 1 and 2.
The basics about the anatomy are similar but it is also important to realize there is wide variation between species and even breeds when it comes to the external and middle ear. The ear starts with the pinna, the most dorsal and lateral aspect of the head. The ear canal then travels to the tympanum and middle ear which are all enclosed by bones of the skull. The tympanum and end of the ear canal are located ventral and caudal to the eyeball but almost on the same plane medial to lateral. The ear is then divided into three major portions the external ear, middle ear and inner ear. The external ear main purpose is to collect sound waves and transmit them to the tympanic membrane. To dermatologists the anatomy most important to learn relates to the external and middle ear as well as the communications to the inner ear.
The external ear is formed from two pieces of cartilage and a boney canal that are covered by skin which ends at a specialized epithelial structure the tympanic membrane. From the external orifice the cartilage is rolled onto itself and goes distally with the central open area forming the lumen of the ear canal. The external ear canal is variable in length (5-10cm) and classically divided into the vertical and horizontal portions. As one proceeds down the vertical canal there is another ridge or fold in the cartilage that is called the auricular projection. It creates the "corner" around which one must proceed to allow access down the canal and when otitis is present the lumen ridge is often inflamed and when pushed against by otoscope cones, especially the edge of the cone, may result in pain and the dog resisting examination. The size of the lumen ridge varies between breeds and between individuals within breeds. During otoscopic examination the lumen ridge is moved and decreased from blocking access to the ear canal lumen by pulling on the pinna and straightening the lumen ridge.