NEUROMUSCULAR CASE OF THE MONTH - APRIL 2006

Malignant peripheral nerve sheath tumor in a 5 year old MC Standard
Poodle.
Contributed by Dr.Beatrix Nanai and Dr. Ron Lyman
Animal Emergency & Referral Center
Ft.Pierce, FL 34982

Clinical History
The dog presented with a 10 day history of left thoracic limb lameness.

Physical Examination
On initial presentation the dog showed left thoracic limb lower motor neuron paraparesis, although the dog was weight bearing and could advance the limb. Severe atrophy of the infraspinatus and supraspinatus muscles was noted with the remainder of the musculature normal in appearance.  There was moderate caudal cervical hyperesthesia.  No mass or swelling could be palpated in the axillary area.

Diagnostic Testing

Myelogram – Very mild ventral extradural compressions at C5-6, C6-7

Electromyography – Positive sharp waves and fibrillation potentials focally in the left supraspinatus and infraspinatus muscles (Fig. 1A).  All other muscles evaluated were normal. The motor nerve conduction velocity could not be measured on the left thoracic limb.

MRI – Hyperintensity on T2 at the level of C7 showing an enlarged nerve root (Fig. 1B)

Assesment

A peripheral nerve sheath tumor was suspected. The owner declined amputation at that time and treatment with gabapentin (10 mg/kg TID), piroxicam (0.3 mg/kg SID), and omeprazole (1 mg/kg SID) was initiated.

Second Presentation 3 weeks later

The dog was represented 3 weeks later and was non-weight bearing (Fig. 1C).  Muscle atrophy was more pronounced (Fig. 1D) and affected all of the left thoracic limb muscles including the biceps and triceps muscles. The dog was unable to advance the limb. There was still no evidence of swelling or pain in the axillary region. At this time the owner requested amputation and hemilaminectomy.

Clinical Outcome The left thoracic limb including the scapula was amputated and a hemilaminectomy performed at C6-7.The C7 nerve root appeared of appropriate size within the spinal canal but was thick and firm directly outside the spinal canal. Histopathology at the time of amputation showed active denervation within the muscle biopsies and severe nerve fiber loss, cellular infiltration, and foci of lymphocytic infiltration within the subscapular nerve (Fig. 2A,B). Differentials at this time included malignant peripheral nerve sheath tumor or, due to the amount of lymphocytic infiltration, a hypertrophic neuritis. A second opinion at that time confirmed a malignant peripheral nerve sheath tumor (Dr. Brian Summers). Most peripheral nervous system tumors in the dog are malignant and undifferentiated, and are best designated malignant peripheral nerve sheath tumors without specification of the tumor type. These tumors usually have a bad prognosis as they commonly extend into the vertebral canal or the thorax. Approximately 3 months after surgery the dog developed a left Horner’s syndrome and cervical hyperesthesia. One month later, a large mass was evident at the former surgery site. The dog was euthanized.

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