To provide effective care, veterinarians must be knowledgeable about middle-ear anatomy and physiology, including key differences between species.
According to Bradley L. Njaa, professor in the College of Veterinary Medicine at Kansas State University in Manhattan, Kansas, the ossicles (malleus, incus, and stapes) within the middle ear are responsible for sound transduction from low-impedance air to high-impedance perilymph in the inner ear. One way to help achieve sound transduction involves having a large ratio of the surface area of the tympanic membrane to that of the footplate of the stapes. In an interview with American Veterinarian®, Dr. Njaa noted that “the actual difference is a ratio of 20:1 from the tympanic membrane to the head of the stapes.”
Presenting at the American College of Veterinary Pathologists 2017 Annual Meeting in Vancouver, British Columbia, Canada, Dr. Njaa also reviewed some anatomic and biological features of the middle ear in domestic animals, highlighting how some of these features influence management of middle-ear disease in veterinary patients.
He described how the middle ear comprises the air-filled tympanic cavity, which is surrounded by bone, including the ventrally located tympanic bullae. The tympanic bullae are septate in pigs, cattle, camelids, and goats, he noted, containing multiple air-filled compartments with interconnected bony septa. But the tympanic bullae in dogs and sheep contain a single air-filled compartment surrounded by bone.
Dogs and cats also have a bony ridge known as the septum bulla within the tympanic bulla. In cats, the septum bulla almost completely separates the tympanic cavity into two compartments (dorsomedial and ventrolateral), while in dogs the separation is incomplete. Dr. Njaa stressed that surgeons need to be aware of these anatomic differences, especially when managing cases in which they need to open and drain the tympanic bullae.
He also explained that while cattle, pigs, and many other animals have tympanic bullae at birth, dogs and cats do not have these bony structures at birth.
Dogs and cats do have a complete tympanic ring at birth, with the tympanic membrane stretched across it, said Dr. Njaa. The tympanic membrane is suspended in air and separates the tympanic cavity from the external ear canal. “This is a mostly avascular, thin membrane with only blood vessels along the outer rim and in the region of the embedded manubrium of the malleus in the pars tensa,” he told American Veterinarian®. “However, the pars flaccida appears to be a rather well-vascularized membrane.”
When sound waves hit the tympanic membrane, they stimulate the three auditory ossicles, which form a chain suspended in the tympanic cavity. These bones articulate with each other as a lever system, amplifying sound waves that ultimately produce vibrations in the inner ear and allow animals to hear.
The tympanic membrane is composed of the pars tensa and pars flaccida. The pars tensa comprises the largest area of the membrane, and functions in sound transmission. The manubrium, the longest appendage of the malleus, is also embedded in the pars tensa.
Although the tympanic membrane is very thin, it is “a wonderfully functional part of the ear,” stressed Dr. Njaa. Histologically, the pars tensa is composed of three layers of tissue: an outer layer of keratinized squamous epithelium (continuous with the external ear canal epithelium), a middle layer of fibrovascular connective tissue, and an inner layer of low cuboidal to nonkeratinizing squamous epithelium (this faces the inner ear).
The tympanic membrane is susceptible to perforation, most commonly secondary to conditions such as otitis media, neoplasms, or traumatic injury. Dr. Njaa highlighted the unique ability of this structure to heal quickly while simultaneously retaining its thinness for functionality, noting that it undergoes a unique healing process known as epithelial migration.
During the healing process, “the squamous epithelium of the outer portion of the tympanic membrane bridges the gap first,” he said. The fibrous portion advances next, and then the inner lining of the middle ear completes the healing process. This process contrasts with what occurs in other tissues such as the skin, in which development of granulation tissue occurs first and allows for subsequent migration of epithelium to close the defect. In the tympanic membrane, “the epithelium starts the process,” said Dr. Njaa.
Veterinarians must be aware of the anatomy and physiology of the middle ear, including key differences between the species, because these have important implications for treatment and management of middle ear disease, emphasized Dr. Njaa.