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SouthPaws Veterinary Referral Center
6136 Brandon Avenue
Springfield, Virginia 22150
Tel: (703) 569-0300
Fax: (703) 866-4962


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Summer/Fall 1998 Newsletter
Peripheral Nerve Sheath Tumors
Dan Brehm, VMD (Surgery)
Betsy Dayrell-Hart, DVM (Neurology)

One of the more difficult diagnostic challenges you can face in veterinary practice is the peripheral nerve sheath tumor (PNST). These tumors are derived from the cells surrounding the axons of peripheral nerves. They have been known by a variety of names, including neurofibromas, neurofibrosarcomas, and schwannomas, but are now referred to as PNST due to their presumed common cell of origin (the Schwann cell).

What makes PNSTs so much of a diagnostic challenge is the similarity of their presenting clinical signs to many other, more common, orthopaedic disorders. The most common clinical sign associated with PNST is chronic, progressive, unilateral forelimb lameness. In a 1995 study of 51 cases of PNST, 78.4% of dogs presented with this sign. This tumor is typically seen in older dogs (average age was 8.7 years), but has been seen in dogs as young as 3 years. Because this tumor is usually seen in older dogs, it is very common to misinterpret it as elbow or shoulder arthritis, two conditions also commonly seen in older dogs. Pain on palpation near the tumor site was seen in 48.7% of dogs. Since most of these tumors grown in the axillary region, it is again common to mistake the pain as derived from the shoulder. Only 37% of dogs had a palpable mass (these are typically found only on deep palpation of the axillary region, which sometimes requires sedation for efficient palpation). One sign that is commonly seen with PNST and less commonly seen with chronic arthritis is muscle atrophy. About 93% of dogs with PNST had moderate to severe muscle atrophy of the affected limb.

Diagnosis of PNST often requires multiple tools. The first step is to take radiographs to look for obvious orthopaedic abnormalities, such as fractures or bone tumors. If the radiographs do not fit the clinical picture, we recommend electromyography (EMG). In the 1995 study, 100% of cases in which an EMG was performed demonstrated abnormal, spontaneous electrical activity, indicative of neurological compromise to the muscles of the affected limb. Electromyography is not specific for PNST, but is is a sensitive test to help differentiate between denervation and disuse muscle atrophy (usually associated with neurological and orthopaedic causes of lameness, respectively). If an EMG is abnormal, additional tests are necessary to help better define the nature and scope of the probably tumor. Myelography can be very useful to identify extension of the tumor into the spinal canal At SouthPaws, we have been utilizing MR imaging more frequently when dealing with PNST. This test costs about the same as a myelogram, and has been very good at demonstrating both intra-spinal extension of PNSTs as well as better defining the peripheral limits of the tumor. We have been very excited about the results from MR scans and about the benefits they provide when planning the surgical approaches used to resect the tumor. The best hope we have to successfully treat PNSTs is early, accurate diagnosis, which electromyography and magnetic resonance imaging can assist in providing. Both electromyography and magnetic resonance imaging (as well as myelography) are available through the SouthPaws neurology service.

SouthPaws has an active and interactive oncology unit, combining medical and surgical oncology services, which allows a multi-disciplinary approach to the cancer patient. It is very common to have many of the services at SouthPaws participate in the diagnosis and treatment of our cancer patients. We view this approach as vital to providing the best care possible. Our goal has always been to provide our cancer patients - as well as all of our other patients - with as long and good a quality of life as can be achieved. SouthPaws has a strong and experienced oncology team; we are, as always, ready and willing to help referring veterinarians and their patients in any way we can.