Common Ear Diseases in Dogs: Diagnosis and Management E-Book
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- Herausgeber: Bentham Science Publishers
- Kategorie: Fachliteratur
- Sprache: Englisch
Common Ear Diseases in Dogs: Diagnosis and Management is an essential resource for veterinarians, veterinary students, and researchers focusing on ear health in dogs. This comprehensive guide covers the anatomy, physiology, microbiology, and pathology of canine ears, offering a thorough understanding of ear structure and function. It covers diagnosing, treating, and managing various ear conditions, including otitis externa, otitis media, aural hematomas, traumatic injuries, and more. Chapters explore cutting-edge diagnostic techniques, surgical interventions like ear canal ablation, and pain management strategies, making it a valuable tool for clinical practice. Written by subject matter experts, the book bridges foundational knowledge with advanced clinical applications, helping readers develop expertise in ear disease management.
Key Features:
- Provides an in-depth understanding of ear anatomy, physiology, and pathology in dogs.
- Covers diagnosis, treatment, and management of common ear conditions, including infections, injuries, and neurological issues.
- Features practical guides on advanced procedures like laser surgery and ear canal ablation.
Readership:
Ideal for undergraduate and graduate students, veterinary practitioners, researchers, and academicians.
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Seitenzahl: 606
Veröffentlichungsjahr: 2025
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PREFACE
Common Ear Diseases in dogs: Diagnosis, and Management 1e book cover otic disease as a serious ailment at the forefront of veterinary medicine. The book covers updated information for practicing veterinarians in identifying small animal ear diseases. It also illustrates the anatomy of the ear, examination techniques, pathophysiology, and treatment strategy to prevent the disease. Various predisposing factors, primary causes, and perpetuating factors associated with ear diseases in dogs are elaborately described in the book. Various therapeutic options with new drugs, and nutritional products may prove to be valuable in the prevention and treatment of ear diseases in dogs. The book covers an extensive study of the diseases of the pinna, external ear canal, middle ear, and inner ear. Various photos are included in the book for easy identification of diseases. The book is an invaluable resource for both veterinarians in training and in practice for gathering knowledge about the ear diseases of dogs. The contributors are specialized in their knowledge for the writing of the individual chapter. This book is especially intended for pet practitioners, academics, researchers, veterinarians, and DVM graduate students engaged with a special interest in pet animal health, and management. I hope that this book serves new paradigms for the stimulus to further research in clarifying the pathomechanisms, diagnosis, and treatment of ear diseases in dogs. I expect that the reader will find this book interesting as well as up-to-date information about ear diseases and will utilize the knowledge in the research and teaching to the new generation. I always welcome constructive feedback and encouragement from my veterinarian colleagues all over the world.
List of Contributors
Introduction
Apoorva Mishra1,*,Randhir Singh1,Babita Das1,Shobha Jawre1Abstract
This chapter discussed some common ear illnesses such as otitis, haematoma, tumors etc. and establishes the groundwork for understanding the significance of ear health in dogs. We will go more into each ear ailment and examine diagnosis and treatment options in the upcoming chapters. It is crucial for any veterinarian to comprehend the anatomy of a dog's ear, identify common ear illnesses, and know how to diagnose and treat them.
INTRODUCTION
The ears are a crucial part of a dog's anatomy, serving not only as sensory organs but also playing a significant role in maintaining balance. Otitis externa, or inflammation of the skin lining the ear canal, is the most common cause of canine ear illness and can result in otitis media, a secondary infection of the middle ear chamber. The majority of the time, ear inflammation is a symptom of a more widespread skin condition. As a result, dogs with ear issues frequently lick or chew at their feet or experience irritation elsewhere. In addition to the ear illness, more severely infected dogs may also exhibit obvious signs of skin allergies throughout other body regions. Because of the environment inside the ear, the ears are frequently more seriously impacted by this generalized skin irritation than other parts of the body. The hypersensitivity or skin allergy first results in a mild degree of inflammatory processes, which promotes the growth of bacteria and yeast organisms that are ordinarily found on the skin. Most skin locations in dogs with moderate cases can escape considerable overgrowth of these organisms, but the warm, moist environment inside the ear canal creates the perfect conditions for these organisms to proliferate and subsequently exacerbate inflammation. As the organisms proliferate, more inflammation and skin damage are brought on by
their presence, creating a vicious cycle. Unfortunately, due to their unique structure and susceptibility to various environmental factors, dogs are prone to a range of ear diseases. In this chapter, we will delve into the world of canine ear health, exploring the anatomy of a dog's ear, common ear diseases, their causes, symptoms, and preventive measures [1].
