Nerve sheath tumors (NST) are rare in dogs. NSTs may be benign or malignant and originate from the periaxonal Schwann cells (schwannoma) and fibroblasts (neurofibroma/neurofibrosarcoma). NSTs tend to be more common around areas of former injury, generally in the skin and mouth, and are most often found in the docked tail of a dog. The earlier the diagnosis and the more aggressive the surgery, the better the chance for avoiding amputation of a limb. NSTs do not spread but do have a high rate of recurrence in the originally affected area.
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Diagnostic Procedures
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The diagnostic procedure includes a thorough physical examination of the dog by a veterinarian. A vet will check for lameness in the legs, peripheral nerve disorder, plumps and masses, a droopy eyelid or one-sided facial paralysis and decreased pupil size.
The vet will take a blood chemical profile, a complete blood count, urinalysis and an electrolyte panel. A computed tomography (CT) or a magnetic resonance imaging (MRI) is necessary for providing the exact extent and location of the disease.
The vet may use an ultrasound and immunohistochemical analysis to diagnose distal malignant peripheral nerve sheath tumors. Ultrasounds alone can't differentiate between peripheral nerve sheath tumors and a normal or abnormal lymph node. A doctor will use ultrasonography to project a beam of 90 degrees on the surface of the lesion and the nerve. The affected nerve will show increased echogenicity compared to other surrounding nerves.
A vet may use myelography to check the spinal cord and pinpoint the exact location of the lesion. It is most helpful when combined with a CT scan to assess the spinal cord and vertebral column. The scan will illuminate the degree of spinal cord compression and nerve root involvement
An electromyogram is a measurement of muscle activity and is beneficial because it shows abnormal muscle activity in the case of a schwannoma.
The dog will be anesthetized and a vet may take a biopsy of the tumor (guided by x-rays or ultrasound) to be tested. The dog will only be referred to an orthopedic surgeon or neurosurgeon if the spinal cord or main peripheral nerves of the legs or tail are involved.
The age of the dog, extent of the tumor growth and quality of life are taken into consideration when deciding how to proceed with treatment. Recommended treatments involve tumor removal, amputation and radiation.
Tumor Removal
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Removal of the tumor is the most common treatment for an NST. Depending on the size and location of the tumor, an organ may have to be removed along with the tumor. Amputation of a leg or tail may occur. If the tumor or the area around the tumor is not safe to remove, radiation along with surgery is necessary.
A spine operation called a laminectomy removes the portion of the vertebral bone that is necessary when dealing with a schwannoma involving the nerve roots. Radiotherapy may be required depending on the size of the tumor and its location.
The causes for amputation are usually failure to catch the tumor in its early stages, and the high likelihood of recurrence; 72 percent of dogs with a malignant NST have a recurrence of the tumor. The closer the tumor's location to the paw, the greater the chance of complete recovery in the dog.
Follow-up Radiation and Prognosis
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When NSTs occur in the limbs, removing the entire tumor through surgery in this area can be difficult. The limbs provide very little skin and subcutaneous tissue to work with, and a great variety of vital structures (like blood vessels, nerves, tendons and ligaments) are involved.
A veterinarian will typically perform the surgery to remove as much of the growth as possible and then follow up with radiation therapy to ensure that any remaining microscopic tumor cells are destroyed.
Recurrence of the tumor can occur as early as two to five months after surgery, and dogs can be re-operated on, have an affected limb amputated or be euthanized depending on the severity of recurrence. Some dogs go into complete remission.
The survival time for dogs with malignant peripheral nerve sheath tumors is two years. Dogs diagnosed with benign peripheral nerve sheath tumors have an excellent prognosis for enjoying a long, natural lifespan.
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