Ear Anatomy...Terminology...Etiology...Diagnosis...Treatment

Otitis Media...Ototoxicity...Micellaneous

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Ear Anatomy

The Anatomy of the ear is somewhat complex....

The images above illustrate some basic anatomy of the ear canal (vertical and horizontal portions), the position of the tympanic membrane separating the external and middle ears, the presence of the auditory ossicles (bones), the bulla of the middle ear, the oval and round windows and the inner ear structures concerned with hearing and balance.

Some of the pictures to the left require that you hold the mouse down in order to read information about the picture or to move the text out of the way for better viewing. The images change automatically, but this can be interrupted using the "Pause" button (the lower button). I apologize for the relatively low quality of some images...better images would take too long to download on a modem connection.

The ears are composed of external pinna (outside portion of the ear) and a skin-lined canal (the external ear canal) descending (vertical and horizontal portions) to the ear drum (the tympanic membrane). This portion comprises the external ear. Other compartments of the ear, beginning after the tympanic membrane, are sequentially named the middle ear and the inner ear.

The horizontal ear canal is lined by specialized skin approximately 1mm thick, rich in sebaceous glands associated with hair follicles (oily secretions) and cerumen producing glands, which secrete a mixture of degenerating epithelial cells in a fatty [lipid] base and are deeper into the skin. Cerumen secreting gland are found in increasing numbers closest to the ear drum (the tympanic membrane). Glands secrete either "dryish" or "moist" cerumen. Cerumen resists moisture and thus, under normal conditions functions to keep the ear canals relatively dry. In addition, it is believed that these secretions contain enzymes involved with antimicrobial activity.

The middle ear begins on the other side of the tympanic membrane (ear drum). It contains boney elements...the ossicles which transmit vibrations ...and a large, air-filled boney chamber, the bulla through which low-pitch sound is also transmitted. The bulla is lined with a mucus membrane (not skin). Reflex contraction of the muscles attached to the auditory ossicicles in response to extremely loud noises actually dampens the movement of the ossicles and the transmission of potentially damaging sounds. Aside from ear-related structures, some nerves traverse the bulla and inflammation or other pathology within the middle ear can result in neurologic signs not directly related to hearing or balance. Information from the middle ear is transmitted through the oval and round windows to the structures of the inner ear.

The inner ear, housed in the petrosal temporal bone, is responsible for receiving auditory signals and for maintaining balance. A portion of the petrosal bone is in close proximity to the cerebrum of the brain and another portion is juxtapost to the cerebellum. The middle portion of the cerebella portion houses the internal acoustic meatus containing, in turn, portions of the osseous facial canal, (in which the facial (seventh cranial) nerve traverses), a component for the vestibular (balance-related) part of the eighth cranial nerve and a third area housing the cochlear component of the eighth cranial nerve, the nerve which transmits auditory signals to the auditory centers of the brain. The coclear is a winding, fluid-filled structure containing hairs connected to the cochlear nerve (the portion of the eighth cranial nerve concerned with hearing). The vibration of fluid as a result of sound bends specific hairs (certain hairs bend in response to certain frequencies); this hair-bending transmits the nerve impulses via the connected coclear nerve endings to the brain where they are converted to and perceived as sound.

The actual neurological network and the physics of sound and balance control involving the boney labryinths, coclear apparatus, utricles and saccules is extremely complex and beyond a more detailed elucidation here.

Moist cerumen

Terminology

(please review anatomy, above)

Aural: refers to the ear.