Anatomy of a Dog's Ear
Before we delve into the specifics of ear diseases, it's essential to understand the basic anatomy of a dog's ear. A dog's ear consists of three main parts:
Outer Ear (Pinna)
The visible part of the ear is called the pinna, which varies in shape and size among different breeds. It's designed to capture sound waves and funnel them into the ear canal.
Middle Ear
The middle ear begins with the ear canal and includes the eardrum (tympanic membrane) and a system of small bones (ossicles). Sound waves are transmitted through the ear canal to the eardrum, where they are converted into vibrations.
Inner Ear
Beyond the eardrum lies the inner ear, which contains the cochlea, responsible for converting vibrations into electrical signals sent to the brain. The inner ear also plays a crucial role in balance and orientation.
The canine ear canal is more vertical than that of a human, forming an L-shape that tends to hold in fluid. This makes dogs more prone to ear infections. Ear infections are typically caused by bacteria, yeast, or a combination of both. In puppies, ear mites can also be a source of infection.
The pinna, middle ear, inner ear, and external ear canal form the canine ear. The cartilage of the auricle and annulus constitute the external ear. At the external ear canal opening, the pinna's auricular cartilage takes on the appearance of a funnel. After approximately one inch, the auricular and annular cartilage that makes up the horizontal ear canal emerges from the vertical ear canal. The three auditory ossicles, the tympanic membrane, and the air-filled tympanic cavity are part of the middle ear [2]. The pars flaccida and pars tensa make up the semitransparent tympanic membrane. The little epitympanic recess, the huge ventral bulla, and the tympanic bulla proper make up the tympanic cavity. The cochlea is located on the promontory, which is the medial wall of the tympanic cavity. A thin membrane covers the cochlear (round) window, which is situated in the caudolateral section of the promontory. The stapes footplate is linked to a thin diaphragm that covers the vestibular (oval) window, which is situated on the dorsolateral surface of the promontory. The auditory tube is a brief canal that connects the rostral region of the tympanic cavity proper to the nasopharynx. The bones that carry and intensify air vibrations from the tympanic membrane to the inner ear are called auditory ossicles (Fig. 1). The petrous part of the temporal bone contains a bony labyrinth that houses the inner ear. The membrane labyrinth, with its auditory and vestibular organs, is housed within the bone labyrinth [3].
Fig. (1)) Internal structure of ear of dog.Common Ear Diseases in Dogs
Now, let's explore some of the most common ear diseases that affect dogs. There are three types of ear infections, known formally as otitis externa, media, and interna. The most common is otitis externa, in which inflammation affects the layer of cells lining the external portion of the ear canal. Otitis media and interna refer to infections of the middle and inner ear canal, respectively. These infections often result from the spread of infection from the external ear. Otitis media and interna can be very serious and may result in deafness, facial paralysis, and vestibular signs. That’s why it’s important to prevent infections and seek early treatment when problems arise [4].
Otitis Externa
This is the inflammation of the external ear canal, often caused by factors such as allergies, moisture, foreign bodies, or ear mites. Symptoms may include redness, itching, discharge, and a foul odor.
Otitis Media
Otitis media refers to inflammation of the middle ear. It's typically a result of untreated otitis externa or the spread of infection. Dogs with otitis media may show signs of pain, head tilting, and a reluctance to chew or open their mouths.
Otitis Interna
This is a severe infection of the inner ear, causing vestibular issues, such as loss of balance, head tilting, and abnormal eye movements. It can be caused by untreated otitis media or systemic infections.
Aural/ Ear Hematomas
Hematomas occur when blood vessels rupture within the ear flap due to vigorous shaking or head scratching. This leads to the accumulation of blood and a swollen, painful ear flap. Surgery is the most common treatment of choice for recurrent or persistent hematoma in dogs and cats (Figs. 2a, 2b). The most commonly reported approach is a linear incision with sutures. Dogs with aural hematomas have a decent to excellent prognosis provided the perpetuating causes are removed [5].
Fig. (2)) Aural haematoma in a dog (a) linear incision on ear pinna (b) horizontal mattress suture placement.It's crucial to keep in mind that the formation of an aural hematoma is not a primary condition until there has been ear trauma. To successfully resolve the hematoma, the primary, predisposing, and perpetuating factors that lead to the formation of otitis media and/or aural hematomas must be addressed.
