Assessment Anatomic diagnosis

Etiologic diagnosis—Neoplasia originating from the trigeminal nerves (eg, lymphoma) or extending from adjacent structures should be considered as a differential diagnosis for chronic progressive bilateral masticatory muscle atrophy and a dropped jaw in a geriatric patient. For this dog, a neoplasm originating from the brainstem or extending from adjacent structures (eg, meninges or bone) or metastases seemed less likely because of the lack of other specific brainstem signs (eg, abnormal mentation, proprioceptive deficits, gait abnormalities, and multiple cranial nerve deficits). Nevertheless, some slow-growing neoplasms, such as meningiomas, can markedly compress the brainstem without any obvious central nervous disturbances. Inflammatory or infectious processes (eg, trigeminal neuritis or neuritis secondary to infectious diseases [eg, Toxoplasma gondii or Neospora caninum infection]) should also be considered. A CBC and serum biochemical analyses were performed at the time of the referral evaluation and revealed no remarkable abnormalities. The dog was negative for serum anti-T gondii and anti-N caninum antibodies. The diagnostic plan included MRI and CT of the head, CT of the thorax, and examination of a CSF sample.

Diagnostic test findings—Magnetic resonance imaging was performed with a 1.5-T permanent magnet.^a The dog was positioned in dorsal recumbency. Transverse, dorsal, and sagittal T2-weighted images; transverse T1-weighted images; transverse fluid-attenuated inversion recovery (FLAIR) images; and transverse T2^* images of the head were obtained. Transverse, dorsal, and sagittal T1-weighted images were also obtained after IV administration of gadopentate dimeglumine.^b The MRI examination revealed a large, lobulated mass lesion most likely originating from the base of the skull and the right tympanic bulla, which extended into the nasopharynx and the caudal fossa causing mild compression of the brainstem. Compared with gray matter, the lesion was heterogeneously hyperintense on T2-weighted images, mildly hyperintense on FLAIR images, and iso- to mildly hypointense on T1-weighted images; it had strong peripheral contrast enhancement (Figure 1). Computed tomography of the head confirmed the presence of the mass, which appeared heterogeneously hyperattenuating with some areas of suspected osteolysis and thickening of the right tympanic bulla. Computed tomography of the thorax did not reveal any abnormalities. Taking into consideration the dog's signalment, progression of clinical signs, and MRI and CT findings, a mesenchymal neoplasm (eg, osteosarcoma or chondrosarcoma) was suspected. The dog was anesthetized, and during examination of the oropharyngeal region, palpation of the mass through the soft palate was possible. Fine-needle aspirates specimens and a punch biopsy specimen were obtained and sent for histologic examination. Histologic findings included a characteristic pattern of a tumor of bone with multiple lobules formed by cartilage that was focally and extensively undergoing mineralization and bordered by thin septa of spindle cells and some mitotic figures. The diagnosis was a multilobular tumor of bone (MTB).

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Midsagittal T2-weighted MRI image (A), midsagittal Tl-weighted postcontrast MRI image (B), parasagittal CT reconstruction image (C), transverse T2-weighted MRI image (D), transverse Tl-weighted MRI image (E), transverse Tl-weighted postcontrast MRI image (F), and bone window transverse CT image (G) of the head of a dog with suspected bilateral keratoconjunctivitis sicca of 2 months' duration, progressive bilateral masticatory muscle atrophy of 3 months' duration, and a dropped jaw of 2 weeks' duration. A mass lesion is present in the nasopharynx (arrows) and invades the floor of the caudal cranial fossa (arrowhead). The lesion is heterogeneously hyperintense on T2-weighted images, and marked peripheral contrast enhancement of the mass is visible on Tl-weighted images after contrast medium administration. On CT images, the mass appears highly hyperattenuated, suggestive of extensive calcification.

Citation: Journal of the American Veterinary Medical Association 253, 3; 10.2460/javma.253.3.269

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An MTB is an uncommon bony tumor that usually originates from flat bones of the skull and is rarely detected in other body locations.² This tumor has been described as affecting the cranial vault (frontal, temporal, parietal, and occipital bones), face (maxilla and zygomatic arch), mandible, and hard palate.^1–5 To the authors' knowledge, this is the first report of an MTB that originated from a tympanic bulla, crossed the midline, and caused bilateral compression of the trigeminal nerves and pons, with subsequent dropped jaw and decreased reflex tear production.

Multilobular tumors of bone most commonly develop in medium- to large-breed, middle-aged to older dogs. This neoplasm is a slow-growing, relatively nonaggressive yet potentially locally invasive tumor, which typically does not affect the surrounding soft tissues.² For this reason, clinical signs usually appear when the MTB has attained a considerable size. Metastasis is possible, especially to the lungs, although the time to metastasis is relatively long, compared with that for other bony tumors.²

In the dog of the present report, the tumor most likely caused severe compression of both trigeminal nerves and partially the pons. The trigeminal nerve, through its ophthalmic branch, is also responsible for the sensory innervation of the cornea. Lack of corneal sensation leads to reduced reflex tear production. A dropped jaw usually results from bilateral trigeminal nerve disease in which there is paresis or paralysis of the masticatory muscles.⁶ The most common cause of dropped jaw is idiopathic trigeminal neuritis, although neoplastic processes, such as lymphoma, can also be a cause.⁶ In addition to the dropped jaw, the compression of the soft palate and pharynx complicated adequate mastication of food and caused the abnormal sounds noticed during examination of the dog.

For diagnosis of an MTB, multiple imaging techniques can be used including MRI, CT, and survey radiography. Magnetic resonance imaging is considered the gold-standard imaging technique because of the high-quality definition of soft tissues in the images. However, because an MTB is a bony tumor, CT may provide a better definition of the skeletal extension of the neoplasm. On survey radiographic views, an MTB has the appearance of multilobular masses with mineralization and lysis.¹ The MRI and CT findings for the dog of the present report were similar to features described for previous cases.^1,3

For dogs with an MTB, surgical excision with complete surgical margins may provide good long-term palliation; however, complete resection might be difficult to achieve because of the infiltrative nature of this tumor and its location.² The grade of the tumor was found to have an effect on the median survival time and time to metastasis.² The usefulness of adjuvant treatments, such as chemotherapy or radiation therapy, is still unclear.⁷

After diagnosis, the dog of the present report was discharged from the hospital and was to receive ocular treatment for the descemetocele. Although no improvement of the clinical signs was noticed and the dog required help to chew and swallow food properly, its condition remained stable for 3 weeks. Eventually, the dog stopped eating and was euthanized owing to its poor quality of life.