United Veterinary Clinic
The nerve sheath is a layer of myelin and connective tissue that surrounds and insulates fibers in the peripheral nerves. A nerve sheath tumor is an abnormal growth within the cells of this covering. Nerve sheath tumors include schwannomas, neurilemmomas, and neurofibromas. The trigeminal nerve is the most frequently affected cranial nerve. This results in unilateral atrophy of the temporalis and masseter muscles and facial dysesthesia or anesthesia. Eventually, brain-stem compression can develop.
Signalmen: 12 years old, female, castrated Labrador retriever
History: The owner noticed that dog`s head has not normal shape.
Case presenting signs: Chronic right trigeminal nerve deficit with atrophy of the temporalis and masseter muscles. Reduced facial sensation, absent palpebral reflex with normal menace response reaction and reduced right corneal sensation and enophtalmus.
Clinical examination: The overall condition of the dog was normal with normal appetite, good muscle and body condition except the right temporalis and masseter muscles. Internal body temperature 38,8 ; Normal respiratory and heart rate; Color of mucous membranes – pink; CRT – 1,5 sec.
Mentation and behavior- Normal
Gait – Normal
Cranial nerves – right trigeminal nerve deficit
There was no change in conscious proprioception and bladder function was normal.
Spinal reflexes were normal.
Neuroanatomic localization: R Trigeminal nerve
Case work-up: CBC – without changes; Biochemistry – Elevation of Liver enzymes (ASAT 69 IU/L; ALAT 90 IU/L)
Contrast MRI study of the head was performed with GE MRI 1.5 Tesla.
There is a large extra-axial T1 hypointense, T2 hyperintense tubular mass that arises at the origin of the right trigeminal nerve and extends rostrally through the trigeminal canal of the temporal bone. The right oval foramen is enlarged because of involving the mandibular branch. Atrophy and denervation of the masticatory muscles (temporalis, masseter and pterygoid muscles) is present with T1-, T2-hyperintesity, reduction of the muscle mass and replacement by fatty tissue. Post contrast images shows marked contrast enhancement of the right trigeminal nerve compared with the left (mild enhancement of the left trigeminal nerve is physiologic).
Right Trigeminal nerve sheath tumor
Intracranial peripheral nerve sheath tumors are relatively uncommon tumors in dogs. Clinically, dogs with intracranial PNST have one or more of the following clinical signs: ipsilateral masticatory muscle atrophy, loss of facial sensation, and Horner’s syndrome. Signs from intracranial brainstem compression can also occur. Radiation therapy is a commonly used modality in the treatment of intracranial PNST. Stereotactic radiation therapy (SRT) is one method used to deliver a curative dose of external beam radiation therapy. This precise and conformal treatment directly targets the radiation at the tumor with rapid dose drop-off, which allows for very high doses of radiation to be administered without increasing toxicity to adjacent normal tissues.