Otitis media-interna is an inflammatory process of the deepest parts of the auditory canal, which can lead to neurological signs such as vestibular signs, Horner’s syndrome and facial nerve paralysis.
The most common cause of otitis media-interna is extension of an infection of the middle ear (otitis media) to the structures of the inner ear (otitis interna). An animal shows vestibular signs when the inflammation of the middle ear directly or indirectly affects the function of the membranous labyrinth. Signs of vestibular syndrome (head tilting, vestibular ataxia, pathological nystagmus and ventral strabismus) due to an inner ear infection can occur at any age, in all breeds and in both sexes of dogs and cats. The Cocker Spaniel and other breeds with long ears and a tendency to chronic otitis are frequently affected. Likewise, Poodles with chronic otitis or pharyngitis are also commonly affected.
Besides vestibular signs, affected animals can also show signs of otitis externa, including head shaking, rubbing and scratching of the ears, and pain. Otitis media can develop in the absence of signs of otitis externa. In the early stages of otitis media-interna there may be hyperirritability of the sympathetic nervous pathways of the eye, leading to mydriasis. The animal may yawn frequently. Subsequently the animal may develop a peripheral vestibular syndrome, ipsilateral paralysis of the facial nerve and/or ipsilateral Horner’s syndrome. There may also be keratitis sicca because of the involvement of parasympathetic fibres of the facial nerve. Furthermore, the facial nerve paralysis can lead to drooling of food, water or saliva from the corner of the mouth on the affected side, because of paralysis of the lips.
Inflammation of the middle and inner ear may be bilateral, but if the disorder of the vestibular function is not identical, the signs will be asymmetrical and predominate in one direction. There are three ways in which inflammation of the middle ear can occur. The most frequent way is through the tympanic membrane. Debris, hairs, foreign bodies and disorders of the auditory canal can cause inflammation, necrosis and rupture of the tympanic membrane, enabling extension into the tympanic cavity. The second way is through the Eustachian tubes that connect the nasopharynx with the middle ear; this explains why bacteria typical of the respiratory tract can be isolated from the tympanic bullae. The last way is by haematogenous spread; in fact, free pathogens can reach the middle ear in the blood, although this seems uncommon. As far as concerns infections of the inner ear, these too usually develop as a result of the spread of inflammation from the middle ear, in the same ways as just described.
Bacteria are normally considered the main aetiological agents of otitis media. Staphylococcus and Streptococcus are among the most commonly isolated micro-organisms, but they have also been isolated from the ears of healthy dogs. Other bacteria that have been isolated from animals with middle ear infections are Pseudomonas, Proteus, Clostridium and Escherichia coli. Infections by yeasts, such as Malassezia canis or Candida spp., are uncommon causes of otitis media, as are fungal infections. Otodectes cynotis, a ubiquitous mite of the psoroptidae family, can occasionally lead to rupture of the tympanic membrane and the development of otitis media, especially in the cat. Trauma, polyps, neoplasms and foreign bodies (such as bits of grass) can cause inflammation of the middle ear and related symptoms. Neoplasms, through both expansion and invasion, can enter the inner ear, causing the signs of involvement of this anatomical area.
DIAGNOSIS
Otoscopy
A careful examination of the structures of the ear must always be carried out when there is a suspicion of peripheral vestibular syndrome in order to determine whether or not there are lesions in the area. However, in order to be able to examine the structures of the middle ear and inner ear in detail, collateral investigations must be used, because these parts of the ear are lodged within the petrous part of the temporal bone.
General anaesthesia must be used in order to perform a satisfactory otoscopic examination, otherwise there is the risk of gaining little information from this investigation which, it is worth mentioning, is easy to perform, cheap and a valid diagnostic instrument.
Inspection of the external auditory canal and the tympanic membrane can exclude the presence of a foreign body in the ear, which could carry an infection into the deeper structures and thereby produce vestibular syndrome, as being the cause of a head tilt. Other possible findings during this diagnostic investigation are diseases or factors predisposing to otitis, such as parasites, excessive secretion of wax, tumours, hyperplasia of skin structures and hypertrichosis. The presence of ulcers, masses, debris, hyperkeratosis or hyperplasia of the surrounding tissues can impair inspection of the tympanic membrane. It is, therefore, advisable to wash the ear with a tepid solution of saline before examining the tympanum. If stenosis of the auditory canal prevents introduction of the otoscope, other more sophisticated techniques can be used, such as video-otoscopy with the introduction of a fibre-optic probe.
