Otitis Externa: An Essential Guide to Diagnosis and Treatment

166 Pages·2014·26.618 MB·English

by  Richard G Harvey, Sue Paterson

OTITIS EXTERNA
– AN ESSENTIAL GUIDE TO
DIAGNOSIS AND TREATMENT
Richard G. Harvey
BVSc, DVD, Dip.ECVD, FSB, PhD, MRCVS
The Veterinary Centre
Cheylesmore, Coventry, UK
Sue Paterson
MA, VetMB, DVD, DipECVD, MRCVS
RCVS and European Specialist in Veterinary Dermatology
Rutland House Referral Hospital
St Helens, Merseyside, UK
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CONTENTS
Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . v
Abbreviations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .vi
1 The Normal Ear ........................................... 1
Introduction ....................................................1
Pinna ..........................................................2
External auditory meatus .........................................4
Middle ear ......................................................4
Microscopic structure of the external ear canal .......................6
Microclimate of the external ear canal ..............................9
Cerumen in normal and otitic ears ................................12
2 Approach to the Diagnosis of Otitis Externa .................. 13
Introduction ...................................................13
Signalment ....................................................14
History .......................................................14
Physical examination ...........................................15
Otic examination ...............................................16
Otoscopic appearance of the external ear canal and tympanum ........17
Cytological characteristics of normal and abnormal ears .............25
Biopsy ........................................................34
Imaging .......................................................35
3 Aetiology and Pathogenesis of Otitis Externa ................. 45
Concept of primary and secondary factors, predisposing factors, and
perpetuating change ............................................45
Primary causes of otitis externa ...................................48
Secondary causes of otitis externa .................................63
Perpetuating factors ............................................65
4 Ear Cleaning ............................................. 74
Introduction ...................................................74
Assessing the integrity of the tympanum ...........................75
Assessing the discharge .........................................75
Assessing the canal .............................................78
Cleaning process ...............................................79
Home cleaning .................................................80
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iv
5 Medical Management of Ear Disease ........................ 81
Dealing with primary triggers ....................................81
Dealing with secondary causes ...................................86
Chronic or recurrent otitis externa in dogs ..........................95
6 Otitis Media ............................................ 104
Aetiology ....................................................104
Clinical signs .................................................105
Major differential diagnoses ....................................107
Diagnosis ....................................................107
Management .................................................111
Treatment of nasopharyngeal polyps in cats .......................113
Prognosis ....................................................114
7 Ototoxicity and Other Side-Effects of Otic Medication ......... 115
Introduction ..................................................115
Meatal and tympanic inflammation ..............................116
Effects within the middle ear ....................................116
Ototoxicity ...................................................117
Systemic effects of otic medication ...............................119
Ototoxicity of systemic drugs ...................................119
Summary ....................................................120
8 Aural Ablation and Bulla Osteotomy ....................... 121
Introduction ..................................................121
Presurgical investigations ......................................122
Vertical canal ablation ..........................................123
Bulla osteotomy ...............................................137
Special problems relating to surgery of the middle ear in cats ........140
References ................................................ 141
Index .................................................... 159
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PREFACE
v
The investigation and management of ear disease will occupy a
significant proportion of a veterinary clinician’s time. Otitis externa, in
particular, is likely to be seen by a busy small animal clinician at least
once a day. Chronic, and chronic recurrent, otitis externa is frustrating
and costly to the owner and often painful for the dog or cat.
The concept behind this book is to provide a comprehensive source of
