What Is Your Neurologic Diagnosis?

Cydney T. Fires 1Friendship Hospital for Animals, 4105 Brandywine St NW, Washington, DC 20016

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Leontine Benedicenti 2Department of Neurology and Neurosurgery, School of Veterinary Medicine, University of Pennsylvania, Philadelphia, PA 19104

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A 5-year-old 10-kg (22-lb) neutered male West Highland White Terrier was evaluated because of acute unilateral masticatory muscle wasting. Prior to evaluation, the dog was drooling and scratching its right ear. A physical examination was performed by the referring veterinarian, who noted a swollen and stenotic external right ear canal, an enophthalmic right eye, and a historical grade 3/6 left apical systolic heart murmur. Cytologic examination of swab specimens from both ear canals revealed yeast and cocci; the right ear was more severely affected than the left. The dog's ears were treated with an anti-inflammatory, anti-pruritic, antifungal, and antibacterial ointmenta on the medial aspect of both pinnae every 12 hours for 10 days and a topical anti-inflammatory and antimicrobial solutionb for the otitis externa in the right ear every 12 hours for 10 days. Eight days later, the referring veterinarian noticed severe atrophy of the right masseter muscle and prescribed gabapentin (100 mg, PO, q 12 h) for 10 days. Three days after the muscle wasting had been noticed, the dog was assessed by a board-certified veterinary neurologist.

What is the problem? Where is the lesion? What are the most probable causes of this problem? What is your plan to establish a diagnosis? Please turn the page.

Assessment Anatomic diagnosis

ProblemRule out location
Severe unilateral masseter and temporalis muscle atrophy on the right side of the faceMandibular branch of the right cranial nerve V (trigeminal nerve)
Decreased sensation in response to stimulation of the nasal mucosa in the right narisOphthalmic branch of the right cranial nerve V (trigeminal nerve)

Likely location of 1 lesion

Given the dog's unilateral deficits and otherwise unremarkable neurologic examination findings, a peripheral neuropathy of the right cranial nerve V (trigeminal nerve) affecting both the ophthalmic and mandibular branches was considered most likely. The involvement of the ophthalmic branch was suspected because of the decreased response to stimulation of the nasal mucosa on the right side. Involvement of the mandibular branch was suspected because of severe muscle atrophy of the right masticatory muscles; the mandibular branch provides both motor and sensory function to the temporalis, masseter, medial and lateral pterygoid, and digastric (rostral aspect) muscles.

Etiologic diagnosis—Differential diagnoses for acute unilateral trigeminal nerve paralysis1 included neoplasia (peripheral nerve sheath tumor [PNST], meningioma, or lymphosarcoma) or ischemic infarction affecting the motor nucleus of the trigeminal nerve. Although some inflammatory diseases (idiopathic trigeminal neuritis) were considered possible causes for the unilateral masticatory muscle atrophy, others (eg, masticatory muscle myositis) were considered less likely because of the unilateral nature of the dog's disease. Additionally, infectious diseases (eg, infection with Neospora caninum or Toxoplasma gondii) were considered less likely because of the unilateral and focal nature of the dog's disease. Diagnostic imaging including thoracic radiography to rule out pulmonary metastatic disease and MRI of the head to identify any structural abnormalities within the brain and surrounding tissues were performed.

Diagnostic test findings—Clinicopathologic analyses had been performed by the primary veterinarian. Results of a CBC were unremarkable other than mild thrombocytopenia (192,000 platelets/μL; reference range, 170,000 to 490,000 platelets/μL). Although a manual platelet count was not performed, platelet clumping was a possible cause. No kidney-, liver-, or electrolyte-related biochemical abnormalities were detected. In a case report by Kent et al,2 a dog with PNST had clinicopathologic test results that were within reference ranges, providing some evidence that dogs with PNST may not have detectable clinicopathologic abnormalities. For a Beagle with systemic lymphosarcoma, an inflammatory leukogram characterized by mature neutrophilia, left shift, and lymphocytosis was revealed by a CBC and reactive lymphocytes were identified on examination of a blood smear.3 The same changes are not often evident in dogs when lymphosarcoma is localized to the CNS only.3

The dog of the present report underwent 3-view thoracic radiography, which revealed mild left-sided cardiomegaly but no evidence of pulmonary metastasis. During anesthesia, MRIc of the dog's brain was performed and various image sequences were obtained as follows: sagittal T2-weighted images; transverse T2-weighted, T2*-gradient echo, and fluid attenuated inversion recovery images; transverse T1-weighted images with and without IV gadolinium administration; transverse diffusion-weighted images; transverse apparent diffusion coefficient map; and dorsal T1-weighted images with and without IV gadolinium administration.