At otoscopic examination, the normal tympanic membrane appears translucent and slightly concave. A pars tensa and a pars flaccida can be distinguished. These two parts of the membrane have different appearances on direct visual examination: the pars flaccida is a soft, opaque, white or red-coloured part of the membrane on which small blood vessels can be seen, while the pars tensa is a shiny, transparent part of the membrane, which may also appear pearly coloured and translucent. The base of the malleus, surrounded by small vessels, can be seen through the pars tensa.
Middle ear diseases should be suspected if the tympanic membrane is damaged, raised, dark or opaque and red, white or blue. Focal areas of black indicate perforations of the tympanum, while dark, thickened areas represent scar tissue or adhesions and bear witness to previous episodes of inflammation. Hypervascularisation of the tympanum indicates ongoing repair processes. A completely absent tympanum and difficulty in distinguishing the middle ear from the inner ear are signs of an acute or active, chronic infection, rendering the spread of the infection obvious. In the case that there are no signs of otitis externa, it should be possible to see any fluid collections present within the tympanic cavity given that the tympanic membrane is semi-transparent. Large collections may make the tympanum protrude externally. It should, however, be emphasized that a normal tympanic membrane does not exclude the possibility of an underlying otitis media-interna. In fact, the tympanic membrane may appear normal either because, once the insult has ceased, repair processes have restored the membrane to its normal state or because the infection may have been spread in the blood or arrived through the Eustachian tube.
If the membrane is injured it can be useful to take a sample of the material present for direct cytological examination and bacteriological studies; alternatively, if a fluid collection is suspected and the membrane is whole, a myringotomy can be performed. This minimally invasive procedure involves making an incision in the tympanic membrane, leaving only a small hole that heals very quickly. This procedure has both diagnostic and therapeutic value, since it allows removal of the collection of pathological matter as well as lavage of the structures involved. Topical medications should be applied with care since many disinfectants and chemical agents can provoke damage. Furthermore, before performing a myringotomy, the external auditory canal must be washed carefully to avoid iatrogenic spread of pathogens into the middle ear.
Conventional radiology
Before the introduction of more advanced imaging techniques into veterinary practice, such as computed tomography (CT) and magnetic resonance imaging (MRI), conventional radiology was widely used in the diagnostic protocol for otitis media-interna. This technique is much less frequently used nowadays, essentially because of its laboriousness and poor sensitivity. General anaesthesia is necessary to position the tympanic bullae suitably during the X-rays. In fact, the position of the head is crucial in order to obtain a good view of the structures, given that overlapping of hard tissues of the skull has a marked effect on the images. Five classical views have been described: dorso-ventral, latero-lateral, open-mouth rostro-caudal (at various angles depending on the conformation of the occipital protuberance) and lateral oblique with right and left inclinations of 20°.
Thus, more than one projection must be performed to obtain a complete examination of the tympanic bullae and the auditory canals. The dorso-ventral view enables comparison of the tympanic bullae and the external auditory canals but, compared with the open-mouth rostro-caudal view, there is greater superimposition of soft tissues over the bullae. The dorso-ventral view is preferred to the ventro-dorsal view because the mandible facilitates better positioning without rotating the head, compared to resting on the cranial vault. Both bullae and their respective auditory canals can be seen with the dorso-ventral view; furthermore, the middle ear is closer to the X-ray film thus reducing artefacts. The disadvantage of this projection is that the temporal bones partially overlie the bullae, making interpretation of the films more difficult.
In the latero-lateral view, the radiogenic beam is pointed at the area of the bullae. This view can be used to evaluate the nasopharyngeal region, but the bullae are overlaid although not superimposed by other tissues.
The open-mouth rostro-caudal view requires that the patient is placed in dorsal decubitus with the atlanto-occipital joint flexed and with the hard palate forming an angle of approximately 60° with the film. The mandible is pushed caudally and the central axis of the radiogenic beam must divide the angle of the temporo-mandibular joint in half. This view enables contemporaneous vision of both tympanic bullae and auditory canals without excessive overlie of tissues, as occurs with the dorso-ventral view (Fig. 1).
