From Ear to Brain: A Short Pathway to Disaster

Michael Podell MSc, DVM, Diplomate ACVIM (Neurology)

Infectious otitis media and interna are a common clinical problem in dogs and cats. The majority of cases are due to a progression of otitis externa via the tympanic membrane. Rupture of the tympanic membrane, however, is not a necessity for extensional infection. An insidious, progressive infectious process may lead to a variety of clinical problems, ranging from the benign ear scratching to a life-threatening otogenic intracranial infection. The latter can rapidly lead to cerebral abscess formation, meningitis and ultimately, a fatal outcome.

The general functions of the vestibular system are to maintain posture, regulate tone in antigravity muscles and correct for changes in body posture and balance. More specifically, this special proprioceptive system serves to control involuntary eye movements and to integrate eye movements with changes in head and neck position. The peripheral anatomic components include the external ear canal, tympanic membrane, tympanic bullae and associated middle ear structures and the inner ear. The inner ear is composed of the petrous temporal bone, vestibular ganglion, and vestibular nerve. The petrous temporal bone contains the bony labryrinth and the membranous labryrinth, which contains the vestibular receptors.

Case Study
“Kazmir” 7 year old MC DSH cat
History: Kazmir presented for evaluation of right head tilt for 2 months followed by a 2 week history of progressive ataxia and imbalance and decreased ability to chew and swallow over the past 3 days. Prior treatment consisted of enrofloxacin therapy for 3 weeks and prednisone therapy for 2 weeks.

 

Neurologic exam:
Mentation: Alert but quiet
Head posture: Right head tilt
Gait: Tetra-ataxia, grade 3 with falling to the right
Cranial nerve exam: Absent right facial sensation and lip movement; dropped jaw and decreased gag reflex; positional vertical downbeat nystagmus
Postural exam: Decreased RF and RH conscious proprioception
Nociception exam: Pain on palpation of right bullae
Neurolocalization:
Central peripheral vestibular disease with involvement of cranial nerves V, VII, VIII, IX and X

Clinical Signs Overview

The initial clinical signs of ear infection include excessive scratching, rubbing and possible head tilt from pain. With otitis media, the degree of head tilt may progress, ipsilateral facial muscle paresis or paralysis and ipsilateral Horner’s syndrome (miosis, enophthalmus, and ptosis) may also occur. Signs of vestibular disease can only occur with involvement of the inner ear structures, vestibular nerve or central vestibular nuclei in the brain stem. Thus, it is imperative to distinguish the clinical signs of peripheral versus central vestibular disease as the initial clinical guide for further diagnostic testing and treatment. (Table 1)

Table 1. Signs Associated With Peripheral Versus Central Vestibular Disease

CLINICAL SIGNS	PERIPHERAL	CENTRAL
Head Position	Head tilt toward the lesion	Paradoxical vestibular syndrome: Lesion of the caudal cerebellar peduncle with a head tilt opposite the side of the lesion with all other signs ipsilateral to the lesion.
Nystagmus	Sustained(more common): Horizonatal to rotary; fast phase away from the lesion; Non-direction changing	Positional(more common): Pure vertical or horizontal; Rapid onset; >30 sec duration +/- Direction changing
Gait	Asymmetric ataxia Circling, falling to lesion No conscious proprioceptive deficits	Asymmetric ataxia; Circling, falling to lesion Ipsilateral hemiparesis, hypermetria, +/- conscious proprioceptive deficits
Strabismus	Positional and ventro-lateral	Positional and ventro-lateral
Cranial Nerves	VII ipsilateral to lesion	V, VII, VIII= cerebellopontine angle
Pupillary Changes	Horner’s ipsilateral to lesion (miosis)	Present with rostral brain stem lesions
Mental Status	Alert but disoriented	Changes in arousability;Disorientation

 

Diagnostic Tests

The diagnostic testing should progress from neurologic examination, otoscopic and oral examination to more advanced imaging tests. Assessment of the oral cavity to evaluate for polyps, tumors or infections spreading down the eustachian tube from the middle ear is especially important in cats. Magnetic resonance imaging (MRI) is the recommended advanced imaging modality to evaluate the location and severity of peripheral nerve lesions due to the ability to diagnose early signs of otogenic intracranial infection. (Figure 1) Aerobic and anaerobic cultures of the outer and middle ear are essential for selection of appropriate antimicrobial therapy. Myringotomy via iatrogenic rupture of the tympanic membrane to allow for aspiration of fluid for bacterial culture is recommended when there is confirmation of otitis media only.