information on the relevant structure, function, medicine, and surgery
of the ear, from Otodectes cynotis infection to Pseudomonas aeruginosa
infection, by way of atopy.
We hope that students and clinicians, in particular, will find it so
useful that their copy becomes battered and stained, the ultimate test
of practical relevance.
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ABBREVIATIONS
vi
BAER brainstem auditory evoked response
CT computed tomography
DMSO dimethylsulfoxide
EDTA ethylenediamine tetra-acetic acid
GABA gamma-amino butyric acid
Ig immunoglobulin
LBO lateral bulla osteotomy
MIC minimum inhibitory concentration
MRI magnetic resonance imaging
MRSA methicillin-resistant Staphylococcus aureus
PCMX parachlorometaxylenol
PVA polyvinyl alcohol
TECA total ear canal ablation
tris tromethamine
VO video-otoscope
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1 THE NORMAL EAR
1
KEY POINTS INTRODUCTION
Q The external ear comprises the pinna The ear of the dog and cat is composed of
and the external acoustic meatus. It three parts: the external ear, the middle ear,
serves to collect and locate the origin and the inner ear (Figure 1.1)1–3. Together
of sound waves. these components allow the animal to
Q The middle ear comprises the locate a sound and the direction from which
tympanic membrane, the ossicles, it emanates, to orientate the head in relation
the auditory tube, and the tympanic to gravity, and to measure acceleration and
cavity. It serves to transduce incoming rotation of the head. Selective breeding, of
airborne sound waves into waves in a dogs in particular, has resulted in a wide
liquid medium. variation in relative size and shape of the
Q The inner ear comprises the cochlea, components of the external ear. Compare,
the vestibule, and the semicircular for example, the French Bulldog, the
canals. It interprets sound and serves Cocker Spaniel, the German Shepherd
to relate the head to gravity, allowing Dog, the St. Bernard, and the Persian cat.
the visual system to compensate for The pinnal shape and carriage, the diameter
movement and to perceive both linear of the external ear canal, the degree of
and rotational acceleration. hair and amount of soft tissue within the
external ear canal, and the shape of the
Vestibular nerve
Cochlear nerve Semicircular canals
Cochlea
Ear canal
Incus
Malleus
Tympanic
Stapes membrane
Fig. 1.1 The anatomic relationship of the components of the external, middle, and inner ear
remains constant in relation to each other and the skull.
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2 The Normal Ear
skull within which the middle and inner The pinna is composed of a sheet of
ear lie, vary from one breed to another. cartilage covered on both sides by skin
Despite this anatomic variation the essen­ (Figures 1.2, 1.3), which is more firmly
tial relationship between the various com­ adherent on the concave aspect than on the
ponents of the external, middle, and inner convex aspect2,3,5. The cartilage sheet which
ear is preserved1. supports the pinna is a flared extension
of the auricular cartilage. Proximally this
PINNA
becomes rolled to form the vertical ear canal
and part of the horizontal ear canal6. The
The evolutionary role of the pinna has major part of the external auditory meatus
been as an aid to sound collection and is contained within the auricular cartilage.
point­of­origin location (Figures 1.2, 1.3). Generally the pinna is haired on the
However, selective breeding of dogs has convex surface and in some breeds, such as
resulted in pinnae which often appear to the Cocker Spaniel and Papillon for example,
have been designed more as lids to prevent markedly so. The concave aspect may be
access by foreign bodies (Figures 1.4, 1.5) lightly haired on the free edges and towards
or as vehicles to carry ornate displays of the tip, but towards the base it becomes
exuberant growths of hair (Figures 1.6, 1.7). essentially glabrous and is tightly adherent
Despite these changes, the functionality of to the underlying cartilage. A few fine hairs
the ear appears to have been maintained. In are usually present around the entrance to
most breeds of cats the pinna is held erect, the external auditory meatus. In breeds with
with the exception of the Scottish Fold Cat hirsute ear canals, such as Cocker Spaniels,
where the distal portion of the scapha is there may be profuse hair growth along the
folded rostroventrally4. whole length of the ear canal.
Figs. 1.2, 1.3 Archetypal pinna, in this case of a German Shepherd Dog. Note the even
distribution of short hairs on the convex aspect (1.2). There is a variable amount of glabrous,
sparsely-haired skin on the concave aspect (1.3) which is confluent with the epithelial lining of
the external ear canal.
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The Normal Ear 3
Figs. 1.4, 1.5 Examples of the various pinnae which have resulted from selective breeding.
Labrador Retriever (1.4), Cocker Spaniel (1.5).
Figs. 1.6, 1.7 Examples
of the various pinnae
which have resulted
from selective breeding.
Papillon (1.6), Yorkshire
Terrier (1.7).
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