Aural Hematoma: This is manifest when a bloody fluid accumulates between the skin and cartilage of the ear pinna, usually the concave surface and usually the result of self-trauma (scratching, head-shaking)

Cerumen: described above, below moving diagrams . Secretion by special glands in the vertical ear canal which functions to repel moisture and as antimicrobial

Ear Canal: As described above (anatomy),

Epithelial Migration: a clearance mechanism of the skin lining the vertical ear canal. Keratinocytes (surface epidemis cells) originating from the central portion of the tympanic membrane (the ear drum) slide along the superficial basal epithelial surface toward the outside opening of the ear. This process clears the vertical ear canal of wax and ceruminous debris

Otic: refers to the ear

Otitis: refers to inflammation of the ear; Otitis externa refers to inflammation of the external ear canal, including the horizontal and vertical portions; otitis media refers to inflammation of the middle ear and otitis interna refers to inflammation of the inner ear containing the auditory and vestibular portions of the ear that communicate with the brain. When inflammation is confined to the vertical and horizontal ear canals it is referred to as otitis externa. Inflammtion of the middle and inner ear compartments are called otitis media and otitis interna, respectively.

Stenosis: the narrowing of the ear canal or opening due to swelling, hyperplasia of skin and, eventually, thickening and mineralization of cartilage. Most often a response to chronic inflammation. And example of severe stenosis of the external ear opening is seen here.

Stenotic ear...outside opening of ear canal is mostly obstructed

Vestibular: refers to the portions of the inner ear and centers of the brain involved in the maintenance of body orientation and balance

Ear pinna: this is the external, most visual portion of the canine ear. In some dogs it is of the "floppy" variety and in others it is "straight" or "upright". It is composed of a cartilage core and skin. The portion of the pinna that is contiguous with the ear canal is called the concave surface and the other side...with fur on it...is the convex surface. The plural of pinna is "pinnae".

Etiology...Causes of Ear Disease

Diseases in which the pinnae only are affected include the following
...these will not be further addressed:

Primary diseases involving the ear canals (and sometimes the pinnae) most likely originate with allergies (hypersensitivity reactions). However, other primary factors may also be at fault....

Predisposing Causes...factors which contribute to the development of otitis, though they are not the primary cause...generally either contribute to the accumulation of moisture directly or by impeding ventilation:

The ensuing inflammation of the external compartment of the ear, if left unchecked, can extend to the middle and inner ear compartments. Additionally, with chronicity, complications arise, such as stenosis of the ear canal and the deposition of calcium salts within the diseased soft tissue itself ("dystrophic mineralization"). The presence of soft-tissue mineralizaton is an end-stage finding, meaning that even with treatment, it is highly unlikely to result in a return to the normal anatomical condition.

Diagnosis of Otitis Externa

Keep in mind that the desired environment of the ear canal in acidic and dry, and that deviation from this norm is evidence of disease. Diagnosis of otitis alone does not necessarily determine what underlying disease(s) and predisposing factors precipitated this problem.

Diagnosis of otitis externa is straightforward, based on clinical signs (head shaking, scratching at the ear, the visual redness of some portion of the ear surface, odor and/or discharge eminating from the ear, otoscopic evaluation of the ear canal integrity and cytological evaluation of swabs from the ear and ear canal (looking for the presence of inflammatory cells and microorganisms, foreign bodies, masses and/or parasites) and sometimes ear cultures. Diagnosis of middle (otitis media) and inner (otitis interna) ear disease is more difficult but can be accomplished via otoscopy (if tympanic membrane is ruptured, discolored or bulges), radiographs (the tympanic bullae can be evaluated), myringotomy (obtaining material from middle ear by passing instruments though the tympanic membrane), CT or MRI and clinical signs of vestibular (balance) problems along with a full neurological assessment. Again, culture (and antimicrobial-sensitivity) of material obtained is usually done. More about otitis media below...

In addition to the above findings, if the history consists of persistent ear inflammation (at least 6 months in duration), there is a 90% probability that both otitis externa and otitis media are present. In this case, a complete medical resolution is unlikely, and only medical management for the long term is possible. However, if there are also clear anatomical changes, such as stenosis of the ear canal (narrowing due to chronic inflammation and the resulting hyperplastic response in which the skin grows and thickens; the result is severe narrowing of the canal), treatment becomes more difficult . Further, if dystrophic mineralization of the of the ear pinna or canal is present, then the prognosis for even acceptable long-term management is markedly reduced. These ears are "end-stage" and surgical resolution may be the only reasonable option.