Magnetic resonance imaging revealed a broad-based, round, 1-cm-diameter mass without a dural tail in the dog's right midbrain region. The lesion was hyperintense on T2-weighted images and isointense on T1-weighted images; it had moderate heterogeneous enhancement on T1-weighted postcontrast images. Material inside the right osseous bulla was hyperintense on T1-weighted and T2-weighted images but was not enhanced on the T2-weighted postcontrast images. The temporalis, masseter, and pterygoid muscles were severely atrophied unilaterally on the right side of the dog's head (Figure 1). Owing to the presence of an obvious intracranial mass, idiopathic trigeminal neuritis and ischemic infarction were considered less likely, leaving PNST as the primary differential diagnosis for the dog of the present report. With the presumptive diagnosis of PNST, it was recommended that a follow-up examination of the dog with an oncologist be arranged by the owner for stereotactic radiation therapy.

Figure 1—
Figure 1—

Transverse MRI T1-weighted postcontrast image of the brain of a 5-year-old dog that was evaluated because of a 3-day history of right-sided masticatory muscle atrophy. Notice a round lesion (arrow) in the right midbrain region that has moderate heterogeneous contrast enhancement. Contrast-enhanced effusion (star) fills the right tympanic cavity. Atrophy and moderate contrast enhancement of the temporalis, masseter, and pterygoid muscles (in the region denoted by the line) are evident.

Citation: Journal of the American Veterinary Medical Association 256, 1; 10.2460/javma.256.1.47

Comments

Peripheral nerve sheath tumors can have an intracranial location in the brainstem affecting the pons and medulla oblongata or in the trigeminal canal affecting the oval or round foramen or orbital fissure, or can have an extracranial location affecting the trigeminal nerve and its branches.4 Depending on the anatomic location of a PNST, it is possible to have denervation to any and all branches of the trigeminal nerve.5 The trigeminal nerve provides both afferent sensory innervation to the head and face and efferent motor innervation to the muscles of mastication as it branches into the ophthalmic, maxillary, and mandibular nerves.5 The ophthalmic, maxillary, and mandibular nerves supply sensory innervation to the eye and surrounding skin, sensory innervation to the upper jaw, and sensory innervation to the lower jaw and motor innervation to the muscles of mastication, respectively.5 The mandibular branch of the trigeminal nerve also gives rise to the tensor veli palatini nerve, which innervates the tensor veli palatini muscle (TVPM).2 During contraction, this muscle opens the pharyngeal orifice of the auditory tube, thereby facilitating drainage of any fluid that is present in the tympanic cavity, allowing for pressure equilibration between the tympanic cavity and the atmosphere, and protecting the nasopharynx during swallowing.2 Therefore, animals that have trigeminal nerve disease can have clinical signs such as unilateral or bilateral masticatory muscle atrophy, hypoalgesia or analgesia of the skin that the trigeminal nerve innervates, absent palpebral reflexes, reduced corneal sensation, corneal ulcers, torticollis, ipsilateral Horner syndrome and ptosis, ipsilateral hemiparesis, and clinical signs associated with otitis externa or otitis interna such as rubbing of the face.2,4,5 The duration of these clinical signs can span from a few days to several months.5 No specific signalment is overrepresented among dogs with PNSTs.5 The dog of the present report most likely had otitis media that was secondary to neurologic disease attributable to the denervation of the ipsilateral TVPM, which caused bacterial overgrowth inside the tympanic cavity.

