Abstract

Removing brain masses is a team approach involving a wide spectrum of personnel, facilities and instrumentation. Indications are trauma, neoplasia, histological sampling. The aims are curative, debulking, extending life and quality of life. There are some species differences comparing dogs to cats. Intensive postoperative care is needed if this surgery is to be attempted.

Introduction

Affected animals are usually middle aged (median 9 years for dogs and 10 years for cats). Canine and feline patients with meningiomas tend to be older when diagnosed as compared to other neoplasms. No sex predilection for dogs, though male cats are more prone to meningiomas than females. Golden Retrievers and Boxers are more prone than other breeds. Dolichocephalic breeds are more likely to develop meningiomas, brachycephalic more prone to gliomas. Primary tumors are more likely in the cerebrum than the brainstem and cerebellum. Up to 23% of dogs with primary brain tumors also have concurrent unrelated neoplasia of the thoracic and abdominal cavities. All cases of brain neoplasia should therefore be intensely screened for any other signs of neoplasia.

Clinical signs can vary depending on the area of the brain involved. Forebrain masses typically present with seizuring, behavior changes, circling, head pressing, visual deficits, hemi neglect, proprioceptive placing defects or neck pain. Brainstem masses can present with altered consciousness, dysfunction in cranial nerves III–XII, obvious gait and or proprioceptive abnormalities. Cerebellar masses often present with cerebellar ataxia, intention tremors, vestibular dysfunction and menace deficits. Signs can be nonspecific.

Primary and Secondary Tumors

Primary and secondary neoplasia of the central nervous system is relatively commonly encountered in the dog and cat. Primary intracranial neoplasia can originate from the brain parenchyma, the meninges, and the ependyma as well as from vascular structures such as the choroids plexus. Secondary neoplasms involve the brain either via metastasis or direct extension from extraneural sites. Benign and malignant tumor types exist.

Meningiomas are the most commonly encountered primary brain tumor in dogs and cats, and are often benign. They are typically discreet tumors. They most commonly arise from the arachnoid layer and expand inward. They can, however, arise from the dura and pia mater. Metastasis is rare. Feline meningiomas are usually well encapsulated. Canine ones tend to infiltrate the parenchyma, making them more difficult to remove surgically.

Gliomas are primary brain tumors that are reported frequently in dogs and less frequently in cats. They originate from the support cells (astrocytes and oligodendrocytes) of the brain parenchyma. Variants include astrocytoma, oligodendroglioma and the malignant glioblastoma multiforme.

A host of other primary tumors have been infrequently reported in the dog, including choroid plexus tumors, which can be papillomas or carcinomas.

Diagnosis

Suspicion taking into consideration typical signalment. Advanced imaging modalities such as CT or MRI should be used. Certain tumor types have specific characteristic features that can help to distinguish them. Full blood work, chest radiography and abdominal ultrasound should be done routinely. No specific diagnosis can, however, be made without a cytological sample. The accuracy of predicting tumor type based on MRI findings in twenty dogs was only about 65%.

Surgery

The benefits of surgery in these cases of brain masses is to collect histological samples, curative, debulking and extending life and quality of life. A definitive diagnosis allows accurate prognostication and aids in the development of an appropriate therapy plan. Palliative treatment includes managing the peritumoral oedema and seizure activity. This is commonly affected with the use of prednisolone and anticonvulsant agents. Definitive therapy consists of surgical removal or debulking, chemotherapy and megavoltage radiation. Tumor size, location and degree of invasiveness can help the surgeon to determine if resection is viable. Intra-axial tumors are more difficult than extra-axial masses. Surgery is the treatment of choice for feline meningiomas, while it is difficult to completely resect canine meningiomas and microscopic disease is often left behind. Subtyping of canine meningiomas is possible though, which does have prognostic value. Gliomas are difficult to remove, as they are intra axial and debulking with cytological sampling is often only achieved. Choroid plexus tumors are often accessible due to the secondary hydrocephalus associated commonly with them. Ventriculoperitoneal shunting may however be preferable (with or without primary mass resection).

Surgery must be a combined team effort involving:

 Referring veterinarian

 Diagnostic imaging

 Medicine specialist

 Specialist anesthetist

 Specialist surgeon

 Hospital facilities (ICU)

 Nursing staff

 Rehabilitation

Surgical approaches include rostrotentorial, caudotentorial, transfrontal and suboccipital craniectomy or craniotomy. The surgical approach can either use one or a combination of the abovementioned surgical approaches.

Patients can be positioned in a 30 degree heads up position, and care should be taken to avoid inadvertent jugular compression. This can help manage increased intracranial pressures during and after surgery.

A craniectomy can be performed either with a high-speed burr, delineating the margins, which are then levered away from the brain with a periosteal elevator while gently freeing any dura from the bone, or via a craniotomy, which is sequentially enlarged with rongeurs.

Bipolar electrocautery can then be used to coagulate any dural vessels that are bleeding, or that will be severed, when a durectomy is performed.

The tumor may then be removed via gentle blunt and or sharp dissection with a fine Frazier suction tip with controllable degree of suction. Lint free cottonoid pads can be used to aid in dissection and separating normal from abnormal tissue, protecting normal tissue from suction. In infiltrative masses or where oedema or peritumoral haemorrhage is present, damage to normal tissue will be inevitable. Gross tumor margins can be assessed with intra operative ultrasound.

Ultrasonic aspirator can be used if available. Microsurgical or ophthalmic instrumentation is needed. Two to three power optical loupes helps with identification of tissue planes and blood vessels.

The craniotomy defect can be closed with a synthetic prosthesis (methylmethacrylate, titanium mesh) or original bone flap removed (kept moist in saline swab). The defect can however be left "open" as well. Leakage of CSF into surrounding tissue does not seem to be a clinical problem in animals. If bone flap is replaced, try to avoid using orthopedic wire, as it will interfere with future diagnostic imaging if needed.

Bone flap should be stabile within 1 month of replacement. If the dura is replaced, it thickens and attaches to bone flap and underlying brain. When skull defect is not replaced, the defect is covered with a dense connective tissue membrane within 6 weeks that completely separates cerebral cortex from muscle. After 2–3 months, the membrane is strong enough to provide adequate protection to the underlying brain tissue. Muscle is attached to this membrane, though brain not, and is separated from membrane with CSF. A nonsuppurative meningitis and encephalitis can be expected.

Analgesia should be provided and patients cared for in an ICU environment for at least 3 days postextubation. Corticosteroid and any anticonvulsant therapy use should be continued in the postoperative period. Antibiotic therapy should be considered. Serial neurological examinations should be carried out and recorded.

Complications

Complications can vary from uncontrollable brain swelling, deterioration of neurological signs, increased intracranial pressure, seizures, brain herniation, hemorrhage, and infection. Most common long-term complication reported is recurrence of primary neoplasm.

Intensive postoperative nursing care and physical therapy and rehabilitation are usually needed.

References

1.  Tobias KM, Johnstone SA. Chapter 36: Intracranial Neoplasia. In: Veterinary Surgery: Small Animal.

2.  Welch Fossum T. Chapter 42: Surgery of the Brain. In: Small Animal Surgery.

3.  Slatter. Chapter 78: Brain. Textbook of Small Animal Surgery. 3rd ed.