As long as the underlying cause is addressed, an aural hematoma may resolve on its own, however, as a result of this, the pinna and ear canal may undergo potentially serious morphological alterations. Recurrent otitis media may be exacerbated by long-term alterations to the pinna's architecture in cases of severe ear deformity.
Ear Tumors
Ear canal tumors are comparatively rare, making up 2–6% of all tumors in dogs. While less common, dogs can develop benign or malignant tumors in their ears, which may require surgical intervention. Tumors may develop from any of the structures lining or supporting the ear canal including the outer layer of skin, the glands that produce earwax and oil, or any of the bones, connective tissues, muscles, or middle layers of the skin. Tumors of the external ear canal and pinna are more common than tumors of the middle or inner ear. Even though the etiology of these tumors remains more as a research topic, it is believed that chronic ear canal inflammation may produce aberrant tissue growth and development, which in turn may result in the creation of an ear canal tumor. The prognosis for dogs with ear canal tumors is not well understood. Furthermore, it is mostly unknown how different treatments (such as surgery, radiotherapy, or chemotherapy) may help with the regression and remission of ear canal tumors [6].
Apart from these common diseases, dogs may also suffer from some less common infections such as traumatic lacerations of ear, ear polyps, vestibular diseases, ruptured eardrum, para-aural abscess etc. which are discussed in detail in further chapters of this book.
CAUSES OF EAR DISEASES
Ear diseases in dogs can result from various factors, including allergies, parasites, foreign objects, bacterial or yeast infections, and genetic predispositions. It is crucial that the doctor assess involvement of numerous primary, predisposing, and perpetuating variables that may be contributing to ear ailments while evaluating each individual patient impacted.
Primary Factors
Primary factors include otic parasites like Otodectes cyanotis, hypersensitivity diseases (food allergy, atopic dermatitis, contact hypersensitivity), endocrine diseases like hypothyroidism, otic neoplasia, and foreign bodies. These conditions can directly affect the external ear canal and result in otitis. The most frequent primary cause of otitis in dogs is underlying hypersensitivity illness [7].
Predisposing Factors
Predisposing factors are factors that alter the local ear canal environment and create an increased risk for development of otitis externa. Ears with excessive hair, stenotic ears, increased cerumen production in the canals, otic masses, frequent ear cleaning, as well as changes in external environmental temperature and humidity can all act as predisposing factors.
Factors that may predispose dogs to ear infections include:
Moisture, which can create a prime growing environment for bacteria and yeast.Allergies, which lead to ear disease in about 50 percent of dogs with allergic skin disease and 80 percent of dogs with food sensitivities.Endocrine disorders, such as thyroid disease.Autoimmune disorders.Wax buildup.Foreign bodies.Injury to the ear canal.Excessive cleaning.Perpetuating Factors
Perpetuating factors are factors that do not initiate inflammation but lead to exacerbation of the inflammatory process and maintain ear disease even if the primary factor has been identified and corrected. Bacteria such as Staphylococcus and Pseudomonas, and Malassezia yeast are common perpetuating factors. If infection travels to the tympanic bulla, presence of this infection in the middle ear can also act as a perpetuating factor, leading to recurrent external ear infections. Perpetuating factors are often the main reason for treatment failures in dogs affected with recurrent otitis externa [8].
DIAGNOSIS OF EAR DISEASES
If the dog is showing any of the common signs of ear infections, it’s important to diagnose the root cause of it as soon as possible. Quick treatment is necessary not only for dog’s comfort (these conditions can be painful!), but also to prevent the spread of infection to the middle and inner ear.
The veterinarian should take thorough history of the case. This is especially important for first-time infections. Some salient questions to be asked to owners must include:
Duration of any symptoms, such as pain, swelling, discharge, and odor.If the dog has any allergies or other underlying medical conditions.Whether or not, the dog is on medication.What the dog has been eating.How often does the owner clean dog’s ears and which dog ear cleaning products are used for that purpose.If the dog has undergone recent trimming or plucking of hair in dog’s ears.Recent activities, such as baths, grooming, or swimming.If the dog has a history of ear infections, when they occurred, and how they were treated.After obtaining history, the veterinarian should perform a physical examination. In severe cases, sedating the dog to facilitate examination deep within the ear canal is also recommended. Evaluation of both ears should be done simultaneously and it should include:
Visual assessment to look for signs such as redness, swelling, and discharge.Examination with an otoscope, which allows evaluation of the ear canal and eardrum.Gentle palpation of the ear to assess level of pain.Microscopic examination of samples taken by swabbing the ear.Culture of samples from the ear.Biopsies or X-rays in severe or chronic cases.Detecting ear diseases in dogs requires a combination of observation, examination, and, in some cases, specialized testing. When you notice any unusual signs or symptoms in dog's ears, it's essential to go for a proper diagnosis. Common diagnostic methods include:
Physical Examination
Inspecting dog's ears for redness, swelling, discharge, or signs of trauma. The veterinarian may also check for pain reactions during ear palpation.
