Assessment Anatomic diagnosis

Etiologic diagnosis—Differential diagnoses for the right-sided conduction deafness, ocular partial external ophthalmoplegia (ptosis), and decreased ocular menace reflex included a congenital or traumatic anatomic anomaly affecting the right cranial nerves II, III, or VII; occult mass; or foreign body obstruction. An additional differential diagnosis included infectious or inflammatory otitis media or otitis interna (as with viral or bacterial labyrinthitis) with secondary right cranial nerve VII neuropathy along its path through the right internal acoustic meatus. Independent lesions of cranial nerves II, III, VI, and VII, or a regional cavernous sinus lesion resulting in ptosis and decreased menace response were not definitively excluded. The right ocular nystagmus was likely physiologic. Labyrinthine nystagmus was not ruled out. Differential diagnoses for the reduced left ear hearing included late maturation of the cochlea with resultant long latencies of the brainstem auditory response or breed-related sensorineural deafness. Differential diagnoses for the skin mass along the right side of the neck included congenital anomaly with communication to the spinal cord (eg, lipomyelomeningocele, lipomyelocele, myelocystocele, or dermoid sinus) and benign or malignant dermal neoplasm.

Diagnostic investigation included CT and MRI of the head (to evaluate the craniocephalic osseous and soft tissues). Positive contrast CT fistulography of the right cervical skin mass was also performed. Imaging procedures were performed in a staged manner.

Diagnostic test findings—Computed tomography was performed with the dog sedated and in left lateral recumbency, and scout imaging did not reveal implanted or ingested metallic foreign material. Computed tomographic images of the head and neck to the C4 vertebra (without IV contrast agent administration) were acquired with a multi-detector 64-slice CT scanner^a in the transverse plane, in helical mode with the following protocol: 120 kVp; 90 mA; 0.8-second rotation; 1 pitch factor; and slice thickness, 0.625. Sagittal and dorsal 2-D and 3-D planar reconstructions were evaluated. The right tympanic bulla and the tympanic and petrous parts of the temporal bone were severely thickened and misshapen, compared with those structures on the left side, which were normal. There was 70% to 80% reduced gas volume within right tympanic bulla, complete attenuation of the right tympanic cavity, and deformity and attenuation of the right internal acoustic opening, facial canal, bony labyrinth, and promontory. The right external acoustic meatus was also misshapen, asymmetric, and displaced ventrally, compared with the left horizontal external acoustic meatus. There was widening of the right tympanooccipital fissure and mild asymmetry of the occipital suture with a mild, smooth, surrounding periosteal response (Figure 1). Adjacent to the C3 vertebra, there was a dermal and subcutaneous outpouching with soft tissue–dense material. Positive contrast fistulography with repeated image acquisition was performed following intralesional injection of a nonionic iodinated contrast agent^b (58 mg of iodine/kg [26.4 mg/lb]; total dose of iodine, 300 mg). Fistulography revealed tracking of contrast agent from the structure caudally a few millimeters within the subcutaneous soft tissues adjacent to C4 (not shown). No contrast agent was seen near or within the spinal canal or intervertebral foramina, ruling out the presence of an atypically located (type IV) dermoid sinus with communication to the vertebral canal. Immediately following CT, while the dog remained sedated, the conical dermal mass of the right cervical region was removed by blunt and sharp dissection to the level of the deep cervical muscle layer. The tissue was submitted for histologic characterization.

