Canine Otitis Interna Study Guide

Overview and Clinical Importance

Otitis interna is inflammation of the inner ear structures, including the cochlea (responsible for hearing) and the vestibular apparatus (responsible for balance). It represents the most severe form of ear disease in dogs and typically occurs as an extension of otitis media (middle ear infection). This condition is relatively rare compared to otitis externa but carries significant clinical importance due to potential complications including permanent hearing loss, chronic vestibular dysfunction, and life-threatening intracranial extension.

The inner ear is located within the petrous portion of the temporal bone and contains the sensory organs for both hearing (cochlea with organ of Corti) and balance (vestibular labyrinth with semicircular canals, utricle, and saccule). When infection or inflammation extends to these structures, patients develop characteristic peripheral vestibular signs that are important for NAVLE recognition.

Anatomy of the Canine Inner Ear

Bony and Membranous Labyrinth

The inner ear is housed within the bony labyrinth in the petrous portion of the temporal bone. This bony structure contains the membranous labyrinth, which is filled with endolymph and surrounded by perilymph. The inner ear has three functionally related parts:

• Cochlea: A spiral, snail-shell-shaped structure responsible for hearing. Contains the organ of Corti with hair cells that convert mechanical sound waves into neural impulses transmitted via the cochlear division of CN VIII
• Vestibule: Contains the utricle and saccule with sensory maculae that detect linear acceleration and head position relative to gravity
• Semicircular Canals: Three fluid-filled canals (anterior, posterior, lateral) oriented at right angles to each other. Each has an ampulla containing cristae that detect rotational head movement

Key Anatomic Relationships

Etiology and Pathophysiology

Routes of Infection

• Extension from otitis media (most common): Infection spreads through the round window membrane or oval window from a chronically infected middle ear
• Extension from otitis externa: Long-standing external ear infections erode the tympanic membrane, infecting the middle ear, then extend to inner ear
• Hematogenous spread (rare): Bacteria reach the inner ear via bloodstream with intact tympanic membrane
• Iatrogenic: Overly vigorous ear cleaning or use of ototoxic medications through a ruptured tympanic membrane

Common Pathogens

Breed Predispositions

Dogs with long, pendulous ears and narrow ear canals are predisposed to chronic otitis externa, which can progress to otitis media/interna:

• Cocker Spaniels (most commonly affected)
• Basset Hounds
• Bloodhounds
• Beagles
• Cavalier King Charles Spaniels (also prone to primary secretory otitis media/PSOM)
• Shar-Peis (stenotic ear canals)

Clinical Signs and Physical Examination

Peripheral Vestibular Signs

Otitis interna causes dysfunction of the peripheral vestibular system (inner ear and CN VIII). Clinical signs typically have acute onset and include:

Differentiating Peripheral vs. Central Vestibular Disease

This distinction is CRITICAL for NAVLE as prognosis and treatment differ significantly:

Associated Cranial Nerve Deficits

Facial Nerve (CN VII) Paralysis

The facial nerve traverses the petrosal bone adjacent to the inner ear and is commonly affected:

• Inability to blink (reduced palpebral reflex)
• Lip droop on the affected side
• Deviation of nose away from affected side
• Drooling from affected side of mouth
• Keratoconjunctivitis sicca (KCS): Damage to the major petrosal nerve (branch of CN VII) causes decreased lacrimal gland secretion. ALWAYS perform a Schirmer Tear Test!

Horner Syndrome

The sympathetic trunk passes through the middle ear; damage causes:

• Miosis (small pupil)
• Ptosis (drooping upper eyelid)
• Enophthalmos (sunken eye)
• Third eyelid protrusion

Diagnostic Approach

Physical and Neurological Examination

• Complete neurological examination: Document head tilt direction, nystagmus type/direction, postural reactions, cranial nerve function
• Otoscopic examination: Evaluate external ear canal for debris, inflammation, masses; assess tympanic membrane integrity (may require sedation)
• Palpation: Pain on palpation of tympanic bulla; pain when opening mouth (temporomandibular joint proximity)
• Schirmer Tear Test: Baseline STT identifies dogs at risk for KCS; monitors for ototoxic antibiotic effects
• Oral examination: Check for pain with jaw manipulation

Diagnostic Imaging

Myringotomy and Culture

When the tympanic membrane is intact but otitis media is suspected, myringotomy (controlled perforation of the tympanic membrane) allows sampling of middle ear contents for cytology and culture. This is performed under general anesthesia using a spinal needle or myringotomy blade through the caudoventral pars tensa.