As part of the diagnosis, it is important to ascertain the underlying primary disease and if appropriate, the contributing predisposing factors. Sometimes these are obvious (ear mass, ear mites) but other times, there are concurrent problems that may relate to the presence of otitis. These include hormonal imbalances, allergic and contact sensitivities, for example.

Treatment

Treatment of uncomplicated, primary causes of otitis are as follows:

Ear Mites:

    • Thorough cleaning of the ear canal (with or without sedation) hastens the resolution of the problem. See more below...
    • Topical Treatments:
    • Topical insecticide marketed and safe for the killing of otodectus are instilled in the affected ear(s) once or twice daily, usually for up to 14 days.  Some of these products contain ceruminolytics to loosen dry ceruminous debris, antibiotics to treat secondary opportunistic bacterial or yeast infections
    • Topical mineral oil preparations: These float debris to the external ear surface and suffocate mites by blocking the breathing apparatus
    • Systemic Treatments:
    • Ivermectin (Ivomec® 1%)Systemic treatment with ivermectin (--excluding breeds with known sensitivities to this drug) at a dose of 250 µg/kg subcutaneously every 10 days to 14 days, for a total of two or three injections
    • Selamectin (Revolution® ) is a topically applied insecticide that kills fleas, and various species of mites, including Otodectus cyanotis as well as acting as a Heartworm preventative. Though applied topically, it is absorbed systemically
    • Fipronil (Frontline® Top Spot) a topically-applied insectide marketed for flea and tick control in dogs and cats has demonstrated miticidal activity, though it is neither marketed nor approved for this purpose. The medication remains in the oils and oil glands of the skin. Mites that contact the host animal's skin will be killed and thus will not reproduce

Foreign Bodies and Masses

  • Foreign Bodies
  • Foreign bodies, such as foxtails, are most easily removed under anesthesia. Many of these are barbed and cause considerable damage to the ear canal.
  • Ear canal masses are surgically excised where feasible. In some instances, the mass is highly invasive and complete excision is not possible. Some animals may benefit from total ear ablation (removal of the ear canal) as well as adjunct therapy depending on the nature of the mass
  • Follow up treatment with topical and systemic antibiotics as well as antiinflammatory medications is usually warranted. Topical ear medications compounded with some form of a corticosteroid may be useful in quelling local pain and swelling. Sometimes, a systemic antiinflammatory medication is necessary for severe pain or swelling

Skin Disorders

Address Secondary/Predisposing Causes

  • Stenotic ears, excessive glandular secretions
  • Frequently but gently clean and dry the ear canal
  • Swimmers' Ear
  • Gently dry the ear immediately after swimming
  • Hairy Ear(s)
  • Gently and frequently pluck excessive hair from the ear canal.
  • Do NOT randomly or indiscriminatly employ previously prescribed ear medications without the knowledge and approval of your veterinarian

The following interactive animation shows the basic approach to at-home external ear canal cleaning and principles of medicating inflamed and infected ears. The concept of ototoxicity in the presence of a perforated ear drum is also described here and below...

Otitis Media

The preceding information focused upon the most common inflammatory ear problem, otitis externa. However, in many animals, the inflammation progresses from the skin-lined external ear canal (or rarely ascends via the eustacian tube from the pharynx (throat)) into the middle ear compartment, lined with mucus membrane, not skin, producing otitis media.