To obtain the lateral oblique views with an angle of 20°, the patient must be put in lateral decubitus with the tympanic bulla to be viewed closest to the X-ray film, keeping the animal’s mouth closed. In this view the two bullae cannot be compared directly, but since there is less superimposition of soft tissues, the individual bulla under examination is better delineated (Fig. 2). Comparison of the two bullae is, however, important because most patients with otitis media and interna have unilateral involvement and so the pathological and normal situations can be evaluated contemporaneously. The radiographs obtained in this way may show different features depending on the underlying pathology. In the case of otitis externa, there may be abnormal opacity due to soft tissues that replace the air in the external auditory canal and are, therefore, visible.
The radiographic signs associated with middle ear pathologies include opacity of the soft tissues of the tympanic bulla, sclerosis of the wall of the bulla or of the petrous part of the temporal bone, which can be associated with bone proliferation, as well as signs of otitis externa. In particularly severe cases, there may also by lysis of the bulla. Radiographic changes may not be present for several weeks in acute processes, while only sclerosis of the bulla may be present in chronic processes (Fig. 3). The external auditory canal can be visualised better with the use of a positive contrast agent, such as iohexol; this technique, termed canalography, is superior to otoscopy for determining whether the tympanic membrane is perforated or not.
Computed tomography
CT is the diagnostic imaging technique of choice in the case that hard tissues of the skull need to be evaluated; this is particularly the case for the ear and surrounding structures. A CT study of the tympanic bullae allows better visualisation of the external auditory meatus, the middle ear, but also the inner ear. (Fig. 4) Furthermore, it is superior to radiology in demonstrating slight increases in the opacity of soft tissues, whether these are fluid or masses, within the tympanic bullae. The study of the tympanic bullae typically requires continuous transverse images with a slice thickness varying from 1 to 3 mm, acquired using the algorithm for hard tissues. The thinner the slices are, the more the information that can be obtained from the views of the anatomical structures of the middle and inner ear. The administration of iodinated contrast agents is not necessary for this type of investigation unless there are space-occupying lesions or other signs raising the suspicion of a tumour.
The images should be acquired using both soft tissue and hard tissue windows. In this regard it should be remembered that the wall of a tympanic bulla filled with fluid appears artificially thickened compared to that of a bulla full of air. In a well-performed study the two bullae should appear symmetrical, even though small differences may be present, the wall should be thin and well-defined and the lumen must be full of air. The external auditory canal is also full of air and the thickness of the walls of the canal should be the same along the whole canal, without strictures or obstructions (Fig. 5).
The CT features during pathological conditions of the ear are discussed below, starting with the external structures and working towards the internal ones.
- Otitis externa: the CT changes that can be found in cases of otitis externa include mineralisation of the auditory canal and narrowing of the lumen, due to the presence of material with a density typical of soft tissues which occludes the canal completely or partially.
- Otitis media: thickening and irregularity of the walls of the bullae may be see in otitis media, sometimes related to proliferative processes, but also to lysis of the bone and to the presence of material with a density similar to that of soft tissues which collects within the tympanic cavity. Besides these signs, there may be the typical findings of otitis externa, which, as explained above, may involve more internal structures after having crossed the tympanic membrane (Fig. 6).
- Otitis interna: unless there is considerable destruction of the most internal structures, CT is usually of less diagnostic value than MRI, which is considered the imaging technique of choice for evaluating the inner ear (Fig. 7).
Although nasopharyngeal polyps are more common in cats, this type of lesion should not be excluded in dogs when there are concomitant masses with the same density as soft tissues in the nasopharyngeal region and within the tympanic cavity (Fig. 8).
Fig. 6 Fig. 7 Fig. 8
Some published studies have demonstrated that MRI is better than CT for the diagnosis of otitis in the dog. On the basis of these studies it can be hypothesised that MRI will become increasingly used as the investigation of first choice in cases of suspected otitis or vestibular syndrome, because this imaging technique can detect even very small lesions of soft tissues of the auditory canal and of intracranial structures.