Culture and sensitivity testing is essential for guiding antibiotic selection, especially given the prevalence of multidrug-resistant organisms like Pseudomonas and MRSP.

Treatment

Medical Management

The majority of otitis media/interna cases can be successfully managed medically. Treatment duration is prolonged (6-12 weeks) due to poor antibiotic penetration into the bulla.

Systemic Antibiotic Therapy

Supportive Care

• Anti-nausea medications: Maropitant (Cerenia) 1 mg/kg PO/SQ q24h for vestibular-induced nausea
• IV fluid therapy: If patient cannot eat/drink due to nausea or disorientation
• Anti-inflammatory therapy: Prednisolone 0.5-1 mg/kg PO q12-24h for 1-2 weeks to reduce inflammation and protect adjacent nerves
• Pain management: Often underutilized; otitis media/interna can be very painful. Consider gabapentin or NSAIDs
• Ocular protection: Lubricating eye drops/ointment if facial nerve paralysis prevents complete blinking; monitor for corneal ulceration

Topical Therapy

Topical medications can be used if the tympanic membrane is ruptured, but care must be taken to avoid ototoxic agents:

• SAFE for middle ear use: Fluoroquinolones (enrofloxacin), Tris-EDTA, dilute chlorhexidine, saline
• POTENTIALLY OTOTOXIC (avoid): Aminoglycosides (gentamicin, neomycin), polymyxin B, chlorhexidine in high concentrations

Surgical Treatment

Surgery is indicated when medical management fails or in cases of end-stage ear disease. Total Ear Canal Ablation with Bulla Osteotomy (TECA-BO) is the definitive surgical treatment.

Indications for TECA-BO

• Chronic otitis externa/media refractory to medical therapy
• Severe ear canal stenosis and calcification
• Ear canal neoplasia (ceruminous gland adenocarcinoma)
• Aural cholesteatoma
• Recurrent middle ear infection despite prolonged antibiotics

Surgical Complications

• Facial nerve paralysis: Occurs in 5-10% of cases; usually temporary but permanent in 10-15% of affected patients
• Horner syndrome: Usually transient
• Hearing loss: Expected; most patients have diminished hearing pre-operatively
• Abscess/fistula formation: 5-10% of cases; may require revision surgery
• Vestibular dysfunction: If inner ear is damaged during bulla curettage

Prognosis

• With appropriate medical treatment: Good prognosis for resolution of vestibular signs
• Vestibular signs: Typically improve within 2-6 weeks; smaller dogs recover faster than larger dogs
• Residual head tilt: May be permanent in some cases, especially if treatment is delayed
• Facial nerve paralysis: May persist if still present 3-4 weeks after treatment initiation
• Hearing loss: Often permanent if inner ear damage occurred
• Complications: Meningitis or brain abscess (rare) carries guarded to poor prognosis

Peripheral vs. Central - "PERIPHERAL = PRESERVED": Peripheral vestibular disease has Preserved Postural reactions, Preserved Mentation, and Predictable nystagmus (horizontal/rotary only, direction doesn't change).

Nystagmus Direction: "Fast away, Fall same way" - The fast phase of nystagmus goes AWAY from the lesion, but the patient FALLS TOWARD the lesion.

Duration Mnemonic: "Ears need MONTHS, not WEEKS" - Remember that otitis media/interna requires 6-12 weeks (often 2-4 months) of antibiotic therapy, much longer than most infections.

Associated Deficits: "VII + Horner = Middle/Inner ear" - When you see vestibular signs PLUS facial nerve paralysis and/or Horner syndrome, think otitis media/interna. Idiopathic vestibular disease does NOT cause these cranial nerve deficits.