Progressive Head Muscle Atrophy in a Dog

History

A 9-year-old castrated golden retriever was evaluated for a 9-month history of progressive muscle atrophy of the right side of the head. The client initially noted the dog rubbing the right side of its face with his paw and against objects. In addition, the right eye appeared irritated consistent with keratitis and the right palpebral fissure appeared smaller than the left. Artificial tears (right eye q4h) and topical gentamicin ophthalmic solution (right eye q12h) were currently being applied. The client was referred initially to a veterinary ophthalmologist who in turn referred the client to a neurologist.

Physical Examination

Dramatic atrophy of the muscles of the right side of the head (masseter, temporalis, and pterygoid muscles). The right eye could not be adequately visualized because of pronounced enophthalmos and elevation of the nictitating membrane.

Neurologic Examination

• Mentation: Alert, responsive, appropriate

• Gait: Ambulatory with normal strength, no ataxia or paresis noted

• Postural reactions: Proprioceptive placing was normal in the left thoracic and left pelvic limbs but was delayed in the right thoracic and pelvic limbs.

• Cranial nerves:

  • Enophthalmos of the right eye

  • Menace response absent in the right eye, and enophthalmos obscured the right globe

  • Menace response normal in the left eye

  • Palpebral reflex present in left eye, absent in right eye

  • Atrophy of the right masticatory muscles: masseter, temporalis, pterygoid

  • Absent nasal sensation on the right, normal on the left

  • Normal ability to open and close the mouth

• Remainder of cranial nerves were normal.

• Spinal reflexes:

  • Normal myotatic reflexes in all four limbs

  • Normal perineal reflex     

  • Normal cutaneous trunci reflex

• Spinal palpation: No spinal hyperpathia noted

Lesion Localization

Right trigeminal nerve and pons

Differential Diagnoses

• Neoplasia

• Infectious or noninfectious inflammatory disease

• AV Malformation

• Intracranial arachnoid cyst

Diagnostics

• Hematology, serum chemistry panel, urinalysis, thyroid profile (total T4, free T4, and cTSH), and thoracic radiography were all normal.

• MRI of the head revealed atrophy of the muscles of the right side, as compared with the left, including the masseter, temporalis, digastricus, and pterygoid muscles. The trigeminal nerve, including all three branches, was enlarged—T2 hypointense ventrally and laterally and hyperintense dorsomedially, T1 isointense and strongly enhanced following contrast administration. The enlargement extended proximally through the associated foramina to enter the skull.

• As noted, the lesion involved all three branches of the trigeminal nerve: ophthalmic, maxillary, and mandibular. The ophthalmic branch provides general somatic afferent (GSA; sensory) fibers to the orbit. The maxillary branch is also a sensory (GSA) nerve providing sensation to the maxillary teeth, the palate, and the skin of the upper aspect of the head and face. The mandibular branch is a mixed sensory (GSA) and motor (GSE) nerve that provides motor branches to the muscles of mastication, the ventral throat, and the muscles of the palate. It also provides sensory branches to the mandibular dentition and skin of the mandible. When long tract signs (ie, postural reaction deficits in ipsilateral limbs) are present, the pons may also be affected.

Imaging Diagnosis

Trigeminal nerve sheath tumor. Lymphoma, hemorrhage, and sarcoma of the trigeminal nerve are all less likely. Neuritis can cause nerve swelling, but the size of the lesion makes this extremely unlikely.

Outcome

Unfortunately, trigeminal nerve sheath tumors tend to be poorly responsive to therapy, including surgery, radiation and/or chemotherapy. Further definitive therapy was not pursued by the owner.