Otitis externa sometimes complicated, frustrating
Otitis externa is an inflammatory condition with or without concurrent infection and is the most common dermatological disease in a busy veterinary practice. Thus it is the most common claim forwarded to veterinary insurance companies. For the most part, the majority of cases are simple, and treatment clears the disease in a few days.
However, many cases are recurrent, and the management of those cases is frustrating and often veterinarians seek the aid of a specialist.
This article will focus on the more complicated cases, and will clarify and discuss the pathogenesis, treatment and preventive measures. I will not discuss the uncommon causes of otitis such as parasites, foreign bodies and neoplasia. The emphasis of this article will be on the dog with severe, complicated or recurrent bacterial otitis with a focus on the pathology and treatment.Normal anatomy The canine ear varies in the size, shape and conformation from breed to breed. The external ear is comprised of the pinnae that can be large and pendulous or erect. It is speculated that pendulous pinnae predispose to otitis externa. However, even erect pinnal breeds such as the German Shepherd are affected by otitis externa. The ear has some hair (some breeds have excessive hair), modified apocrine sweat glands (cerumen glands) and sebaceous glands. A small amount of cerumen should be present on the surface of the canals and leading down to the tympanum membrane. Some dogs have small or stenotic canals (such as Shar-peis) as a breed-related problem.
The canals are divided into vertical portions leading into the horizontal canal that ends at the tympanum membrane. The tympanum membranes separate the external ear canal from the middle ear and leads ventrally into the bulla. The tympanum membrane should be a bit opaque, not completely cloudy and should be relatively smooth.
The microbiology of the ear canal is very similar to the hair and skin in dogs. In my recent article concerning pyoderma, the normal bacterial flora was elucidated. Staphylococcus, E. coli, Corynebacteria, and even transient bacteria such as Proteus, Enterococcus and Pseudomonas can be found. Malassezia as a yeast organism is also found normally on the pinnal and in the canal in dogs.
Predisposing causes It is speculated that predisposing factors are not the primary cause of otitis externa/media but either trigger or increase the risk of their development.
Conformation is the most common predisposing cause and include the large and pendulous pinnae seen in certain breeds such as the Cocker Spaniel or Springer Spaniel. These breeds are also predisposed to other causes of otitis, so careful history and physical examination must be performed before one blames anatomy as the sole cause of otitis.
Excessive hair in the ear canal (or on the medial aspect of the pinnae) has also been implicated as a predisposing cause to otitis.
Breeds such as Poodles and certain Asian breeds are considered the bulk of the breeds affected with excessive hair and most definitely benefit from routine plucking of hair from the vertical canals.
I am very careful when I perform hair removal, because certain cases seem to worsen after this procedure. Moreover, some specialists seem to believe removal of hair can worsen and not benefit these patients. I uncommonly observe these cases, but when I do I am very careful in selecting patients for hair removal.
Lastly, the one predisposing factor that can complicate and trigger otitis is swimming. Swimming has definitely been linked to otitis in human patients, and it seems that certain dogs are also affected with a similar problem. The real dilemma that I have is the most common breeds that swim are also the more common breeds affected by allergy: the most common primary cause of otitis.
There are many primary causes of otitis in dogs, including parasites such as Otodectes, Demodex, Sarcoptes, seborrhea, autoimmune diseases, non-yeast fungal infections and glandular disorders. The most common primary cause of otitis is atopic dermatitis. Because of its high incidence, atopic dermatitis is more frequently associated with otitis externa than any other disease.
It has been reported that 3 percent to 10 percent of atopic dermatitis cases can present soley with otitis externa and less so with unilateral otitis externa.
Generally speaking, if you examine closely the medial aspect of the pinnae of early cases of otitis externa with atopic dermatitis, you will see subtle erythema on the central and medial aspect (concave surface just where the pinnae might "fold" in half) of the pinnae.