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Representative craniocephalic CT and MR images highlighting changes associated with the ear canals, tympanic bullae, cochlear fluid, and bony labyrinths of a 9-week-old Border Collie evaluated for right conduction deafness and reduced hearing of the left ear. A—Transverse plane CT image of the head. Notice the asymmetry of the tympanic bullae and cavities. B—Transverse plane CT image of the head (more caudal location than that of the image in panel A). There is severe thickening and deformity of the right tympanic bulla with almost complete obliteration of the right bony labyrinth (arrow). Notice for comparison, the normal left bony labyrinth. The internal acoustic meatus (arrowhead) divides into 3 canals (including the facial canal) to transmit cranial nerves VII and VIII and the labyrinthine artery to the bony labyrinth (not shown). C—Dorsal plane CT image of the head. Notice the deformity of the right bulla and asymmetry of the external acoustic meatuses (arrow). A small, incidental dermal mass (asterisk) was confirmed to be a choristoma and not a communicating dermoid sinus. Partial volume averaging artifact, not a fracture of C1, is denoted by the arrowhead. The CT images were obtained in a bone window (window level, 300 Hounsfield units; window width, 1,500 Hounsfield units). D—Transverse plane MR image (acquired with a FIESTA sequence) of the head at the approximate same level as the image in panel A. Panel D is more rostral than panel E. Notice the signal void, deformed right bony labyrinth, and lack of normal cochlear hyperintense fluid signal intensity. For comparison, the left semicircular canal has normal (bright white) hyperintense fluid signal intensity fluid (arrow). The FIESTA sequence is ideal for evaluating details of the inner ear and the course of the cranial nerve branches. E—Transverse plane MR image (acquired with a FIESTA sequence) of the head at the approximate same level as the image in panel B (more caudal location than that of the image in panel D). There is asymmetry of gas-related susceptibility artifact with decreased gas volume seen on the right. F—Dorsal plane T2-weighted MR image of the head. Notice the malformed right tympanic bulla and ear canal, compared with those structures in panel C.

Citation: Journal of the American Veterinary Medical Association 251, 10; 10.2460/javma.251.10.1133

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For MRI, the dog was administered propofol IV once and anesthesia was maintained via inhalation of an admixture of isoflurane^c and oxygen. Magnetic resonance imaging was performed with a 3.0-T magnet.^d Multiplanar images of the brain to the level of the C2 vertebra were acquired as follows: T2-weighted images in the dorsal, sagittal, and transverse planes; T1-weighted images in the sagittal and transverse planes; fluid-attenuated inversion recovery (FLAIR) images in the sagittal and transverse planes; T2*-gradient recall echo (T2*) images in the transverse plane; fast imaging employing steady-state acquisition (FIESTA) images in the transverse plane; and diffusion-weighted images before and after IV administration of a paramagnetic contrast agent^e (0.1 mmol/kg [0.045 mmol/lb]; total dose, 0.52 mmol).

On T2-weighted images, there was an almost complete absence of hyperintense fluid within the right semicircular canal, compared with the left semicircular canal. A residual, 2.3-mm-wide × 1-mm-long, elliptically shaped region of T2-weighted hyperintense fluid remained within the abnormally shaped right semicircular canal (Figure 1). The right facial nerve was subjectively thickened, tortuous, and asymmetric, compared with the left facial nerve (not shown). The lateral ventricles were mildly asymmetric, with the left being larger than the right. Deformity and attenuation of the right internal acoustic opening, facial canal, bony labyrinth, and promontory were noted. Osseous asymmetry of the right and left tympanic bullae with attenuation of the right vertical and horizontal external acoustic meatuses; malformed, thick tympanic and petrous parts of the right temporal bone; and laterolateral and dorsoventral foreshortening of the right petrous temporal bone were also seen on MRI images. The bone of the dorsal aspect of the right temporoparietal region was thickened and depressed, compared with its counterpart on the left. The signal void of gas within the right tympanic bulla was again noted to be less than that of the left (Figure 1). The left tympanic bulla appeared normal. Lack of T1-FLAIR high signal intensity within the residually malformed, attenuated cochlear apparatus and bulla and along the auditory pathway ruled out the presence of subacute hemorrhage. No evidence of acute parenchymal hemorrhage was noted on the T2* sequence (not shown). Normal patterns of brain enhancement and a normal lack of bullae enhancement were seen following administration of the contrast agent.

The absence of perilymph and endolymph fluid signals within the right cochlear apparatus was consistent with a perilymph fistula. Combined with the dog's signalment, history, and calvarial structural abnormalities, perilymph fistula attributable to trauma with resultant malformation of the right aspect of the calvarium (probable healing skull fractures), right osseous tympanic bulla, and right cochlear apparatus and probable disruption of the right occipital suture was considered most likely despite the lack of a known historical traumatic event. Congenital malformation and concurrent viral or bacterial labyrinthitis were not definitively excluded, but were considered less likely. Histologic examination of sections of the cervical dermal mass revealed normal skin with multiple hair follicles with a hyaline cartilage core, which extended deeply into the dermal component of the tissue. Findings were consistent with a cartilaginous choristoma. Cartilaginous choristomata are rare in dogs, with one recently reported case.¹ This benign tumor was an incidental finding in the dog of the present report.