Common Symptoms of Dog Ear Infections
Some dogs show no symptoms of ear infection aside from a buildup of wax and discharge in the ear canal. But ear infections often cause significant discomfort and affected dogs may show signs such as:
Head shakingScratching at the affected earDark dischargeOdorRedness and swelling of the ear canalItchinessCrusting or scabs in the ears: Alopecia, excoriation, crusting, erythema, and hyperpigmentation are possible alterations of the ear pinna. Hyperemia, ulceration, ceruminous or suppurative discharge, tumors, stenosis, glandular alterations, or foreign bodies can all be seen in the external ear canal (Fig. 3).
Fig. (3)) Scab formation or crusting in ears.In an afflicted ear, multiple aberrant findings are typically observed. An important aspect of the otoscopic examination is the assessment of the tympanic membrane, yet it can be challenging to do so in presence of otitis externa. It makes sense to postpone evaluating the tympanic membrane until after modifications linked to active otitis have been made [9].
Ear Cytology
This involves taking a sample from the ear canal to examine under a microscope. It helps identify the type of microorganisms present, such as yeast or bacteria, guiding treatment decisions.
Ear Culture and Sensitivity
In cases of chronic or recurring ear infections, a culture and sensitivity test may be performed to determine the specific bacteria involved and which antibiotics will be most effective.
Imaging
X-rays or advanced imaging like CT scans may be necessary if inner ear issues or tumors are suspected. It is possible to assess the ear canal and tympanic bulla radiographically, surgically, and otoscopically. Another imaging modality being considered for the diagnosis of otitis media is ultrasound, which is noninvasive and reasonably priced. Furthermore, there are sophisticated imaging methods available for imaging the ear, such as CT and MRI (Figs. 4a, 4b) . The ear canal and tympanic bulla can be examined noninvasively with these imaging techniques, although they require general anesthesia. To diagnose otic neoplasia, vestibular illness, otitis media, and congenital ear abnormalities, radiographs, CT scans, and MRIs may be used [10]. An infectious or inflammatory process as opposed to a neoplastic one, the degree of involvement in the middle or inner ear, peripheral as opposed to central vestibular disease, the chronicity of the disease process, involvement of nearby structures, and postsurgical complications are just a few of the important details about ear disease that can be obtained through imaging of the ear canal [11].
Fig. (4)) Computed Tomographic examination (a) dorsoventral positioning of dog under general anesthesia (b) computed tomogram showing radiolucent right and left ear canal.Biopsy
For suspected ear tumors, a biopsy may be needed to determine if they are benign or malignant [12]. The ear punch biopsy also provides a reliable method for sampling dogs with recurrent chronic ear disease irresponsive to treatment and proves useful for detecting benign and malignant growths of ear.
Treatment of Ear Diseases
The treatment approach for ear diseases in dogs depends on the underlying cause and the specific condition. Here are some common treatment methods:
Medications
Ear infections are often treated with topical or oral antibiotics or antifungal medications. Veterinarian must prescribe the appropriate medication based on the diagnosis.
Cleaning
Thorough ear cleaning is crucial for treating and preventing ear diseases. Veterinarian must demonstrate the proper technique for at-home cleanings or perform a deep cleaning under sedation if needed.
Surgery
In cases of severe ear hematomas or ear tumors, surgical intervention may be necessary. Surgery aims to drain blood from hematomas or remove tumors.
Lifestyle Adjustments
If allergies are the root cause, veterinarian may recommend dietary changes, environmental modifications, or allergy medications to manage the condition.
Long-Term Management
Chronic ear conditions may require ongoing management, including regular check-ups, ear cleaning, and medication to control symptoms and prevent recurrence.
Preventive Measures for Ear Diseases
Preventing ear diseases in dogs involves a combination of proactive measures. To help prevent majority of mentioned diseases in this chapter, following measures should be considered:
Regular Cleaning
Ear cleaning is recommended to prevent wax buildup and moisture retention.
Regular Veterinary Visits
Schedule regular check-ups to catch early signs of ear issues.
Allergen Management
If dog has allergies, the inflammatory triggers must be identified, and appropriate management strategies should be implemented.
