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Nerve Sheath Tumors

Description- A nerve sheath tumor is a type of tumor arising from the nervous system (nervous system neoplasm) which is primarily made up of the myelin (it forms a layer around the axon of a neuron) surrounding nerves. On the other hand, peripheral neural sheath tumors are those originating from the peripheral nervous system (it extends outside the central nervous system consisting of the brain and spinal cord).

Peripheral neural sheath lesions can be divided into both benign and malignant. The non-malignant ones can be further divided into schwannoma, neurofibromas and hemangiopericytoma. The names may be different but they presumably originate in the schwann cells surrounding the nerve axon. The malignant peripheral nerve sheath tumors (MPNSTs) are cancerous in nature. The lesions may appear as white, firm nodules. They tend to be locally aggressive. Although rare, these tumors can cause potential damage. However, peripheral nerve sheath tumors do not metastasize through the lymphatic system. Neural sheath tumors are most commonly found in aged animals. Early detection is very important for better diagnosis.

Causes- Although the etiology is unknown, they are believed to develop in areas around former injury. Normally schwan cells from which these tumors originate help in the restoration of tissues and cells damaged during injury. It is thought that during the process of repair, tumorogenesis takes place. However, there is no published information supporting the fact.

Symptoms- It is very difficult to diagnose malignant peripheral nerve sheath tumors of the thoracic limb (forelimb).Clinically, most patients exhibit chronic progressive thoracic limb lameness, which cannot be distinguished from musculoskeletal lameness. A palpable axillary mass is seen in some of the cases.

Normally the clinical signs include severe, unexplained, intractable pain, chronic, progressive forelimb lameness and muscle atrophy, lameness in the hind limbs, monoparesis (partial loss of movement of one extremity), ataxia (lack of coordination of muscle movements) and proprioceptive deficits (condition in which the dog is not aware of its movement and posture), peripheral nerve disorder (from self-mutilation), palpable mass (mass can be felt by touch examination), hypotonia (condition that causes reduced muscle strength), hyporeflexia (condition caused by absence of reflexes). Horner’s syndrome (symptom caused by damage to the sympathetic nervous system) and paresis are generally caused if the spinal cord is suppressed. If the schwannoma is in the neck, only one side of the face will be affected and eyelids would be droopy.

Other symptoms include decreased pupil size and slight elevation of the lower eyelid. The reported duration of time before diagnosis has been found to be between 2-24 months.

Diagnostic techniques- The diagnostic techniques include a thorough physical examination of your dog. This comprises a blood chemical profile, a complete blood count, urinalysis and an electrolyte panel. A computed tomography (CT) or, ideally, a magnetic resonance imaging (MRI) provides the most accurate information regarding the extent and location of the disease. An electromyogram is essential because (a measurement of muscle activity) it shows abnormal muscle activity in the event of a schwannoma.

Ultrasonography and immunohistochemical analysis are important for diagnosis of distal malignant peripheral nerve sheath tumors. The tumor characteristics are normally hypoechoic to mixed echogenic. Ultrasonography alone is not reliable in differentiating peripheral nerve sheath tumors from a normal or abnormal lymph node. Therefore, the ideal method of assessment is to identify a nerve associated with the malignant peripheral neural sheath tumor. For doing this, doctors take the help of ultrasonography only, but with a difference. Doctors project a beam of 90 degrees on the surface of the lesion and the nerve. The affected nerve will show increased echogenecity compared to other surrounding nerves. Myelography is important as it helps in evaluating the entire spinal cord and also determines the anatomical location of the lesion. However, it is most beneficial when combined with computed tomography because it helps in assessing the whole spinal cord and vertebral column. It can also help in understanding the degree of spinal cord compression and nerve root involvement.

Treatment- Surgical extirpation of the tumor is the treatment of choice for peripheral nerve sheath tumors. Amputation becomes inevitable at times. Local recurrence post surgery is common. A laminectomy (it is a spine operation to remove the portion of the vertebral bone) is indicated with a schwannoma involving the nerve roots. Radiotherapy may be beneficial depending on the size of the tumor and its location.

Prognosis- Malignant peripheral nerve sheath tumors usually have a guarded prognosis because in at least 72% of cases, the disease recurs after surgery. Since these lesions are not detected early, the limbs have to be amputated in most of the cases. The median survival time for dogs with malignant peripheral nerve sheath tumors is 2 years. More closer the tumor is to the paw, greater are the chances of recovery. However, reports suggest that benign peripheral nerve sheath tumors have an excellent prognosis. 

References

Pet MD

Biomed experts

Nerve sheath tumors in the dog- Bradley RL, Withrow SJ, Snyder SP

Tumours Involving the Nerve Sheaths of the Forelimb in Dogs- Targett MP, Dyce J, Houlton JEF

Small Animal Clinical Oncology- Withrow Stephen J, and David M. Vail

Cancer in Dogs and Cats: Medical and Surgical Management- Morrison Wallace B

Tumors in Domestic Animals- Donald J Meuten