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Conduction deafness, caused by the obstruction of sound from reaching the cochlear aqueduct within the inner ear, is most often a result of excess cerumen accumulation or chronic otitis media causing stenosis and occlusion of the external ear canal.² Sound conduction is highly dependent on the translation of sounds from the auditory ossicles in the middle ear through the oval window by way of vibrations. Auditory ossicles send vibrations through the oval window to perilymph fluid within the cochlea or bony labyrinth of the inner ear. Perilymph is analogous to CSF and communicates with the intrathecal space.³ The vibrations cause ripples in the fluid that interact with thin membranes of the cochlear duct (membranous labyrinth) and endolymph fluid.² Specialized nerve (hair) cells of these membranes translate ripples in the perilymph and endolymph into an action potential (or neural signal), which travels to the brain via the cochlear nerve.²

A perilymph fistula is an abnormal communication between the inner ear (cochlea) and the middle ear (space containing the auditory ossicles) with complex pathophysiology.⁴ Two soft tissue membranes—the round and oval windows—separate the inner and middle ear.² The stapes, immediately adjacent to the oval window, distributes sound vibrations to the perilymph by pressing against the membranous window.² The round window, which is also covered by a membrane, allows for pressure changes to occur in an otherwise noncompliant structure as the sound waves travel into the cochlea from the stapes-oval window interface.² Traumatic rupture of the round or oval window membrane may occur by implosive or explosive forces. Implosive rupture is caused by increased air pressure from the auditory tube to the middle ear. Explosive rupture occurs secondary to increased intracranial pressure, which causes increased perilymphatic pressure via the cochlear aqueduct within the inner ear.⁴

For the dog of the present report, the right-sided osseous calvarial malformations, disrupted bony labyrinth, and lack of right cochlear fluid detected via CT and MRI were suggestive of a traumatic event in the first few weeks after birth. Traumatic microfissures of the oval window or explosive rupture of the oval window due to increased intracranial pressure and a secondary fistula between the middle and inner ear were considered most likely. A congenital abnormality was deemed less likely. Secondary CN VII weakness or neuropathy was the likely cause of the dog's ophthalmoplegia. Although the dog of the present report had conduction deafness, Border Collies are predisposed to congenital sensorineural deafness.⁵ The pathogenesis of congenital sensorineural deafness may vary but usually involves degeneration of a component of auditory nerve transmission to the brain (eg, hair cell agenesis or hair cell and neuronal degeneration due to postnatal stria vascularis degeneration).² Conduction deafness is often a result of otitis externa or media, but it can also be caused by ear obstructions, as with masses or polyps.⁵ Unlike the dog of this report, perilymph fistulas in people are often associated with sensorineural deafness.³ However, the dog's positive response to bone stimulation was critical in ruling out sensorineural deafness and prompted evaluation with advanced diagnostic imaging. Lack of perilymph fluid in the cochlear apparatus was the likely cause of the right ear conduction deafness. Had bone stimulation not been performed, the dog's deafness would have been attributed to congenital, breed-related, sensorineural deafness. With regard to the reduced hearing of the left ear, an anatomic cause was not identified and left cochlear perilymph volume was subjectively normal. Delayed hearing maturation, early sensorineural deafness, and intermittent fistulation of the left cochlear apparatus, despite normal anatomic features, were considered as causes for reduced hearing of the left ear. No interventional treatments were instituted. At a 6-month follow-up via telephone conversation, the owner reported that the dog of the present report was compensating well with the hearing deficits through its ability to respond to nonverbal cues. The dog was performing well in agility competitions.

In combination, MRI and CT enabled excellent assessment of the dog's middle and inner ears. This imaging approach aided in ruling out lesions of cranial nerve III and cavernous sinus syndrome. In animals, determination of the cause of deafness can be particularly challenging given the limited ability to assess animals for subtle signs of inner or middle ear disease such as tinnitus, vertigo, and aural fullness. For the dog of this report, MRI identified the occult cause of right-sided deafness and provided an accurate assessment of the cochlear fluid apparatus and bony labyrinth. Perilymph fistula should be considered in animals with conduction or sensorineural deafness and those with a history of trauma and an associated onset of deafness, and when CT fails to provide a definitive diagnosis. More frequent application of MRI for the evaluation of deafness in nonhuman animals would enhance surveillance for this disease.