Parasite Control
Maintain a year-round parasite control program to prevent ear mites and other parasites.
Prompt Treatment
Seek prompt treatment for any signs of ear discomfort, such as itching, redness, or discharge, to prevent conditions from worsening.
Gentle Handling
Handle dog's ears gently to avoid injury or discomfort.
Most uncomplicated ear infections resolve within 1–2 weeks, once appropriate treatment begins. But severe infections or those due to underlying conditions may take months to resolve or may become chronic problems. In cases of severe chronic disease where other treatments have failed, surgeries such as a Total Ear Canal Ablation (TECA) is recommended. A TECA surgery removes the ear canal, thus removing the diseased tissue and preventing the recurrence of infection [13]. TECA is often performed for the following reasons:
Severe, incurable, or recurrent otitis externa (infection or inflammation of the ear canal), with little relief provided by medical treatment.Chronic ear disease-related ear canal constriction that makes it difficult to properly treat the ear canal.Unsuccessful ear canal disease management surgeries, such as Lateral Wall Resection (LWR) or Vertical Canal Ablation.Tumors limited to the canal of the ear.Otitis externa that is not improving with medication and has developed into otitis media, or middle ear inflammation or infection.Having trouble applying topical therapy for persistent otitis externa.It is important to provide a detailed follow up to pet owners recommended recheck appointments. Lapses in treatment may lead to the recurrence of the infection. It is especially important to finish the full course of medication, even if dog appears to be getting better. Failure to finish the full course of treatment may lead to additional problems such as resistant infections.
CONCLUSION
In this chapter, we've laid the foundation for understanding the importance of ear health in dogs and introduced some common ear diseases. In the following chapters, we will delve deeper into each ear condition, exploring their diagnosis and treatment options. Remember that a proactive approach to ear health can significantly improve the quality of life for these furry companions.
Understanding the anatomy of a dog's ear, recognizing common ear diseases, and knowing how to diagnose and treat them are essential for responsible pet ownership. In the subsequent chapters, we will explore each ear condition in greater detail, providing comprehensive information to help you keep your canine companions ears healthy and free from disease. Remember that a proactive approach to ear care can lead to a happier and healthier life for these beloved pets.
REFERENCES
[1]Huang HP, Little CJL, McNeil PE. Histological changes in the external ear canal of dogs with otitis externa. Vet Dermatol 2009; 20(5-6): 422-8.[http://dx.doi.org/10.1111/j.1365-3164.2009.00853.x] [PMID: 20178479][2]Zamankhan Malayeri H, Jamshidi S, Zahraei Salehi T. Identification and antimicrobial susceptibility patterns of bacteria causing otitis externa in dogs. Vet Res Commun 2010; 34(5): 435-44.[http://dx.doi.org/10.1007/s11259-010-9417-y] [PMID: 20526674][3]Saridomichelakis MN, Farmaki R, Leontides LS, Koutinas AF. Aetiology of canine otitis externa: a retrospective study of 100 cases. Vet Dermatol 2007; 18(5): 341-7.[http://dx.doi.org/10.1111/j.1365-3164.2007.00619.x] [PMID: 17845622][4]Morgan JL, Coulter DB, Marshall AE, Goetsch DD. Effects of neomycin on the waveform of auditory-evoked brain stem potentials in dogs. Am J Vet Res 1980; 41(7): 1077-81.[PMID: 7436102][5]Hewitt J, Bajwa J. Aural hematoma and it’s treatment: A review. Can Vet J 2020; 61(3): 313-5.[PMID: 32165757][6]Little C, Pearson G, Lane J. Neoplasia involving the middle ear cavity of dogs. Vet Rec 1989; 124(3): 54-7.[http://dx.doi.org/10.1136/vr.124.3.54] [PMID: 2919494][7]Cole LK, Rajala-Schultz PJ, Lorch G. Conductive hearing loss in four dogs associated with the use of ointment-based otic medications. Vet Dermatol 2018; 29(4): 341-e120.[http://dx.doi.org/10.1111/vde.12542] [PMID: 29664150][8]Eger CE, Lindsay P. Effects of otitis on hearing in dogs characterised by brainstem auditory evoked response testing. J Small Anim Pract 1997; 38(9): 380-6.[http://dx.doi.org/10.1111/j.1748-5827.1997.tb03490.x] [PMID: 9322176][9]Griffiths LG, Sullivan M, O’Neill T, Reid SWJ. Ultrasonography versus radiography for detection of fluid in the canine tympanic bulla. Vet Radiol Ultrasound 2003; 44(2): 210-3.[http://dx.doi.org/10.1111/j.1740-8261.2003.tb01273.x] [PMID: 12718358][10]M.G. Bischoff, S.K. Kneller / Vet Clin Small Anim 34 (2004) 437–458.