Magnetic resonance imaging
Until the recent past, the same considerations made regarding visualisation of abnormalities of the middle ear also applied to the study of the structures of the inner ear; in fact, in most affected subjects, otitis interna does not produce X-ray changes, and injury to these most internal structures are difficult to detect with CT. For this reason, the diagnosis of otitis interna was made on the basis of clinical signs and radiographic evidence of otitis media. MRI (particularly with instruments using high magnetic fields) has become the diagnostic imaging technique most suitable for evaluating intracranial structures and the inner ear, thanks to the possibility of acquiring sections in any plane of space and to its greater sensitivity in detecting pathological states of soft tissues. The alterations detectable by MRI are described below, starting with the most external structures and proceeding to the most internal ones (Fig. 9).
External ear: MRI is not used to study the external ear, given that there are easier and cheaper diagnostic techniques. However, during the study of intracranial structures, changes to the external auditory canal can be detected. Normally the canal is full of air and, therefore, appears black. Narrowing of the external auditory canal, due to thickening of the walls, can be detected in both T1-weighted images and in T2-weighted ones. Further MRI findings that may raise the suspicion of otitis externa include fibrotic tissue and exudations, which produce signals within the auditory canal, while any mineralisation of the soft tissues is difficult to differentiate from normal auricular cartilage unless major changes are present.
Middle ear: fluid collections in the tympanic bulla appear hyperintense in T2-weighted images and isointense with respect to the cerebral parenchyma in T1-weighted images. If air is present there is usually a clearly visible air-fluid meniscus separating the two media. However, the presence of material can be an incidental finding, especially in some breeds such as the Cavalier King Charles Spaniel and the Boxer. After administration of the contrast agent, pathologies of the middle ear are usually more evident along the internal margin of the bulla, because of the presence of vascularised tissue. The T1-weighted images after administration of the contrast agent can distinguish whether the material that fills the bulla is related to proliferation of soft tissues or whether it is a fluid collection. Sclerosis of the wall of the tympanic bulla cannot be detected if mild and if the bulla contains air, because of signal attenuation by both the bone and the air. In chronic conditions hypointense areas, representing fibrotic tissue, can be seen in both the T1-weighted images and in the T2-weighted ones in which they are better shown. However, the bone changes that can occur during chronic inflammatory processes are generally more difficult to detect with MRI, especially if the changes are only mild (Fig. 10).
Fig. 9 Fig. 10
Inner ear: The structures of the inner ear are extremely small and slices of less than 2 mm thickness are necessary (sections are normally 3.5-5 mm thick). This way of acquiring images is called “volume acquisition” and, especially in T2-weighted images, is particularly useful for investigating the structures of the labyrinth in considerable anatomical detail. For example, during chronic otitis interna, fibrous obliteration of the space containing endolymph and perilymph is visualised as an absence of signals in T2-weighted images; that said, volume acquisition is only possible with the latest generation magnetic resonance instruments, which are still rare in normal veterinary centres. Evaluation of T1-weighted images after administration of contrast agent can reveal anomalies within the labyrinth such as labyrinthitis. Administration of contrast agent may be useful during the acute phases of labyrinthitis; in fact gadolinium can highlight the membranous labyrinth through its passage in the endolymph, following lesions to the blood-perilymph barrier, or through its uptake by inflamed tissues. The uptake of contrast agent by the meninges has been reported in T1-weighted images and has been described secondary to the presence of otitis interna.
An inflammatory process of the most internal part of the ear can also lead to involvement of the cerebral parenchyma; for this reason it may also be possible to detect changes in the neurocranium compatible with cerebral abscesses, meningitis and inflammatory lesions.
TREATMENT
As far as concerns the therapeutic choices for otitis media-interna, when possible, the antibiotic used must be selected on the basis of the results of an antibiogram, given that long-term (4-6 weeks) systemic treatment is necessary because the pathological material is collected in a bone structure which is also less vascularised compared to other tissues. Lavage of the tympanic bulla, together with removal of the pathological material present via myringotomy, is a valuable aid to antibiotic treatment.
Given that infections of the middle and inner ear are often the result of chronic inflammatory processes of the external auditory canal, it is essential to treat the otitis externa with topical therapy and lavage of the canal.
Surgical treatment may be necessary in cases of otitis refractory to antibiotic therapy. The proposed surgical techniques include: resection of the auditory canal, total ablation of the auditory canal and bullectomy.
Nasopharyngeal polyps are often associated with middle ear infections in the cat, in that they are one of the factors predisposing to the development of otitis; it may, therefore, be necessary to remove such polyps surgically.