A presumptive diagnosis of PNST can be made on the basis of the findings of an MRI examination.2 In dogs with intracranial PNSTs, MRI reveals contrast enhancement of the affected trigeminal nerve and atrophy and increased signal intensity of the ipsilateral muscles of mastication on T1-weighted and T2-weighted images, changes that are attributable to denervation of the trigeminal nerve.2,4 On T1-weighted and T2-weighted images, effusion can often be visualized in the tympanic cavity on the affected side, which looks homogeneously hyperintense, compared with the contralateral gas-filled, signal-void tympanic cavity.4 Depending on the chronicity of the disease, the tympanic cavity effusion may or may not appear as contrast enhancement of the tympanic cavity on postcontrast T1-weighted images.2,4 Acute tympanic cavity effusion consists primarily of water and will therefore not appear contrast enhanced on T1-weighted images obtained after contrast medium administration.4 Over time, this water is reabsorbed and replaced by mucus, which has a high protein content and appears as hyperintense material in the tympanic cavity on T1-weighted images.4 The MRI findings for the dog of the present report were consistent with an intracranial PNST affecting the pons and medulla oblongata. In dogs suspected to have PNSTs, electromyography and CSF sample analysis can also be performed. On electromyograms, fibrillation potentials and positive sharp waves provide evidence for denervation in the masticatory muscles of animals with PNSTs.5 Cerebrospinal fluid samples from animals with PNSTs may have a high protein content, compared with that found in CSF samples from healthy animals.5

During necropsy, the ophthalmic, maxillary, and mandibular branches of the trigeminal nerve can be traced as they exit the calvarium through the single orbital fissure, bilateral round foramina, and bilateral oval foramina, respectively.2 In dogs with a right-sided intracranial PNST, gross enlargement of the trigeminal nerve and its branches and the associated foramina (oval foramen, round foramen, and orbital fissure) is evident.2 Muscles of mastication on the ipsilateral side of the PNST may appear pale and atrophied, compared with the muscles of mastication on the contralateral side.2 In animals with a PNST that have tympanic cavity effusion, the TVPM appears atrophied grossly and the pharyngeal opening of the ipsilateral auditory tube may be more concave than that on the contralateral side.6 Histologic examination of tissue samples of the lesion provides a definitive diagnosis of PNST.4,6 Microscopically, a PNST is composed of spindle-shaped cells and neurons incorporated into interlacing bundles.5 Neoplastic cells have indistinct borders with a pale eosinophilic cytoplasm and oval or elongated nuclei with stippled chromatin.2 Microscopically, the affected TVPM myofibers are atrophied and distorted and may contain thickened bands of collagen dissecting through the muscle fascicles.2 The myofiber cytoplasm has a basophilic appearance, compared with myofibers of the contralateral TVPM.2 These microscopic findings are consistent with neurogenic atrophy.

Stereotactic radiotherapy is a safe radiation technique for treatment of dogs with intracranial tumors.6 Surgical excision of a PNST is no longer the treatment of choice because of the potential for multiple immediate postoperative complications including ventromedial strabismus, positional nystagmus, hemiparesis, seizures, and both neurotropic keratitis and keratoconjunctivitis sicca of the ipsilateral eye.5 Stereotactic radiotherapy is a successful treatment option for dogs with PNSTs. Among 8 dogs with PNSTs that underwent stereotactic radiotherapy, there were no immediate treatment-related adverse effects, and median disease-specific survival time was 745 days.6 Long-term adverse effects of treatment in those dogs included seizures (starting between days 235 and 855 after treatment), nonspecific edema in the brain, and nonclinical effusion of the tympanic bulla.6 Unlike conventional radiation protocols that involve irradiation of both neoplastic and normal tissues at low doses every day for weeks, stereotactic radiotherapy is an efficient and powerful form of radiation therapy that can be used to deliver high-fraction doses of radiation only to neoplastic cells, thus sparing normal tissues.6 With this type of therapy, patients undergo far fewer sessions of anesthesia and radiation, which is of benefit to the pets and clients; overall, stereotactic radiotherapy provides a useful alternative to fractionated radiation protocols.6

Footnotes

a.

EnteDerm Ointment, VetOne, Boise, Idaho.

b.

Tresaderm Solution, Merial LLC, Duluth, Ga.

c.

Brivio MR355, 1.5-T MRI unit, GE Healthcare, Milwaukee, Wis.

References

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