[11]Belmudes A, Pressanti C, Barthez PY, Castilla-Castaño E, Fabries L, Cadiergues MC. Computed tomographic findings in 205 dogs with clinical signs compatible with middle ear disease: a retrospective study. Vet Dermatol 2018; 29(1): 45-e20.[http://dx.doi.org/10.1111/vde.12503] [PMID: 28994490][12]Hnilica KA. Otitis externa small animal dermatology: a color atlas and therapeutic Guide (3rd ed.) 3rd ed.2011395-8.[13]Cole LK, Kwochka KW, Kowalski JJ, Hillier A. Microbial flora and antimicrobial susceptibility patterns of isolated pathogens from the horizontal ear canal and middle ear in dogs with otitis media. J Am Vet Med Assoc 1998; 212(4): 534-8.[http://dx.doi.org/10.2460/javma.1998.212.04.534] [PMID: 9491161]Normal Structure and Function of Ears
Jasvinder Singh Sasan1,*,Shalini Suri1,Kamal Sarma1Abstract
Hearing is one of the fundamental sense. Ear also known as the vestibulocochlear organ, is subdivided into three parts namely external, middle and inner ear. Auricle and external acoustic meatus comprise external ear. Sound waves are transmitted from the external ear to the middle ear. In dogs, breed-specific variances of external ear is noticeable. The auricle has a funnel-like shape which helps in sound collection. Auricle is divided into the proximal conchal cavity and distally located scapha. Anthelix divides the conchal cavity from the scapha and is located close to the conchal cavity. External acoustic meatus is made up of a proximal osseous portion and a distal cartilaginous portion. The cartilaginous portion of carnivores is relatively long and curved which hampers the passage of the straight otoscope for examination. The tympanic membrane consist of two parts, namely the pars flaccida and the pars tensa. The middle ear comprises the auditory ossicles (malleus, incus and stapes), muscles and auditory tube. Tympanic cavity is contained in the petrous temporal bone and has dorsal (epitympanicum), middle (mesotympanicum) and ventral (hypotympanicum) section. Auricular ossicles are located in the dorsal portion. The tympanic membrane is located on the lateral wall of the middle portion. The tympanic bulla is known as the ventral hypotympanicum. Internal ear has membranous and osseous labyrinth. The membrane labyrinth is filled with endolymph and includes the vestibular labyrinth which houses the receptor organ for balance and cochlear labyrinth containing the organ of hearing. The osseous labyrinth consists of vestibule, semicircular canals and cochlea. While defects in the inner ear may result in sensorineural hearing loss, defects in the outer, middle, and middle ear can cause conductive hearing loss. Therefore, it is crucial to research the anatomy and physiology of the ear. This chapter's main objective is to explore the fundamental anatomy and physiology of numerous components of the canine ear that plays a vital role in hearing.
INTRODUCTION
Ear houses both the hearing and the balancing organs thus known as the vestibulocochlear organ [1]. The cochlea receives and converts mechanical impulses from sound waves into electrical signals, and the vestibular organ provides animal a sense of position and movement in relation to gravity. The inner ear contains the receptors for both organs. The vestibulocochlear nerve is the physical and physiological relationship between the two organs. The ear can be divided into three subdivisions (Fig. 1):
Fig. (1)) Schematic diagram showing external acoustic meatus, middle ear and inner ear. External earMiddle earInternal earThe organ of balance (vestibular system) is restricted to the internal ear.
External ear
The external ear consists of the following parts:
Auricle along with the auricular cartilage.External acoustic meatus.Tympanic membrane.When upright, the ears can be used independently to locate and gather sound. The tympanic membrane, located deep within the external acoustic meatus, receives sound waves via the external acoustic meatus.
Auricle
Auricle is also known as the ear leather to dog fanciers [2]. The size and shape of the external ear in domesticated animals varies widely between species and breeds (Fig. 2). In dogs, breed-specific variances are particularly noticeable. Konig and Liebich [1] reported breed-specific ear forms as summarized in Table 1.
Fig. (2)) Showing shape of auricle in different breeds of dog.The auricle, which resembles a funnel, acts as a sound collecting structure. The auricular muscles move it to focus and gather sound. The muscles that move the auricle are coated in skin. The auricular muscles surround and insert onto the auricle. The external ear is turned and moved up and down.