Presentation:

Pathophysiology

Ototoxicity

As explained in the animated sequence above, when serious otitis externa results in damage to the tympanic membrane (aka ear drum), most routinely employed ear cleansers and medications may access the inner ear via the vestibular and coclear windows of the middle ear. These can damage the middle and inner ear structures, which can result in pain, deafness, balance, head tilt and other neurological problems. Some of the common agents in ear cleaners and medicated ear preparations are ototoxic or of unknown ototoxicity

Agent

Purpose

Ototoxicity

Propylene glycol

carrier vehicle and weak cerumenolytic; present in many otic preparations

causes granulation, boney and inflammatory changes in the bulla and functional changes in the inner ear (cochlea)
Glycerin carrier vehicle and weak cerumenolytic; present in many otic preparations no studies; however, glycerin possesses chemical properties similar to propylene glycol...
Polyethylene glycol 400 and dimethyl formamimide carrier/solvent adverse effects on middle ear mucosa and on inner ear
Other ceruminolytics and solvents: carbamide peroxide, dioctyl sodium sulfosuccinate, ethanol (alcohol), triethanolamine, toluene solvents and ceruminolytics and foaming action few specific studies, but anectodal reports of ototoxicity
Astingents: lactic acid, malic acid, benzoic acid, aluminum silicate, aluminum acetate astringents; function to dry the ear safety of these is unknown
Aminoglycosides antibiotics: gentamycin, amikacin, tobramycin, neomycin, kanamycin, streptomycin antimicrobials destroy auditory and vestibular hair cells (these affect perception of sound frequency and balance, respectively); mechanism is unknown. Deleterious effects may take months after treatment to manifest, and then may be progressive for up to a year
Other antimicrobials: bacitracin, chloramphenicol, chlortetracycline, colistin, erythromycin, gramicidn, minocycline, polymyxin B, tetracycline, ticarcillin, vancomycin, viomycin, griseofulvicin, amphotericin B antimicrobials inflammatory and osseous changes to the middle ear compartment
Parasiticides insecticides...destroy parasites (e.g. ear mites) anectodal resports of middle ear effects when penetrate the tympanic membrane; also can cause inflammatory problems to external ear canal

Some topical treatments are generally regarded as safe in the presence of a ruptured tympanic membrane (though in many cases, scientific proof of safety is lacking). Agents deemed relatively safe are listed below.

Relatively* Safe Ear Agents

Antibiotics
Carbenicillin
Ceftazidime
Cefmenoxime
Ciprofloxacin
Enrofloxacin
Fosfomycin
Ofloxacin
Antiinflammatory Agents
Dexamethasone
Triamcinolone
Antifungals
Clotrimazole
Nystatin
Tolnaftate
Ceruminolytic Agents and Solvents
Isopropyl myristate
Squalene
* No guarantee of safety is implied since there are limited data supporting safety claims

In general, only a small percentage of animals treated with any of the above "ototoxic" agents in the presence of a damaged tympanic membrane actually demonstrate clinical signs of ototoxicity. One commercial preparation** containing known ototoxins was tested on healthy animals in which the tympanic membrane was deliberately punctured. Apart from the implications of animal cruelty and questionable ethics manifest by these studies (a whole other topic), these clinical studies appeared to show that the commercial formulation was reasonably "safe". Of course, the effects of these substances on diseased/inflamed ears was not studied, so the implications of safety when otitis media and a ruptured tympanic membrane are simultaneously present are at best dubious.

**Otomax® (Shering-Plough™)

Miscellaneous

This page has focused upon the most common ear problems in dogs...inflammation and infection. There are other presidposing factors related to otitis such as vitamin A and/or zinc deficiency, hormonal imbalances and disorders of skin development (keratinization disorders) which are not detailed here. More often, these are associated with clinical signs referrable to areas besided ears.

Additionally, damage and rupture of the tympanic membrane has been mentioned here several times. It is important to realize that a perforated ear drum can be repaired in most instances, provided the underlying problem is addressed and the blood supply and germinal epithelial cells (located by the auditory ossicles) are not destroyed. Ordinarily, complete restoration requires anywhere from 3 to 16 weeks. This process is retarded in the presence of otitis media, because the accumulation of prurulent liquid ("pus") in the middle ear applies pressure against the perforated ear drum...the path of least resistance, unless the eustacian tube is completely patent, in which case the liquid instead flows that way. If otitis media is severe and protracted, the malleus bone...(an auditory ossicle) which is the center of the germinal epithelium is, itself, destroyed and there is no possibility of repair

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