Figure 1. Case study MRI scan for Kazmir
The scan demonstrates diffuse contrast enhancement of tissue in the middle and inner ear (open arrow) and focal contrast enhancing mass lesion the right pons at the cerebellopontine angle (closed arrow). The differential diagnoses for these findings are neoplasia (lymphoma, carcinoma, sarcoma) versus extensional otogenic infection with brainstem abscessation.

 

Treatment of Otitis Media/Interna
Initial treatment is the appropriate selection of antimicrobial therapy based on culture and sensitivity results. Long-term antibiotic treatment of 4-6 weeks is recommended regardless of surgical intervention. Empirical treatment with a first-generation cephalosporin is not recommended due to the high prevalence of reported resistant bacterial infections. Vestibular depressant therapy is useful for patient comfort and to enhance return of self-support of nutrition and hydration. (Table 2) Steroid therapy is contraindicated for treatment of infectious otitis interna due to the potential to precipitate progression to otogenic intracranial infection. Lavage of the tympanic bulla combined with medical management can also be an effective option for treatment of otitis media in dogs. Surgical intervention with ventral bullae osteotomy is recommended with evidence of polyp formation, recurrent episodes of otitis media, or failure of medical management. (Figure 2)

Table 2. Pharmacologic Treatment of Vestibular Signs

Drug	Dose	Pharmacologic Class	Potential Adverse Reactions
Vestibular Suppressants
Diazepam	1 – 5 mg po tid to qid	Benzodiazepine	Sedation
Lorazepam	0.25 to 0.5 mg po bid	Benzodiazepine	Sedation
Diphenhydramine	25-50 mg po tid	Antihistamine	Sedation
Meclizine	12.5-50 mg po bid	Antihistamine and anticholinergic	Sedation; dry mouth; dysuria
Antiemetics
Metoclopramide	1-2 mg/kg/24 hr CRI IV	Dopamine and 5HT antagonist	Tremor Restlessness
Prochlorperazine	0.5 mg SC TID	Phenothiazine	Sedation; seizure
Ondanestron	0.5 mg/kg IV q 24 hr	5HT3 antagonist	Diarrhea
Maropitant citrate (Cerenia)	1mg/kg SC q 24 hr	Neurokinin receptor antagonist	Diarrhea; injection site pain

 

Otogenic Intracranial Infection
Extensional infection from otitis media/interna can lead to a serious, life-threatening, otogenic intracranial infection in people, dogs, and cats.(5,6) A history of preceding otitis externa is common prior to a more acute onset of signs of brain stem disease, to include head tilt, circling, nystagmus, altered consciousness and ipsilateral cranial nerve deficits of the cerebellopontine angle (V, VII, and VIII). Rapid decompensation may occur as meningeal and cerebrospinal fluid dissemination occurs. Diagnosis is obtained by MRI scanning, cerebrospinal fluid analysis and culture, and external and middle ear cultures. Immediate treatment includes reduction of intracranial pressure with mannitol (1 gm/kg IV), and broad spectrum IV antibiotic therapy. The recommended empirical treatment pending microbial culture and sensitivity results is ceftriaxone (50 mg/kg IV bid over 30 minutes), enrofloxacin (5 mg/kg IV bid), and clindamycin (15 mg/kg IV bid). Ultimately, surgical exploration and ventral bullae osteotomy is recommended to remove the septic focus. (Figure 2). In this author’s experience, surgical morbidity and mortality is reduced if surgery is attempted after 48- 72 hours of antibiotic therapy. Long-term prognosis can be favorable with early initiation of appropriate antibiotic therapy, surgical removal of the septic focus and long-term oral antibiotic therapy (2-3 months).
In summary, the goal is to prevent otogenic intracranial infection through early detection and rapid institution of appropriate antibiotic therapy followed by surgical removal of the septic focus. Once intracranial disease is present, however, patient morbidity and mortality can be reduced with aggressive antimicrobial approach prior to surgical intervention.

Case Study Treatment and Outcome

A ventral bullae osteotomy was performed (A) and an inflammatory polyp and purulent fluid was removed. (B)

Kazmir was treated with intravenous ceftriaxone, enrofloxacin, and clindamycin prior to and after surgery. A PEG feeding tube was placed and Kazmir did well for several months, but unfortunately did not regain the ability to swallow on his own.

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