The auricle's funnel-like distal opening narrows to form a tube at its proximal end. The skin-covered auricular cartilage determines the auricle's size and shape. Numerous foramina penetrate the auricular cartilage, allowing blood vessels and nerves to travel from the convex surface to the concave surface [3]. On the concave surface, the hairs are sparse and thin except few to guard the opening to the external auditory meatus. In dogs with pendulous ears in particular, the convex surface is covered in typical fur and has dense, long hairs.
The auricle on its concave surface is divided into two parts, namely proximal concha or conchal cavity and distally located flattened scapha (Fig. 3).
The proximal part of the auricle, where it funnels into the external acoustic meatus, is known as the conchal cavity (cavum conchae). On the concave surface of the auricle, there is a transverse cartilage fold called the anthelix which divides the conchal cavity from the scapha and is located close to the distal section of the conchal cavity. The conchal tube's base is lined by an additional cartilaginous band called the anular cartilage (cartilago anularis), which connects to the osseous external acoustic meatus. The cutaneous marginal pouch (saccus cutaneus marginalis) is visible in the caudal edge of cats and dogs (Fig. 4). In a dog of ordinary size, the anular cartilage is around 2 cm long. The conchal section of the auricular cartilage is rolled into a tube at the entry of the external auditory meatus, and there are several cartilaginous extensions there. A thick, blunt, quadrangular plate of cartilage called the tragus protrudes from the rostral edge of this opening. The antitragus, a thin, long cartilage projection next to the tragus on this rostral boundary, is present. The intertragic incisure (incisura intertragica), which divides the antitragus from the tragus, is present [4].
Fig. (3)) Auricle of Labrador dog showing scapha and conchal cavity separated by antihelix. Fig. (4)) Cutaneous marginal pouch (arrow) in Comboi (A) and Rajapalyam (B) breed of dog.External Acoustic Meatus
It is the canal that runs from the base of the auricle to the tympanic membrane and is made up of a proximal osseous portion and a distal cartilaginous portion. The cartilaginous portion of carnivores is curved and relatively long, with its beginning portion oriented downward and its subsequent horizontal portion directed medially [1]. As it is curved, it hampers the passage of the straight otoscope for examination [2].
The anular cartilage connects the comparatively short osseous portion to the concha's basal region. The flexibility of external ear is due to configuration of a separate ring connecting the auricle and the external acoustic meatus. Stratified squamous epithelium contains sebaceous and tubular ceruminous glands. These glands secrete earwax (cerumen). These glands are located throughout the external acoustic meatus in carnivores [1].
Tympanic Membrane
The middle ear is separated from the external acoustic meatus by the tympanic membrane, commonly known as the eardrum. It transfers sound waves to the middle ear's auditory ossicles. It is a thin, oval-shaped sheet which is semi-transparent and is composed of three layers:
Outer stratified squamous epidermis derived from ectoderm of first pharyngeal groove.Middle layer of fibrous connective tissue.Inner layer of mucosa which is of pharyngeal pouch origin.The tympanic membrane can be divided into two parts: the pars flaccida and the pars tensa [1]. The malleus's short lateral process and the tympanic incisure's edges are separated by the pars flaccida, a small dorsal triangular area. The remaining portion of the membrane is known as the pars tensa. In the dog tympanum's connective tissue layer, Wakuri et al. [5] discovered outer radial and inner circular fibres in the pars tensa. The umbo membranae tympani is the name for the most depressed point, which is located opposite the distal end of the manubrium. The head of the malleus articulates with incus, body of neighbouring auditory ossicle, whereas the handle of the malleus is implanted in the tympanic membrane. Tympanic membrane converts incoming sound waves into mechanical impulses, which the auditory ossicles then transmit to inner ear. Sensory nerve fibres innervate richly vascularize the tympanic membrane.
To explore microangiology in relation to surgical procedures, Maher [6] removed the tympanic membranes and adnexa from neonatal dogs. He discovered that the canine tympanic membrane's anatomy and vascular supply are remarkably comparable to that of humans. There were two sources of vascular supply: intrinsic sources from the deep auricular and rostral tympanic branches of the maxillary artery, and extrinsic ones from the stylomastoid branch of the caudal auricular artery.
