3.4 NEUROLOGICAL DISEASES OF OLD AGE CHRONIC 'OLD DOG' ENCEPHALITIS NEOPLASIA
Central nervous system

Brain tumours can occur at any age but the incidence is very rare in aniunder 5 years of age (Skerritt 1989). They are usually focal lesions and cause neurological signs directly related to the site at which they occur.

Localisation of brain tumours can be achieved from a complete neurological examination and the use of modern imaging techniques particomputerised tomography (CT scans) and magnetic resonance imaging (MRI). Plain radiography is rarely helpful unless the tumour involves the bony parts of the cranium causing osteolysis or new bone deposition, and the use of contrast studies (positive or negative) alone is less reliable than CT or MRI. Many brain tumours produce hot spots' that can be detected by scintigraphy and photon emission computed tomography

Meningiomas are the most common form of brain tumour in dogs and cats. In the dog these and gliomas are often locally invasive, and gliomas are particularly aggressive. Some brain tumours are accessible to surgery and, if benign and well described, may be successfully removed, e.g. meningiomas in cats, but the prognosis is guarded. Radiotherapy and chemotherapy have also been reported to lead to remission in some patients.



Spine

Spinal tumours can occur at any age but (with the exception of lymphoare most prevalent in older animals. They may cause pain or neusigns which are usually insidious in onset. Spinal tumours are uncommon in dogs but account for up to 50% of cats with spinal disease, though many of these are lymphosarcoma in young individuals.

CSF examination is sometimes helpful, but myelography is usually needed to identify the site of the lesion.

Extradural tumours are usually primary bone neoplasms and occasecondary (e.g. from a prostatic carcinoma). In cats lymphois the most common extradural tumour. Radiographic findings may include new bone deposition, bone loss or vertebral collapse. Surgery is possible for some tumours and medical treatment and/or radiotherapy for others.

Intradural neoplasms are usually either:


(1) meningiomas which in dogs occur most often in the cervical spine where they may be amenable to surgery; or
(2) nerve sheath tumours including neurofibromas and neurosarcomas which often occur in the brachial plexus and are difficult to remove surgically.


Primary intramedullary tumours are the least common. They are usually glial cell tumours or result from secondary metastatic spread.

The prognosis for patients with spinal tumours is guarded.



Peripheral nerves

Neoplasia of the brachial plexus are most prevalent in middle-aged and old animals and in dogs they are most often nerve sheath tumours, e.g. schwannomas, neuromas and neurofibromas. They are usually slow growing but are locally invasive and though they rarely spread to the lungs the prognosis is poor.

In dogs a mean age of 7.4 years has been reported for brachial plexus tumours (Sharp 1989) Most of the dogs were medium or large breeds and they all presented initially with unilateral intractable foreleg lameness or paresis with muscle atrophy and pain. The spinatus muscles over the scapular were most often involved. Over 45% had radiological or clinical evidence of spinal cord compression or invasion and often a small mass was palpable in the axilla. Homer's syndrome may occur in conjunction with these tumours in both dogs and cats.



Diagnosis

Biopsy of the tumour tissue which is hard and discoloured grey or off-white is possible during exploration of the plexus (Sharp 1989).



Treatment

In dogs with spinal cord involvement dorsal laminectomy is recommended to confirm the diagnosis and assess for surgical removal by a craniolateral approach (Sharp 1988). If the proximal border of the tumour can be identified high limb amputation with removal of local spinal nerves is the treatment of choice as local excision will usually result in severe neurolodeficits and local recurrence of the tumour.



Prognosis

Guarded. Local recurrence is common.