Middle ear
The middle ear comprises the:
Air filled tympanic cavity (cavum tympani),Auditory ossicles (ossicula auditus) andAuditory tube (tuba auditiva, Eustachian tube).Tympanic Cavity (Cavum Tympani)
The petrous temporal bone contains the tympanic cavity. It has a dorsal, middle, and ventral section. The auricular ossicles are located in the dorsal portion known as the epitympanicum [1]. The tympanic membrane is located on the lateral wall of the middle portion, or mesotympanicum, which enters via auditory tube into the nasopharynx rostrally. The tympanic bulla (bulla tympanica), also known as the ventral hypotympanicum, is an inflated, bulbous protrusion of temporal bone which in some species, is separated into a huge cell area [1]. The floor and a sizable portion of the lateral walls of the tympanic cavity are formed by the tympanic bulla. In canines, tympanic bulla is located medial to the muscular process of the mandible and great cornu of the hyoid bone [7]. The tympanic membrane is incorporated into the lateral wall of the tympanic cavity, and there are two windows on the medial side.
The base of the stapes occupies the oval vestibular window (fenestra vestibuli), which connects the tympanic cavity with the inner ear. It is situated rostrodorsally. The cochlear window (foramen cochleae), which is more caudally located and has a rounded form, connects to the cochlea cavity. The secondary tympanic membrane closes it. The promontorium, a bony protrusion from medial wall of tympanic cavity, houses cochlea. The tympanic cavity is lined by a single-layered epithelium that also covers the tympanic membrane and auricular ossicles.
The tympanic cavity's long axis measures around 15 mm in length and forms a caudolateral angle of roughly 45 degrees with the sagittal plane. The dimensions of the breadth and depth are roughly comparable, ranging from 8 to 10 mm. The tympanic membrane is slanted ventromedially [4].
The chorda tympani, which originates from the facial nerve and travels through the tympanic cavity medial to the malleus before joining the lingual nerve, is one of the nerves that can be seen running through the tympanic cavity. The tympanic plexus arising from the tympanic nerve (n. tympanicus) of the glossopharyngeal nerve lies on the promontory and supplies the tympanic mucosa and the minor petrosal nerve to the otic ganglion. This tympanic plexus also receives support from the internal carotid plexus' caroticotympanic nerves.
Auditory Ossicles
The three auditory ossicles transmit the vibrations that travel from the tympanic membrane through the tympanic cavity and into the inner ear (Fig. 5):
Hammer (malleus),Anvil (incus) andStirrup (stapes).These tiny bones, which stretch from the tympanic membrane to the vestibular window, are connected to one another by syndesmoses to form a chain.
The malleus, made up of the head, neck, and manubrium, is the most lateral of the auditory ossicles. The three-sided manubrium is embedded within the tympanic membrane. The neck, which extends above the tympanic membrane, connects it to the head. The head of the malleus articulates with the body of the incus. The muscular projection of the malleus, which extends medially and somewhat rostrally from the base of the manubrium, provided a little hook at its end for the attachment of the m. tensor tympani. The lengthy rostral process is deeply enmeshed in the tympanic membrane. Short lateral process is located opposite to the muscular process at an angle of approximately 90 degrees with the rostral process [4]. This is the most dorsal attachment of the manubrium to the tympanic membrane.
Incus is considerably smaller than malleolus. Its form is comparable to a bicuspid tooth with diverging roots in a human. Incus is made up of a body and two limbs, i.e. long and short limb. The short limb points caudally. Lenticular bone articulates with head of stapes.
The stapes is made up of a base, two limbs, a head, a neck, and a muscular process. It is horizontally positioned, with the base pointing medially. Through the lenticular process, the head articulates with the incus. The vestibular window's edge is covered by a fibrocartilaginous ring, which the base articulates with. The stapes, the smallest bone in the body with a length of about 2 mm, is the innermost ossicle [4]. The concave or opposing sides of the rostral and caudal limbs are hollowed out. A single crus shows as a narrow semicircle of bone when cut in half. The muscular process provides attachment site for stapedius muscle.
Fig. (5)) Schematic diagram showing auditory ossicles of dog.Several ligaments attaches the ossicles to the wall of the tympanic cavity [8]. Lateral ligament of the malleus connects the lateral process of the malleus to the margins of the tympanic notch. Dorsal ligament of the malleus joins the head of the malleus to a small area on the roof of the epitympanic recess. Rostral ligament of the malleus attaches the rostral process of the malleus and the osseous tympanic ring just ventral to the canal by which the chorda tympani leave the tympanic cavity. Dorsal ligament of incus attaches the body of the incus to the roof of the epitympanic recess. The caudal ligament of the incus attaches the short limb to the fossa. An anular ligament attaches base of stapes to the cartilage that lines vestibular window.
